Should alternator be permanently connected to battery?

[Esmerelda]

I am currently rewiring out battery system to include a VSR to give priority charging to the engine battery. With the current system, the alternator is connected to the starter motor, which then runs along one thick cable to the common of a 1/2/both switch. With the VSR system, is it better to separate the alternator and starter motor wires so that the starter motor can be isolated via an on/off switch and have the alternator permanently connected to the battery... or is it better to have both starter motor and alternator going to the on/off switch so that when that switch is off, the battery is completely isolated. Are there significant benefits / disadvantages of each system as they both seem to have their supporters.

Any sensible views appreciated

 

[jac]

Sounds similar to the system that we had until last weekend.

Some alternators need 12v to the b+ stud to actually charge hence the wire to starter, thence to battery so however you wire it you will need to ensure that is place.

 

[pvb]

If you're fitting a VSR, the simplest and safest solution is to have 2 on/off switches - one to isolate the starter battery, one to isolate the domestic battery. With this arrangement, you can safely isolate both batteries when you leave the boat. You could conceivably use the existing 1/2/Both switch as one of the on/off switches.

 

[Esmerelda]

The intention is to use 2 on/off switches, what I am unsure about is whether the alternator should be permanently connected to the engine battery or whether it should only be connected when the switch is on - and if alternator is permanently connected to battery, what would be the drawbacks...

 

[pvb]

The intention is to use 2 on/off switches, what I am unsure about is whether the alternator should be permanently connected to the engine battery or whether it should only be connected when the switch is on - and if alternator is permanently connected to battery, what would be the drawbacks...

If the alternator is permanently connected to the battery, you can't isolate the battery. In the event of a wiring fault, there could be a fire, unless you fuse the alternator cable close to the battery.
.

 

[the_branflake]

If you're fitting a VSR, the simplest and safest solution is to have 2 on/off switches - one to isolate the starter battery, one to isolate the domestic battery. With this arrangement, you can safely isolate both batteries when you leave the boat. You could conceivably use the existing 1/2/Both switch as one of the on/off switches.

I did as pvb has suggested, got the VSR and two on/off switches so that both batteries can be isolated should they need to be. When we arrive switch both on, when we leave both off.

 

[BabaYaga]

A fuse would be mandatory in that case, but it would have to be quite large to cope with the starting current, 250 A or so. One drawback of not being able to isolate the battery is the possibility that a limited wire fault or short developed that created a lot of heat and a fire risk without actually blowing the fuse. Don't know how likely that is.
On my boat the alternator is permanently connected to the (fused) domestic battery bank. The relay puts the starter battery online. The advantage of this setup is that much less current flows through the relay. There is also an emergency switch by which the engine can be started from the domestic bank. I think the idea of "priority charging" of the starter battery needs some consideration. In practice, the duration may be counted in seconds.

 

[prv]

If the alternator is permanently connected to the battery, you can't isolate the battery. In the event of a wiring fault, there could be a fire, unless you fuse the alternator cable close to the battery.
.

This is exactly what I did. A megafuse in a holder bolted directly to the battery terminal, then an unbroken wire to the alternator. I made sure that this wire was well insulated (including a rubber cap over the terminal on the alternator); if I was working in that area and needed to isolate it I could remove the terminal from the battery (or remove the fuse).

Pete