Starter battery being drained.

[tyce]

With the isolator switch off something still drains my battery. If I leave it for a few days it will be flat, however if I disconnect the terminal and leave it for a week it holds it's charge perfectly so it is not a faulty battery.
The battery starts only the engine, the house battery's are separate but can be linked in an emergency to start the engine. The engine has a fancy little charger connected to the alternator.
So where do I start looking and what tests should I do and what if any is the likely culprit.

 

[pvb]

Has this only happened since the "fancy little charger" was installed?

If not, it could be a fault in the alternator regulator. If this happens, you may find the alternator feels slightly warm when the rest of the engine is cold.

 

[GrahamM376]

Has this only happened since the "fancy little charger" was installed?

If not, it could be a fault in the alternator regulator. If this happens, you may find the alternator feels slightly warm when the rest of the engine is cold.

Starting point is to check for current draw with ammeter and then start disconnecting wires until no draw. Do you by any chance have a solar panel connected with no diode or controller in line?

 

[tyce]

Thanks for replys, fancy charger has always been there and no solar panel.
The bit I don't understand is, I thought when the isolator is off it's isolated from the alternator and everything else is this correct.

 

[pvb]

The bit I don't understand is, I thought when the isolator is off it's isolated from the alternator and everything else is this correct.

All depends on how it's been wired.

 

[VicS]

Thanks for replys, fancy charger has always been there and no solar panel.
The bit I don't understand is, I thought when the isolator is off it's isolated from the alternator and everything else is this correct.

This is what you need to check. As pvb says it all depends on how it has been wired

Something must be connected which is not disconnected by the isolator. Look to see what wires are connected to the battery side of the switch, the battery connection or to a busbar, connector block or terminal post in between.

Once you find some candidates you can set about investigating which is carrying any current and what it/they supply. May be something switched on you did not know or maybe something has developed a fault.

If the battery goes flat in 3 days you are looking for perhaps a significant fraction of an amp, which may be enough to make something warm, not just a few milliamps.

Your alternator may bypass the switch, very likely if you have some form of split charging system.

 

[davidej]

I don't know if you have a shore power charger installed but if you do this may be the problem

Our shorepower charger connected to the battery side of the isolator (so it would charge even when the isolotor was in the off position) -it developed a problem and drained the starter battery

 

[tyce]

Thanks everyone, I will do a little probing and see what I find. I will report back.

 

[Stemar]

I came back to the boat one day to find both batteries completely flat. It turned out that a diode in the alternator had failed, making the field energising wire from the alternator to the no charge wrning light live, and, therefore energising the spilt charge relay, which is why both batteries were run down.

The starter battery recovered from 0v and lasted a couple more years, but the domestic one didn't It also cost me a reconditioned alternator. I'm a lot more careful about isolating the battery when I leave the boat now!