Wednesday, Jul 16, 2003 at 10:33
I've built a Jaycar 12V battery manager kit - haven't installed it yet. The reverse current problem via the protection diodes is well documented in the kit instructions.
".....the MOSFET internal protection diodes will allow a 'reverse' current to flow if the auxilliary battery's terminal voltage is significantly higher than that of the main battery. While this is obviously not the case during normal charging operations, there can be circumstances where the auxilliary battery is fully charged yet the main battery's voltage has fallen due to a high current load - such as the vehicle's starter motor.
If the auxilliary battery can deliver a high current (most deep cycle batteries can' by the way), then the MOSFET diode current is only really limited by the conecting lead resistance Rl, and may rise to a high level - say 60amps. This in turn means that the MOSFETs will dissipate around 30watts each, thanks to the 1V drop accross the diodes. If this curent were present for an extended period, the MOSFETs would become rather hot and bothered due to the modest heatsinking capability of the diecast box.
Note thatthe MOSFETs dissipate less than 10W during normal charging operations.
While we were initially a little concerned about the above scenario, and considered a number of circuit configurations that would provide full two-way isolation between the batteries (by the use of a high current relay), we found that the additional expense and circuit complexity of the alternatives was difficult to justify.
And as the condition would only occur if
a) the auxilliary battery was of the high current type.
b) the load was applied for an extended period (unlikely with a vehicle's starter motor).
...and c) the main battery voltage dropped substantially under load (indicating a less than healthy battery), we reasoned that there was not too much cause for concern.
Of course, we'd have to addd that it's extreme folly to set off in your 4WD, camper or car/caravan combination with an unhealthy main battery - or for that matter, a vehicle that's difficult to start and needs to be cranked for an extended period.
Under normal circumstances, the potential for reverse current flow could be considered as somewhat of an advantage in a dual battery system, since any excess charge in the auxilliary battery will be automatically passed to the main battery via the MOSFET diodes. This in turn ensures that the main battery is always in the best possible state of charge for the conditions at the time, and can therefore crank the engine into life......."
Anyway, sorry for the long-winded post, but I thought the reverse current 'problem' should be documented correctly here.
I'm sure there are better (and more expensive) alternatives to the DIY kits. But I'm also sure that the DIY kits offer a good cheap and reliable alternative.