I have a number of NiMH cells that I’ve accumulated over the years, from various brands. Duracell, Rayovac,Tenergy, Eneloop, and most recently EBL. They’re all rated at 1.2v.
But what I’ve observed with charging these cells is that often charge levels that are reached once the charger deems “complete” is upwards of 1.4v (usually 1.42v~1.46v). I’m assuming that these cells are designed to push upwards of over 0.2v and that it’s normal, as in a similar case of 18650’s rated at 3.7v charged up to 4.2v.
Some questions:
Is it generally safe to let NiMH cells charge up to over 1.4v (but under 1.47v)?
Will NiMH cells last longer if charged up to less, like 1.35v?
I have noticed that will some older NiMH cells, that they’ll get very warm, especially as they near the end of the charging cycle. So I’ve gotten into the habit of checking temperature… and if the cells feel very warm, I’ll tend to pull them. Usually the charge level will range from 1.36v to 1.41v. But it does seem like older cells will finish with a higher external temperature. I don’t know if that’s a sign of aging and chemistry wear (i.e. inefficiency).
Last question: How do you know when it’s time to toss a NiMH cell? Do you just wait until it becomes a very poor performer (shorter runtimes than expected), charger can’t get above a certain voltage level, or other observable behavior?
When to toss cells ?
If you look at the cycle ageing of a NiMH cell , you’ll see that the internal resistance starts to degrade before the capacity does and ends up through the roof, while the capacity has only degraded by a reasonable amount.
So when you see poor performance in your flashlights, i.e. a high current device, it’s because the DCIR has worsened, you can relegate this cell to lower current devices and use them for a very long time there.
I’ll toss them when I put them in whatever device that I don’t want alkaleaks to ruin and notice that the voltage is too low. I have many cheapo NiMH AA/AAAs that are charged up and ready to go. But every once and awhile 1 will have really low voltage when it gets around to being used.
That hasn’t happened to any of my trusty eneloops yet.
1.4 to 1.5 volts is quite fine for NiMh cells fresh off the charger. Depending on the charger I use it is between 1.42 and 1.5 volts. ( Maha C9000 or C808M, SkyRC MC3000)
Time to pitch cells is when the internal resistance (IR) gets too high to support the device.
What charger are you using? Many have the ability to check the IR. I can quote what I get, but none of the chargers are great at measuring this. I find that over around 180 mOhms on the C9000 is pushing it for higher drain devices. But 200 is ok for low drain devices like pocket radios. Just less runtime.
Thanks! That’s great to hear that there’s no real “hard stop” point with NiMH and that when performance degrades too much for acceptable use in high drain devices, just relegate them to easy tasks like remotes (helps guard against dangerous “alkaleaks”). Thanks for the links!
Good to know. I’m using a Fenix ARE-D2 charger. It’s really great with Liion. I usually charge my NiMH with some NiMH-only chargers I amassed over the years, but just recently started trying out the Fenix. It’s supposed to be “Rated output power: 3W (Ni-MH/Ni-Cd), 8.4W (Li-ion).”
