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Do you need to fully discharge Ni-MH batteries before recharging them?

Lucas Jarvis , Jun 19, 2005; 01:38 a.m.

I've just used some Ni-MH rechargeable batteries in my flash unit, but replaced them before they were dead because the refresh was getting a little to slow for me. I'd like to charge them up, but I'm not sure if I should drain them first. Anyone know what I should do?


Jim Strutz - Anchorage, AK , Jun 19, 2005; 01:48 a.m.

It matters little if they are fully drained or not before charging. A bigger issue is not overcharging, or overheating them. On the other hand, they aren't that expensive, so if you damage a set by charging them too fast/hot it's not a big deal.

H. P. , Jun 19, 2005; 04:42 a.m.

NiMH batteries suffer less from memory effect than NiCads but they do get it. If you're a heavy user of either type it's worth getting a charger that has an auto-discharge cycle which should make the batteries last longer in use and keep them working for longer too.

Ray - , Jun 19, 2005; 04:45 a.m.

I got my batts and charger from a specialist.

The invoice said, use them fully in the device b4 charging. This will equalise the different voltage each batt may have. Then charge them.

Up to you if you decide to fully discharge before charge. If the time is avail .. why not.

But remember .. unless you got some really high quality batts and a good charger ... pick and give. The mah rating on the batts may well differ if you got a fancy batt charger and it does a mah rating test for each individual batt. If your charger has continous refresh mode - continous discharge/charge mode repeatedly then you can use this to maximise the capacity of the batt. It will stop discharge/charging when after 2 cycles there is no differ of mah rating. For care remember to use slow mode if you can select diff currents .. or perhaps your charger has temperature monitor builtin?


Ray - , Jun 19, 2005; 04:49 a.m.

If you forget does not really matter. You can equalise the voltage later on when the device is used. You can discharge later on ...

All in all does not matter. Discharge then charge each couple of cycles to treat them nice :-) Other features are nice to have ....

Roger Krueger , Jun 20, 2005; 10:40 a.m.

The invoice said, use them fully in the device b4 charging. This will equalise the different voltage each batt may have. Then charge them.

That does nothing to equalize voltage--any difference there is will be there no matter how far you discharge. If you have them in a dumb device (flashlight, etc.) and discharge them all the way you'll reverse the weakest cell and likely ruin it.

This is, however, a great way to make people wear out their batteries faster so they'll buy more.

NiMH batteries suffer less from memory effect than NiCads but they do get it.

Memory effect in both chemistries is very small. You may lose a few %, but nothing anyone but but an RC racer will notice. Dumb chargers can cause problems by overcharging batteries put in them with a partial charge, but a good charger should be fine. More iimportant is to make sure the batteries stay matched, especially since most chargers--even pretty good ones--monitor the group of batteries put in, not each one individually. Every time I've a set of batteries lose capacity it's been one cell going bad, the rest were fine.

I've found I still prefer NiCD for flashes. NiMHs dirty secret is that they deliver only 50-70% of their capacity under extreme current conditions--exactly what they get in a big flash. NiCd deliver better max current and faster refresh rates. They also survive more charge cycles before they die.

Ron Chappel , Jun 21, 2005; 05:56 a.m.

I totally agree with roger on two important points:

Memory effect is basically a fallacy.It does occour but only in extreme conditions-exactly repeated charge/discharge cycles for example.

Nicad are still the kings of current.Of course it depends on specific models/brands etc .
I even read that satelites still use Nicads because they can because of the extreme loads they can endure (but i'm not sure how true/up to the minute this is)

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