Charging two batteries at once
QOur 20 year old flexible solar panels have been regulated by a Sunware FOX-350 until recently when a mother board fault started over-charging the two service batteries. Repair was possible but replacement with the updated FOX-360 made more sense.
Both the FOX-350 and 360 charge each battery in turn. Once Battery 1 reaches a predetermined voltage, (or as with the 360 after a set period of time), battery 2 receives the charge from the panels. Once both batteries are fully charged the regulator gradually reduces the charge amperage to zero. This arrangement requires the two batteries to be isolated apart from a common neutral.
Research into alternative regulators revealed they were only suitable for a single battery or two batteries that are connected in parallel, which I understand is the preferred arrangement.
Clearly these were not suitable for my separate batteries which are selected using a rotary switch.
My question is why is it better to connect two service batteries in parallel? This arrangement could result in flat batteries and no alternative or back up.
An unexpected benefit of the new the FOX-360 is that the charge rate in amps is now higher than with the original FOX-350. Richard Bull
DUNCAN KENT REPLIES: I’m assuming you mean you have two batteries for domestic use, not one for domestic and another for engine starting.
As you say, the most common arrangement for a domestic battery bank is to have several batteries connected in parallel (+ve to +ve; -ve to -ve) to form a single battery bank. This is much simpler than having two separate banks in that all the capacity is available to use should it be required for heavy-duty equipment, rather than limiting yourself to the maximum current available from just one battery.
With separate banks you might be halfway through running the anchor windlass, for instance, when bank 1 becomes fully discharged (effectively below 12.15V for a lead-acid battery) and you then have to manually transfer to bank 2 by turning a switch.
Secondly, lead-acid batteries don’t like being discharged too deeply and by separating the banks you are doubling the chance of draining one battery dangerously low unless you also have low voltage protection installed.
Batteries connected in parallel will charge together as a single battery, there is no need to prioritise half of it. It is much more straightforward (and most likely cheaper) to charge a single bank from any source and you will have far more choice of solar charge controllers.
While the Sunware FOX-360 might be an ‘enhanced’ PWM solar controller, the newer technology MPPT controllers are far more efficient than any PWM device, though more expensive.
That said, if your flexible panels are 20 years old you may not notice much difference. Most MPPT solar controllers only have a single battery charge output, but if you really want to continue having two banks then I’d recommend you interconnect them using a simple voltage sensing relay (VSR). That way they remain separated when not being charged but automatically connected together when any charging source is present.