NiMH 3.0A Charge Results - Is This Concerning?

[pinem56]

I have an iSDT D2 Mk II charger and decided to charge a fully spent (was resting around 7.6v) 8.4v-7c-3300 mAH NiMH battery at 3.0A for the first time today. Previously I have charged this battery at 0.5, 1.0 and 2.0A rates (this would be the 4th time charging the battery). The only options on the charger for charging NiMH batteries, is amps and "final condition" which default is "- delta 8 mV", whatever that means (the pathetic manual provides no clue). Near the very end of the charge cycle, the voltage was reading around 10.6V. The picture below was taken about 10 secs after the charging stopped. The battery was warm but not hot (it is noted that previous charges at lower rates, the battery was cool at the end of the charge cycle). Once the battery cooled down, the resting voltage dropped to 9.9v.

Should I be concerned about the high final voltage and capacity number being significantly above 3300 mAH? Is this going to greatly reduce lifespan, or should this be expected?

 

[slick2500]

That sounds about right if you are charging at higher amps it will actually charge the cells more than at lower amperages. The final voltage per cell should be between 1.4-1.5v per cell or 9.8-10.5v total for a 7 cell nimh battery.

 

[gotnitro]

+1 ^^

Wow, an NiMh Q..

If you get a couple cells that are heating up more than the others while charging at about 80% cap, charge it at a lower rate. As mentioned, NiMh like higher charge rates. If a cell pops, it sounds like an M80 and can send shrapnel everywhere including blowing up another cell. They say they don't get memory like NiCd but I'll beg to differ lol.

Side note, are you racing in a spec class and/or have you considered converting to LiPo? NiMh has crazy volt sag in comparison.

 

[slick2500]

+1 ^^

Wow, an NiMh Q..

If you get a couple cells that are heating up more than the others while charging at about 80% cap, charge it at a lower rate. As mentioned, NiMh like higher charge rates. If a cell pops, it sounds like an M80 and can send shrapnel everywhere including blowing up another cell. They say they don't get memory like NiCd but I'll beg to differ lol.

Side note, are you racing in a spec class and/or have you considered converting to LiPo? NiMh has crazy volt sag in comparison.

I have so many dead nimh batteries because I never use them.

 

[pinem56]

+1 ^^

Wow, an NiMh Q..

If you get a couple cells that are heating up more than the others while charging at about 80% cap, charge it at a lower rate. As mentioned, NiMh like higher charge rates. If a cell pops, it sounds like an M80 and can send shrapnel everywhere including blowing up another cell. They say they don't get memory like NiCd but I'll beg to differ lol.

Side note, are you racing in a spec class and/or have you considered converting to LiPo? NiMh has crazy volt sag in comparison.

Not racing, just got the kids Granite Megas, and want to use the NiMH batteries that came with them as long as possible before possibly upgrading to LiPo. I like the lower risk nature of the NiMH batteries, and while the run time isn't great, not necessarily a bug when dealing with brushed motors/esc that are prone to overheating. I am actually thinking about actually buying a second NiMH battery for each truck, to alleviate the short run time issue.

I am also going to try to wring out as much from the stock motor/esc, and have installed fans on both, and a heatsink on the motor. I also reduced the pinion tooth count to 12t from 14t, as the current full speed on 50% throttle is still a bit too much for them. So far the motor/esc has been running pretty cool, when running them around the basement.

 

[SrC]

That sounds about right if you are charging at higher amps it will actually charge the cells more than at lower amperages. The final voltage per cell should be between 1.4-1.5v per cell or 9.8-10.5v total for a 7 cell nimh battery.

+1
___________________________

And a warm Nimpack after its charged at a near 1C rate is Normal. A Delta Peak alogorithim is used to charge Nims and Nicads. Leave it at 8, the default setting. Safest. Yeah Most Lipo chargers that also do Nims, rarely say much about charging Nimpacks. Actually there is not much to know about charging them.
But Read up on this if you want to learn more about charging nim packs before modifying the Delta Peak setting. And Best performance will always be when charged at the Rated Amps. Charging slowly at a .2 amp rate for example will provide little punch. And won't get warm at all. But the discharge rate will be flatter, the pack slightly running for longer. There is a trade off.
With your 3,300 mah Nimpack, I would charge at the full 3.3 Amp rate. (maximum) To get the best performance out of the pack.
Best to store the Nimpacks in a "discharged state" untill you want to charge and use them again. Unlike Lipos. New Nimpacks will take several cycles charged at the same rate before they offer peak performance. Don't recharge Warm recently used Nimacks. Let them cool first.

 

[pinem56]

Thanks a bunch, guys, for the information. It is truly appreciated. It has been surprisingly difficult to find answers to all my questions on charging/discharging NiMH batteries. I have read about formation charges, and doing at least 3 full charge/discharge cycles before putting them to work, but trying to find what rate to charge and discharge them at, how to store them, and all-around care, has been challenging. I think the only other item of information I am still lacking is, what is the recommended rate for discharging batteries? I have been doing 0.5A, as I figure I can't hurt anything at that rate.

So summary, as far as I understand it:

First formation charge at 0.1C (0.3A for 3300mAh pack)
Discharge/recharge at ??C/1C (??A/3.3A) at least 3 times to start.
Discharge to 1.0V per cell for storage (7.0V for 7-cell pack) [old rule-of-thumb was 0.9V, but 1.0V is safer, from what I have read]
Fully charged voltage should be 1.4V to 1.5V per cell
LVC voltage should be around 1.1V (+/- 0.1V) per cell, but it will vary significantly on electronics configuration of RC car, and how well balanced the cells are in the pack.