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Split charge on third battery (ie Starter and Aux 1 & adding Aux 2


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Now I've got my alternator working properly I'm contemplating a semi-permanent installation of a third battery to enable sustained use of the Eberspacher heater and inverter in my vehicle for midwinter outings - some work, some pleasure, but all requiring heat and power. Previous outings have seen me run out of power on the Aux battery and needing to run the engine for a spell on really cold overnights (as in -10 or so).

I've been given a spare leisure/deep cycle battery which is identical make/type to the one I currently have fitted - 86Amp hour. My vehicle battery management is done by a Swiss IBS Dual battery system which works very well.

The IBS unit is capable of handling a third battery and their website shows a schematic for adding one with the addition of their SBS unit (Single Battery System) monitor and a relay, the DBR in the drawing below which is a 12v 200A unit. The relay is an eye-watering £175.01 (!) and has some fancy electronics in it and dainty LED's. Information here. I already have the main relay shown between Main & Aux 1 batteries as this came with the original IBS kit.

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Am I right in assuming that what the schematic shows - using their fancy DBR relay to add an Aux 2 battery - can be achieved with this instead - a £14 relay off ebay without the LED's?

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Alternatively, and more simply, do I just wire the second Aux battery in parallel to get the same 12v current but a longer Amp Hour delivery?

I'm assuming the IBS unit will 'see' the battery pair as one battery but with a larger capacity, and charge accordingly. The IBS main unit works so that when the engine is running the main and aux batteries are linked together for parallel charging from the alternator. When the engine is stopped the main and aux batteries are disconnected automatically allowing the starter battery to be 'protected' and the power for heater, lights, etc all run off the Aux battery.

Or am I missing something that will bite me?

(My electrical knowledge is basic, but gradually increasing! So bear with me!)

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Hmm..... Not sure I'd go for that set up TBH.

A lot of weight, a lot of potential for problems with 3 batteries and the attached wiring and electronics..

IMHO the night heater should not drain the battery at all. The thing uses very little amps wise and we have ours on loads and loads of chilly nights (like -25 to -35 C.) in the Alps. The inverter is bound to take quite a bit of amps although things like a laptop etc. don't ned a lot of juice..

Have a look at that inverter as it seems to consume a lot of amps...

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Hmm..... Not sure I'd go for that set up TBH.

A lot of weight, a lot of potential for problems with 3 batteries and the attached wiring and electronics..

IMHO the night heater should not drain the battery at all. The thing uses very little amps wise and we have ours on loads and loads of chilly nights (like -25 to -35 C.) in the Alps. The inverter is bound to take quite a bit of amps although things like a laptop etc. don't ned a lot of juice..

Have a look at that inverter as it seems to consume a lot of amps...

Hi Arjan - the inverter is only charging camera batteries and the like, not running high-draw items. It's possible there's a fault still with the charging circuit as the Eberspacher seems to be really dragging the life out of the battery.

Although come to think of it, it's possible that the fault is the heater fan as I have a feeling its not performing as it should. I might be trying to solve a problem from the wrong end! - feeding more power in rather than solving another issue. The Eberspacher seems to initiate its 'low-voltage cut out' much sooner than it should.

How many hours running do you get with the Eberspacher? (Mine is a D4W)

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Fit a 115AH battery in place of your current 85.

Space is tight as it is which is why I could only fit in a 86AH! The battery compartment lid barely fits.

I did contemplate building a new battery box to accommodate a bigger battery, but then snagged a second one the same as I've got (if I want it). Hence the pondering over using it.

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Although come to think of it, it's possible that the fault is the heater fan as I have a feeling its not performing as it should. I might be trying to solve a problem from the wrong end! - feeding more power in rather than solving another issue. The Eberspacher seems to initiate its 'low-voltage cut out' much sooner than it should.

How many hours running do you get with the Eberspacher? (Mine is a D4W)

The battery cutout should be around 10.2v, , the current draw after initial start up will be for the blower motor, the wp and of course the blower on your fan matrix.

Power on start up about 120w and when running between 50 to 20w + your blower Hope that helps

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The relay you show doesn't appear to have a sense circuit, it's just a coil and the relay contacts. The ones in the first circuit have the voltage sense wire which is what's used to decide if the relay can be operated, closing the contact and thus allowing the alternator to be switched through to the second and third battery to charge them, once the first battery is charging at a reasonable level.

As far as needing a third battery, can't really comment but I don't see why it needs a relay at all unless the idea is to only charge the third when bot the second and first are charging at the correct voltage.

Don't forget that invertors aren't particularly efficient and so consumer power just to work their circuit as well as what the connected devices consume.

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The battery cutout should be around 10.2v, , the current draw after initial start up will be for the blower motor, the wp and of course the blower on your fan matrix.

Power on start up about 120w and when running between 50 to 20w + your blower Hope that helps

Ok cheers for that. I think I need to carefully work out whats actually happening when the heater is on, and get a clearer idea of current draw.

There's less to go wrong with 2 batteries. I have a 110 Ah and a starter battery and a fuse box in my battery box that's slightly modified.

That might be a solution in the longer term. The floor and some of the seatbox needs renewing so fitting a deeper battery box when I repair all that would be a real advantage - as it is I've little room for the nest of wires that's in there.

The relay you show doesn't appear to have a sense circuit, it's just a coil and the relay contacts. The ones in the first circuit have the voltage sense wire which is what's used to decide if the relay can be operated, closing the contact and thus allowing the alternator to be switched through to the second and third battery to charge them, once the first battery is charging at a reasonable level.

As far as needing a third battery, can't really comment but I don't see why it needs a relay at all unless the idea is to only charge the third when bot the second and first are charging at the correct voltage.

Don't forget that invertors aren't particularly efficient and so consumer power just to work their circuit as well as what the connected devices consume.

I was guessing there was a good reason why a £15 relay seemed too easy a solution!

I'm not hell bent on this idea, I just wanted to see if it could be done easily/cheaply but safely. I think now that I have the alternator sorted I need to figure out whether all the ancillary kit is working properly. To be honest, ever since I had the bulkhead swapped by a "well known Land Rover specialist featured in the Haynes Manual" I've had several electrical problems that didn't exist previous to their work, that have proved hard to resolve, but which I'm slowly tracking down.

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