Battery pack voltage keeps dropping without use

Question

I have a battery pack for a circa 2008 Black & Decker 9.6 V electrical screwdriver. It was not holding a charge so I replaced its batteries. It had eight NiCd 1200 mAh batteries. I replaced them with new NiMh 1500 mAh ones (these were available to me). I charged them with the original charger for 6 hours. I measured the battery pack open circuit voltage at 10.96 V afterwards.

Next day I repeated the measurement, and it was 10.75 V. I never used the device and in fact, did not install the battery pack on the device.

• Measurement two days after charging: 10.65 V
• Measurement five days after charging: 10.56 V

Is this normal for the type of batteries used?

Answer 1

Nickel metal hydride batteries have a relatively high self discharge rate. Battery University says that they lose 10 to 15 percent of charge over the first 24 hours. That loss of charge will result in a lower battery voltage.

BU provides this self-discharge rate diagram for nickel based batteries:

Note that the diagram is charge state, not voltage. Voltage is related to charge state, but not in a simple way. About all you can say is that lower charge means lower voltage.

In other words:

Yes, it is to be expected that the voltage slowly drops with NiMh batteries.