My Nimh batteries only powers small devices now

I have 8 Energizers and 4 Powerex Nimh batteries that can no longer power my Nikon 4600 digital camera. I don't think I've even recharged them more than 100 cycles over two years. The batteries can power smaller devices like a walkman or flashlight, however, but performance is lower compared to a healthy battery. They can still be recharged and voltage at full charge is 1.45 volts but goes below 1.3 volts within a few days.

The reverberating answer I get is that the batteries have developed increased internal resistance. What does this mean?

what type of batteries (capacity) and which charger are you using to charge them?
Higher internal resistance means that the batteries themselves are adding more load in circuits
so they cannot put out as much power. kinda like a huge fan blade on an engine the faster the
engine spins the more the fan robs you of power and the less power goes to moving the car itself.

Energizer 1850 and Powerex 2400 I bought two years ago. I used a Maha MH-C401FS charger as well.

1850.... that is pretty old, what charge rate were you using? (Am not familiar with the maha chargers rates). If you were charging them fast and heating them up it could have stressed them leading to this condition.

Higher internal resistance in NiMH batteries, also known as voltage depression, can be a temporary and reversible change. In order to get the performance back a refresh cycle or two is needed. The best refresh cycle involves a slow discharge down to 0.9 V followed by a 16 hour charge at 0.1C (which would be 200 mA for a 2000 mAh cell). This can be repeated until no more improvement is gained.

If you don't have a charger that can do the 16 hour slow charge, then several cycles of full discharge and regular charging are the next best substitute.

To do the discharge if you don't have an automatic discharge feature on a charger, you can connect each battery individually to a small flashlight bulb and leave it overnight. It is important to discharge the batteries one by one and not in a two cell flashlight for example.

It is also possible for batteries to develop permanent voltage depression, caused by loss of electrolyte and/or degradation of the electrodes. If this happens, you just have to buy new batteries. If you do buy new ones, Eneloops would be a good choice.

Also besides what was said above, have the batteries been segregated into groups? If not it is possible that sometimes a half dead battery got put in a set with a more full battery. When the weak battery dies it gets reverse charged, this will damage the battery and cause the high internal resistance situation as described. I try to always keep my rechargeables in groups, not mix them and make sure that only freshly charged batteries ever go into a device.

The batteries were grouped and never mixed. I've discharged and recharged the batteries several dozen times by now with no improvements. This is the reason I don't want to buy Energizers any longer but to have it happen to the Powerexs was disappointing since it's highly regarded by batteryholics. I do have Eneloops and Duracell LSD and I'll see what happens in a year or two.

The Maha Charger slow charges at 300 mAh and has thermal sensors.

Hello Pinoy,

The 401FS charger has a default setting of charging at 1000 mA (for AA cells) and a slow charge setting of 300 mA. It also uses -dV to terminate the charge.

In order to get a strong termination signal when using -dV termination, you need to charge in the 0.5 - 1.0C range. Using the 1000 mA charge rate would have worked much better.

My guess is that charging at 300 mA failed to produce a strong end of charge signal, and over time the constant overcharging turned your cells to crap.

The difficulty with that charger is that the cells tend to get hot when charging at 1000 mA, so people choose the lower setting. Heat also damages the cells, so you are kind of caught between a rock and a harder place...

May I suggest the C9000 charger...

Tom

Cool. I did not know about the -ΔV working at 1A. Where did you find this out?

Instead of using thermal shut-off or -ΔV couldn't they just terminate the charge by reading the battery voltage? In either of the first two methods charge is terminated after the fact - meaning the battery is already overcharged or reverse charging.

The Maha C9000 is a nice looking charger. Someone also recommended hobby chargers. Have you any opinions on those?

pinoy said:

Cool. I did not know about the -ΔV working at 1A. Where did you find this out?

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That's a bit like walking into a brewery and asking if anyone there knows how to make beer.

Instead of using thermal shut-off or -ΔV couldn't they just terminate the charge by reading the battery voltage? In either of the first two methods charge is terminated after the fact - meaning the battery is already overcharged or reverse charging.

Click to expand...

You can read the voltage, but with NiMH cells it is hard to get a reliable full charge that way. However...

The Maha C9000 is a nice looking charger. Someone also recommended hobby chargers. Have you any opinions on those?

Click to expand...

...as it happens, the C9000 does terminate the charge on most cells by reading the voltage. It means that those cells get slightly less than a full charge, but they also remain very cool and undamaged.

Even if an NiMH battery does got overcharged a bit it doesn't hurt very much. NiMH cells are built to shrug off over charging and they are quite tolerant of the normal excess charge delivered by a properly designed charger.

Mr Happy said:

Higher internal resistance in NiMH batteries, also known as voltage depression, can be a temporary and reversible change. . . . .

Click to expand...

If you want to stop this happening in future -
1. Store the batteries in a discharged state
2. If you must store them in a charged state, at least once every three months, slowly discharge to 1.0 volt per cell and slowly recharge.