Does anyone have any advice on repairing cordless tool batteries ?
I've got about 6 various ones that either don't hold good charge or don't charge at all, is there a way of testing individual cells and replacing faulty ones or is there a firm that repairs them?

Any advice much appreciated

like keith never used them
http://www.re-cell.co.uk/ or
http://www.eurobatteries.com/ or
http://a1-battery.co.uk/POWER_TOOLS_BATTERY/index.htm or
http://www.strikalite.co.uk/prodcat_type/1/ALL/0/NiCad_Batteries.html


what you can do if youve got more than one pack the same capacity batteries is measure each individual cell [volts] which will in a good cell reach about 1.5 v write the volts on each cell
take the best battery remove the worst cells and replace with good cells from the other battery

having said that a duff cell may stop or reduce the the voltage off the other cells

It's only hard if you pussy around with a baby soldering iron that's not got enough thermal mass.    I can even solder cells together end-to-end with no wire in between!  You can't even see the +ve end of one cell when it's done.

You need a really big soldering iron ideally 75-100W with a lot of copper in the bit. I've made bits using 10mm diameter copper rod before.

You must roughen up the ends of the cell with emery paper.

Apply a flux paste.

Tin the cell and then quench with a wet cloth quickly to stop the cell over heating.

Then sweat solder the wire on. Quench quickly again.

The wrapper on the cell will probably be damaged by the soldering process as it's a heat shrink sleeve. You can get replacement sleeving from model car racing shops or on the web.

This isn't my handiwork but this is what you can do with a big enough iron. Note the cells in the back are joined end to end without wire...click to enlarge.



Try... http://www.overlander.co.uk for cells.

12 rechargable cells will only deliver around 14-15V (=12 x 1.2V) so calling it an 18V battery is a little optimistic.

The little black thing is probably a resettable thermal fuse. Designed to prevent the battery overheating either on charge or in use. If it has any polarization marks (eg a + sign or a band near one end) then it's probably a diode - either to protect aginst reverse polarity charging or as part of the charging circuit. Make a not of where its is physically located in case you want to make a new battery pack.

When they are new they have a slightly better curve than big_all suggests. More like this..



When they get older the curve gets steeper and the voltage falls off more steeply. It also fall further under load than when new.

Yes NiCad and NiMH cells have a very flat curve compared to lead acid. In fact it's so flat (particularly with new cells) that it's next to impossible to use the voltage as an indication of how much energy is left in the battery. It can only really tell you if the battery is nearly empty of nearly full. In the middle it's not reliable to use the voltage. The voltage can also go up and down more with temperature and age of the cell.

Laptop computers measure the current going into and out of the battery and then use a small microprocessor to work out how much is left in there. Thats how they are able to display a sliding bar type fuel guage. Even they get it wrong sometimes.

Formula 1 racing cars use much the same idea. They know how much fuel they put in and they measure how much the engine has used - the rest is maths.

Found this comparison of different cell curves..



Edit: Thats from here..

http://www.mpoweruk.com/performance.htm