Battery bms shutoff

[Pat911]

The following image is courtesy of Victron Energy.



Yellow square is the shunt. Charger and controller should be connected where the blue Victron device is.

Connecting positive and negative at opposite ends of the pack helps balance current between all batteries. If connected to one single battery, that first battery would see higher peak currents than the others.

Cheers
Pat.

[DaveMJ]

Another diagram:

Meter.jpg

Additional batteries are connected Pos to Pos and Neg to Neg per Pat911's diagram.


Combined battery negatives are connected to the B- end of shunt.

Charger and Load connect to P- end of shunt.

[augiedoggy]

Originally Posted by WalterM6

Here is the reason the charger has to be plugged in before the BMS will turn on again. The BMS will open the discharge switch if a individual battery cell falls below 2.5v. But it won't turn back on again until that cell recovers to 2.7v It happened to me yesterday. It confused me for a while because my lowest battery was setting above 2.5v.(2.637V) What I did to fix it was temporally reduce the recovery voltage to 2.55v and the BMS turned on w/o connecting a charger. This probably won't help with the store bought batteries because you don't have access to your BMS settings but it's worth mentioning. But it will work for DIY batteries.

This is also a scenerio where having a solar panel on the roof (or as the roof) will correct this issue fairly quickly in daylight conditions as it will get the cell back over the 2.5v threshold in short order allowing the bms to rest in short order. I believe a balancer can help with this also if you have a weaker cell as it will transfer power from adjacent cells.

Also this may only be useful for those with single batteries or diy batteries where replacing the bms is an option but they do make BMS units with built in shunts and remote hardware amp meters in addition to the bluetooth info.
I recently bought one for the battery im building. it also allows a continuous current draw of 240amps which could be advantageous to those having to turn down their current on the controllers to prevent overdraw disconnects. these types of smart bms units often also giv you a record of what kind of current your pulling and how often which is useful.

[Pat911]

Originally Posted by augiedoggy

I believe a balancer can help with this also if you have a weaker cell as it will transfer power from adjacent cells.

You don’t want a balancer operating on a discharged battery if you’re normal balancing strategy is to top balance, which is 99% of cases. Bottom balancing is an option but very rarely used.

If your pack is top balanced there will always be one cell that is discharged first. Your cells will never be identical. If you then start moving energy between cells when discharged, you will mess up your top balance and when recharging, that one cell that you pumped energy in to will hit the high volt cutoff first and cause all the other cells to stop charging.

Cheers
Pat.

[621gears]

Originally Posted by Pat911

The following image is courtesy of Victron Energy.



Yellow square is the shunt. Charger and controller should be connected where the blue Victron device is.

Connecting positive and negative at opposite ends of the pack helps balance current between all batteries. If connected to one single battery, that first battery would see higher peak currents than the others.

Cheers
Pat.

Pat, I have the same setup as the diagram (except 6-batteries), the shunt is attached to the negative terminal directly above the yellow box (the last terminal in the series). The meter is calibrated for 18ah in order to give accurate reading.

[Pat911]

So the b- on the shunt is connected to the negative of one battery only and the negatives of the other batteries are connected to the p- of the shunt, effectively bypassing the shunt for those 5 batteries?

That’s the only way configuring with 18Ah will work. That is definitely the wrong way to configure it. Since the shunt is a low value resistor, the battery it is connected to will see less load than the other 5 batteries because it now has a higher cable resistance due to the added shunt. The current drawn will not be shared equally between batteries.

Are you able to rewire it as per the diagram above?