# NiMh Charging - What voltage?



## zmoz (Mar 25, 2003)

I'm working on a little experiment - a gas powered flashlight. /ubbthreads/images/graemlins/ooo.gif When charging NiMh AA batteries (2 in series) what voltage do I need to provide? And what would be the maximum current I could give for the quickest charge?


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## FreeBSDboy (Mar 25, 2003)

Well, charging NiMH batteries is not that simple. Quick charging them especially so. If you're not carefull about your charge procedures you will ruin your battiers. There are three methods for charge termination in NiMH batteries: Time, negative delta V, and temperature.

Time only works well for low charge rates (like c/10). Most quickchargers use two of the three methods (some use all three somultaniously). So, if you are planning on charging above a rate of 180ma or so, you'll need come kind of electric circuit controling the charge. If the cells are too drained, the controler needs to start at a lower charge rate, then step up to the fast charge rate (Maybe C/4-ish 0.5 amp or so maybe) Then slow down to a trickle either when the temperature gets too high, or when the batteries aproach full charge.

Must chargers are current driven, not voltage driven. You will need a power supply capable of constant current variable voltage output. If you have a constant electrical voltage from your gas (engine?) then you'll need to have some kind of regulator circuit to provide the necessary power.

Just out of curiosity, what kind of gas are you planning on driving the flashlight with? A model airplane engine? How much power do you expect to get out of one of those? 

[EDIT] you will not be able to get a charge time of less than a couple hours. Charging with gas driven engine is probably not practical over that timeframe.


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## zmoz (Mar 25, 2003)

I am going to use a model airplane engine. /ubbthreads/images/graemlins/smile.gif It is aproximately .10hp which at 100% efficiency would be able to produce 74 watts of power, or about 24 amps at 3 volts! I don't want the batteries to reach full charge, but I want them to get as charged as they can be in the 3 minute run time of the engine. (that is, 1 tank of fuel, which is 8cc) The batteries will then be used by some LEDs. So...what voltage/amperage should I provide to get the maximum charge in about 3 minutes?


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## InTheDark (Mar 25, 2003)

FreeBSDboy,
I was following your thread on building your own charger, at least for the very first page. It got way over my head by the second page. I'm trying to make the same thing you are, to charge 12 NimH AA's in series. However, I want it powered by 12V car battery. Would it be possible to use a simple step up to up the voltage to about 18V and use that to trickle charge my batteries? I'm not too worry about overcharging, since the only time it'll actually be charging is when my car is on (sort of a human timer) Or should I look into a constant current step up?


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## FreeBSDboy (Mar 25, 2003)

zmoz: I have no clue. You're talking about short term high charge currents. However, I think that any problem you do run into is going to be thermal. If you charge at too high a rate, the insides of the battery heat up to quickly and are not able to disapate the heat. The result is leaky dead batteries.

Lets say 74 watts at 100% eff. But the best you'll get is probably 50%. So 37 watts from the conversion from mechanical to electrical. Charging batteries is only 80% efficient, so really only 30 watts. At 3 volts is 10 amps. times (3min/60min) is only 500mah. So even if you could charge at a rate that high without killing the batteries, you'd still need 4 tanks of gas to charge go from empty to full.

But I think you're going to kill any NiMH or NiCD batteries with a charge rate that high anyway. Sorry.

The best you could do is probably a 1 hour charge rate. Even still that is 20 tanks of gas.

InTheDark, either PM me, start a new thread, or make a posting in my other charger thread and we can continue the discussion.


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## zmoz (Mar 25, 2003)

Again...I'm not trying to fill the batteries, just get the most I can get. 1 LED is only going to use about 20-30mah, and I only want about an hour of run time. What I'm asking here is what is the highest amperage I can _safely_ charge 2 1800mah batteries in series, and what voltage should it be at?


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## FreeBSDboy (Mar 25, 2003)

I took a look at the panasonic datasheets for their batteries as an example. They state a maximum of 1200mah not to exceed a cell temperature of 40 degrees C (113F). For whatever battieries you are using I would find the datasheet for them and see what the manufacturer states as the rapid charge rate. I would suggest not exceding that rate. That should give you about an hour of runtime per tank of fuel.

I must also caution you not to run the cells below 1V nominal voltage (2v in 2 series cells). Below that you risk reversing one of the cells.

You know, you might be better off using an ultracapacitor instead of batteries. You could also charge it faster than that small engine can provide power. It would cost more though. Check http://www.maxwell.com they have a broad product line.

Hope I could help.


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## zmoz (Mar 25, 2003)

Ok...duracell says for a fast charge, charge at "1C Rate". That means a maximum of 1.8amps for an 1800mah battery right? Is that at 3v?


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## FreeBSDboy (Mar 25, 2003)

I dunno. The voltage needed to get a 1.8amp rate will change depending on the internal resistance of the batteries, which will change depending on how charged they are. The right way to do it is with a current regulated power supply, which varies the voltage to keep the current the same.

Also, it occurs to me that 3V might not have enough overhead to charge a 2 cell pack at that rate.

Internal resistance of .5 ohm (is this right? I'm really not sure) means you would need a forward voltage in the 4-4.2V range to get that kind of charge current for two cells in series.

The best you could do with 3V is a charge current somewhere in the neighborhood of 500ma. That's assuming your power supply is limited to 3V.


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## zmoz (Mar 25, 2003)

I can get whatever voltage I want, as I will be winding the generator myself. So around 4 volts @ 1.8 amps would be good?


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## FreeBSDboy (Mar 25, 2003)

Asuming I have the internal resistance figure correct. Might want to verify that.

I'm asuming you are also running this through a bridge rectifier to get DC instead of AC from the generator? You'll also need a filter capacitor if you do it that way. And remember that you'll approximately double the output voltage after you rectify it.

Also, are you making provisions to isolate the generator from the batteries when the generator is not running? A relay might do the trick. Otherwise power will leak through the diodes in the bridge from the batteries when the generator is not turning.

You also might consider building a constant current regulator based on a LM317. It's a relatively simple circuit and it can regulate the charge current to any value you set. You'll need a higher input voltage to run it though (probably around 8Vdc)


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## itsme1234 (Mar 26, 2003)

InTheDark:

The best sollution for you is to split 12 NiMh in 2x6 or 3x4 (if you can). Then you only need one resitor for each group (can be a bulb). I have this setup and I'm very happy; to charge 4 NiMh I have 2 small bulbs (4 or 5 W, 12 W) in parallel. Two bulbs used as a resistor in this setup will give me 600+ mA from 12 V, but I have a small DC-DC converter from 12 V to 3-4.5-6-7.5-9-12 V (used for my cd player and cell phone as well). Everithing is very simple and cheap, but also very flexibile, because I can have a dumb fast charger if I need, but I can also trickle charge if I have time and if I care for my NiMh. Also I can use a wall-wart to power this "charger" (and I can use the same wall-wart to recharge my mp3 player, cell phone and palm pilot). I really hate to have more than one charger with me when I travel !


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## Frank Schwab (Mar 26, 2003)

For your design purposes, the fully-charged voltage of a Ni-MH cell is around 1.4-1.5V. However, if you're charging at a high rate, the voltage during charging will be above this. I can't tell you how much higher.

So, you might want to run a little experiment with a NiMH battery and a variable-current power supply. Set the power supply to 1.8A, and chart the voltage on the battery as it charges. If the battery gets hot to the touch, stop charging. You could try the same test with 10A (for a very short test!) to see what voltage you'd need to push current into the battery at that rate. Start your charging tests with the battery discharged (voltage < 1.2 V).

I don't recommend this method of charging, but it's kinda cool to think about. For example, those mini-RC cars apparently charge their NiMH batteries in 45 seconds! That's like an 80C charge rate. I think they are doing something similar to what you're trying to do - they aren't trying to maximize the amount of charge stored, they are simply dumping as much into the battery as they can in a very short window (nobody wants to wait an hour between sessions playing with those little cars, even if it meant that they ran twice as long), and assuming that they won't overcharge the batteries.


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## itsme1234 (Mar 26, 2003)

Please don't do this at home 
Seriously, at the very minimum you should protect your face/eyes.

[ QUOTE ]
*Frank Schwab said:*
You could try the same test with 10A (for a very short test!) to see what voltage you'd need to push current into the battery at that rate.


[/ QUOTE ]


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