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Thread: dual batteries & dual voltage sensing relays

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    Expert pollenface's Avatar
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    dual batteries & dual voltage sensing relays

    After adding a 190w panel to the roof and a Victron 75/15 smartsolar MPPT to specifically charge the Aux battery, I added a 2nd voltage sensing relay to help maintain the cranking battery when the patrol is parked in the sun for long periods.

    So both batteries will connect to the other when their voltage is over 13.3v & disconnect when under 12.8v.

    I'm in the process of running 8mm2 cable to the fridge to help minimize voltage drop when compressor is on.

    I like it, all I need now is a decent aux battery
    2008 CRD Auto Wagon

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    Travelling Podologist Cuppa's Avatar
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    Sounding good, but I'd suggest that 8mm2 may be a bit light unless the fridge is pretty close to the battery. I would use a minimum of 8AWG which is 10mm2. Also if you are expecting cable induced voltage drop when you have the compressor running, do you have the compressor & fridge on the same circuit? If so I would advise a separate circuit for each.

    I have just run two new circuits in our truck. One just for Starlink running direct of 12v (12v to 47v converter) from a circuit breaker rated at 16 amps, & the other to run an Engel freezer & my Cpap machine from a 10 amp circuit breaker. Both using 8AWG/8B&S cable.

    I'm kinda hoping that you have used the same gauge cable & wrongly assumed it is 8mm2?

    2006 4.2TDi ex-Telstra Remote area Camper.
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    Expert pollenface's Avatar
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    Quote Originally Posted by Cuppa View Post
    Sounding good, but I'd suggest that 8mm2 may be a bit light unless the fridge is pretty close to the battery. I would use a minimum of 8AWG which is 10mm2. Also if you are expecting cable induced voltage drop when you have the compressor running, do you have the compressor & fridge on the same circuit? If so I would advise a separate circuit for each.

    I have just run two new circuits in our truck. One just for Starlink running direct of 12v (12v to 47v converter) from a circuit breaker rated at 16 amps, & the other to run an Engel freezer & my Cpap machine from a 10 amp circuit breaker. Both using 8AWG/8B&S cable.

    I'm kinda hoping that you have used the same gauge cable & wrongly assumed it is 8mm2?
    sorry by compressor I meant the one in the fridge.

    I used to have a 3rd battery in the back right next to the fridge with a separate dc-dc charger but used that battery in another project and now want to simplify things in the car and just stick with the two under the bonnet.

    I've already bought the 8AWG cable so if I have issues I'll have to either double it up or get bigger stuff
    Last edited by pollenface; 12th June 2024 at 09:34 AM.
    2008 CRD Auto Wagon

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    Travelling Podologist Cuppa's Avatar
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    Quote Originally Posted by pollenface View Post
    sorry by compressor I meant the one in the fridge.

    I used to have a 3rd battery in the back right next to the fridge with a separate dc-dc charger but used that battery in another project and now want to simplify things in the car and just stick with the two under the bonnet.

    I've already bought the 8AWG cable so if I have issues I'll have to either double it up or get bigger stuff
    Ah my mistake re compressor. Doh!
    If it's 8AWG & not 8mm2 it'll be fine.

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    Expert pollenface's Avatar
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    Quote Originally Posted by Cuppa View Post
    Ah my mistake re compressor. Doh!
    If it's 8AWG & not 8mm2 it'll be fine.
    apparently 8awg is 8.3mm2, 7awg is 10.5mm2
    https://www.rapidtables.com/calc/wire/awg-to-mm.html
    2008 CRD Auto Wagon

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    Travelling Podologist Cuppa's Avatar
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    Quote Originally Posted by pollenface View Post
    apparently 8awg is 8.3mm2, 7awg is 10.5mm2
    https://www.rapidtables.com/calc/wire/awg-to-mm.html
    Hmmmm, I may have got it wrong based upon this…. https://www.multicable.com/resources...ce-awg-to-mm2/

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    Expert Brissieboy's Avatar
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    Quote Originally Posted by Cuppa View Post
    Hmmmm, I may have got it wrong based upon this…. https://www.multicable.com/resources...ce-awg-to-mm2/
    There is a lot of confusion and mis-use with wire gauges and sizes. According to the old standard, 8 SWG is 12.97 mm2, 8 AWG (also B&S) is 8.36 mm2. There is also 8 BWG (not common now) 14.79 mm2.
    The only way to be absolutely sure what you have is to measure and count strands and calculate the total cross sectional area.
    Then there is the 'automotive' way of referring to size by 3mm, 4mm etc which doesn't appear to directly relate to either wire diameter, copper diameter or cross sectional area.
    The best way to purchase electrical wire is to go by copper cross sectional area only and ignore all the rest.

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    I've got a voltage drop calculator on my phone which shows AWG/B&S and mm2 ... so i understand that at least
    2008 CRD Auto Wagon

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