I am installing a 2nd 12v battery into my van to run eletronic devices. The 2nd battery is a bank of 2x LiFePO4 12v 120Ah connected in parallel.

My three questions are surrounding the circuit diagram pictured below:

  1. Is running a ground from the negative pole of Lithium 2 to the negative pole of the starter battery sufficient to earth the system?
  2. Is the green Inverter ground wire earthed if I connect to the negative BusBar (going to negative of battery bank), and what should the rating of the negative BusBar be?
  3. What should the amp rating of the positive BusBar be?

I understand cable needs to thick enough to carry the amps in that circuit, e.g. the circuit with fuse block needs to include wire rated for 100A. What about the interconnecting batteries? Should this be rated for the largest circuit, i.e. the 550A from one of the positive BusBars?.

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  • \$\begingroup\$ In vehicle minus is usually a vehicle chassis. \$\endgroup\$
    – user263983
    Mar 23 '21 at 12:45
  • \$\begingroup\$ You're planning on connecting two chargers to the same batteries? \$\endgroup\$
    – Finbarr
    Mar 23 '21 at 12:49
  • \$\begingroup\$ Why two +ve bus bars? It would be simpler to have 1. The current rating? Sufficient so that they can handle the maximum current without getting too hot. Remember - the fuse protects the wire, not the load. The bus bars should be the point of distribution, so the -ve wire to the existing battery should connect to the -ve bus bar. You probably want fuses on each of the batteries to protect the wire. \$\endgroup\$
    – Kartman
    Mar 23 '21 at 14:04
  • \$\begingroup\$ @Finbarr - yes, one charges from PV, one from the alternator of the vehicle. Just incase we stay parked for a while, or the sun isn't shining. \$\endgroup\$
    – nickot93
    Mar 24 '21 at 0:39
  • \$\begingroup\$ @Kartman - Two +ve bus bars mainly because if they were on the one, the current rating would be around 640A and I wouldn't know where to source a bus bar that can handle such ratings (in Australia). Will fuse between the +ve bus bar/s and the battery (however many there ends up being). Noted re: -ve bus bar. Will connect a wire from -ve of car battery to -ve busbar (rather than -ve of lithium 2), then -ve bus bar to lithium 2 -ve. Hope this is correct and thank you for your reply. \$\endgroup\$
    – nickot93
    Mar 24 '21 at 0:45

I think you try to connect your solar system to the car battery via the second charger. This is not necessary and will not work (Or has your van 24V?). You can just use a relay do disconnect the car battery from the others (for starting etc..). There are special relays for that on the market. You can search for battery disconnect relay.

Wiring the negative poles together is correct. Remember that in the car the chasis is used for negative potential.

The Ground connection of your Inverter should be grounded correctly. So you can wire it to a metal pole, take a hammer and push it into the earth or if you want to connect it to your chassis (12V negativ) you should ground your chasis (metal pole :)

Btw. most people don't wire the gorund connection of their inverter. If you only use devices without metal case/ground connection this should be no problem. But be aware this isn't correct.

Hope this helps you a little bit.

  • \$\begingroup\$ It helps immensley, thank you @Daniel Morgner. The solar charget needs to connect to the lithium bank rather than the car battery. The only positive coming from the car battery will give input to the DCDC charger which outputs current to the lithium battery bank. Do you think this will pose a problem somewhere? I will also ensure the ground connector for the inverter connects to the -ve bus bar (and therefore -ve pole of car battery to ground) as an added measure. \$\endgroup\$
    – nickot93
    Mar 24 '21 at 0:59
  • \$\begingroup\$ Your van has an alternator for charging your 12v car battery. You can just wire the batterys together and disconect them(may via an automatic relay) for starting your car or when your only use your electicity to not discharge your car battery. So normally you connect them only for loading. You should not need an extra charger for that \$\endgroup\$ Mar 24 '21 at 7:36

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