| On 5/14/2004 6:41 AM Ted shuffled out of his cave and grunted these | great (and sometimes not so great) words of knowledge: | | > OK, I've done some searches about charging batteries and found various info | > on charging Nicad and NiMh packs. Having read all that theory I'd like to | > hear some practical information. So now for the questions: | > | > 1. Is there a simple (and safe) method for determining the max charge rate? | | A SAFE charge rate is 1C ie you have a 1200 mah battery, the safe | charge rate is 1.2 amps or less.
Most NiCds will tolerate up to 2 C, and some (like sub-C cells for cars) will do 3 C. But as mentioned, 1 C is a good rate for most NiCd cells. Faster rates WILL wear them out faster.
For NiMH, don't go over 1 C, *ever*, unless it's a sub-C cell and explicitly rated for it. 1/2 C is better.
| > 2. What's the minimum permissible voltage per cell? | | NiCAD and NiMH is .9 (point 9) volts PER CELL. LiPo is USUALLY 2.8 | volts PER CELL
NiCd and NiMH cells don't mind being discharged to zero. The danger is that if you have a battery (made of many cells) and you discharge the entire battery down to zero, that some cells will have gone down to zero before the others, causing the others to `reverse charge' them as they discharge, which is very bad. So for a pack, 0.9 volts/cell is a good guideline. For a single cell (like for a glow driver), go ahead and go down to zero if you want.
For LiPo, don't go much below 3 volts, ever. (But you didn't ask about LiPo.)
| > 3. What's the max permissible voltage per cell? | | NiCAD and NiMH is USUALLY 1.2 - 1.3 volts PER CELL about 1 hour after | being removed from the charger. LiPo will USUALLY read about 4.2 PER | CELL fresh off the charger and then drop to about 3.7 - 3.9 about 1 hour | after being taken off charge.
There is no set max voltage, but do keep the battery cool and don't exceed 2 C for NiCd or 1 C for NiMH. I doubt that any sort of NiCd/NiMH cell (unless it's really old and worn out) would ever require over 1.8 volts/cell to charge at a safe rate (and 1.8 volts/cell is a high estimate -- 1.6 volts/cell seems more reasonable.)
Fresh off the charger, NiCd and NiMH cells read around 1.4 volts for a little while.
| > 4. Is there a realistic method for determining safe discharge rates? | | DO NOT EXCEED 1C That is a SAFE charge rate. SOME manufacturers say | you can safely charge their batteries at a higher rate. IMHO, perhaps | you can, however, you are most likely shortening their life.
He said discharge, not charge.
It depends on the battery. Some won't handle over 2 C well, some can handle 30 C. For AA or AAA cells, it's probably on the low end, and for sub-C cells, it's much higher.
The best method is to ask the manufacturer, but I imagine you could measure the voltage drop as you draw more and more current and pick a rate where the drop (and therefore the heating of the battery) isn't too high. Easier to just ask, though.
Again, the faster you discharge a battery, the faster it'll wear out.
| > I'm asking specific questions for a reason but I won't bias my questions by | > spelling it out.
You'd probably be better spelling it out ...