RM> The sucked cells will be charged to a lower max voltage,
RM> because the "others" voltage will be higher, so the sucked
RM> cells won't get fully charged.

DD> Indeed (although they're not ordinary car batteries - do they
DD> come in 2 volt versions?). 

 Lead-acid is lead-acid no matter where you put them. 

DD> Other disadvantages are:

DD> 1) Installing a second lot of wiring for the 6V;

 Remember to use thick wire...

DD> 2) The cells, solar panels and load are all wired via an
DD> "intelligent" regulator. It keeps track of current/time into
DD> the battery, and out and adjusts charge rate accordingly. It
DD> also disconnects the load if the battery voltage gets too low.
DD> This would not happen with 6V tapped straight off the three
DD> cells. 

 Jeeze! You don't muck around. I'm suspicious of an "intelligent"
device. Actually, it's only as smart as the dickhead who designed it.
Those words you quote sound like they were written by a salesman...
why on earth would they keep track of ampere-hours in and out? Are
your cells lead-acid? If they're NiCad it makes a sort-of sense, but
even then it's pretty silly.

 My guess is that it disconnects if the voltage gets to *high* (to
prevent over-charging), with a limited current that drops to zero as
the battery gets up to 13.8 volts (2.3V per cell).

 It *will* work with a 6V tap and half the cells run flat. The six
cells in series are never the same anyway, and this can be a problem
with NiCad batteries and the fancy gel-cells, but good ole lead-acid
is pretty tolerant of being overcharged. The three non-flat cells
*will* be overcharged at the beginning of the charge cycle, they will
fizz more gas, but at the end of the charge, the bottom three cells
will be fully charged. You can't over-charge a lead-acid battery.
As soon as the charging current falls, so does the voltage. A
single cell can be 2.5V on hard charge, but it slowly falls to 2.25V
just sitting there (it leaks internally) and on use to 2.1V. A flat
battery is 2.0V but it varies with temperature. That's why the
*best*way to test a charge battery is with a hygrometer (the float in
a bulb).

 Over a few years, the life of your top three cells will be reduced,
but you can only expect 5 years out of a battery anyway (whether you
put water in or not). If it worries you, swap the top three and bottom
three cells around every six months.

Regards,
Bob


  

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