Re: o/t battery tender question


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Posted by D. Sherman on Friday, March 28, 2008 at 20:07:26 :

In Reply to: o/t battery tender question posted by Jim in N.Y. on Friday, March 28, 2008 at 19:36:46 :

I "tend" my spare batteries by hooking them all in parallel and connecting them to a small regulated power supply adjusted to 13.8 volts. I generally have half a dozen batteries in storage like that. I tried one of the so-called "float chargers" that's supposed to turn on and off as needed, and didn't like its unpredictable operation. At room temperature, a constant-voltage charge at 13.8 volts (2.3 volts per cell) will keep a battery well-charged without overcharging it. For long-term storage, avoiding overcharging is more important than trying to maintain 100% charge. A lead-acid battery can discharge 50% without permanent damage, so if you only float it at 70-80%, say, that will probably still give you enough juice to start the engine, and which point the battery will get fully recharged. The small power supply I use has a current limit of a couple of amps, so that if I put a completely dead battery on it, it won't be damaged. It will simply take a few days to charge up. I also keep a cheap ($3) digital voltmeter hooked up to the power supply so I can easily check the current charge state of the battery set.

Your 36 volt battery pack would need to be float-charged at 41.4 volts. Your easiest solution would be to look on ebay for a small bench-type power supply with an adjustable output voltage of up to 50 volts. The lowest-current one you find would be more than ample. If you're handy with electronics, it wouldn't be hard to build your own, but with so many used and surplus units available cheaply these days, there's no reason to. A nice plus when using a bench supply is that it usually comes with a volt and amp meter and maybe even an adjustable current limit. When shopping, if you find one that claims to go up to 40 volts, look closer and you'll probably see that it really goes to 45 or 50. High-quality power supplies are designed with considerable headroom above the rated voltage. Also, there are a lot of dual supplies that give you two independent 20 or 25 volt adjustable full scale outputs, or a +20 and -20 output. You can easily wire them in series to get 41.4 volts.

All in all, I like a simple regulated constant-voltage float rather than any gimmicky "automatic" charger that turns itself on and off. There's absolutely no good reason to let a battery repeatedly run down part way before (over?)charging it back up, nor is there any need for any special trick waveforms, pulses, and the like such as some chargers put out. A battery is inherently a DC device, and it prefers to be held at a nice steady DC voltage, that voltage being just enough to keep the chemistry where it should be, without breaking down the electrolyte.



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