This time I’ll show you how to repair faulty NiMH battery instead of buying new one.

How I did it - you can check by looking DIY video or you can follow up instructions bellow.

For this project you will need:

Materials:

Faulty NiMH Battery

2 good NiMH cells for bad ones replacement

Tools:

Soldering iron

Screw driver

Knife

ImaxB6AC V2 or other smart charger

Multimeter

Step 1: The Issue

I have two Bosch NiMH batteries from cordless power tool. One is performing very well, but other not so good. Most of the case there are two reasons of that: first - natural battery wear down or only few cells just die inside battery pack.

Because of NiMH chemistry here won't work zapping method like on NiCd batteries. So bad cells must be identified and replaced.

Both batteries was fully charged with original charger. Let’s check what voltage they hold. Healthy and fully charged 18V battery must hold about 21V. That's 15 cells multiply by 1,4V. 1,4V is each cell top voltage, when they got full charge.

Step 2: Inspection

The good one is with 21,7V. And second battery shows 19V, so there is definitely something wrong. Comparing to good battery there is a gap of 2,7V. By looking at this number and knowing that both batteries are fully charged - I’m guessing, that there are two dead cells inside. Let’s take it apart and check how it looks inside.

One NiMH cell nominal voltage is 1,2V. Fully charged they could have up to 1,45V. So my guess make sense.

Step 3: Identify Bad Cells

Everything looks pretty nice. No leaking or corroded cells. Let’s check voltages of individual cells and identify bad ones. Like I guessed, there are two dead cells with 0,05V and 0,11V. All other are good with ~1,4V.

Step 4: Checking the State of Rest Cells

Before beginning repair process, I used my Imax B6AC V2 charger and discharged 3 randomly picked cells at 2A of load to 1V cut of voltage, because at 1V cell will be almost empty. I did that, because I want to know how much juice still left in good cells and does repairing this battery is worth it at all. All three cells showed around 2Ah of capacity. So all good and this battery is still usable.

Step 5: Cells for Replacement

At local electronic parts shop I bought two NiMH cells. They are rated 2,8Ah of capacity and have spot welded tabs, which will help me to solder cells back. I paid 3 euros for each cell, so this gonna be a really cheap fix.

Also they could be purchased at ebay, amazon or other online shop.

Step 6: Removing Bad Cells and Soldering New Ones

Removed bad cells by using knife and cutters.

Soldered new cells in place.

Final voltage check - does cells was soldered properly.

Step 7: Reassembling

Reassemble everything back in battery housing and put it on charge.
After few minutes charger showed that battery is full. What about voltage? It is as it should be at fully charged healthy battery ~21,7V.

Step 8: Conclusion

Having faulty NiMH battery from cordless power tools, not always mean that you have to throw it away and buy the new one. Most of the case you can change only few bad cells inside and keep it running.

I hope this was helpful!