24v NiMh battery question

[miracleboy]

Sorry if this is posted in the wrong section, I couldn't work out where it best fit. I have a 24v NiMh battery which I think is 20 x 1.2v D batteries connected in a pack. I haven't had this from new, it's been used before.

I have tried to charge it but it doesn't seem to be taking a full charge, the charger says it's done but when I connect it to the equipment I'm using it on I get the low battery light straight away and it won't power the equipment.

Someone said to discharge it and recharge it a few times to revive the battery. Does anyone have any suggestions on how to discharge the battery, my charger doesn't have a discharge facility. I have tried connecting it to the equipment and leaving it on for 24 hours or more but it doesn't seem to drain it.

Many thanks

 

[winston1]

Two 12volt 5 watt car bulbs wired in series will gently discharge it at just under half an amp.

 

[miracleboy]

Thanks for the quick reply. I am not that experienced with this, could you advise how I would do this?

 

[ericmark]

The old NiCad chargers came in different flavours.
1) Trickle charge at such a low rate little damage done if left on a bit too long.
2) Temperature switch in the battery switches off charger.
3) Voltage monitored this looked for the volts to peak then dip again.

With the voltage type faulty batteries would refuse to charge. Also as we went from NiCad to NiMh the amount of voltage dip reduced and many old NiCad chargers will not work with NiMh batteries.

NiMh batteries are a collection of designed rather than one. There is a trade off between how much power and how long it will store the power as a result just looking at 1620 mAh does not tell you that much.

Storage of NiIron batteries was always done discharged opposite to Lead Acid which were stored charged. New NiCad were always supplied discharged but once activated there was a problem in crystal growth could short them out if kept charged as a crystal grows it is melted out of existence as it shorts out so cell still works but if it grows when flat then there is not enough power in the charger to zap it. So once activated NiCad batteries were kept charged. With NiMh there are so many different designs it's hard to say if they should be kept charged or discharged. However the selling point for the NiMh was it did not have the chemical memory associated with the NiCad.

As a battery rather than single cell there is a problem that one cell can have crystal growth and go short circuit and with a charger of the third type (which is considered the best often called Delta V) it will sense all the other cells have charged so switch off. But as we come to use the battery the device looks for total voltage so will show it as discharged.

With single cells one does have a chance putting two cells in parallel can zap crystal growth and put enough charge into the cell for the anti reverse polarity protection to allow them to charge. But will a battery it is near impossible to bring a cell back to life. Often the cell is destroyed due to other cells in the pack reverse charging it.

So 20 x 1.2 volt = 24 volt but straight off charge they will be more like 1.3 volt per cell so around 26 volt if straight off charge you get around 24.7 volt then likely there is a cell down. I would not try discharging with a bulb as likely it will reverse charge some cells and damage them. Any discharge method should have a voltage limit to stop good cells reverse charging poor cells.