split charge relay

[dreadnought]

hi all,i`m going to take out my split charge relay soon(its packed up)and was wondering if i just connected the alternator charging cable direct to the leisure bank and took another cable from the alternator to the starter battery would this work ok,i only have a single alternator, many thanks  

[MtB]

Depends what you mean by "work ok".

 

Yes both batteries will charge and appear to work just as before, but now if you run your domestic batts down to utterly totally flat, you'll have taken the engine battery down too.

[David Mack]

Fit a Voltage Sensitive Relay - the fit and forget solution.

[BEngo]

It is OK as a short term expedient but you really want to replace the original relay with a voltage sensitive relay, connected so the start battery recharges first (takes very little time ) then the domestic battery comes on to charge.

 

N

 

[nicknorman]

Wot mtb says. But you could insert a battery isolator switch in the link between the two batteries, so you can isolate the start battery from the leisure when you stop charging.

Just now, BEngo said:

It is OK as a short term expedient but you really want to replace the original relay with a voltage sensitive relay, connected so the start battery recharges first (takes very little time ) then the domestic battery comes on to charge.

 

N

 

i disagree with this. The reality is that starting an engine uses very little charge, maybe 1Ah or less. And thus as you say recharges very quickly. And then ever after that (with your proposal) the main current going to charge the domestic batteries has to route through the relay, creating heat and voltage drop. Have the VSR by all means, but prioritise the domestics by connecting the alternator supply direct to the domestic batteries (via the domestic battery isolator to keep BSS man happy).

[Tracy D'arth]

6 of 1 and half a dozen of the other.

 

This could run for months.................................................................

[Tony Brooks]

1 hour ago, BEngo said:

It is OK as a short term expedient but you really want to replace the original relay with a voltage sensitive relay, connected so the start battery recharges first (takes very little time ) then the domestic battery comes on to charge.

 

N

 

 

I completely disagree with this and agree with Nick for the reasons he stated. The engine battery will spend most of its life only discharged by a very few Ah so doing it our way passes the minimal current through the relay contacts. In any case the batteries will apportion the available charge between themselves according to what is needed so those few h will be put back in before long.

 

I think the problem is the automotive boys seem to advise doing it as BEngo says but that is, in my view, only to save the fitters having to rewire the charging circuit when they fit the relay.

 

[dreadnought]

1 hour ago, nicknorman said:

Wot mtb says. But you could insert a battery isolator switch in the link between the two batteries, so you can isolate the start battery from the leisure when you stop charging.

i disagree with this. The reality is that starting an engine uses very little charge, maybe 1Ah or less. And thus as you say recharges very quickly. And then ever after that (with your proposal) the main current going to charge the domestic batteries has to route through the relay, creating heat and voltage drop. Have the VSR by all means, but prioritise the domestics by connecting the alternator supply direct to the domestic batteries (via the domestic battery isolator to keep BSS man happy).

so are you saying,alternator to vsr from vsr  to domestic bank as a priority then secondly to starter battery ?

[nicknorman]

9 minutes ago, dreadnought said:

so are you saying,alternator to vsr from vsr  to domestic bank as a priority then secondly to starter battery ?

Not quite as you’ve described it. Alternator to boat side of domestic battery isolator, other side of domestic battery isolator to domestic battery +, domestic battery + to VSR, other side of VSR to engine battery +

[Tony Brooks]

19 minutes ago, dreadnought said:

so are you saying,alternator to vsr from vsr  to domestic bank as a priority then secondly to starter battery ?

 

Simple terms. The two thick ores on your rely transfer to the large terminals on the VSR making sure if it is NOT a bi-directional relay the big terminal that is also sensing goes to the battery the alternator is connected to. transfer the small negative from your rely to the small negative on the VSR. remove or tape back the last thin wire on the old relay unless there are two. In which case just join the and insulate (The original fitted may have just cut the warning lamp wire so continuity needs maintaining).  That will get it working with the minimum of effort.

[ditchcrawler]

3 hours ago, BEngo said:

It is OK as a short term expedient but you really want to replace the original relay with a voltage sensitive relay, connected so the start battery recharges first (takes very little time ) then the domestic battery comes on to charge.

 

N

 

This leads people to believe that the relay will charge the starter battery first and then the domestics, where as in actual fact both banks are in parallel as soon as the relay closes which is normally as soon as the alternator warning light goes out. So regardless or whether you  feed directly to the starter  or the domestics the charging of the batteries is the same. I don't know of any smart ones that charge one first and then the other

[Tony Brooks]

For absolute correctness. A very flat battery that is connected to the alternator, or a heavy load on that battery could well cause the charging voltage to drop to about 13 volts and then slowly climb as the battery charges and the current drops. As VSRs typically energizes between about 13.4 and 13.6 volts the relay may not energize for a period of time so the other battery would not start to be charged until the charging voltage reached that 13.4 to 13.6 volts. However in that scenario I would want the flattest one starting to charge asap. Until a battery starts to fail this situation would rarely occur,  especially as we try not to discharge batteries below 50%. I think Lithiums may hold the voltage down for far longer so those may need a different approach.