Is this normal NiMH behavior?

Timothybil

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I have some Amazon Basics AAA NiMH cells. They are labeled Made In China with a manufacture date of June 2016. They are rated according to the cell at 800mAh. The last two times I have recharged them my Xtar VC4 has read ~925mAh for the recharge. Both times the cells showed 1.2v when charging started.

Is this a normal situation. Is the 800mAh rating just being conservative? Did I get some magic stacking of the tolerances to result in this? How accurate are the Xtar VC4 measurements?
 

kpatz

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Jan 6, 2018
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Some energy is lost to heat when charging cells, so the mAh going in will be greater than what comes out.

Also, some chargers, depending on the termination method used, may overcharge the cells slightly before terminating. This may shorten the life of the cells, depending on how much they are overcharged. Do the cells get warm at the end of the charge? Do they stay warm if kept on the charger?
 

Gauss163

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Yes, that's normal charge behavior for NiMH cells. Unlike Li-ion, NiMH charge (Coulombic) efficiency = charge_out/charge_in is not close to 100% but, rather, is typically closer to 80% for full charges.

For example, AAcycler has done some tests on Eneloop charge (and energy) efficiency. He found they that are about 100% charge efficient up to about 90% SOC, but in icharging the final 10% much energy is lost due to parasitic side reactions, resulting in the full charge efficiency dropping below 80%. Follow the link for graphs.

If your Basics are also 80% efficient then that would imply that they got about 0.8 * 925 = 740mAh. You can confirm that by doing a discharge capacity test.
 
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