Numpty electrical question

[Nick]

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Present set-up is two 110AH leisure batts on a 1-2-both switch with one kept exclusively for engine starting.

I intend to buy a dedicated engine start battery (small car batt with devent CCA prob.) and wire my two existing 110AH leisure batts in parallell. (Partly because I am installing a Webasto so will need more juice).

Anyone got a simple wiring diagram for this battery set-up?

[marisca]

If you peruse the PBO site there was a recent thread on just this very topic. I would suggest that having separate switches for your paralleled batteries is a good idea - if ever one cell fails, and eventually one will, you won't want the good battery flattening itself. So the simplest way is each 110Ah +ve connected to its own simple battery switch, the output from these switches both connected to the "2" terminal of your existing switch; the +ve of your starter battery connected to the "1" terminal; all -ve terminals connected together. This allows you to have any combination of the 3 batteries but beware of selecting "2" with both 110Ah switches off - not a good idea!

[Nick]

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Hmmm . . . seems like a lot of switches - and wiring.

Could you post a diagram?

[marisca]

Exactly the same diagram as VicS's on TOP but I added (not absolutely necessary) switches on the +ve of each 110Ah battery. The switches aren't necessary 'cos you can just disconnect the terminal from a battery if one goes phut and even if the dud cell has dragged both 110Ah down to ~10v you will still have the engine start yin to get you out of trouble.

Incidently, what is this double posting shite about?

[Nick]

Incidently, what is this double posting shite about?

There's a somewhat larger pool of knowledge over there . . . and someone posted a diagram, which is what I was hoping for. They must have taken the time to draw and scan it, so good for them.

On here I get grief for double posting but no diagram

People do seem to be concerned that if the two batteries in the leisure bank aren't in exactly identical condition then disaster looms . . . how worried should I be? If I wanted to fit battery isolator switches then it would mean a heap more heavy duty wiring in a place where there isn't enough space for it. (And I still need a diagram - and a picture of this type of switch would be useful to help me get my head round it).

I presume that with said isolator switches fitted, if the domestic bank was playing up I could isolate either battery and check the voltage separately to see where the problem was ?

[Clyde_Wanderer]

Something to note Nick, apologies if I am teaching my Gran to suck eggs.
Always regard each battery as a Bank,regardless how many batteries are in it, for the purpose of connecting to the selecter switch.
It also helps if you can completly isolate a dedicated starter batt bank in the event that it is flat and you have to use domestic bank to start engine, this way the start bank wont draw down the domestic bank while starting.
It makes the set up a little more complicated but gives you more options.
Here is a rough draft of my batt set up, which ensures that the two banks are never parralled during charging ensuring that if any bank fails its batts arent stealing all the charge, crappy drawing.
C_W

[Nick]

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Thanks Eamonn. It looks great but I am not sure I totally grasp it

As I said, electrics are not my strong point.

[Clyde_Wanderer]

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Thanks Eamonn. It looks great but I am not sure I totally grasp it

As I said, electrics are not my strong point.

It is actually quite simple Nick, just study it for a while removing the X Split unit.
It is wise to have all associated ignition electrics taken from the start batt as opposed to the domestic/service bank as they will only be used while the engine is running, ie you dont need temp/fuel guage etc, while the engine is not been used.
It is said that an alternator can be damaged if engine is started without having selecter switch to either of the banks, this cant happen in my set up as the Batt isolater switch has to be On to start engine in the first place.
I have been asked to completly rewire a Cutlass 27 in the new year, a tedious task.
C_W

[marisca]

Nick

The VicS diag on TOP is simple and gives you pretty well the same operating rules as you have already and just replaces one battery with two in parallel. Switch on "1" for the battery to be saved for engine starting; "2" for all domestics, lighting and going through the night; "Both" for charging both when the donk is going (and I use this for starting as well). The 2 110 Ah batteries paralleled together can be singled by simply removing the single wired terminal from the one you don't want (and making sure it can't touch anything electrical). These batteries will always be at the same voltage so if a cell does go short in one of them the other will discharge down to ~10v too. It will eventually happen but that is true for any system.
VSRs, fuses, extra switches can be incorporated but you are heading towards a rewire. If you are happy with what you have and just want more hours of lights, radios, etc. then go with the VicS diagram. You will need 2 bits of battery cable and terminals like and leave everything else alone.

[marisca]

I've been contemplating the same game but space for an extra battery and its box (suitably restrained) requires thought. Which comes first - the battery size or the box size?

[Nick]

I've been contemplating the same game but space for an extra battery and its box (suitably restrained) requires thought. Which comes first - the battery size or the box size?

If you've got money it's a no-brainer - get a Red Flash starter battery. They are about half the size for the same CCA. I will be looking in Oban for a cheap car battery though and then making a new box to suit.

I am very grateful for your comments on Vic's diagram. The 1-2-both system has worked well for us for a long time now, and I don't have Eamonn's expertise or the desire to do a serious rewire, so will keep it as simple as possible. I will do what you suggest.

(In forgot to say - both battery banks are in any event wired into the wind generator, so both are trickle charged).

[sahona]

I haven't gone looking at all the diagrams referenced above, but don't see what the problem is Nick.
If you already have a 1-both-2-off switch and like it, just move one existing battery across and parallel it to it's mate,
Then procure the motor start battery and put it where you removed the other from.
This of course needn't be done literally,,, just move the cables as you will probably be making another battery box elsewhere.
If you want step by step instructions, post a picture of the existing set-up.

[Nick]

I haven't gone looking at all the diagrams referenced above, but don't see what the problem is Nick.
If you already have a 1-both-2-off switch and like it, just move one existing battery across and parallel it to it's mate,
Then procure the motor start battery and put it where you removed the other from.
This of course needn't be done literally,,, just move the cables as you will probably be making another battery box elsewhere.
If you want step by step instructions, post a picture of the existing set-up.

Thanks Bill,

I think I've got it in my head now . . . but if I get confused I will post a pic of the current set-up. The only real complication is the wind generator, whcih is wired to charge both batteries.

[sahona]

Try separating the input (charging) circuits and output circuits in your mind when designing (or pondering!), unless of course they are the same.
If you already choose which bank (or both) the alternator feeds at any one time by using the big switch, the windmill may be put in the same place as the alternator, however I would have it feeding just the domestic bank all the time, and let the start battery get fed (like your car) when the engine runs. If it gets flat, that's what the Both position is for!
Otherwise a splitter - either relay or FET (victron) will deliver available charge to both banks as required regardless of the 1-b-2-o switch.
As I said if you're happy with the existing regimen just move the batteries as above.

Happy sparking.