Re: [A/S] battery question

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

For optimum paralleling, batteries should be rated at the same specific
gravity at full charge (e.g. 1.260 or 1.280 probably which might be
stated without the decimal point). That way their terminal voltages will
match and they will share the load and the charging a much as possible.

Vehicle alternators are constant voltage devices, they don't reduce the
charging current by detecting a battery has charged, they just hold the
system voltage to their setting and the good lead acid battery accepts
whatever current limited by battery and wiring resistance to raise its
charged voltage to match that of the alternator. Small wires over long
distances will limit the charging from the vehicle system.

Deep cycle batteries tend to be set up to operate at a lower specific
gravity (which makes their full charge terminal voltage a little lower),
but they should charge off the alternator. They might actually tend to
overcharge though an occasional overcharge called equalization is good
for most any battery. Continuous overcharge is not good. It removes
water from the electrolyte by electrolysis and then when excessive to
expose plates, the plate material dries out and often does not recover
to supply battery capacity when rewet. Lots of batteries means the need
to check lots of connections and cells. Connections get corroded by acid
leaks around the posts, charging to full capacity causes some
electrolysis and battery electrolyte level is critical for battery life.
Most automotive tool suppliers still carry automatic battery watering
bottles that cut down on the looking, just pop off the caps and insert
filler nozzle and push. When it stops gurgling the distilled water
(NEVER use any water but distilled water to fill a battery!) lift up and
go to the next cell.

Gerald J.