Paralleling switch and battery fuses – how?

[BabaYaga]

Hi,

I would appreciate your thoughts on the best way to wire in a paralleling switch in a system where both the starter battery and the domestic battery bank are fused.

In my boat the alternator output is wired to the domestic battery bank, via the positive busbar, a shunt (for the V and A-meters) and a 80 A fuse , close to the battery. There is no switch in this charging circuit.
The starter battery has a 250 A fuse close to the positive post. There is also a switch in the wire leading to the starter motor. In order to charge the starter battery, a 70 A relay connects the positive busbar to the load side of the switch in the starter motor circuit as soon as the alternator starts to put out.
Last year I decided to wire in a paralleling emergency switch, so that I could use the domestic battery bank to start the engine, in case my (rather small) starter battery became depleated.
I connected this switch to the battery side of the two fuse holders, so that I would still have the 250 A fuse in the circuit, even if I were to use the domestic batteries for starting.
However, I have started to wonder if this is the best approach. I have seen suggestions in this forum that it is better to wire the emergency switch directly between the domestic bank and the starter motor, bypassing the starter battery. I suppose the reason is to avoid the domestic bank draining into the empty starter battery before the engine is started.
What do you think is the best way? And how do switches with incorporated paralleling, like this:
http://bluesea.com/category/1/products/6011
deal with fusing of different amperage in starter and domestic circuits?

 

[pvb]

I connected this switch to the battery side of the two fuse holders, so that I would still have the 250 A fuse in the circuit, even if I were to use the domestic batteries for starting.

That doesn't make sense.

Ideally, the parallel switch should be wired to the output side of the domestic isolating switch and to the output side of the starter isolating switch. That way, if the starter battery is flat, you can isolate it, then use the parallel switch to connect the domestic bank before starting the engine.

The Blue Sea switch you linked to wouldn't enable you to isolate the starter battery in this way.

 

[BabaYaga]

Ideally, the parallel switch should be wired to the output side of the domestic isolating switch and to the output side of the starter isolating switch. That way, if the starter battery is flat, you can isolate it, then use the parallel switch to connect the domestic bank before starting the engine.

Yes, but would I not then blow the 80 A fuse in the domestic circuit?

 

[pvb]

Yes, but would I not then blow the 80 A fuse in the domestic circuit?

Quite right, thought the 250A was in the domestic. In that case, I'd wire the parallel switch to the battery side of the domestic fuse, but install a second 250A fuse as close to the battery as possible. That way, the wire is protected.

 

[BabaYaga]

install a second 250A fuse as close to the battery as possible.

That could certainly work, although I do not remember having seen a circuit diagram that suggests a dedicated fuse in the paralleling connection. Perhaps I have not looked hard enough?
Basically I am just curious how other people have done, as fusing battery circuits are often recommended and so are emergency paralleling switches. But how to best combine these two is not obvious.

Any other suggestions?

 

[JerryHawkins]

Three switches

I don't like that switch at all. Look at http://bluesea.com/files/resources/sales_sheets/Dual_Circuit_Plus_Battery_Switches_(2,200 KB).pdf
Which shows this switch and the alternatives. I have the three-switch option which allows me to switch the engine and house batteries independently as well as combine them or power starter from 'house' bank or power 'house' from engine bank - best of all worlds. This single switch only allows both loads connected, both off or combined.
If you're not sure what you're doing here, you could do a lot of damage to your boat and yourself. I would suggest contacting a competent electrician in your local area. The Blue Sea systems site has lots of useful information including wiring diagrams which you could follow yourself or show to an electrician to say 'thats what I want'.

 

[VicS]

The simple solution surely is to connect your emergency switch, not as a paralleling switch, but directly from the domestic battery to the starter motor side of the starter battery isolating switch.

That would enable you to use the domestic battery, without the 80amp fuse being involved, with the starter battery isolated if it is completely knackered.

The only draw back is that there will be no fuse in this emergency circuit. It is common not to find main fuses in starter circuits anyway so it not a serious problem, esp for emergency use.

If you must have a fuse then fit a 250 amp one in series with your emergency switch. Thats what pvb is suggesting.

Alternatively rewire the starter battery isolator and fuse to put the isolator first in line followed by the fuse then connect your emergency switch to the junction of the two. The isolator can then isolate the starter battery if necessary .. the existing fuse protects both old and new circuits.

 

[BabaYaga]

Many thanks for your replies so far.
I will have to think though the different options, but am leaning towards putting a second 250 A fuse in the emergency connection between domestic bank and starter switch.