Battery connection for shunt

[Toys_with_time]

I am rewiring my parallel wired 12v AGM house batteries and adding in a shunt to be connected to a battery monitor. Consensus seems to be that battery input (charging) should be to 1st battery and output (draw) from last battery but how should shunt then be wired - just to 1st battery or to both (via Neg Buss bar) or does it make no difference for the negative side?

TwT

[stormalong]

If AGM1 and AGM2 are permanently in parallel option one is correct.

[Wotname]

I would do the negative side as per the modified 'Alternative 1' attached.

I would take all the positive side of the charging sources and the positive of the house to a positive bus bar and then both AGMs positive either direct to the same positive bus bar directly or via a house master switch if so desired.

[Toys_with_time]

Originally Posted by Wotname

I would do the negative side as per the modified 'Alternative 1' attached.

I would take all the positive side of the charging sources and the positive of the house to a positive bus bar and then both AGMs positive either direct to the same positive bus bar directly or via a house master switch if so desired.

I'm also installing Lithium hence the rewiring of the current AGM's. For Lithium batteries what would you recommend? I have presumed I might have to split the Buss bars into 2 as not sure how many connections I can get onto 1 of them given the cable sizes.

TwT

[hjohnson]

Originally Posted by Toys_with_time

I am rewiring my parallel wired 12v AGM house batteries and adding in a shunt to be connected to a battery monitor. Consensus seems to be that battery input (charging) should be to 1st battery and output (draw) from last battery but how should shunt then be wired - just to 1st battery or to both (via Neg Buss bar) or does it make no difference for the negative side?

TwT

The basic rule is that all current to your battery needs to flow through the shunt. The cross-battery thing (ie line goign into negative of battery 1 and out positive of battery 2) is more about ensuring that the cells within the batteries stay balanced. Even millivolts difference can throw this out.

Conversely, the only thing that matters to the shunt is the current flowing through it back to the negative connection as your battery as a whole.

Also, don't forget, that your charger needs to connect through your shunt as well, so that your battery monitor can measure the charging current as well as the discharging current.

[smac999]

Originally Posted by Toys_with_time

I'm also installing Lithium hence the rewiring of the current AGM's. For Lithium batteries what would you recommend? I have presumed I might have to split the Buss bars into 2 as not sure how many connections I can get onto 1 of them given the cable sizes.

TwT

For lithium each battery should have same lengh jumpers to pos and neg bus bars. With 2 batteries you can run both equal lengh jumpers from the battery neg to the shunt. If more then 2 batts you will want a neg battery bus bar between batteries abd shunt. Because you can’t fit many wires onto the shunt. Equal lengh Pos jumpers should each be fused (Mrbf at battery is easiest) and run to a pos bus bar

[Moontide]

All the negative connections should be on the shunt or bussbar upstream from it. Since it is reading current multiple sets of batteries negative connections should be electrically together on one side of the shunt and all loads on the other. Of course if you want to be able to read each bank separately then you would run the negatives to separate shunts (have never seen that)

[Toys_with_time]

Originally Posted by smac999

For lithium each battery should have same lengh jumpers to pos and neg bus bars. With 2 batteries you can run both equal lengh jumpers from the battery neg to the shunt. If more then 2 batts you will want a neg battery bus bar between batteries abd shunt. Because you can’t fit many wires onto the shunt. Equal lengh Pos jumpers should each be fused (Mrbf at battery is easiest) and run to a pos bus bar

Makes sense, thanks. Hadn’t heard of MRBF fuses before but very neat solution to fusing each battery.

[Toys_with_time]

Originally Posted by smac999

For lithium each battery should have same lengh jumpers to pos and neg bus bars. With 2 batteries you can run both equal lengh jumpers from the battery neg to the shunt. If more then 2 batts you will want a neg battery bus bar between batteries abd shunt. Because you can’t fit many wires onto the shunt. Equal lengh Pos jumpers should each be fused (Mrbf at battery is easiest) and run to a pos bus bar

How do you size the fuses for the batteries? Max output per battery is rated at 50 amps which presumably is current limited by the LifePo4 BMS and max total house power draw from the 4 batteries is about 175 amps (they are not used for starting) so not sure if I should use 50 amp MRBF fuses or go slightly higher?