Forget for a moment the issue of connecting a 12 volt load
to a 24 volt system.

Consider only the problem of properly charging two 12 volt
batteries in series with a single 24 volt charger. My
understanding is that unless the two batteries are identical
(goes way beyond same make or capacity) they will not be
charged to the same voltage. Two electrically non-identical
batteries will have different internal resistances (by
assumption)and thus charge to different voltages. As I
mentioned in an earlier post, Ample Power shares this
understanding and offers a 24 volt charger that is in
reality two 12 volt chargers, one connected to each of the
two 12 volt batteries (which are in series, of course).

If my understanding is incorrect, that would mean that a 12
volt marine battery and 8 AAA's in series would all be
properly charged by a 24 volt charger (with current limited
so as not to destroy the AAA's). An attempted reductio ad
absurdum.

My point is simply that with a real-world installation of
two 12 volt batteries in series, the two batteries will
probably measure different voltages. The greater the
differences between the batteries, the greater the potential
problem (no pun).

Without knowing the internal resistances, etc. of the two
batteries, it is not possible to know the precise effect of
placing a different load on one of the batteries. Doing so
would effectively place the added load resistance in
parallel with the internal resistance of the affected
battery. That could make the two batteries either more or
less alike, depending on the values involved.

All of this is hand-waving, of course, because a battery's
internal resistance is really not a constant, but a function
of the battery's chemistry, state-of-charge, etc. It is a
non-linear resistance, in that it varies with current. I
don't believe we miss the mark here by ignoring these
factors, however.

Regards,

Chuck









Pete Styles wrote:
"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...

snip

The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.

Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).


Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.