Matching cable length, contact resistance etc., for a series or
parallel connection is a total myth. The differences caused from these
sources are minor compared to the normal variations found in the
chemistry of the battery cells and can be ignored.

When in series, the same current flows through the whole circuit and
has limitations based on the total resistance of the circuit. An
imbalance in the cables is meaningless regarding to loading the
batteries since the same identical current flows through all and a
slight drop in the total voltage due to a slightly longer cable does
not have any "imbalance" problem.

When in parallel, it is true that a slightly longer cable or a contact
with a few extra milliohms will cause one battery to accept more
current than another however it doesn't cause any imbalance. As the
batteries approach full charge, the current drops until at full charge
it is less than 1% of the initial current. When the current gets to
zero, so does the voltage drop along the cables and connections so
everything is at the same voltage and there was no imbalance caused by
any mismatch.

In fact charging in parallel when you need to use them in series is a
preferable method. The longer the series chain, the more important
equalizing becomes. There will always be one weakest cell in the chain
and it will be the first to get discharged when in use, however when
charging, the strongest cell will be the first to reach full charge and
start gassing. The difference between these is lost capacity in the
battery bank. More reading on the subject of equalizing at
http://www.yandina.com/troll24Help.htm#Q20.

The warnings about ensuring a make before break and having an interlock
safety system are appropriate. One mistake and you will have a dead
short across one or more batteries with very little resistance between
them which can have drastic and dangerous results. If you do attempt
this, please put 250 amp circuit breakers or fuses in series with every
individual battery.

I happen to be very familiar with this subject. We make an automatic
series/parallel switch for doing exactly what you want but it is only
rated for 85 amps, not 200. It is designed for charging 24 volt
trolling motor batteries from 12 volt outboard alternators. See
http://www.yandina.com/trollinfo.htm

Regards,

Ann-Marie Foster,




Meindert Sprang wrote:
"CS" wrote in message
oups.com...

The last 250A continuous DC contactor I bought last year was around
£20. Vetus and Sidepower do this all the time.

But, successful operation depends on equal charging and discharging of the
batteries. In fixed installations, this is achived by making the lengths of
the connecting cables in the parallel sections equal, to make the resistance
in each branch equal. If you do this with relays, there can always be a
slight variation in contact resistance (we're talking milliohms here). This
will eventually shorten the life cycle of the batteries.

Meindert