Battery change-over switch wiring

[Roach1948]

As part of the engine replacement I have decided to re-wire the basic electrics. She is only a day sailor so I don’t have much an electrical drain. Just Nav lights (I rarely sail at night), interior lights and a bilge pump. GPS and VHF are battery powered.

I notice that my Plastimo change over switch (made my AAA) has two field connectors and I understand one these are for wiring in a blocking diode, regulator or Isolator via the alternator. Not entirely sure what each of these does, but do understand that one of allows one battery not to discharge into the other and another allows for charging of the drained battery first. Do I need any of these items such a simple set-up, and if so can I just ignore the field connections on the switch?

 

[Norman_E]

You should arrange matters so that the engine start battery is not discharged by other uses. I think that simply needs the blocking diode, but do not have a circuit diagram.

 

[Guest]

The field disconnects are usually a set of 2 contacts that break before the battery terminals break.

If they are correctly wired into the alternator field circuit they can disable the alternator before the battery is disconnected. The field circuit is not the ignition circuit.

The theory is fine, but in practice I would leave them alone. Suddenly stopping any current in an alternator can cause high voltages.

 

[William_H]

Yes just ignore the field connection terminals. You should have just 2 terminals for +ve of each battery and one terminal for outlet to engine and all power. This makes a simple 1,2,both switch. Just don't change the switch with engine running.
I assume you have 2 batteries. from your description you could probably manage with one battery only but if you already have 2 you might as well retain both.
Usually you will start and run on both. However you should occasionally test each battery by starting on one battery and next time the other. Just to ensure that they will both do the job of starting the engine. Cos one day they will die hopefully at different times but you won't know if using both the other battery does the job until they both are dead.
I think much preferable is 2 individula on off battery switches, but use what you have got.
olewill

 

[Roach1948]

Yes, I only have two batteries. I will leave the field connectors well alone then and set-up the system you advise. Its seems a very simple solution and for my requirements I dont think I need anything more complicated (and expensive)

 

[andy_wilson]

Don\'t entirely agree...

The only reason to start on both is if both batteries are low, but combined can get the engine going.
A better regieme is to mentally have 1 battery for starting and emergencies (VHF & GPS) only (say batt. No.1), and 1 for domestic loads (batt. No 2).
If you wish you could have a starter battery for 1 and a deep cycle for 2, though I prefer to use truck batteries designed for tail lift applications as they are perfectly happy at both, and therefore you build in redundancy.
Always start on 1 so you know it has a good charge (IF it starts the engine that is) and switch to both whilst engine is running. Switch to 2 once engine is stopped.
The rotary switch is designed to allow switching whilst the engine is running, provided you go via BOTH and not via the OFF position.
If you develop a battery fault and take the advice above and always start on BOTH, you run the slight but quite possible risk of a perfectly good Batt. 1 discharging itself rapidly through a faulty Batt. 2 (or vice-versa) and leaving you without power to start the engine.

I agree with the latter part however, if both batteries are designed for engine starting, test the 'domestic' one occasionally to underwrite the redundancy.

 

[Guest]

I have one reservation ..... as a user of the 1-Both-2-Off switch ....

Will has said start on both batterys and dont change switch when engine running. Sorry but I must disagree.

I reckon and this is what I do - Decide which battery will be your engine satrt battery ... mine is #2 .... the other is your domestic ... mine #1. This way you are only looking at one battery destroying itself ! Usually the domestic by being dragged down low on discharge.

Having both on for starting - one day you will forget and bingo - no juice to start engine as you forgot to change switch over ...

Not changing switch while engine is running is only applicable if you go via the OFF postion .... a definite NO NO. But you can change 1 to 2 to both as you like - with a proper battery switch - it is Make before Break ... so alternator is fine.

My regime is : Start on #2 ..... motor out of marina with switch still on #2 .... after 30 mins or more - switch to both to charge up both batterys .... when engine stopped - switch to #1 to keep instruments etc. going and keep #1 for engine. Often I start engine on #1 leaving #2 alone as back-up ... but may just as often reverse sequence on arrival at destination .... this ensures both batterys are exercised and charged. With both - you have no idea which is good / bad / charged or not etc. With my system - suspect battery shows up immediately - leaving you still able to start engine ... from good one that has not been discharged into bad battery !