Battery powered hand tools are very popular and very conveient to use. The problem is often the batteries. Even as rechargeables they don't last forever and are expensive to replace. Sometimes the problem is not that the battery is worn out. There may be a break in a connector inside the case. These can sometimes be repaired using solder.

Supplies

To repair the batteries of my mini chainsaw I used:

Small screwdriver

Soldering iron

Solder

Flux

Rubber band

Step 1: Disassembly

The problem with my batteries is that the connector between the batteries and the circuit board was not made of the correct material to withstand vibration and snapped at the bend. Two battery gangs with two connectors each and three repairs.

The first challenge is to get the screws that hold the case together out. Mine aren't a simple blade or Philips head but are a triangle. I ended up cutting the head off a nail, grinding the point into a triangle, and then putting the nail into a drill to back the screws out. With the screws out, separate the plastic cover and carefully remove the battery/circuit board assembly. When removing the screws you find it helpful to rest the battery on a folded up towel to keep it stabile.

You may want to take a phone photo of the insides of the case so you remember how things go back in.

Step 2: The Repair

Examine the assembly for broken or loose connections. All of my breaks were the ninety degree bend of the connector between the batteries and the circuit board. I used a rubber band to pull the loose ends back together. Get the soldering iron hot and apply soldering flux to the area of the break using a toothpick. Carefully solder the break being careful not to melt the plasic case. Before reassembling the battery, make sure the repair was successful by charging the battery. If it charges, reassemble the insides to the case. I did not use the original screw but replaced them with ones with slotted heads.

I hope this repair works for you on your seemingly dead batteries.