Lost volts

I have a a battery case that holds 6 'C' size batteries.  When charged, six batteries would deliver more than 6 volts for a couple of little motors I have.  I didn't have a C size case for four batteries so I took two batteries out and put packers in.  When checked the voltage it was only 4.5v.  After scratching my head I took the packers out and fiddled with them thinking there was a problem with their contacts. Eventually I checked the voltage of all six batteries out of the case and then in the case and found a 1.1v drop.  It turns out that a contact in the case had corroded and that was the problem.

So if you use a battery case check that it hasn't corroded so it doesn't add resistance you don't want.