Repairing a Bosch battery pack

This page is referenced in the letters section of Good Woodworking magazine issue 147 (May 2004).

Why do this?

Sometimes the battery pack stops holding charge. The genuine Bosch replacement battery is almost as much as a new drill – about £45.If your drill is out of warranty and you don’t wish to spend this much then it is possible to replace the batteries inside the battery pack for about £15. This page tells you how.

Note: This is ‘mate in the pub’ advice and no responsibility will be accepted for any problems experienced after following this suggestion.

Step 1 – Check the battery really is dead.

Before you rip you battery pack apart it’s a good idea to confirm that it really is the battery pack at fault. To do this remove the battery pack and measure the voltage across the + and -  terminals with a voltmeter. As there are eight cells in the pack and the nominal voltage of a NiCad is 1.2V there should be 8 x 1.2V = 9.6V across the terminals. The minimum voltage a NiCad should go down to is 1V so the minimum voltage should be 8V. If its significantly lower than this you probably have a duff battery pack.

Bosh drill with battery

Step 2 – Assemble the bits and pieces you’ll need.

To perform this salvage you’re going to need to break open the battery pack , replace the NiCad cells and glue it back together again. For this you’ll need:A replacement set of 8 sub-C cells. Available from www.cpc.co.uk part no BT01300 (or click here for the whole page)·   
A craft knife (Stanley knife)·
Long-nosed pliers
A good soldering iron
Plastic glue

..And about 1-2 hours of your life.

Step 3 – Open the case.

Using the craft knife carefully cut open the plastic case as indicated.
Where to cut the battery

Step 4 - Remove the battery pack.

Carefully withdraw the battery pack. Be careful not to damage the thermistor connected to the third terminal and taped against a cell. At this stage it’s a good idea to make a note of the battery polarities in the pack and their orientation.

If  your drill is not the same as this model you can establish the battery type by using the table below.

Step 5 - Confirm the failure mode

You might want to check the individual cell voltages since it may be that only one or two cells have failed, and so you only need to buy the cells you need.

It may even be the case (as happened with my brother's drill) that the cells were OK and it was simply a broken connection.

Step 6 – Remove the cell interconnects.

The cell pack is interconnected with welded –on metal strips. By carefully removing this using long-nosed pliers they can be re-used.
Battery cells removed

Step 7 – Rebuild the pack.

Using the salvaged metal interconnect strips start rebuilding the pack by soldering the strips on to the new cells.

Step 8 – Re-fit to the case.

Check the battery pack voltage at the terminals. If it’s OK install the pack in the plastic case. Check the voltage again and if all is OK hold the case in a large vice and re-glue it together.

Step 9 – Test.

After the glue as set, give the re-conditioned pack a good charge and test it in the drill.

Email: andy@thepriors.free-online.co.uk

Finding the battery type

Use this table to find out the type of battery you need. Measure the height and diameter of one cell, look for the closest match under 'Dimensions HeightxDia' and note the type. If you are buying from CPC then look up the order code for the relevant type in the lower half of the table.

For example, I measured my cells at about  42mm dia x 23mm long. This corresponds to a 'Sub c' type. Looking up Sub C gave an order code of BT01300.

Type NominalVoltage (V) Nominal Capacity (mAh) Dimensions HeightxDia.
AAA 1.2 250 44.8x10.5
1/3AA 1.2 180 16.9x14.6
4/5AA 1.2 720 42.5x14.6
AA 1.2 950 50.2x14.6
2/3AF 1.2 700 29.2x17
4/5AF 1.2 1200 43.2x17
AF 1.2 1400 50.2x17
7/5AF 1.2 2000 66.2x17.2
Sub C 1.2 2000 42.1x22.7
D 1.2 4500 61.5x34.2
2xD 2.4 4500 120x34.2
3xD 3.6 4500 180x34.2
Description Order Code
AAA BT00160
1/3AA BT00161
4/5 AA BT00634
AA BT00313
2/3AF BT00165
4/5AF BT00166
AF BT00167
7/5AF BT00168
Sub C 2000 BT01300
D 4500 BT01330
D 4500 2.4V BT01373
D 4500 3.6V BT01374