Current Regulator

You need a battery isolator, Typically 2 diodes back to back, like...

The charge current for a lead acid battery group 24, can be quite high >100 amps, depending upon the alternator. That said, I think 30A is a little under sized. A Properly sized circuit breaker would be better.

I recommend a isolator such as

Cheers

A 12-volt light bulb, which is a nice nonlinear resistor. Car or motorcycle headlamp maybe. It also serves as a charging indicator!

John

always

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.

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.

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
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I had the dual diode isolator on my motor home. Due to the voltage drop in the diode the 2nd battery never seems to reach full charge. Changed to the relay set up and put a 50 amp breaker in series to 2nd battery. every thing work fine now. W W

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Thanks for all the ideas. An isolator is the real answer and if I was designing it from scratch that is what I would use but the relay serves as an isolator albeit a very poor one. I had thought of using a resistance to limit the flow and the bulb is a good idea but I am concerned about the heat generated in the small space where the circuits are installed.

For info. I have draw a wiring diagram of the circuit, as it is, and put it here:

I had thought in terms of a simple circuit using a 2N3055 with a zener in the common base circuit and if you scroll down below the wiring diagram on the web site I have sketched what I mean. Anyone any ideas about this one please?

I was hoping someone would come up with a circuit since I could use such a box myself. I really don't think you can make a decent controller without some current limiting. I did a few back of the envelope designs for this application awhile ago and concluded anything linear makes too much heat. You would probably need a hysteretic current-mode controller. It is best to have a working design since there are plenty of gotchas in switchers.

As I have written a resistive device would radiate too much heat for the confined space I have to install a circuit. I did not think that my problem was very demanding and am surprised that no one has come up with an active circuit so I have installed a large choke in series with the fuse hoping that this will be sufficient to remove the spike, time will tell.

You could use the resistive limiting, and use a circuit to spread the charging out over time so that the temperature does not increase as much.

But I'm curious. If, as you say, the real answer is an isolator, why not use that?

Ed