The charging is a bit confusing
QI have a single alternator, 65-amp Lucas type, and a Cyrix split-charge electronic relay. All my cabling is shrink-wrapped, conduited and supported. Is it okay to connect the Cyrex to the starter permanent live on the switched side of the isolator? If not, can I connect straight to the starter battery. The Cyrix is connected currently with the starter battery as the primary battery.
Would taking the alternator live wires up to the isolator (mounted on instrument /control panel) mean a long run and create a greater possibility of voltage drop? SIMON, via email
ATONY REPLIES... I think the Cyrex is nothing more than a voltage sensitive relay. That means (like any other charge splitting relay) minimising the current passing through the contacts.
The instructions talk about charging the engine battery before the relay closes to charge the domestic bank, but (in my own and several other people’s view) this is of little value. If you minimise the current through the contacts it really does help maximise the relay’s life.
Starters draw a huge current but for such a short time (on engines in reasonable condition) that the discharge is only a few amp hours; so the engine battery will be fully charged probably within half an hour of starting. A delay of probably no more than half an hour while very discharged domestic batteries charge a little is of little consequence to engine battery charging.
If you wire the alternator to the engine battery then as soon as the relay closes you will not only have the full alternator output flowing through the relay contacts, but also an amount of current being provided by the starter battery (that is all but fully charged). Experience shows this can burn out the relay contacts, and the larger the alternator, the more likely. You avoid this problem by sending the alternator charge to the battery that will be the most deeply discharged, so the bulk of the charging current never passes through the relay. So connect the alternator output to the domestic master switch.
If by isolator you mean the battery master switches, then I think it is a terrible idea to put them on the instrument panel. It means long cable runs with volt-drop problems, as well as being easy to access and therefore to turn off accidentally with the engine running, which could ruin the alternator. Keep them close to the batteries and hidden, apart from the mandatory notice.