Can batteries in series be reverse charged if the circuit is open?

Question

Suppose I have four AA batteries hooked up in series and I forgot about them and one of them eventually got completely empty due to self discharge. Also suppose that these batteries are not connected to any external circuit. Can that empty cell be reversed charge by the other cells?

My understanding of batteries is that the chemical reaction which releases the stored energy cannot proceed unless the batteries are connected to a load so that the electrons release by the reaction can flow through the external circuit. So in my example the batteries should be fine. But maybe something can occur in the empty cell that somehow make a current flow even though there is no external load to complete the circuit?

The reason for my question is that I am wondering if it is safe to leave four rechargeable NiMH batteries (in series to provide 5V) in an DIY device I built if I have a switch that disconnected the batteries from the circuit.

Answer 1

NiMH cells in series will be fine. Self discharge is a chemical reaction within the cell, and does not depend on the internal resistance (although they are somewhat correlated, especially at end of charge). So, polarity reversal can occur only if cells are connected in parallel.

Self discharge is also somewhat self limiting. It reduces as charge falls, due to electrode passivation (which also contributes to increasing internal resistance).

Easy precautions and good practice:

1. Make sure cells are same type and from same manufacturer.
2. State of charge is similar when installed, and none are close to depletion.
3. Most importantly, charge each cell individually.

Answer 2

While in an open circuit no current can flow so no cells can be reverse charged. once the circuit is completed however it can happen. but your 5V circuit would indicate low voltage before any damage could be done.