# Thread: NiMh Charging - What voltage?

1. ## NiMh Charging - What voltage?

I'm working on a little experiment - a gas powered flashlight. [img]/ubbthreads/images/graemlins/ooo.gif[/img] 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?

2. ## Re: NiMh Charging - What voltage?

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.

3. ## Re: NiMh Charging - What voltage?

I am going to use a model airplane engine. [img]/ubbthreads/images/graemlins/smile.gif[/img] 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?

4. ## Re: NiMh Charging - What voltage?

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?

5. ## Re: NiMh Charging - What voltage?

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.

6. ## Re: NiMh Charging - What voltage?

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?

7. ## Re: NiMh Charging - What voltage?

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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