Connecting two LFP banks

[Yahu]

Good morning.
My battery bank for BowThruster died... so...
I've ordered more LFP cells and want to make a connection like on the diagram.

I have house batter ~700Ah LFP (H.BATT)
I will add Fron Battery Bank (FBB)

FBB will be used only for bowthruster (BT) as a high amp load (UP to 500A) together with house bank via Victron BP220.

How to protect a bowthruster battery:
Use any BMS like DALY on negative and:
connect BP220 "-" cable to the BMS load output line as on the diagram - overcharge protection.

mount additional relay on signal line also grounded to the BMS load output line as on the diagram


A result:
1. if BMS will "decide" to cut load - BT will not work... simple.

2. but... if BMS will decide to cutt off charging it will open BP220 and I won't be able to use FBB capacity to use as additional House Battery until V will drop... hmmmm...

Now my House Battery is charged only to 14,1V so if I will set up Front Battery Bank BMS to higher level probably i will never cut this bank unless there are unbalanced/malfuctioning cells or situation like this but it will come back to normal when it will manage to balance cellls.

What is making me thinking. If the moment of connectiong BP220 will happend when there will be a bigger V difference between bank we can meet a high current between banks what is not recommended... Of course on the line ~20m of cable 95mm2 it shouldn't be a problem and resistance will not allow current to increase dramaticcaly but... there is always a question
Anyone any practical experience?

And a second question:
Does BP220 can work both directions? What are current limitations when it works from OUT -> IN?

[ if it doesn't solution would be to put 2x BP220 connected in parallel but with opposite directions of IN and OUT ]

What to you think?

 

[jwelter99]

BP's are one way, and if you try to feed current in reverse it will damage/destroy them.

For this reason you also can not just put them in parallel to get bi-directional control - you would need to somehow isolate the one that is "backwards"

If you want full bi-directional control use a contactor or one of the SSR products setup to do that. If it's just to charge one bank from the other a B2B charger is the best solution to deal with the different voltages.

 

[Yahu]

BP's are one way, and if you try to feed current in reverse it will damage/destroy them.

For this reason you also can not just put them in parallel to get bi-directional control - you would need to somehow isolate the one that is "backwards"

If you want full bi-directional control use a contactor or one of the SSR products setup to do that. If it's just to charge one bank from the other a B2B charger is the best solution to deal with the different voltages.

Thank you for a message. You're right that easiest way is to use SSR and.... I've got one spare from blue system 120A and one 220A from victor! Hah! ?

I want to use both banks in parallel to feed inverter and bow thruster so B2B charger is not an option - it would have to be "both ways charging" depending on discharge level of banks.

Thank you for advice about BP.