Fast or slow, which is better for NiMh?

[weedle256]

I have an 8 pack of AA NiMh batteries and one of those universal smart chargers.

Is it better to charge at .9A or 1.8A?

Also, is it bad that one or two of the cells get warmer than the rest? How can I tell if they are bad? I have a DMM... they all come out about the same voltage but...

 

[radellaf]

As far as I can tell, in theory, you're fine with slow (c/10) charging, or even c/2 charging with modern cells. 1 hour might be bad in and of itself, not sure, and 15 min, yeah, sacrificing long-term life. Efficiency (mAh in/out) is lower at slow rates, not sure how c/5 compares to c/1 though.

In theory.

Chargers don't terminate charging perfectly.

Charge termination criteria are easier for a charger to sense at higher rates.

But, damage to the cell is much worse at higher rates.

So, like the MH-9000 manual says, less than c/2 (or c/3 depending on version of manual) may make a smart charger terminate late, but faster than c/1 may be harmful. At those rates just about any overcharge is really bad.

Then again, that charger I don't think has the best charge termination sensing. No good reason it shouldn't be made, like the 401FS Maha slow mode (my favorite so far), to be able to terminate charge at the right point even with a 5-6 hour charge rate.

So, my present wisdom, a good charger at as slow a rate as you'll tolerate, though nothing i think to be gained with longer than 3-5 hour rates.

I'd rather run 24 hour c/10 charges the whole cell's life than a few bad 1-hour runs.

 

[mdocod]

I totally agree with radellaf on this one. Slow is good but harder to detect "end-of-charge" indications, so there's a delicate balance. I like my Vanson for AA cells because it's about a 3-6 hour charge in most cases. It detects end-of-charge on AAA and AA cells reliably, but on larger cells, like the 10,000mah D cells that take like 20 hours, I almost have to just monitor it to know when it's done. When they've been on the charger for about 15+ hours, I often just remove the cell, and test voltage. it's it's up over 1.4 and warm to the touch, I call it good.

 

[cave dave]

I like it fast but my girlfriend likes it slow. We compromise and pump the juice at around 500mA. End of cycle detection has never been a problem.

Sorry couldn't resist 😛

 

[SilverFox]

Hello Weedle,

I will simply point out that a 16 hour 0.1C initial charge is used to evenly distribute the electrolyte within the cell. This is usually done with a brand new cell, and after a cell has been in extended storage.

After that, the battery manufacturers recommend, when your charger utilizes negative delta voltage termination, charging in the range of 0.5C - 1.0C. This charging rate gives the largest, and most reliable end of charge termination signal.

Tom