Making sense of battery installation on new boat...

[weaver_fish]

I have very limited knowledge of boat electrics and can't make sense of the battery switch set up on my new (to me) boat.

Has a domestic and engine battery with two simple switches. The photo shows the back of these switches. What is the purpose of the small square Bosch thing?

Here we go...

What's it all about

 

[orizaba]

looks like a split charging relay to me. in other words it splits the charge from your alternator to your start and leisure batteries independantly

 

[DaveS]

It looks like a relay. To make sense of what it's for, however, you really need to trace out the wires, see what's connected to what, and (preferrably) draw a diagram. It might, for example, be wired to parallel the batteries when the engine's running, by taking a feed from the oil pressure or something - or it could be something completely different. Without knowing the actual schematic, however, it is pure speculation.

 

[jwilson]

Doesn't look like a diode. Could be a voltage sensitive relay, that only connects the two batteries in parallel when the voltage on the engine battery rises to around 13 volts or so - thus charging the critical (engine) battery first before connecting to and topping up the domestic battery.

 

[orizaba]

must say john wilson that was my take on it looking at the wires coming off the relay and the wires going onto the battery isolater switchs,they look to be the same but its pure speculation i admit,but probabally a fair assumption

 

[Oldhand]

I believe it is a plain relay. At a guess from the pictures it paralells the 2 batteries, probably when an output from the alternator is present, but depends on where the black wires are connected.

 

[weaver_fish]

I shall do some tracing of wires tomorrow and report back forthwith!

 

[johna]

As others have said it appears to be a split charge relay. The pics do not show the complete route of the green wires but I assume they just link the two switches together. If so they look of a lower amp carrying capacity than the starter battery switch cables and while the intention is that they only carry a charge from the alternator if the relay contacts became welded together then those green cables could be carrying the whole or part of the engine starting current. I would suggest that a circuit breaker rated to cope with the max alternator output is inserted in the green circuit if there is not already one there to protect against relay failure.

 

[William_H]

i really object to the term "charge splitting relay.' because it is misleading. "Charge splitting diodes might be OK but not a relay.

The relay is used to apply charge to an other wise inherently isolated battery system. So why not "charging relay" (Not that I could ever hope to redirect the progress of the English language with the passage of time)

Anyway it does look like a plain relay as fitted to cars for headlights or horn. If the domestic battery had a circuit breaker thne if the charge was directed on the side of the CB away from the battery that would protect the wiring from start current if the relay should weld its contacts.
The relay can as already stated be driven by an oil pressure switch or even a rectifier off the aux terminal of the alternator. Which only gives output when the alt. is charging.

So using a multimeter check the domestic battery voltage before and after start up of the engine. It should be around 12.5 volts or so going up to 13.5 or more with engine running.
Now do the same test with one of the wires of the relay disconnected. If the domestic battery does not rise in voltage ie no charge then the relay is indeed a charging relay. (A good system to my mind)
If that is the case it will be desirable to track down the wires that go to the coil of the relay. One may go to -ve common wire. The other may go to the engine panel or to the alternator. if you can find a aux terminal on the back of the alternator you could remove the wire. If the engine battery gets charge but the domestic doesn't then that is the clue. Like wise if you find a likely looking wire that goes to the engine panel and or oil pressure switch. good luck olewill

 

[whipper_snapper]

Indeed. The only thing that's missing IMHO is a 3rd 'paralleling' switch to allow you to use the domestic batteries to start the engine in an emergency.

 

[halcyon]

Well since 1970's I've always known them as

split charge relays.

As in allowing a charge source to charge two or more battery banks.

I think charge spliting relays is a typo error, that someone has copied.

Brian

 

[johna]

William
From what I can see you are the only person to use the phrase "charge splitting relay". Most have used the expression "split charge relay"