When to recharge NiMH batteries

Dave E

I power a lot of my projects with NiMH batteries. Usually a pack of 4 'AA' batteries in series. After charging, the series voltage is around 5.3 volts but I never really know what voltage level indicates the time to recharge.

My projects generally run continuously for days on a pack of 4 so the current draw is generally low and constant except for a few blinking LEDs.
There is a lot of info on other websites but most of that info is very in-depth and looks more like they are geared to people who want to build chargers or make circuits to monitor the batteries to get the last bit of juice out of them. All I want is a simple idea to follow so that I do not over drain or damage the batteries in some way.

This may be asking a lot or maybe there isn't a simple solution.
Just looking for any rules of thumb, if there are any.

Thanks for any input.

Dave

Beau Schwabe

Dave E,

I've always heard that 0.9V per cell with NiMH is considered dead.... Doing a little bit of a search helped confirm this.

See page 9 and 10 of this link....

focus.ti.com/lit/ml/slap106/slap106.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

jmalaysia

A fully charged cell measures 1.35-1.4 V (unloaded), and supplies a nominal average 1.2 V/cell during discharge, down to about 1.0-1.1 V/cell (further discharge may cause permanent damage).

NiMH·has fairly significant internal leakage,· which makes them undesireable for low current long term applications.·The self-discharge is 5-10% on the first day, and stabilizes around 0.5-1% per day.

Anyway,· recharge when the voltage drops to 4.4 volts.

Dave E

Thanks Beau and jmalaysia!
That's exactly what I was looking for.

Dave

Leon

If they are allowed to discharge too much, one might discharge more than the others and be subjected to a reverse charge, which will knacker it.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

LoopyByteloose

In general, rechargable batteries prefer to be recharged above 50% of their full charge voltage. All batteries are better stored in a fully charged state.

It is all about chemistry. When the batteries are discharged, they are at one extreme of their cycle and this extreme is hardest for them to recover from. Hence we have to apply voltage to recharge [noparse][[/noparse]move them to the other side of the chemical balance].

Unlike lead acid batteries that are known as storage cells and hold a charge for a long time, the NiMH and NiCads don't hold a charge well for more than a few months. So they require more attention even if not in use.

Another rule of thumb is that any battery - in or out of use - has a useful life of roughly 3 years. After all, there are a lot of active corrosive chemicals inside. The cooler the storage, the better as it slows the aging.

In sum, don't stock pile batteries you are not using. If you must stockpile batteries, put them in the refrigerator to slow down the chemistry and extend their useful life.

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···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 6/4/2008 6:05:16 AM GMT