Cambridge Underground 1992 p 34

Result of Bosch Drill Battery Duration Tests

Mark McLean

CUCC's cordless hammer drill does not perform nearly as well below ground as it does above. On a Churchill window ledge, something like twenty holes may be drilled in a piece of limestone, whereas down a cold, damp Austrian cave, less than ten can be obtained.

I have performed some tests to try to determine what factors cause this reduction in drill performance. The drill battery is a 24V Ni-cad, rated at 1.2 Ah. I used a pair of 60W car headlamps in series as a dummy load and discharged the battery under various conditions and after various storage conditions.

The 120W load will draw a current of about 5A, similar to the drill running under light load. After subjecting it to the test conditions I then discharged it through the headlamps and measured the time it took for the battery voltage to drop to first 20V and then 15V (time measured as minutes:seconds).


Amount of
trickle charge
to 20V
to 15V
20-0 mins1 hour 14:4016:51
20-0 mins>8 hours 14:2515:40
4445 mins2 hours 10:2013:12
4445 mins>8 hours 12:3014:20
20208 days>8 hours 10:5511:55
2048 days>8 hours 11:1111:37


Hmm, the difficult bit. The two clear results are:

1) That the battery has quite an appreciable self discharge rate. After eight days storage at room temperature, 25% of the charge had gone.

2) That discharging the battery while it is cold considerably reduces the power available. At 4 degrees C, around 20% less power was available. Put together, these two results can largely account for the reduced battery power found in practice.

[The remaining loss (20-30%) may well be due to the leakage rate being higher in the damp cave conditions (This effect has been reported by the CNCC) - Ed]

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