Effects of joining the middle points of N parallel (2-battery serial sets)
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The scheme risks the possibility of situation (c) which would not be OK.

Leaving out the centre-tap connection makes it foolproof in that regard.







simulate this circuit

Figure 2. A better solution.

How it works:


Don't connect the centre holder mid-point.
Cut the link in the outer holders and cross-connect them as shown so that the two outer cells are in series and the two inner cells are in series.
Now the outer batteries only work if the correct pairs are inserted.
Running with three or five batteries leaves the odd one out of circuit. Again this is safe.




Figure 3. Wiring diagrm.