woodys3b

My Feedback: (13)

I'm new to this so excuse the basic question. I am charging 3000 mah NiMH batteries with a liniar peak charger. I've read that heat is the enemy with these batteries. At just 2 amps, they get pretty warm by the time they peak and charging takes an hour and change. Am I charging too fast or can they take more? Should I stop the charge and let the pack cool down and then continue?

Thanks
Andy

RaVen1357

They are fine. You can actually charge them at 3 amps with no problems to speed up the process.

Heat during charging isn't a monstrous problem. What you should be concerned about is heat from running.

guver

My Feedback: (325)

If you are getting excess heat and using 2 amps then it is probably coming from "overchargeing" rather than the chargeing itself. If so then the heat will not spike up until the very end of charge (or overcharge)

My guess is that they are not overheating. Maybe 120 deg F max.

zaphod42

My Feedback: (3)

If your charger is intended for NICDs but not for NIMHs then it will overheat them at the end. Chargers intended for just NICDs terminate the charge when the cell voltage starts dropping after reaching the peak. NIMHs don't like that - most of your heat will build up right at the end very quickly. If your charger isn't NIMH vompatible you can get one pretty cheap or you can just keep a close eye on your batteries and remove them when the voltage gain starts leveling off. A pretty good rule of thumb about the heat is you should be able to comfortably hold the pack at its hottest point. Any more than that and you are risking damage.

woodys3b

My Feedback: (13)

I really appreciate the help guys. My charger does both Ni Cad and Ni MH. The last set of batteries I charged I dropped the amps down to 1.5 and they got just as warm at the end of the charge as they did with 3 amps. These are 3000 mah batteries and they do reach their peak setting off the beeper on my charger so I don't think I'm over charging them. They are pretty warm in my hand but not uncomfortable to hold. Should I consider treating them like 2500 mah batteries and not put a full charge on them or just let them get warm? BTW, I am really impressed with the run time on these batteries. Last electric car I had was a Frog in the late 80's. These run at least twice as long as the old NiCads.

Andy

zaphod42

My Feedback: (3)

It's fine for them to get warm at the end of the charge cycle - if you can still hold them they're OK. If you can you will want to let them cool before you run them - they make more power when they're cool. I take a cooler with me when I go race and stick them in the ice after they charge then re-peak them after they cool - I can get another 300 - 500 mah into them that way. (Make sure to put them into a zip lok bag or something so they don't get wet if you try this trick.) I charge at a 3.5 amp rate and my batteries usually last all of the racing season (every Sunday from April through October).

RC-Drift UK

It's not the charging rate that makes them charge, it's the delta cutoff voltage. You should be able to charge your NiMh's at 4-5A no problem as long as the delta is set at 0.01V per cell (whereas NiCd's are typically set to 0.03V per cell)

e.g. 4 cell pack, delta cutoff 0.04V, charge rate 1-1.5C (i.e. 3000's charge at 4.5A, 3600's charge at 5.4A) should just see your cells being luke warm at charge termination

It's discharging that really generates the high damaging temps

guver

My Feedback: (325)

I think your batts are not getting too hot at all. It all sounds normal.
Letting them cool off will make them run flat (less power) they wil be the fastest hot off the charger.

zaphod42

My Feedback: (3)

Letting them cool will make for less voltage but more current and more runtime and if you re-peak them after they cool you will get most of the extra voltage back. It will also mean less total heat in the battery at the end of the run which will help extend their life. Also, I wouldn't recommend charging NIMHs at a 5 amp rate unless they are matched packs - the cheaper stick packs will not take well to more than about 4 amps. If you can set the delta voltage then RC-Drift has the settings nailed for best termination.

Mercruisin

hi, dont mean to butt in here, but i just recently bought 7 sanyo 3600 nimh. got them home, soldered them together with a 230 watt soldering gun, charged it for a while, tried it out, ran alright but didnt think there was as much acceleration as with my little 1400 trinity pack. then i charged it again, it got pretty warm. next day i ran it, ran alright for about 30 seconds then slowed down pretty quickly. i checked the voltages of the cells, the middle one was at 0.02 v. i thought this was very strange since the rest of them were between 1.25 and 1.3. so, i took that cell out, made it a 6 cell and charged it again. it got pretty warm again running on 1 amp charge. i checked the pack voltage, it was decent, let it cool, and then tried to run it again. wouldnt even turn the motor. now i'm kinda pissed cuz they were $20 each cell (yeah, i know i got ripped off cuz i've already found them cheaper). when soldering the pack i was sure not to build up too much heat, so i dont think thats my problem. could it be my charger? its a peak charger that does nicd and nimh. its a $50 charger made by hot bodies (1-2-4 amp charge rates). if anybody has any idea as to what might be wrong, i would greatly appreciate some help. since it was quite a bit of money.

zaphod42

My Feedback: (3)

If the cells were completely dead when you got them (most likely they were) it will take several cycles of charge/discharge before they start acting right. When I get a new pack I usually try to run it right off (it will usually die in 15 or 20 seconds if it runs at all) then I charge it at a low rate (I use about 250 mA) until it's full (on a 3600 pack that will take about 14 hours). Keep an eye on the temps for the last few hours so it doesn't overheat. After that you will need to cycle it a few times before it starts taking a full charge every time and running right. Sorry to hear about the dead cell - bummer! A better charger would help - especially one that you can set the charge current and the delta voltage on. I have a Novak Millenium - I'm pretty happy with it (you can't set the delta V but other than that it's OK). There are better chargers out there (check out some of the other threads here) but a decent charger can be had used for a reasonable price (I got the Millenium for $50 off of the buy/sell forum here).

Mercruisin

thanks! i'll keep on trying. didnt want to go any further cuz i was afraid of wrecking something.

lcdaniels6

I use a slot car fan .It has a murcery switch in it .Works great for cooling your batterys or cooling off your charger.
Local slotcar track should have it.

guver

My Feedback: (325)

I think perhaps your cells were never equalised (as in the same state of charge/discharge) and sounds like the most empty one had been reversed because it was in series with some other ones. If the cell in question is not ruined already then there's 2 good ways to equalise them

1 slow charge them all as a pack at c/10 for 14-16 hours
2 discharge each cell to the same point individually and then charge them.

there could also be more ways to do it.