NiMH battery charging

[TexMechsRobot]

I have purchased this battery and have no idea how to safely charge and discharge it! I have done some digging but the information seems to be wildly varying from C/10 rate of charge to charging at the C rating. There are also warnings about how much charge to put into it. I have several questions that I can't seem to run down answers to and would appreciate answers. Thanks in advance!

1. How much current should I use to charge the battery?
2. How long does it take?
3. How do I know when it is done?
4. Do I need to discharge it back to a safe storage level like I do with lipos?
5. What is the safe storage level?

I think that will at least get me started and help uncover anything else I'm not sure about.

 

[PsyBorg]

What charger are you using? Most nicad nimh batteyrs have a "Trickle" charger that dumps minimal amperage into them that can easily take 12+ hours. I know on my B6 clone it has a program for nimh / nicad that is a peak detector and will manage charging on its own. I set the current to 1c rate on that the same way I do my lipos and it takes the standard hour to fully charge for a total depleted state on the 1800 mah one I use in my electric starter for my nitro truck.

Nicad /nimh dont have that storage issue like lipos but they will get a memory if not fully discharged on occasion. If you only use say 500 of 1800 mah and keep cycling that way it will effectively become a 500 mah battery over time. I used to deep cycle mine about every two moths by doing a charge - discharge on a programmable charger set to 5 cycles to keep them at peak performance when I was regularly using them. I have let them set for literally years with no issues. They just need that deep cycling to "wake them up" again.

 

[TexMechsRobot]

Thanks PsyBorg. I'm using this charger.

It does have a NiMH setting and also has a cycle setting where I can tell it to run through a few full cycles. I'm working from home today so I'll give it a shot while I can keep an eye on it. Do NiMH's have a big fire risk like lithium batteries?

 

[PsyBorg]

Kind of but not as high a chance nor as violently. As long as you are not doing high rate of charge for extended periods its highly unlikely. The battery will get fairly warm so don't worry too much. If it gets to where you want to pull away then its time to stop charging. Most chargers run their programs defaulted two hours max. You should not be more then 40 - 60 minutes on a "Conditioned" battery newer or batteries that have sat can act differently. Usually when I have one that has sat for long term it will only take a few hundred mah the first time and then gradually build back up to near full capacity in that 5 cycle charge / discharge.

That charger should have no problems. Set the charge current to 1c like normal lipo charging and let it do its thing. It should auto sense the number of cells and adjust the voltage automatically. Just look at the status when you press start to make sure it matches the pack voltage. Other then that keep an eye on heat but like I said they can get quite warm when charging a fully depleted pack.

 

[TexMechsRobot]

Yeah I started cycling today and it takes a while. I had to remove the charge timer default because it timed out the first time. A full charge/discharge cycle takes about 4 hours. Seems safe enough to me. It got warm on the charge but I wouldn't call it hot by any means.

 

[PsyBorg]

A single charge is taking 4 hours? Did you set the charge rate to 1c or do you mean the full 5 cycle charge / discharge process took 4 hours?

How did the mah removed vs the mah put back in compare on the cycles?

 

[TexMechsRobot]

I only got through 1 complete cycle and I have 2 more running now. It's charging at 400mAh and discharging at 100mAh. I don't have the ability to change those numbers using the auto cycle function on my charger. The full process charge/discharge took about 4 hours.

 

[TexMechsRobot]

It went through 2 full charge/discharge cycles yesterday afternoon where I was able to record the numbers.

Cycle 1: Charge 344mAh Discharge 295mAh
Cycle 2: Charge 350mAh Discharge 299mAh

I found it strange that it would charge almost 350mAh for a 300mAh capacity battery and still only discharge 300mAh. Is the extra 50mAh just lost energy in heat?

Seems like I've been able to successfully learn how to charge the battery though and that was the goal. Thanks for your help PsyBorg!

 

[PsyBorg]

That happens because you charge at a higher rate and Nicad /nimah are not as accurate in capacity as a lipo. It is also due to the charger charging higher current but draining at a lower current. You will find that in real use that is much closer because of a higher current faster draining. There is also the one minute resting in between the cycles so some recovery will happen. Also I would be wary since you removed a safety feature with the time outs. I would extend them out but not let it run free specially if you are charging while you are sleeping.

But glad it all worked out and you now know how things work. Enjoy the hobby more with the new found knowledge.