Ross said:
You need a "smart" dual battery isolator. A simple "back to back
diode" isolator is no good, it does exactly the same as your existing
relay controlled unit.
Guest make a range of smart isolators starting around 70A rating and
you couldn't build your own any cheaper. Most marine or RV dealers
will sell battery isolators.
eg.
http://www.northeastmarineelectronics.com/index.asp?PageAction=VIEWPROD&ProdID=4108
I have a similar setup that cross connects the engine and house battery
on a small yacht. I never have any trouble with it BUT it differs from
the OP's in two crucial details:
1. The relay is rated 80A and fused at 70A (greater than the alternator
rated output). All the wiring is rated over 80A.
2. The alternator output goes to the house battery, as it the one that
needs the most charging.
(This may be more trouble to set up than its worth on a motorhome - the
engine battery can end up low quite easily if the lights are left on
with the house battery still fully charged so the combiner has to be
designed to handle EITHER battery flat with the other one charged. Also
fitting wiring between the batteries and from the alternator to the
house battery capable of handling the full alternator output and
protecting that wiring against short circuits may be a quite difficult
undertaking. The alternator MUST either be machine sensed or the sense
wire moved to the house battery. If the capacity of the house battery is
much greater than the engine battery its worth it though for better and
quicker charging)
Commercial units usually have quite strict restrictions on the MAXIMUM
wire size and MINIMUM length to use which ensure there is sufficient
resistance in circuit to keep the surge current below their switching
rating.
Assuming the existing unit is controlled by the ignition switch,v a flat
battery at nominally 10V (one isn't supposed to take a lead acid battery
below 10.5V) paralleled with a fully charged one at an absolute maximum
of 13.8V only gives 3.8V differential. It needs a total resistance of
0.1266 ohm to limit the current to 30A.
Even 0.1 ohm in series would probably cure the OP's problem.
(WARNING, at 30A a 0.1 ohm resistor will dissipate 90 watts - This will
be a problem if one of the batteries has a shorted cell. Its a lot
simpler to get rid of the heat safely over several meters of wiring
rather than at a single component.)
There are only two occasions (neglecting short circuits) that the fuse
should blow:
If you are trying to pull STARTING current through the relay, e.g. the
house battery is fully charged and the engine one is bad or has a loose
connection.
If your house battery has a shorted cell.
If the existing unit is controlled by either the alternator warning lamp
output or senses the charging voltage, then the battery being charged
gets up to nearly 15V and you either need a little more resistance in
the circuit (0.166 ohm) or a relay rated higher than the alternator
output (the alternator cant hold the voltage on a battery over its
resting level at currents higher than the alternators rated output)
Fitting a Guest unit (or equivalent) rated greater than the vehicles
alternator output, *AND* rewiring to the correct specs for the new unit
would probably be the easiest and safest solution though.
