Battery isolation relay

[beneteau_305_553]

I have a Volvo 2002 with twin battery installation and a diode splitter to isolate each battery. The volt drop through the diode however is about 1 volt so the batteries never fully charge.

The previous owner gave me a isolation relay so that the domestic battery is isolated from the engine starting battery when the engine is not running ensuring that the starting battery is always fully charged.

I could connect the control side of the solenoid to the blue/red cable on the switched side of the ignition switch but the cabling is quite long.

Does anyone know if another cable can be used so that the relay only switches when the alternator actually starts charging?

Richard

 

[Trevor_swfyc]

Richard,

I have a Volvo 2002 with twin batteries they should already be set up so that the starter battery does not run down if you leave the domestic battery discharging.
Otherwise what was the point of the twin battery system?
Have you put a meter on the batteries to verify the starter battery is discharging with the domestic battery. The voltage on charge of my batteries is 14.2 volts they settle back to 13.6 when the charge / engine is turned off.
Hope this info is of some help.
Trevor

 

[Geoffs]

I've been looking into this issue as well. Firstly, of course, you need to make sure that your relay is capable of switching the charging current for the domestic battery.

Some alternators, mainly automotive, have an output used to switch an ignition light, not usually found on a marine altenator.

So another switching source is needed. You can use a feed from the ignition, as you say, but this will connect the two battery systems together during starting. not ideal.

If you have an oil pressure switch this could be used to switch the relay, thus delaying connection until oil pressure is established.

Another option is to use a delay timer, via the ingition circuit. This will connect the systems after a fixed period, allowing engine starting.

Yet another idea, use a simple battery condition monitor, as produced by Maplin Electronics (about £10). This will monitor the starting engine battery, and switch the relay when the voltage rises, i.e., when charging.

Just a few ideas I have had, hope they make some sense, I've not tried any yet.

 

[pvb]

Battery sensing...

It would be fairly easy to modify your alternator so that its regulator senses the voltage at the battery (thus compensating automatically for the voltage drop across the diode). You'd need to take the alternator off and have an auto-electrician modify it. Then you just run a single wire from the alternator to the domestic battery.

A better solution is to fit a "smart" regulator such as the Adverc, which will help ensure that your domestic battery recharges in the minimum time.

I tend to dislike relay-based solutions. With a relay, if the domestic battery is quite flat and the starting battery fully charged, when the relay closes there can be quite a large current flow between the batteries, which isn't particularly good for either of them!

 

[Guest]

Hi,

I have VP 2002 on my cat, with a 'duel charge relay' (blue solinoid) and it has worked well since I have had the boat (8 years). I think it is delco remy and it is used to charge caravan batteries. This relay will only allow the engine battery to be connected to the domestic batteries if there is about 12v in the engine battery. Less than that, then the engine battery is isolated and all the charge goes into it. There are 2 small connectors which go to the engine battery via the start switch, this is to power the relay when there is 12v, and 2 large connectors; this is 12v positive in from the altenator, and 12 v positive out -to the domestics. There is also another small connector for powering the engine room fan.

I live on board so there is lots of use and I carry a spare as every couple of years the contacts inside need cleaning - a 5 minute job which can be tackled at my leisure.

This is a cheap and foolproof system.

David J