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[on:]

I have about 6 sets of 4 NiMh AA batteries that I use in various gadgets around the house.

All those 6 sets of batteries were working fine till about a week ago. Now, although I charge each sett completely, batteries discharge in just few minutes of use.

Few weeks ago, I was in a harry, and I mixed some half charged with totally empty batteries in the charger. Both of those batteries stopped working as before. Could it be that charger is now broken and that it screwed other good batteries?

[Reply #1 on:]

You may want to try discharging each one completely then try charging them.

How many does your charger require to be able to charge?  If you need to charge in pairs or groups of 4 and you have some fully charged and some not, then the charger may just be getting confused and not charging them all the way.

Also make sure you charge ones with the same capacity together.  I have several AA's that range from 1800 mAh up to 2600 mAh

[Reply #2 on:]

Thanks, they come in groups of 4.

How do I discharge them completely?

[Reply #3 on:]

All those 6 sets of batteries were working fine till about a week ago

NiMh batteries are pretty reliable so I am thinking you might have some problem with the charger. What's the voltage reading on the batteries? 4 NiMH should be around 5.2-5.3 V when fully charged and should be around 5.0 V during the normal discharge cycle.

You can make a makeshift charger with a regular power adapter (anything greater than 6V) with a small resistor for temporary use. This way you can test whether the problem is with the charger or the batteries. Good luck.

[Reply #4 on:]

Actually I have 2 chargers. And results are the same on both of them.

What resistor size should I use on the makeshift charger?

How do I completely discharge the batteries?

[Reply #5 on:]

NiMh batteries do not exhibit memory effects and thus do not need to be completely discharged before recharging. A resistor (has to be at least 1/2 watt otherwise it will get really hot) of 50 to 200 ohm should be a good start if you are using a wall adapter say 9-12V.

[Reply #6 on:]

am I to connect a single AA battery in a series with this resistor?

[Reply #7 on:]

You can connect as many AA's in series as you want as long as the power supply voltage is higher than the overall battery voltage. And to choose a suitable resistor this equation should help:

(Vo - n * 1.25) / I

For instance, if your power supply output is 9V and you have 4 AA's in series and you need 50mA charge current you will roughly need a resistor of (9 - 4 * 1.25) / 0.1 = 80 ohm...