Testing 6V Deep Cycle Batteries

pnichols wrote:
Hmmm .... I'll have to think about that for a minute: In what way could a bad 6 volt battery in series with a good 6 volt battery in any electrical way, shape, or form damage the good battery?

One simple and obvious example: if the bad battery had a shorted cell, then all other cells (including those in the good battery) would get overcharged when charging. In the other direction, it may be the case that the newer battery discharges faster than the old (lead acid batteries take a few hundred cycles to reach full capacity), so the cells undergo deeper discharges.

For all batteries (i.e. more than one cell), any mismatch between cells will cause some to experience more extreme swings than others. Those cells will have shorter lifetimes. Hence the recommendation to not mix new with old.