Best Charge Rate For NiMH

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Witterings
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Best Charge Rate For NiMH

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??

BlueSwordM
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Ideal rate is between 0,5C and 1,0C.

Going above 1,0C is not recommended for heat reasons, unless you have good passive/active cooling.

Below 0,5C means that termination can not be achieved correctly, and therefore, it actually hurts cycle life a bit more than at 0,5C. Not noticeable though.

TLDR: Optimal rates are between 0,5C and 1,0C. Going below isn’t bad, just not optimal.

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Pete7874
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BlueSwordM wrote:
TLDR: Optimal rates are between 0,5C and 1,0C. Going below isn’t bad, just not optimal.

For an Eneloop Pro (2500) that would mean charge rate of at least 1250 mA.

I don’t know too many NiMH chargers that can push more than 1000 mA. Does that make them bad?

whatsthepoint
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Use a charger designed to charge NIMH AA and AAA

In a Sky NC2500 I set the charging to 0.5A for AA and AAA NIMH.

I don't charge them in chargers made for Li-ion even if the charger specs say NIMH supported.

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thefreeman
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BlueSwordM wrote:
Ideal rate is between 0,5C and 1,0C.

Going above 1,0C is not recommended for heat reasons, unless you have good passive/active cooling.

Below 0,5C means that termination can not be achieved correctly, and therefore, it actually hurts cycle life a bit more than at 0,5C. Not noticeable though.

TLDR: Optimal rates are between 0,5C and 1,0C. Going below isn’t bad, just not optimal.

Plenty of good chargers reviewed by HJK seem to terminate properly at rate significantly lower than C/2, good chargers also have a temperature detection in combinaison, for example the opus bt-c700 seem to have no issue at 400mA (halves to 200 at the end) or 200 (100) on AA eneloop. I’m not sure this recomendation is still valid, maybe before with less sensitive chargers.

It also doesn’t matter if the charger use voltage termination like the maha C9000, lii-500 and others.

NorthernHarrier
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thefreeman wrote:
BlueSwordM wrote:
Ideal rate is between 0,5C and 1,0C.

Going above 1,0C is not recommended for heat reasons, unless you have good passive/active cooling.

Below 0,5C means that termination can not be achieved correctly, and therefore, it actually hurts cycle life a bit more than at 0,5C. Not noticeable though.

TLDR: Optimal rates are between 0,5C and 1,0C. Going below isn’t bad, just not optimal.

Plenty of good chargers reviewed by HJK seem to terminate properly at rate significantly lower than C/2, good chargers also have a temperature detection in combinaison, for example the opus bt-c700 seem to have no issue at 400mA (halves to 200 at the end) or 200 (100) on AA eneloop. I’m not sure this recomendation is still valid, maybe before with less sensitive chargers.

It also doesn’t matter if the charger use voltage termination like the maha C9000, lii-500 and others.

While it certainly is true that there are good chargers reviewed by HJK that terminate charging of NiMH cells at rates below 0.5C, there are also very good chargers that might not, in some cases. For example, I just received a MC3000 this week, which HJK clearly adjudged to be an excellent charger. However, it is noted in the HJK review that it is best to use a charging rate of around 0.5C with NiMH cells in the MC3000, as it was found that the charging of these cells might not terminate correctly at rates below 0.3C.

I think all of the basic points raised by previous posts in this thread are 100% correct, but I just wanted to add the above. I think 0.5C is a good recommendation and perfectly fine for the health of NiMH cells. The new Eneloop cells are quite robust and can easily handle that rate in a quality charger. Of course, if you are charging an older, or well-used, or lower-quality cell, you should always take special precaution to err on the side of moderation in charging current rate, and use a quality charger with appropriate safety features.

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HKJ
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A rate of 0.5C or around 1A usually works fine with NiMH and -dv/dt termination. Voltage termination is very problematic, it changes with the temperature and with the chemistry and not all cells uses the same NiMH chemistry.
As some people knowns I have designed my own charger (Requires computer control and have no display or other user interface, article will be posted when I have finished it). It has never failed on a -dv/dt charger (With 0dv/dt and a timed backup), but voltage termination (At 1.52V, ut voltage did not reach 1.48V) has failed on one battery.
When combining -dv/dt and 0dv/dt termination it is a question of time, the -dv/dt will terminate significantly faster than 0dv/dt (At low charge rate neither will work).

I did look at NiMh charging some time ago: https://lygte-info.dk/info/batteryChargingNiMH%20UK.html

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/

Enderman
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I wouldn’t go above 1C, they can get hot.