Battery isolator

If you want to avoid complexity (after all, its just for emergencies) - just carry a jump-lead...

Alternatively, just wire a switch in (with jump-lead thick wire) to do what the jump-lead would do (+ve to +ve).

There's other ways to do it that might avoid a flat start battery draining you domestics, but typically, the improvement in minimal (unless the start battery is *really* flat, in which case, you are still in the marina - cos it happened over a long period of time...)
 
If you are looking for a good quality solution then have a look at the switch banks produced by BEP Marine - I have a block of 3 switches (domestic, engine and both) + a voltage sensing relay to control battery charging. Merlin Power store are one outlet in the UK, there may be others (no links to any of the above...)
 
My preference...

[ QUOTE ]
I have 2 separate isolators but am considering a combined switch

[/ QUOTE ]In that case, my preference would be to install a third isolator switch of the same type as your current starter battery isolator switch. This will be your emergency combining switch. Connect it from the positive of the domestic batteries to the starter side of the existing starter battery isolator switch.

Depending on how far away the domestic batteries are, you might need to use heavier cable than you currently have on your starter circuit. You also need to ensure that the negative return to the domestic batteries is equally heavy cable.

If you got an identical switch, and if the isolator switches have removeable handles, take the handle out of the new emergency switch and put it in a safe place. Then, if you need to start using the domestic batteries, turn off the starter battery switch, remove the handle, put it in the emergency switch and turn it on, thus connecting the domestic batteries directly to the starter circuit. If you do it this way, you'll avoid losing charge from the domestics into a flat starter battery.

Please don't get a 1-2-Both switch - they should have no place on a boat these days.
 
Go for the 1-2-both-off switch, simple and straight forward. Gives you all the options at the flick of a switch. Just a note of caution the switch should be such that it makes before it breaks. So that you can safely go from 1 to 2 to both, or back with out having the alternator running without a battery to charge. This will save your alternator from blowing the diodes. So when you are changing the switch setting with the engine running NEVER go through OFF.
 
Go for the 1/2/both/off switch only if you are going to shell out for a good quality one (high amp rating that is). I have had one of those go recently and also a simple isolator. I have since donew away with the 1/2/both/off and have an good quality isolator for both the engine and domestic battery. very simple.
 
I second PVB on this; get a third isolator switch which has a removal handle.
I had this on my last boat ............... only ever used it for a test but it is a simple solution.

Alan.
 
Far easier to use the split charge relay.
Press a button by the engine start / helm and the split charge relay cuts in linking the batteries for starting, if engine does not start the link drops out to protect the service bank. Gives remote operation, existing wires, and does not have to be perminantly linked while you go off to start the engine or turned back off.

Brian
 
The only problem with all these is that none of them tend to work in an emergency unless your domestic bank is so big it practically ignores the starter batt, even jump leads will fail if your engine batt has a short. If you connect your good battery to a dead one to start the engine you finish up with 2 dead batteries very fast as the the dud shorts out the good one. Starter motors take very little power (about 1.5a/h per min) but need a good battery that will hold a voltage under starter loads to get the engine turning fast enough. The simple solution is to DISCONECT the start batt and run the engine from the house bank via jump leads untill you can repair the fault.

The only time you should need this is because the engine batt has failed or the engin is dead and you have flattend the batt trying to start it, in both cases it is a wast of energy to try to put anything into the engine batt. If you can discharge the engine batt by any means other than through the starter motor change the system so you can't!
 
Hello Roly Voya I would kind of disagree with your sentiments. Technically correct but in my experience with cars when you jump start a battery you simply paralell the new battery to the old (dead) one. You usually hit the starter button soon after. So the donor battery doesn't get time to be discharged. I think the best answer to the post is to provide a heavy jumper lead with clips to go onto the terminals.

This assumes that the negatives of all the batteries are commoned with heavy wire. It also assumes that there is a suitable charging arrangement for the domestic battery.

A VSR to my mend is the best method. As suggested some VSRs have a relay capable of handling the emergency starter current from the domestic battery and so have an emergency jump start switch but some may have a relay only built for max charge current so beware.

Using a 1,2.both switch is fine but it becomes very easy to always start on both. (jump start whether it is needed or not) The problem is that you will never know if the engine battery or indeed the domestic battery has died until both are dead. and you are stuck.
good luck olewill
 
I agree with you, it was a simple answer to a general point.
It is easy to just turn off the engine battery isolator, then press a link button when you want to start the engine, connecting it to the service battery. The relay will lock in running the engine from the service battery, and charge the battery. You can if you wish always switch the engine battery isolator back on.
On our boat I have a service battery in the port and starboard forward saloon lockers, the engine battery is at the rear of the cabin. So to fit a jump lead would be a serious problem.
It also depends on the boat design, it may be twin engined with the services taken from bank, so a flat engine start battery is quite possible. It may be the batteries are be in the engine room and thus not easy to get to. Or you may have spent £1,000,000 buying the boat, and having to fit a jump leads may not go down well. In whitch case when you press the link switch, the duff battery is remotely isolated.

It's just a matter of cost and complexity, if you think you may want link start, and need a split charge relay, why not fit one that has link start, costs no more, and uses what you have.

Brian
 
Yes but usually if a car battery is flat its because the lights or something have been left on and its discharged but the battery is OK just flat. Normally you jump start it from another car with the engine running so are putting enough charge into the battery to get it going. If you use a mobile battery it will normally work because the car battery will pick up enough volts to get the engine going before it goes too flat. This is fine on a car which only has one battery that runs services and starting. My argument is that on a boat because the engine start battery is ONLY used to start the engine you will never get a flat battery unless something has failed, there should simply be no way it can discharge, thats why you have a seperate system because you cant call the garage for a jump start. So if you turn the key and nothing happens its NOT a flat battery, assuming the alternator etc are ok its a dead battery so pumping charge ito it does nothing exept heat it up. You need to ditch that battery and hook up a new one. The problem with linked relays is that you would normally try this first whith the result that if there is an internal short on the engine battery (very common failure) buy the time you have tried and failed to start the engine on a 'Both together' system you may have flattend the house battery as well. Even if you do succeed and the engine starts you are now running the alternator into a shorted battery which wont do it any good and may set fire to something in the system as both the alternator and the house battery are now dumping into a short. So lots of reasons it might not work some of which are dangerous - far better to get it completely out of the system. I know its quite a common system but then so is electrical failure!!
 
If going for separate isolators, tie the little red actuator handles on the the switches with fishing line and fit a sliding cover over the whole face of the area. Then little or big feet won't switch the current off or on accidently.Personally I favour not having combined 1+2+both as when these fail, and they do, it's more difficult to react quickly to sort out the problem.Also switches of either type left in the off position can corrode internally on the contact faces and not make any contact in the spring. solution is to disconnect at the battery + poles when laying up, tape the cable in a safe location.Then leave the switches closed for the winter.May work for the combined switches too.If the red handle type ones give trouble, the have the advantage that they can , by removing a pin, be disassembled cleaned, contacts coated in vaseline and reassembled.
Loadsa money, near a chandlery? disregard this post!!Out in the real world, high seas etc. situation often a little different. like 100nm off Galway, Force 7, boat in sudden darkness.Did I tell you about the women's handbags? mmm. One fine leather model is used for the mechanical tools.Labeled "mech tools" . Another marked "electrical tools" qualities of fine leather seem to protect contents somewhat from ravages of salt air.Any I get a few stares if working on the dockside!
Have Fun !
 
Hello Roly it is good to get a decent argument. Of course the important thing for anyone is to understand that you are correct. My argument is perhaps with the frequency of shorted cells in car batteries. My experience is simply a cell failing to deliver the starting current. Every flat battery i have had with a range of cars has really been time for a new battery,
Anyway not a problem for me with a pull start outboard thatm most always gets left home. olewill
 
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