# How can I measure NiMh battery capacity?



## Darkstream (Dec 2, 2004)

Hi, new on the block but I have read here before.

Is there a simple way to measure the mah capacity of NiMh batteries?

I have read articles where they were discharged against a known load, and the voltage and temperature measured every few seconds automatically. I do not have the time or equipment to do that (I have a digital voltmeter/ammeter), and most of my electrical knowledge is SO old and mostly forgotten.

It's just that I bought some new batteries and wanted to check they were charging to the stated capacity.


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## MrAl (Dec 2, 2004)

Hi Darkstream,

There's only one real way to measure capacity, and you've
pretty much got it already except you dont really have
to measure temperature.
Measure voltage with a known resistive load and you can
calculate everything from there.
If you're willing to build a constant current load you
can save some time by simply letting the cell discharge
to maybe 1.000 or 0.900 volts and then one calculation
gets you to the total charge capacity for the cell.
Most people dont feel like building this (although it's
not too much of a circuit) so they use a resistor and
measure voltage about every 30 minutes. Over a 5 hour
discharge this gives you about 10 readings from which
you can calculate the total capacity.

There's really no other way to do it.

If you're interested in the details let me know.

Take care,
Al


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## gwbaltzell (Dec 2, 2004)

While the temperature doesn't have to be measured the surrounding temp. should be controlled. I think specs. call for 72° F. And if I recall correctly cutoff voltage is 1.0
Also new cells don't reach their full capacity until they have been cycled several times (10 full cycles I think).


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## rdshores (Dec 2, 2004)

How about posting a circuit for the constant current load?


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## MrAl (Dec 2, 2004)

Hi there,

You can start with this circuit:





In this schematic, disconnect the Luxeon. This leaves
the collector of Q1 open.
Change R3 to 1.5k ohms.
Power the circuit from approx 5vdc (replace 'Battery'
in schematic or use some batteries).
Connect the cell to be tested (+) terminal to the
collector of Q1, connect the (-) terminal to ground.
Using a freshly charged cell first, turn power on and
measure current (probably around 390ma).
Turn off, then use a partially depleted cell that measures
around 0.9v when connected, and again measure the current.
To make sure it's working properly, the current should
be the same in both of these cases (around 390ma).

Connect the freshly charged battery cell and monitor
voltage and start a stopwatch or clock. As soon
as the voltage drops to 0.9v (or 1.0v if you're testing
to only 1.0v) stop the clock.
To calculate charge capacity, multiply the time in hours
times the current.

Let's say you measure 400ma and the voltage drops to
1.0v in exactly 5 hours. Multiplying 5 times 400 gives
us 2000, which means the charge capacity is 2000mAh.

The only thing you have to remember is that since charge
capacities vary widely you'll have to keep a close eye
on the voltmeter once you get past about 4 hours. If you
miss the exact time when the voltage drops, you wont know
what time period to multiply by and you'll have to charge
the cell again and repeat the test. Might be a good
idea to start a timer with a buzzer to go off after
maybe three and a half hours so you can start watching
the voltmeter.

NOTES

If you can verify the 0.1 ohm resistor is really 0.1 ohms,
you can measure the voltage across it to calculate the
current so you wont need to use a current meter.

You should replace Q1 with a general purpose NPN transistor
like 2N2222 because the transistor shown in the schematic
wont be able to handle the full power of the cell at
full charge.

If anyone builds this up and uses it please post some
notes on your results if you can...thanks.


Take care,
Al


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## Doug Owen (Dec 2, 2004)

MrAl, I think yer working too hard. How about a 317 and resistor in series with your supply the the cell under test?

If you add three diodes (two to 'doide or' the cell or the third diode) and you get protection against draining the cell down below .7 or so volts (in case you get distracted....).

Five parts, under a buck. You need the external supply, of course in either case.

Doug Owen


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## evan9162 (Dec 2, 2004)

nm...


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## MrAl (Dec 3, 2004)

Hi again,

Doug:
Yes, sounds like a good idea. Im sure most people have
at least one 500ma DC wall wart laying around /ubbthreads/images/graemlins/smile.gif

Maybe someone feels like drawing this up and posting it?

Take care,
Al


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## legtu (Dec 8, 2004)

I'm interested in this circuit. A constant current load based on the LM317. I've tried searching but can't seem to find anything that would work with less then 3 batteries. /ubbthreads/images/graemlins/help.gif

Does anyone have this kind of circuit? I have a couple of rechargeables that I'd want to test and check for capacity and load capability. I'd assume that an external power supply is needed to compensate or power-up the 317 while testing a batt.


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## jtr1962 (Dec 8, 2004)

You can use a low dropout adjustable regulator and have something that will work down to maybe 2 volts. Or use a circuit with a MOSFET and a low voltage op-amp. Some op-amps will run on less than one volt. This would be good to discharge one cell without bothering with an external power supply.


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## Doug Owen (Dec 8, 2004)

[ QUOTE ]
*legtu said:*
I'm interested in this circuit. A constant current load based on the LM317. I've tried searching but can't seem to find anything that would work with less then 3 batteries. /ubbthreads/images/graemlins/help.gif

Does anyone have this kind of circuit? I have a couple of rechargeables that I'd want to test and check for capacity and load capability. I'd assume that an external power supply is needed to compensate or power-up the 317 while testing a batt. 

[/ QUOTE ]

Yes, you need an external supply at least 3 Volts more than the cell under test. The supply, cell under test, and current source/sink all go in series (any order, watch polarities). A "wall wort" (even unregulated) will work fine.

Doug Owen


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## MrAl (Dec 8, 2004)

Hi there,

Here's the other circuit we've been talking about:





You can vary the discharge current by changing the
6 ohm resistor. The current is approximately:
I=1.23/R, with I in amps and R in ohms.

If you're going to monitor the cells almost constantly
then you dont need the three diodes. If you dont use
the diodes and you miss the end of discharge point
the cell will reverse charge and probably get damaged,
so a little caution is adviseable.

You may need a heat sink on the LM317.

Connect the circuit then measure the discharge current and 
go from there...



Take care,
Al


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## udaman (Dec 8, 2004)

[ QUOTE ]
*MrAl said:*
Hi again,

Doug:
Yes, sounds like a good idea. Im sure most people have
at least one 500ma DC wall wart laying around /ubbthreads/images/graemlins/smile.gif

Maybe someone feels like drawing this up and posting it?

Take care,
Al 

[/ QUOTE ]

Alright Doug O, MrAl, stop yer yapping and build the darned thing already (j/k /ubbthreads/images/graemlins/wink.gif )! Specify the parts with links to sources if not commonly available at the local electronics store (real one, I don't mean Radio Shack); build it yourselves, start testing cells, and post your results!

[ QUOTE ]
*jtr1962 said:*

You can use a low dropout adjustable regulator and have something that will work down to maybe 2 volts. Or use a circuit with a MOSFET and a low voltage op-amp. Some op-amps will run on less than one volt. This would be good to discharge one cell without bothering with an external power supply. 

[/ QUOTE ]

You too jtr, build one, start testing cells, post results. So with the same brand/capacity cells being tested, we should get exactly the same capacity results for each of these design models, correct /ubbthreads/images/graemlins/grin.gif ?

If they are that easy and inexpensive, all of you should be able to put one together in an hour or less, correct?


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## BatteryCharger (Dec 8, 2004)

[ QUOTE ]
*MrAl said:*
Hi there,

Here's the other circuit we've been talking about:






[/ QUOTE ]

I think I'll try building that tonight. Does the voltage of the wall wart matter much?


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## MrAl (Dec 8, 2004)

Hi there,

Uda:
I guess you could test a few too then right? /ubbthreads/images/graemlins/smile.gif

BatteryCharger:
The higher the voltage, the more power the LM317 has to
dissipate, so if you keep it down to 5vdc it might
work without a heat sink on it.
At 10v dc for example, the LM317 will need a heat sink
because it will have to dissipate almost 2 watts.
Aside from that, it doesnt really matter.

Oh yeah, the 6 ohms can be had with a 5 ohm in series
with a 1 ohm, of course, or use maybe a 6.2 ohm.

Take care,
Al


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## BatteryCharger (Dec 9, 2004)

It works! Hacked one together with an old computer power supply, a pot taken out of an old stereo, and a C clamp to hold the battery. /ubbthreads/images/graemlins/grin.gif If I can hack one together in under an hour from parts in my garage, ANYONE can make one. /ubbthreads/images/graemlins/smile.gif I'll have to wait until tomarrow to test some batteries...


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## Geheim (Dec 9, 2004)

We, well I, need pictures if you do not mind once you have it setup and working.

Chad


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## MrAl (Dec 9, 2004)

Hi there,

Oh that's great BatteryCharger...what current did
you set it for?
Would like VERY much to hear about your battery testing too,
to compare to my own results.

Geheim:
What kind of pics you need?

Take care,
Al


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## BatteryCharger (Dec 9, 2004)

Last night I tried it at 600ma for an 1800mah battery. Got 1550mah, but I don't know how long the battery has been sitting. I would say a heat sink is definately necessary, the one I used got too hot to touch. Right now I'm testing another 1800mah battery hot off the charger, at 360ma. It's going to take me about 150 hours to test all the batteries I want to test... /ubbthreads/images/graemlins/icon15.gif


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## Geheim (Dec 9, 2004)

Hi Al,


Pics of the setup. Like the one BatteryCharger made. 

Thanks,
Chad


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## BatteryCharger (Dec 9, 2004)

Just finished testing a ~2 year old 1800mah Dynacharge battery fresh off the charger. 1476mah. /ubbthreads/images/graemlins/rolleyes.gif

There's really not much to take a picture of...just a tangle of wires, some bad soldering, and a pile of electrical tape. /ubbthreads/images/graemlins/grin.gif


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## Doug Owen (Dec 10, 2004)

[ QUOTE ]
*MrAl said:*
Oh yeah, the 6 ohms can be had with a 5 ohm in series
with a 1 ohm, of course, or use maybe a 6.2 ohm.


[/ QUOTE ]

How about two 12 ohmers in parallel? Put a switch in series with one and you get a choice of 100 or 200 mA...

Doug Owen


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## Doug Owen (Dec 10, 2004)

[ QUOTE ]
*BatteryCharger said:*
It's going to take me about 150 hours to test all the batteries I want to test... /ubbthreads/images/graemlins/icon15.gif 

[/ QUOTE ]

Hey, repeat the doides and cell (holder), raise the wall wort voltage say 2 Volts per station and you can run several at once. Empty holders are OK, each stage is independent.

Doug Owen


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## Doug Owen (Dec 10, 2004)

[ QUOTE ]
*BatteryCharger said:*
There's really not much to take a picture of...just a tangle of wires, some bad soldering, and a pile of electrical tape. /ubbthreads/images/graemlins/grin.gif 

[/ QUOTE ]

Add a popsicle stick and a couple paper clips and it sounds like my kinda project....

Doug Owen


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## BatteryCharger (Dec 10, 2004)

[ QUOTE ]
*Doug Owen said:*
[ QUOTE ]
*BatteryCharger said:*
It's going to take me about 150 hours to test all the batteries I want to test... /ubbthreads/images/graemlins/icon15.gif 

[/ QUOTE ]

Hey, repeat the doides and cell (holder), raise the wall wort voltage say 2 Volts per station and you can run several at once. Empty holders are OK, each stage is independent.

Doug Owen 

[/ QUOTE ]

Yeah, I think I'm going to make a 4 battery version so I can test a whole set at once. So far all of my 1800mah Duracell's have been around 1300-1500mah. /ubbthreads/images/graemlins/rolleyes.gif


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## rdshores (Dec 10, 2004)

In the great battery shootout HERE four batteries were hooked in series for the test. Couldn't you do this with the Lm317 and resistor as the load. Leave out the wall wart altogether. Put a 1 ohm resistor in series to measure the actual current load.


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## rdshores (Dec 10, 2004)

Here is the simple circuit I have been using to test my batteries. 
Measure the voltage across the 1ohm for the current.


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## BatteryCharger (Dec 11, 2004)

[ QUOTE ]
*rdshores said:*
In the great battery shootout HERE four batteries were hooked in series for the test. Couldn't you do this with the Lm317 and resistor as the load. Leave out the wall wart altogether. Put a 1 ohm resistor in series to measure the actual current load. 

[/ QUOTE ]

The point here is to test them individually.


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## jtr1962 (Dec 11, 2004)

[ QUOTE ]
*BatteryCharger said:*
The point here is to test them individually. 

[/ QUOTE ]

And you can do that by testing a few at a time. Here's my setup where I test 8 cells at a time with no external power supply:






I use an 0.2C discharge current set via those BCD switches and resistors on the far upper right. The other black thing next to it is an LM317 with a heat sink configured as a constant current regulator. Since I have yet to find a cell that didn't make it to at least 70% rated capacity so I can leave the setup unattended for the first 3.5 hours at least (usually the first 4 once I get the first capacity reading). When this first 3.5 to 4 hours is up I check the voltages of each cell. Those that are around 1.1 volts are maybe 5 to 10 minutes from reaching the cutoff point of one volt and need to be checked more frequently than the rest. When a cell reaches maybe 1.02 volts it's within a minute of cutoff and I concentrate solely on that cell and whichever others are also around 1.02. When the cell reaches 1.000 volts I note the time from my stopwatch to the hour, minute, and nearest 10 seconds (I'm not anal enough to worry about anything less than that), quickly remove the cell so I don't deep discharge it, and replace it with a fully charged cell from a batch not being tested to keep the total voltage reasonably constant. A chart I made simplifies converting my time readings to mAh. Since I can do the "quick change" manuever is maybe two seconds I don't even bother stopping my stopwatch. Cells rarely reach end of discharge at exactly the same time. The worst so far I've had happen is jumping between three cells in the same minute. Usually I have a minute or two between cells reaching end of discharge so it's fairly relaxed. Usually all the cells finish within 20 minutes of each other so this procedure doesn't take an inordinate amount of my time. With this method I measure the capacity of 8 cells in a five hour or less period to the nearest mAh. While these cells are discharging the next eight are charging for the next test. I've ran 24 2300 mAh cells through 11 cycles so far. When I reach #20 I'll post my results here.

Next up are my 50 1800 mAh cells. /ubbthreads/images/graemlins/grin.gif Since I recently purchased two electronic counters on eBay, each of which has two independent LCD counters, I plan to build a four-cell tester which has a variable discharge rate from 1 to 1999 mA, an automatic 1.000 volt discharge cutoff, and direct readouts on the counters of capacity to the nearest tenth of an mAh. More on that in a couple of weeks. I'm too busy with my taxi light project for now. /ubbthreads/images/graemlins/wink.gif


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## Doug Owen (Dec 11, 2004)

[ QUOTE ]
*BatteryCharger said:*

Yeah, I think I'm going to make a 4 battery version so I can test a whole set at once. So far all of my 1800mah Duracell's have been around 1300-1500mah. /ubbthreads/images/graemlins/rolleyes.gif 

[/ QUOTE ]

Remember, capacity will depend on (discharge) rates. Retest at a lower rate, the numbers should improve.

Doug Owen


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## BatteryCharger (Dec 11, 2004)

[ QUOTE ]
*Doug Owen said:*
[ QUOTE ]
*BatteryCharger said:*

Yeah, I think I'm going to make a 4 battery version so I can test a whole set at once. So far all of my 1800mah Duracell's have been around 1300-1500mah. /ubbthreads/images/graemlins/rolleyes.gif 

[/ QUOTE ]

Remember, capacity will depend on (discharge) rates. Retest at a lower rate, the numbers should improve.

Doug Owen 

[/ QUOTE ]

That was at a 5 hour rate, (360ma) which is what the manufacturers use, right? The first one I tested wast about 1550mah at a 3 hour rate. I need to refine my testing setup a little bit before I do anymore tests...I keep getting confused about which batteries are doing what. /ubbthreads/images/graemlins/smile.gif


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## MrAl (Dec 11, 2004)

Here's a little chart that shows what load current to use
for a given cell's published rating...
The first number is the cell capacity, in mAh's and the
second number is the required load current for the test.

1400, 280
1600, 320
1800, 360
2000, 400
2200, 440
2300, 460
2400, 480
2500, 500
2600, 520
2700, 540
2800, 560
2900, 580
3000, 600

Thus for a 2000mAh cell we would need a load current of
400ma.

One small note here:
If you're testing more than one cell in series it should
work fine as long as you're using a constant current
like the LM317 circuit we've talked about. One thing
to remember however is that each cell will reach the
cutoff point at a different time so you will have to
monitor each cell voltage independantly. If you can quickly
remove the cell once it's reached the lower voltage limit
the constant current source will adjust so that the current
still remains at the set point, but you'll need the diodes
installed for every cell as Doug O was talking about, and
you'll have to make sure you have an adequate heat sink
on the LM317.
Also, you can probably adjust the current level before
plugging in any real cells if you're using the diodes.

Some caution is required if you're going to use the idea
where you use multiple cells to create a high enough
voltage for the LM317 so you dont have to use an external
5v (or whatever) supply. As the cells run down during
the test, the voltage also runs down and may fall below
the required min for the LM317 to operate properly.
This means the current will decrease and the test will be
rendered invalid. Ensuring that there are enough cells
to keep the voltage high enough (without the ex ps) might
not be enough either, because when some cells fall below
the cutoff point you're going to want to halt the test
on these cells alone while the test on the other cells
continues. This means the voltage ultimately drops as
cells get depleted.
One way around this is to have other cells ready to plug
in to take the place of the depleted ones, but eventually
you may run out of cells so an idea would be to keep 
three cells charged that you dont have to test so you
can plug them in when needed to maintain enough voltage
for the LM317.

Take care,
Al


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## Doug Owen (Dec 12, 2004)

Also keep in mind you're also measuring how fully your charger charges before terminating.....not necessarily the max available. 

Note how the makers charge for testing.

Doug Owen


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## cmassicotte (Dec 12, 2004)

Ummmm.... Those circuts look beautiful... real works of art...

But I am clueless in terms of knowing how to read them, much less build something based on them. I have no electrical knowlege or training.

Is there a device out there that one could purchase (say from Radio Shack) that would measure capacity? I suppose you need the multimeter and this device?

Edit: Can you also measure the capacity - or charge - of regular batteries with this set up?

Thanks

Chuck


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## MrAl (Dec 12, 2004)

Hi there,

cmassicotte (Chuck):
If you're pretty sure you want to build one of these
testers i can draw you a diagram showing the individual
parts the way they appear when you actually see them
rather than in elect. schematical form. This way you
can look at the diagram and immediately know how
to connect all the parts because they'll look pretty
much like the actual real life part.
Alternately, i can supply you with drawings of all
the parts used and the equivalent schematic form
so you'll know how to connect by looking at the
schematic (like the ones shown in this thread).
There's not that much to it really, and once you
know the schematic forms you can read other
schematics too.
You'll still need to purchase the parts, so you'll have
to find a place that sells resistors and an LM317 or
order from an online distributer like Digikey.
Sound interesting to you?

You can measure the charge of regular batteries with 
this setup, but unfortunately once you do the complete
test the cell is worthless if it cant be recharged
because doing the test means running the cell all the
way down until it has no useful power left. Alkalines
for example cant be recharged.

Take care,
Al


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## Geheim (Dec 13, 2004)

Hello Al,

I have a pretty good idea on how to construct this tester, however, having a more detail layout will be helpful to me. 

Thanks,
Chad


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## BatteryCharger (Dec 13, 2004)

Here's the "idiot's guide" I made for myself to follow:


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## BatteryCharger (Dec 13, 2004)

Just finished two 2000mah Lenmar batteries - about 1700mah. Interestingly, the voltage curve on these was much less flat than my Duracells.


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## rdshores (Dec 14, 2004)

[ QUOTE ]
Just finished two 2000mah Lenmar batteries - about 1700mah. Interestingly, the voltage curve on these was much less flat than my Duracells. 


[/ QUOTE ] 
At what current draw did you test these?


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## BatteryCharger (Dec 14, 2004)

[ QUOTE ]
*rdshores said:*
[ QUOTE ]
Just finished two 2000mah Lenmar batteries - about 1700mah. Interestingly, the voltage curve on these was much less flat than my Duracells. 


[/ QUOTE ] 

At what current draw did you test these? 

[/ QUOTE ]

400ma.


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## rdshores (Dec 15, 2004)

[ QUOTE ]
*BatteryCharger said:*
[ QUOTE ]
*rdshores said:*
[ QUOTE ]
Just finished two 2000mah Lenmar batteries - about 1700mah. Interestingly, the voltage curve on these was much less flat than my Duracells. 


[/ QUOTE ] 

At what current draw did you test these? 

[/ QUOTE ]

400ma. 

[/ QUOTE ]

That seems like quite a difference from the advertised 2000mah.


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## legtu (Dec 17, 2004)

Here's a picture of my setup:






Full-sized pix

I'm using my constant-current charger as the constant-current drain in the setup. /ubbthreads/images/graemlins/smile.gif

My almost 2-yr. old semi-abused GP 1300's are down to ~1178mah and ~1047mah. Both were tested with a ~260(261.8)ma drain. I'll wait for the Panasonic 2100mah's to arrive before I continue testing. /ubbthreads/images/graemlins/grin.gif


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## BatteryCharger (Dec 17, 2004)

I refined my setup a little more yesterday.




I accidently left some batteries too long and they reverse charged - meter said about -0.45v. Charged them back up and they seem to be working ok, testing them again right now and they're up to about 1000mah. (1800mah batteries)


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## BatteryCharger (Dec 21, 2004)

I finally finished testing a few of each type of battery I have. I used a 380ma load for all 4 batteries, measuring the voltage every 15 minutes.

1800mah Dynacharge = ~1600mah
2000mah Lenmar = ~1700mah
1500mah Radioshack = ~1200mah
1600mah Ray-o-vac = ~1500mah




So, in conclusion, battery manufacturers are all a bunch of big fat liars. /ubbthreads/images/graemlins/rolleyes.gif


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## koala (Dec 21, 2004)

I don't understand... I used two 1/2watts 10ohm 1% in parallel making it ~5ohm. Wired it up according to MrAl's schematic with all the three diodes. My wallwart is a lightweight small switching power supply with an output of 5v 1A. My dmm reads 5.09volts when underload. I am getting a cell discharge current of 600ma. According to I=1.23/R I should be getting 0.246A discharge, am I missing something here?

How much current should the circuit consume from the wallwart? My wallwart outputting around the same or less amount of current the cell this varies with the resistance.


EDIT: OK CRAP I GOT LM337 instead of 317 /ubbthreads/images/graemlins/banghead.gif /ubbthreads/images/graemlins/rant.gif /ubbthreads/images/graemlins/thinking.gif /ubbthreads/images/graemlins/dedhorse.gif

- /ubbthreads/images/graemlins/confused.gif vince.


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## MrAl (Dec 21, 2004)

Hi again,

BatteryCharger:
He he, now you see what i was talking about all this time /ubbthreads/images/graemlins/smile.gif
All but one (1) cell i tested showed lower than published
ratings too, meaning NiMH cells are probably overrated for
the most part. I've still found them very useful, but
i would have been much happier if the manufacturers didnt
lie about it!
Thanks a lot for the data and graph too.

Vince:
Oh geeze, sorry to hear about that :-(
It's good that you finally found the problem however so
now you can correct it and start testing, testing, testing
/ubbthreads/images/graemlins/smile.gif
I reviewed the data sheet just to make sure the formula
I=1.23/R
was correct, and it is, so once you get the right part
it should work as expected.
You can test it starting without a cell and noting the
current through the 5 ohm resistor, then pop in a cell
and test the current again -- the current shouldnt change.
You can also remove the cell and then short D3 and the
current should be the same again.

Please let us know how it works out, and if you test
any cells how well (or bad) they worked. We'll all
compare notes to reveal the true rating of NiMH cells!


BTW, for anyone testing cells:
As Doug O was talking about, make sure your cells are
fully charged before starting the test or else you'll
get false results. If you need to insert the cell
a second time into the charger to make sure it's really
full then take that little extra time, as you'll want
to get an accurate picture of the ratings of your cells
rather than some number that really reflects the workings
of your charger instead.


Take care,
Al


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## koala (Dec 21, 2004)

Okay, I manage to steal a 317 from an existing circuit, It's late now, most shops are closed unless I rob them. Can't wait till tommorow /ubbthreads/images/graemlins/grin.gif. So it's all working very fine indeed. I am testing some 3 years old yellow Eveready NiCd. I think they are around 600mAh. The discharge current is ~250ma. Should take 1 or 2 hours. Sorry I can't test the higher cap NiMH, getting late now will do tommorow.

The way I am going choose the mA load will be abit different. Since I will be using my cells for my lights, I will test according to my lights mA consumption. For eg. an CMG Infinity Ultra draws around 200mA, EletroLumens XM-3 draws 840ma and so on.

Okay so I yank out the cell as soon as the dmm reads 1.0v? Is this accurate? since there's load voltage there. The cell voltage return to 1.2v open circuit.

Vince.


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## MrAl (Dec 21, 2004)

Hi there Vince,

Oh, if you want to test at 200ma that's ok too i guess,
but 840ma might be a little high for comparison to the
manufacturers data. If you can see your way clear to
test once at 400ma that would help. One test would
do it.

The manufacturers state 0.9v cutoff, but i think there
isnt much difference between 0.9 and 1.0, at least not
with my cells. I think i found about 5 minutes difference
between 1.0 and 0.9 volts, which doesnt change the rating
much.

Take care,
Al


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## koala (Jan 13, 2005)

Hi all,

I realise that this thread just become "How to measure battery capacity with constant current discharge". This is a fun and easy way but what about other methods?

I tried to rig up the other type where the load is a 5watt 2.3Ohm resistor connected in series with a 1.2v NiMH cell. I know through ohm's VIR law, I get around a discharge current of 522mAh regardless all the tolerance in the circuit. The cell voltage will drop together with the current so the voltage is sampled every second until the cell reaches end of discharge voltage.

To calculate the total discharged capacity, voltage is read every second then run through the algorithm below

I = V/R
so Total = Total + (I/3600)

-OR-

I = V/R
Total = Total + I
then at the end of test, Total = Total/3600

Is this correct? Or am I dreaming? I know there's maths and dv/dt to do this but it's way too long I left highschool.

-vince.


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## evan9162 (Jan 13, 2005)

If you want to sample less often than 1 second, do Average(I1, I2) * (t2-t1) (where t2 and t1 are in hours), and sum up all those calculations. Then you don't have to sample so often, or even have a regular sampling rate - though the more samples the better.


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## Doug Owen (Jan 13, 2005)

This is, in fact one of the ways NiMH cells are rated. Check the spec sheets. Often it's intended to simulate real usage, so there's a duty cycle component (the cell is allowed to rest for some part of the time).

This measures lifetime under those conditions, not capacity. Capacity continues to be defined as constant current full time discharge.

Otherwise your math is right. I'd suggest you want to take say ten readings over the discharge time (every 6 minutes if it's an hour, every half hour if it's five....), average the delivered power and credit yourself for that for the full time. NiMH tends to be pretty flat.....

Doug Owen


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## koala (Jan 13, 2005)

Thanks, you guys /ubbthreads/images/graemlins/buttrock.gif


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## MrAl (Jan 13, 2005)

Hi again,

So Vince, what have you been measuring on your cells?

Take care,
Al


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## koala (Jan 13, 2005)

MrAl - I don't quite get your question but I take it as how I am doing it. I hope I am not hijacking the thread, I try to elaborate as close to the thread topic as possible.

The problem with LM317 is component cost. Say I want to build a discharger with 4 channels(4 batteries) each with *individual* discharging settings and mode I will end with with 4x LM317 plus number of discharge mode(numerous resistors x4) and of course powersupply to run. With resistors, it's simple series connection but the work goes back in to the algorithm to calculate the current.

The set of 5watt resistors I will be using..

Min 1.0v - Max 1.4v
0.75ohm - discharge current of 1300mA - 1800mA (1500ma)
1.2 ohm - discharge current of 833mA - 1166mA (1000ma)
2.2 ohm - discharge current of 454mA - 636mA (500ma)
4.7 ohm - discharge current of 213mA - 297mA (250ma)
12 ohm - discharge current of 83mA - 116mA (100ma)

I am working on a Maxim 8 channel 12bit A/D converter with serial interface that talks to a Atmel AVR. The avr reads the maxim, and once detects the end of discharge voltage, the working mosfet will be disabled thus breaking the connection between the battery and resistor. This will help over come the over-discharging problem. I have risk ruining a couple of cells without constant monitoring.

You may think the Maxim 12bit is overkill but this will also be my NiMH charger that will be used to detect -delta in the 3-25mV range. This is out of topic.

With a microprocessor, I can also choose to use PWM and mosfets for discharging but writing the software is out of my abilities. I rather spend a few dollars getting the resistors rather than having problems debugging codes. There is another way to save cost, that is to use round robin style. There will be one set of resistors instead of 4 sets, each 4 channels will share the same set of resistors. Of course sharing means each channel will only get a slice of time for a total shared time. Therefore the discharging process of n+1 cells will take longer than 1 cell. For my way, resistors are cheap I am going to get 4 sets.

So what this does is the microprocessor will do the algorithm, calculate the total discharge current and monitor the battery voltage. Then there's a simple 16charx2lines LCD display that shows the total discharge time, total discharge capacity and average discharging current. Most importantly I don't have to manually monitor my dmm to see if we are there yet.

vince.


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## Doug Owen (Jan 14, 2005)

Impressive system. While it doesn't measure capacity (as defined) it should give a useful measure. I guess I lost the question? The math? Current is measured voltage divided by total resistance. Power is that voltage squared divided by R.

You could also rethink your circuit a mite. Make the loads emitter followers rather than FETs (cheaper even?). Now you have true current sinks (fairly stable if 'done well'). Addition of a dollars worth of op amps could improve it even more.

And you'd be measuring capacity as it's defined. With considerable style......

Doug Owen


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## MrAl (Jan 14, 2005)

Hello again,

Vince:
Actually i was asking about what you have found out
about some of your cells as to Ah capacity.
I've found most of my NiMH cells test out lower
than published rating.

Take care,
Al


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## koala (Jan 14, 2005)

[ QUOTE ]
You could also rethink your circuit a mite. Make the loads emitter followers rather than FETs (cheaper even?). Now you have true current sinks (fairly stable if 'done well'). Addition of a dollars worth of op amps could improve it even more.

[/ QUOTE ]

Doug - I know FETs can be made to discharge cells, can you elaborate what you mean by?


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## koala (Jan 14, 2005)

MrAl,

I used a Energizer(Sanyo) 1700mAh cell. This cell has been cycled numerous time, probably 15, age of cell is around 1 year old.

The cell is charged with a current of (1700ma +- 100ma) to minus delta peak then trickle charged for 2 hours. This is done via my TG1000 charger.

The cell is discharged through a 5watt 2.2ohm 5% resistor. I find it to be 2.3ohm. The voltage in mV is sampled every second from start till 900mV. The data is as below..


1 <font color="red">1344 mV</font> 14/01/2005 10:08:38 PM <font color="blue">584.3478261 ma</font><- ma comes from I = mV/2.3ohm
2 <font color="red">1344 mV</font> 14/01/2005 10:08:39 PM <font color="blue">584.3478261 ma</font>
3 <font color="red">1343 mV</font> 14/01/2005 10:08:40 PM <font color="blue">583.9130435 ma</font>
4 <font color="red">1343 mV</font> 14/01/2005 10:08:41 PM <font color="blue">583.9130435 ma</font>
.
.
.
9700 <font color="red">902 mV</font> 15/01/2005 12:50:17 AM <font color="blue">392.173913 ma</font>
9701 <font color="red">901 mV</font> 15/01/2005 12:50:18 AM <font color="blue">391.7391304 ma</font>
9702 <font color="red">901 mV</font> 15/01/2005 12:50:19 AM <font color="blue">391.7391304 ma</font>
9703 <font color="red">900 mV</font> 15/01/2005 12:50:20 AM <font color="blue">391.3043478 ma</font>

Total discharge current = <font color="blue">5035060</font>/3600 = <font color="blue">*1398.628mAh*</font>

So approximately a total of <font color="blue">1400mAh</font> for a 1700mAh cell with an average load of 500ma. With a runtime of 9703 seconds /ubbthreads/images/graemlins/grin.gif.

Here's something familiar




I'm pretty stunted by the spike near the middle... /ubbthreads/images/graemlins/thinking.gif

Tommorow, I will try discharging the same cell with my new /ubbthreads/images/graemlins/grin.gif DuraTrax IntelliPeak Ice and compare how my homebrew fair. I will also run another test with a 4.7ohm resistor. This should give around an average of 250ma discharge current. I have two of the cell, I will do a compelete test run on both of them if time permits.

-vince.


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## Doug Owen (Jan 15, 2005)

[ QUOTE ]
*koala said:*
I'm pretty stunted by the spike near the middle... /ubbthreads/images/graemlins/thinking.gif



[/ QUOTE ]

As a first guess, a (vibration or thermally induced) change in contact resistances? I bet tapping the rig with a pencil will give spikes like that. 

I'm more interested in the long flat part in front of that....that I don't think 'can happen'???

Anyway, fun stuff, thanks for sharing it.

Doug Owen


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## koala (Jan 15, 2005)

You are right regarding the contact problem. I used test clips to clip both the wire and resistor leads together. I think I may have move the mouse and hit the leads or something.

Regarding the long flat discharge curve, I didn't notice till you mention. Graphs do alot of visual representation. I checked the data. It seems to be alternating between 1221mV,1220mV then 1220mV,1291mV. Then 1218mV is the most consistent then followed by 1217mV.


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## Doug Owen (Jan 16, 2005)

Cool. I assume you meant 1219 mV not 1291? As long as the data is moving a count or two, things didn't stall, the curve must be mighty flat around there....

Thanks again for sharing the information.

Doug Owen


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## koala (Jan 16, 2005)

Yes it's 1219mV my mistake /ubbthreads/images/graemlins/grin.gif.

I did test again during the weekend with two more cells. At ~500ma draw they all end up near 1400mAh. Regarding the spikes and flat discharge curve, it's definetely a contact problem. I can't duplicate anymore discharge curve with a spike and flat period like that. So I guess it must me my mouse knocking on the circuit.

-vince


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