NiMh charge rates?

[Kirker]

I have read that you should charge NiMh batteries at half the amp rate as NiCds the FIRST time you charge them. After that you can charge NiMhs at the same amp rate as NiCds. Yesterday I read that you should charge NiMhs at half the amp rate ALWAYS. What gives?

:confused:

[evst 360]

I think you should charge them at a low charge rate (1-2 amps) for the first charges, and then go by the mah rating. Whern you go by the mah rating, divide the capacity by 1000. For example, a 3300 should be charged at 3.3 amps, and a 3800 can be charged at 3.8 amps

[BoatDoc]

and i'm here to contradict that!!! charge at 1/10 the capacity for the very first charge. .33 amps for 3300's, .42 amps for 4200's etc. after that, i like to charge the the capacity of the cells, or sometimes higher...3.3 amps for 3300's... you can go higher if you want to push the cells a bit. for sport running, though, charging at capacity is a good rule of thumb.

[evst 360]

and i'm here to contradict that!!! charge at 1/10 the capacity for the very first charge. .33 amps for 3300's, .42 amps for 4200's etc.
Wow, I learn something new every day. :cool:

[Kirker]

Well I suppose I split the difference, kinda. I charged my NiMh 4200s at 2.1 amps the first time. Then in theory (that I've heard) charge the NiMh 4200s at 8.4 amps from then on. However my charger has a max rate of 5 amps so thats the setting I used the second time. I think in the future I'll use a 4.2 amp setting, that seems to jive with several suggestions I've heard. See what I mean about batteries/charging theories, they're like a**holes, everybody's got one!! LOL :D

[evst 360]

Then in theory (that I've heard) charge the NiMh 4200s at 8.4 amps from then on. However my charger has a max rate of 5 amps so thats the setting I used the second time. I think in the future I'll use a 4.2 amp setting, that seems to jive with several suggestions I've heard.
8.4 amps would be a bit high for any nimh battery on the market today. I would never recommend charging any batteries above 6 amps, for battery life. So 4.2 amps is good and will not damage the cells. :cool:

[Fluid]

The newer 4200/4300 NiMH cells are far more fragile than the 3300s and 3700s we've had the last few years. They are prone to venting at relatively low temperatures and never fully recover. That is the reason to charge at no more than 1/2 amp for the very first charge. Because no pack is balanced when first assembled (and because all the cells self-discharge at different rates) if you charge at 2-4 amps the first time you will overheat the cells that peak first ejo;r you are trying to peak the rest. The result is the very real risk of causing the hotter cells to vent, permanently damaging them.

By charging at a low rate the first time (it can take 18-20 hours) you insure that the cells that peak first do not get too hot. Typically they will hit 80-90F at 1/2 amp, which is fine. Temperatures over 140-150 are dangerous to cell life, and charging them at 2-4 amps the first time is risky - but hey, they're your cells! :huh:


.

[Kirker]

Well like I said, I charged them at 2.1 amps the first time because that was the written recommandation in the charger manual so what's done is done. I can say though that the battery temps weren't anywhere near 140-150 degrees, I felt them several times. In fact if they were over 90 I'd be suprised. Plus it only took 1850 to 1900 mAh the first time to top them off (4200 packs) so that may have played a factor.

I suppose more importantly is the 4.2 amp charge rate from here on. Does this seem good to you Fluid? Others so far seem to think so. Would you agree?

:confused:

[BoatDoc]

listen to jay (fluid) when it comes to cells. he's probably forgotten more than a lot of us will ever know about FE. cell care is vital if you plan on racing. a weak pack can be the difference between last place and 1st place.

[Kirker]

Oh I'm not trying to dis' Fluid at all, or anybody else. All I meant was that I charged at 2.1 amps the first time because that's what the manual said to do, it's all I had to go on. I wouldn't do it again knowing what I know now but I didn't know that then. I don't think I damaged my cells but even if I did, it's water under the bridge at this point. That was all I was saying. I think it's probably like Fluid said at one point, cells are changing quickly and charger manuals and such are slow to catch up. At this point I'm more concerned with the right now which would be charging at capacity (4200 at 4.2 amps). BoatDoc and the other gentleman seemed to feel that was a good rate. I was just wondering if Fluid and/or any others agreed as well? :cool:

[BoatDoc]

Oh I'm not trying to dis' Fluid at all,

i know that...i was just letting you know that he has a wealth of knowlege in our hobby/sport.