Best Charge Rate For NiMH

Witterings

Enlightened
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Dec 15, 2015
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There seems to be quite a bit of conflicting information so I thought I see what people’s thoughts are here regarding the best charge rate for NiMH’s.
Some say that unless it’s at 0.5c it’ll be too low to detect when the battery’s full and others say that’s too high and a more ideal rate and should be more like 0.25 / 0.33c .. older chargers all used to be even lower giving more of a trickle charge.
Interested to see what people think and how their opinions are backed up??
 

SweD

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Oct 14, 2008
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I think you'll get as many different answers as there are answers. :)
My less than professional idea on the subject is that it largely depends on the charger, more so than the cells themselves.
If the charge current is too low for the charger to see it, then yes, a larger current would be great.
The most "concensus-like" sweet spot answers I've seen is that somewhere between 0.5C and 1C, would probably cater for both negative delta V termination, as well as being slow enough so as to not cook them.

Personally, using a C9000 from Maha, I always go for 0.5C, and let it sit out the extra 2 hours of top of charge. Considering the charger terminates on Voltage and not delta V, it's mostly a case of how much time I have to give it.
My bottom line hunch is ... I really don't think it matters much, as long as you don't go Really Really low, and use a charger that only terminates on -dV, I think it's mostly a nerd-thing that we here like to discuss for the sake of intellectual stimulation. :-D

As always, mileage will vary, and others will chime in as needed. :)

Regards,
/Dennis
 

HKJ

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Charge speed depend on termination method. with -dv/dt (or 0dv/dt) you need some current or there will not be any termination signal. The 0.5C is usually a good value, but the signal is present at lower currents.
I did test this long time ago: https://lygte-info.dk/info/batteryChargingNiMH UK.html

Another way is to use a low current timed charge, this is not a very good method, but some cheap chargers use it.

Voltage termination is tricky and usually the bet method is to terminate early and follow it with a low current timed charge (A top-off charge).

With series packs you has to use a top-off charge (At least occasionally), it is used to balance the cells.
 

archimedes

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I've found that some older cells fail to terminate properly at 0.5C on my C9000.

I haven't had problems with that since bumping up to 0.6C - 0.7C for those.
 

terjee

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Somewhere around this forum, there’s a remark from a guy that spoke to an actual eneloop engineer (iirc), about charging eneloops. Based on that, iirc, 1A for AA and .5A for AA is fine, and won’t cause excessive wear. I don’t like providing second hand info like this without a link, but I figured it worthy of mention in case anyone feels like googling and see if they can find the thread.

Also a quick comment from me, not from that thread: in addition to missing the cutoff mark due to low charge current, you can also miss it if you’re starting to charge an already fully charged battery.

This is a rather old article, and doesn’t refer specifically to modern batteries like Eneloops and “NiMH2”, but you might find it interesting:

https://www.powerstream.com/NiMH.htm
 

Witterings

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Dec 15, 2015
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A big thank you to everyone for their input and HKJ .. cheers for the link some very useful info in there!!
 
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