So I have one bad battery in my boat.  The other three batteries are good, so I thought I would just try and replace the bad one, especially since I use AGM (glass mat) batteries that cost around $230 each.  I use AGM batteries because (1) they are completely sealed and maintenance free and (2) AGM batteries are constructed differently than typical car batteries and can therefore withstand the heavy jolts and vibrations from a boat on the water.

So I called a buddy of mine who runs a battery shop to see what he had available.  I run Deka batteries, he does not sell Deka.  He said since I only need one battery, I should absolutely stick with Deka.  Apparently each battery has internal resistance particular to its manufacturer, and mixing brands on a series circuit of 36 volts can create problems.  He said I will either damage the new one, or damage the remaining two.  Voltage takes the path of least resistance, so whichever battery has the lower resistance will end up getting overcharged while the remaining batteries are charging.  So I guess I will get another Deka.

Moral of the story: Do not mix battery brands in a series circuit. 

If you are a boater, you likely have at least 3 batteries on your boat, maybe 4.  Unfortunately, battery issues usually do not happen in your garage, they occur at the lake while fishing, and probably during a tournament.  It is possible to have a battery that charges fully to 13 volts, but no longer has the stamina to hold that charge sufficiently.  Here is a way to test that battery.  This is a carbon-pile load tester that I got on Amazon for $20.  I tested all 4 of my batteries and found one that was not holding a charge, no wonder my trolling motor was acting sluggish.  You can also use this to test the charging system.  I discovered the alternator on my truck was not working properly.  This is a must have for any boater.