Odd electrical fault

[Bewildered]

A couple of days ago I had a flat starter battery (had the boat 4 years and the battery was on the boat when purchased) jump started the engine and allowed to charge. Battery charged up but wouldn’t hold charge overnight.
Disconnected the battery to move into a better position to check fluid levels, as I moved the battery the now loose positive lead touched ground and gave of a spark. I thought this was odd so investigated why a disconnected lead should have power.

I measured 12v on the disconnected positive cable. Recently had my split charge diode replaced with a voltage sensitive relay so I thought perhaps that was the reason. I had 12v on either side of the relay but I didn’t have continuity between the terminals. So this seemed even odder.
Traced the cable back to the isolation switch but I could measure 12v on the cable with the switch in the on or the off position. 
The isolation switch has a cable in and a cable out but also a third feed which comes from the battery charger, so the process of elimination points in this direction.

The charger is a sterling with 4 output terminals 1 negative which is common to all batteries and 3 positives. Positive terminals 1&2 are strapped together and feed the leisure bank, terminal 3 feeds the starter battery via the isolation switch. I disconnected terminal 3 and could no longer read 12v at the battery cable end. Terminals 1,2&3 all read 12v and I had perfect continuity between 1,2&3.

 

So unless I am interpreting this wrong, not only do I need a new starter battery but I also have a fault with the charger. As far as I can tell there is a short circuit inside the charger that has been linking all batteries as one big bank. I think I am correct in saying the outputs are supposed to be independent of each other, hence strapping them externally to configure to your preference.

 

Haven't used the battery charger for a couple of months (not since having a high output alternator fitted) it was working when I last used it, that is to say it was charging. But I have no idea how long it has had the short circuit. This is possibly why the starter battery has failed if it doubling up as a leisure battery via the charger.

 

thoughts and advice welcome

 

Edited August 28, 2020 by Bewildered

[Jen-in-Wellies]

With no starter battery in the circuit there could be a high resistance path to 12V via the charger and/or voltage sensitive relay. This could lead to 12V being measured by a high impedance multimeter. Doesn't necessarily mean that either the charger, or VSR is borked. The 12V you are seeing may not be able to supply any current. A small spark doesn't need much current, or could be caused by a discharging capacitor in the charger. Geriatric starter batteries can and do fail suddenly all on their own. I wouldn't go trying to diagnose other possible faults without a good starter battery in the circuit. You may be calling something a fault that is just normal behaviour of an incomplete circuit.

Jen

Edited August 28, 2020 by Jen-in-Wellies

[WotEver]

^^^^^ Wot Jen said

[Tony Brooks]

just to point out that with a VSR you don't need a multi-output charger as long as the VSR is bi-directional or is sensed off the battery connected to the charger/solar. Usually the domestics. I think I would just connect all three charger outputs to the domestic bank, make sure the alternator and any solar is also connected to the domestic bank and ensure the VSR (unless bi-directional) is sensing from the domestic bank. Then whenever the charge source is delivering in excess of 13.6 to 13.8 volts the VSR will close and charge both banks.

 

If you have solar then unless you disconnected it or it was dark the VSR is likely to be closed.

 

You shoudl find 12V on both main termianls of the VSR because the engine and domestic battery negatives are linked. It 12V engine battery on one side and 12V domestic battery on the other.

 

[Tracy D'arth]

Excellent advice from Tony, just replace the battery and link all the charger terminals 1,2,3 to the cabin batteries.

[Bewildered]

3 hours ago, Jen-in-Wellies said:

With no starter battery in the circuit there could be a high resistance path to 12V via the charger and/or voltage sensitive relay. This could lead to 12V being measured by a high impedance multimeter. Doesn't necessarily mean that either the charger, or VSR is borked. The 12V you are seeing may not be able to supply any current. A small spark doesn't need much current, or could be caused by a discharging capacitor in the charger. Geriatric starter batteries can and do fail suddenly all on their own. I wouldn't go trying to diagnose other possible faults without a good starter battery in the circuit. You may be calling something a fault that is just normal behaviour of an incomplete circuit.

Jen

Thank you Jen, this makes sense; perhaps it is me being silly expecting that when I disconnect a battery the disconnected lead should be dead and not measuring power. My only concern is that I measured zero Ohms across the charger terminals not high impedance. 

 

2 hours ago, Tony Brooks said:

just to point out that with a VSR you don't need a multi-output charger as long as the VSR is bi-directional or is sensed off the battery connected to the charger/solar. Usually the domestics. I think I would just connect all three charger outputs to the domestic bank, make sure the alternator and any solar is also connected to the domestic bank and ensure the VSR (unless bi-directional) is sensing from the domestic bank. Then whenever the charge source is delivering in excess of 13.6 to 13.8 volts the VSR will close and charge both banks.

 

If you have solar then unless you disconnected it or it was dark the VSR is likely to be closed.

 

You shoudl find 12V on both main termianls of the VSR because the engine and domestic battery negatives are linked. It 12V engine battery on one side and 12V domestic battery on the other.

 

Thanks Tony, I hadn’t even considered that now that I’ve had the VSR fitted I no longer need my charger configured for multi output. I will strap all three outputs together for the leisure bank, this will alleviate my concerns about the continuity between the charger terminals.

 

I spent years working with low voltage electrics but boat electrics just do my head in, my own fault for expecting that when I disconnect something it should actually be disconnected on a boat.

 

[Tony Brooks]

18 minutes ago, Bewildered said:

I spent years working with low voltage electrics but boat electrics just do my head in, my own fault for expecting that when I disconnect something it should actually be disconnected on a boat.

 

That common negative bond takes some getting used to when doing stuff like you are.

 

Please make sure all charge sources run to the domestic bank including the alternator and ensure the VSR is sensing from that bank. All too often the split charging systems are wired for the ease of the installer rather than for optimum performance and life so they leave the alternator feed to the engine battery (starter terminal usually)  rather then changing the feed to the domestic bank. This ensures high current flows through the relay contacts and that tends to burn them out.

[Bewildered]

40 minutes ago, Tony Brooks said:

That common negative bond takes some getting used to when doing stuff like you are.

 

Please make sure all charge sources run to the domestic bank including the alternator and ensure the VSR is sensing from that bank. All too often the split charging systems are wired for the ease of the installer rather than for optimum performance and life so they leave the alternator feed to the engine battery (starter terminal usually)  rather then changing the feed to the domestic bank. This ensures high current flows through the relay contacts and that tends to burn them out.

Fortunately the VSR was fitted by a competent engineer. All charge sources are fed to the domestic bank and the small negative sensor wire from the VSR is also fed from the domestic. 

All I need to do is unscrew my engine isolator switch and remove the battery charger link cable from it, I can then use this short fly lead as the strap for my charger between terminals 2&3 on the charger