Dissilussioned with NiMHs

Hi there,

If your device does not work well with NiMH cells because of the lower
voltage of this cell chemistry (1.2 volts vs 1.5 volts for alkies) then
you should be able to get another device working for a decent run
time once the cells die in your primary device (GPS or whatever).
This is a test to see if they really are not working in your device OR
they are really running down all the way.

All is not lost if your device does not work on NiMH that well because
of the lower 1.2v nominal voltage however. Another solution is to
contruct your very own battery pack that contains one or two 'extra'
cells so that when you use NiMH you actually get one or two more
cells in series thereby raising the total voltage getting to the device
by just enough to make it work for a reasonable amount of time.
The idea is to get one or more battery holders and wire them in series.
This gives you two leads, plus and minus. You also make a compartment
adapter that fits into the battery compartment the way the cells normally
fit in, except instead of batteries you use maybe one or more wooden
dowels with a metal tab at each end. Mark one tab positive and one
negative, then wire your newly created battery holder to this adapter.
If your device runs on 8 cells, then with alkies that would be 8 times
1.5 volts or 12 volts, and with NiMH cells that would require 10 cells
(10 times 1.2 volts) but you might get away with 9 cells.

Another idea is to use a 12v lead acid battery and a home made battery
adapter. You can get 12v lead acid batteries that would run for
hours and hours compared to any alkaline or NiMH you could find.

FWIW, Garmin gps 60-series work well with everything, namely Alkaline, NiMH, Lithium...

My Garmin GPS 60C works very well with Nimh cells.

hunterlar3 said:

I think I'll order 8 Sanyo 2700's and give them a try. If they don't work well either...I give up.

Click to expand...

I would really recommend skipping the 2700's and trying the Sanyo Eneloop or other LSD battery instead. Buy a 4 pack and try them out straight from the package in your GPS. If they work great in that application buy 4 more and try the eight in your other device.

After that its time to invest in a decent charger. Some charges will fry a battery. I suspect you have a couple bad batteries in you collection and its bringing the whole set down. A set of eight batteries its only as good as the weakest battery.

hunterlar3 said:

In summary, you're saying that the 2 applications I'm concerned with are voltage sensitive and intollerant of the voltage drop that NiHM batteries deliver in use?

I think I'll order 8 Sanyo 2700's and give them a try. If they don't work well either...I give up.

Click to expand...

I'm with Cave Dave. Tom's testing (see his NiMH shootout) shows that Eneloops hold their voltage better than Sanyo's 2700 cells at all test rates (500mA up). If you want a better voltage go for the Eneloops, with a good charger, of course.

hunterlar3 said:

Also, GPS only serves a purpose if you can read the display.

Click to expand...

LCD is it? You know, one of those display thingees affected by cold?

MrAl said:

All is not lost if your device does not work on NiMH that well because of the lower 1.2v nominal voltage however.

Click to expand...

... because you will still be able to use the only quarter-used alkalines in other devices.

tarponbill said:

My guess is the device electronics is not designed for the lower voltages of NiMH. I found this to be the case with a camera and GPS. The problem is usually more severe with higher numbers of cells. Call the manufacturer and ask.

8x1.5(12v) is far greater than 8x1.2(9.6v) -- both my devices use 4xAA and they cut out after just a few hours use on NiMH when they would go all day with alkalines. You need to stick with devices designed for NiMH, I notice that most new cameras now spec NiMH, as well as many other devices.

The battery brand is not the issue, it's the device power supply IMHO.

Click to expand...

Please take a look at my essay on the topic. Comments welcome.

c_c

I looked at the essay, no argument with it, it still doesn't show why my older devices won't work for 1/2 the time that the battery charges say they will. It could be internal impedance differences between cell types, it could be my charger, it could be the device power supply designs, but it still doesn't work. I put a resistor load on my cells and did a discharge - charge cycle and the cells deliver what they should within tolerance. Both the older camera and GPS cut out when they should work -- I have several Fluke meters to check things out with, so the data is real. It's curious that both devices are 4 cell 6v, one AA and one AAA. I emailed Garmin, they say the GPS was not designed for rechargeables, use only alkalines. So they know what is wrong, just don't want to own up to it.

The wife's camera, it just quits, the NiMH Hybrid batteries should have about 60% power remaining when it stops working, the manufacturer couldn't be contacted. The fact that the camera can be turned off, allowing the cells to rest, and then it will run again for a short while tells me the power supplies in the camera just don't work with NiMH chemistry. After three tries I gave up, the wife was beginning to say the batteries were no good ---

The GPS particularly pi$$ed me off since I had paid a lot for it and planned on using it with rechargeables. It eats a round of AA alkaline cells after only about 10 hours run time as is. The time remaining indicator on the GPS shows about 60% for a fresh charged set of NiMH Hybrids, about what you would expect given the voltage differences from alkalines. It also seems that the device's time remaining meter reflects the design of the unit, new alkaline cells show 100%. NiMH gives about 4-5 hours run time, no where near what I would expect.

When your device doesn't work for the length of time it should with NiMH, it's not a myth. I am just going to use it like they are with alkalines until either they crap, or I decide to buy new gear. Just my bad luck.

I think the problem is the choice of NiMH cells. I have a pile of hardly-used Energizer cells which have a high internal resistance as well as very fast self-discharge. I've also had several brands of cells which went very bad with little use -- I had three sets of two which I rotated in a camera that got occasional use. A couple of years of that, and all three sets were at a fraction of normal capacity. Repeated cycling wouldn't recover them. Those cells and the Energizers sure didn't behave like the Eneloop used for my essay.

So far I've had very good luck with Eneloops. I recommend that you try just a few of them and see how well they do. I'll bet they run the GPS just fine, too, although the battery gauge will be goofy. (As you can see from the essay graphs, the discharge curves of alkaline an NiMH cells are quite different. A battery gauge intended for an alkaline cell won't read at all right when it's looking at NiMH. My older GPS receivers would typically show full for a short time, then sit at 3/4 for nearly the whole rest of the discharge period. Then they'd tank out in a blink.)

There always is the possibility that a device isn't well designed and actually requires a high cell voltage to work. But you'd find it to have a very short battery life, and the cells you'd remove would work fine in most other devices. But this is the exception and not the rule -- you won't find many devices in this category.

c_c

That's a great essay c_c. Do you think you should mention that Eneloops hold their voltage better than other NiMH cells?

And tarponbill, it sounds like your NiMH cells aren't very good. (Do you think your wife might possibly be right?) Did you take a note of the voltage under load while testing them?

TorchBoy said:

That's a great essay c_c. Do you think you should mention that Eneloops hold their voltage better than other NiMH cells? . . .

Click to expand...

Thanks. But no, I don't want to load the essay with off-topic material, and that really has nothing to do with the subject at hand. (To be honest, neither do the total energy capacity figures, which I did include. But I felt they helped explain the differences in discharge times.) Someone else (I'm ashamed to say I don't recall who) did and posted a study of NiMH self discharge, which should be of great interest to anyone interested in that topic.

I just started one of my many bad 2300 mAh Energizers discharging. I'll post the differences between it and the Eneloop at a couple of currents on this thread when I've got the data.

The Energizers don't look too bad when discharged on my Maha MH-C9000, because it measures the voltage with the load disconnected. Consequently, the effect of the high internal resistance doesn't show. I've read that later production models (mine is an early one) don't do this.

c_c

Curious_character said:

I just started one of my many bad 2300 mAh Energizers discharging. I'll post the differences between it and the Eneloop at a couple of currents on this thread when I've got the data.

Click to expand...

How long since being charged?

Curious_character said:

I've read that later production models (mine is an early one) don't do this.

Click to expand...

That's true. The later versions measure the voltage under load, and stop at 0.9V. It avoids the problem the early algorithm has that bad cells may not show up, because the set discharge current actually reduces as the cell discharges, giving an artificially high result.

TorchBoy said:

How long since being charged?

Click to expand...

The Eneloops I used in my essay rested overnight. The Energizer being tested now rested only about an hour.

That's true. The later versions measure the voltage under load, and stop at 0.9V. It avoids the problem the early algorithm has that bad cells may not show up, because the set discharge current actually reduces as the cell discharges, giving an artificially high result.

Click to expand...

On the other hand, I've also read that the newer ones use a high current pulse for discharge, and measure the voltage during the pulse. If this is true, then some cells could look considerably worse than they would at a constant current.

c_c

Most of the discharge time, NiMH cells stay at 1.2 V under a low load such as a GPS. Once they reach 1.1 V they are pretty much discharged. So I assume that your GPS shuts off at 2x 1.1 = 2.2 V. This is the case with my Garmin Geko.

Now consider that you're using your GPS with alkalines: Their discharge curve is linear (unlike NiMH), and their useful voltage range is 1.5 V down to 1.0 V which means at 2.2 V (= 2x 1.1 V) you are throwing away alkalines that still contain 20% of their energy. If your device could reasonably well discharge alkalines it would be able to use NiMHs, too.

So the fault is in the design of the equipment. I have once contacted Garmin about the battery run time of the Geko and it turned out everything is pretty much on the limit. My Geko gave me a lot of headache due to it's short battery runtime. I stopped using it and went back to the 4x AA GPS12 which not only makes better use of alkalines but works smoothly with NiMH cells, too.

I'd personally not bother and replace the equipment that doesn't run on NiMHs rather than waste time on getting a flawed electronic design to work.

Here's the result of the 2300 mAh Energizer discharge at 1 amp:



It's not as bad as I had remembered. My notes show that I measured its capacity at 500 mA to be just under 2000 mAh to a 1.0 volt end point. But then I dropped the current to 100 mA and got another 200 mAh from it. The constant offset from the Eneloop at the 1 A constant current discharge shows that it has an extra 75 milliohms of internal resistance. So it requires a lower end point voltage (by about 75 mV at 1A) to get most of the energy from it. Total energy delivered by the Eneloop was 2.10 Wh, and by the Energizer 2.15 Wh. The Eneloop is a nearly new cell. The Energizer is several years old, but I doubt that it was cycled more than 15 or so times during that period.

c_c

Curious_character said:

It's not as bad as I had remembered.

Click to expand...

Maybe you're remembering high self discharge. Can we come back in a week for another graph after it has sat idle for that time?

Edit: Actually, that discharge curve is rather close to 1.1V for much of its discharge. It wouldn't take much more drop for it to dip under, and if the digital camera (a similarly high current device) wants 1.1V per cell...

I'll try and do another test in a week.

If the offset is caused by a simple internal resistance, then the drop at 2 amps would be twice the drop at 1 amp. Digital cameras can take a lot of current, especially when running a flash.

c_c

Rather than going through a lot of alkaline cells (and for better overall performance especially in the cold weather), use Energizer E2 Lithium primary cells. They cost a good bit more up front but last a LOT longer than alkalines if the current draw is anywhere near moderate or higher levels.
Also, they are rated as 1.5V, but actually float closer to 1.7V for a lot longer than alkalines, so your devices that appear voltage sensitive will be very happy with them as a substitute. Might just try a full set of new alkalines, and lithiums, on a long weekend, and compare how long they last each, and see if it's worth the cost difference (it probably will be).

They are rated at 3 amp-hours at a 1amp current draw from the spec sheet, but I've been running a bike light at 1 amp constant-current with 4 of them for well over 3 hours in total and it's still running brightly (in fact I found it "on" in the bike bag more than once in addition to a couple of 1-hour bike rides, and camping usage, etc)... frankly I don't know when they are going to die, I bought 8 so I'd have spares that I expected to use up long ago, which are still in the package.

Anyhow considering you can't just pop to the corner store when out hunting, I'd try these out, I think you'll be VERY surprised how much better they are.

Christexan said:

Also, they are rated as 1.5V, but actually float closer to 1.7V for a lot longer than alkalines...
They are rated at 3 amp-hours at a 1amp current draw from the spec sheet, ...

Click to expand...

I quite like the graphs in the latest spec sheet, although they don't actually come out and state the capacity. In the old sheet it was 3Ah to 1.0V at (just) 200mA. But the capacity graph at different loads in the new sheet provides some really useful comparisons that make choosing a battery that much easier. I found the 1A discharge graph particularly interesting, where the voltage drops suddenly to a bit under 1.4V, but after that it just holds on, and it's not long before the alkaline voltage dips under it. Quite a different shape initially.

I second the idea of adding space for additional batteries, if it's possible for the device. I have a radio scanner that was designed like this -- 10 slots and you use 8 alks or 10 rechargeables (at the time that was NiCD -- also 1.2 volts). I also had a portable radio "boom box" that took 4 D cells and added a compartment for a fifth so I could use rechargeables. In the latter case, the added voltage wasn't really necessary but it extended the runtime nicely. Voltage of 5 rechargeables is very close to 4 alks. If your devices aren't amenable to adding extra batteries, perhaps they have a DC-in jack. In which case you could cheaply build an external pack and feed the power with a cable.

Nubo said:

. . .Voltage of 5 rechargeables is very close to 4 alks. . .

Click to expand...

From the graphs in the essay, you can see that the voltage of four rechargeables is greater than the voltage of four alkalines for most of the discharge period at the current levels I measured. At lower currents, it takes longer for the voltages to cross, but cross they do. Your scanner was poorly designed, and was causing you to pull the alkaline cells when they still had a lot of energy in them.

c_c