Design/Planning Help Please

[Heap64]

I have been following for a bit now and trying to take it all in. I'm very interested in building (2) 12v 280ah batteries using the popular cells in the battery threads. I'm leaning to 2 separate batteries and keeping it 12v for the camper. Down the road I plan to add a Victron 12v 3000w inverter/charger. I know that is a high amp draw. Probably best to leverage a contactor right? How does this all fit with 2 batteries, and a single inverter/charger link? Hopefully I said this right. Lastly if both my batteries are in one case or next to each other. Is it possible for one BMS to handle both? I'm assuming no, but thought I would ask. Be easy on me.

 

[nosys70]

you can't do that.
first , you cannot connect 2 batteries together if they do not have the same voltage.
if you go out with the camper and come back with an empty battery, you cannot connect it to the other one that stays at home and is fully charged because one battery will violently discharge into the other.
the 2nd reason is you need at least a BMS for each battery or no BMS at all.
out of that , you could just use the camper to recharge the battery.

 

[Heap64]

Maybe I said this way wrong? Both batteries will be 12v and 280ah. Both will live in the camper. My thought was build them exactly the same and parallel the two batteries just like buying two Battle Borns. The two separate BMS and relays are what I'm struggling with understanding.

 

[JoeHam]

I don’t know much about BMS’s but in your case I would want to serial connect and get a 24v inverter. You will save on wiring etc. that way.

 

[Heap64]

That has crossed my mind to make it simpler. I would not have any redundancy and would need to add a DC/DC converter for 12v stuff.

 

[FilterGuy]

If you keep it two separate batteries, you need two BMSs (That is my opinion. Some people go without BMSs at all, but I would not)
I am waiting on batteries to do almost exactly what you are talking about. (but smaller cells).

I am doing it this way because I have two BMSs that would work but they are not rated for the current of a 2P4S layout.

The alternative (and my preferred method) is to go parallel first. 2P4S

I understand the 'redundancy' argument, but keep in mind that with two BMSs, there are twice as many complex circuits to break.

 

[John Frum]

You are going to need big cables for 300 amps, 3/0 or larger depending on length.
2 bms's in parralel can share the load.
Its my understanding the current ratings for most bms's are way optimistic.
My wild guess is to de-rate them by 50 percent.
What 12v loads do you have?
What is the biggest 12 volt load?
Perhaps a buck converter is worth considering.

 

[Heap64]

You are going to need big cables for 300 amps, 3/0 or larger depending on length.
2 bms's in parralel can share the load.
Its my understanding the current ratings for most bms's are way optimistic.
My wild guess is to de-rate them by 50 percent.
What 12v loads do you have?
What is the biggest 12 volt load?
Perhaps a buck converter is worth considering.

I honestly don't know, I would need to look into that if I went down that road. Lights and fans nothing major, but the electric Jack and power slide out would be pretty big draws. A bit for furnace and fridge controllers.