8s2p with two BMS Q

[Devo1982]

Hi everyone,

This is my current 8s battery Bank with one BMS connected to my hybrid inverter. The 24v 230ah "pack" isn't really big enough so I've ordered another 8 cells and another BMS. My plan is to create an identical "pack" to the one pictured with its own BMS and then parallel the two. My question is about where the two wires from my inverter should connect. Right now the positive goes to main pack positive and the negative goes to the BMS P- but how should that be once I've got another "pack" and another BMS?

Thanks in advance...

 

[meetyg]

I would suggest you get some busbars, and connect both batteries to the busbars, then connect the inverter too.

Don't forget to get proper fuses for each battery too.

 

[Rednecktek]

The other option is to connect positive to positive on the batteries, negative to negative, then come from your inverter positive to battery 1 and the negative to battery 2.

 

[Devo1982]

But both BMS P- will be joined to the inverter negative, correct? And then a BMS B- to each bank -?

 

[Devo1982]

I would suggest you get some busbars, and connect both batteries to the busbars, then connect the inverter too.

Don't forget to get proper fuses for each battery too.

I hadn't thought about fuses. Where would each go? And what considerations in terms of amperage?

 

[Rednecktek]

But both BMS P- will be joined to the inverter negative, correct? And then a BMS B- to each bank -?

Yes, each pack of cells and BMS constitutes a single battery. Treat them as such and you'll be good.

 

[Rednecktek]

I hadn't thought about fuses. Where would each go? And what considerations in terms of amperage?

Inverter size ÷ voltage × 1.25 = fuse size.

Eg: 2400w inverter ÷ 24v = 100a × 1.25 = 125a fuse on each battery positive terminal.

 

[Devo1982]

Inverter size ÷ voltage × 1.25 = fuse size.

Eg: 2400w inverter ÷ 24v = 100a × 1.25 = 125a fuse on each battery positive terminal.

Sorry to ask a stupid question but I don't want to mess this up. The two positive terminals will be joined by a busbar and then the inverter positive will be connected to one of them. Where will the fuses go? One between battery bank B + and battery bank A + and then battery bank A + and the inverter?

 

[Rednecktek]

The fuses will go on each battery, their job is to protect the wires between the battery terminal and everything else. If you have to offline a battery or something goes wrong, you'll still have full protection from the other battery. Likewise if the batteries drain unevenly and one goes flat early, the other one has the capacity and protection to pick up the load.

 

[Devo1982]

The fuses will go on each battery, their job is to protect the wires between the battery terminal and everything else. If you have to offline a battery or something goes wrong, you'll still have full protection from the other battery. Likewise if the batteries drain unevenly and one goes flat early, the other one has the capacity and protection to pick up the load.

On the positive of each battery, right?

 

[Devo1982]

Sorry for the crap diagram but this is what I'll have, in theory, when the two banks are connected. The positive wire from the inverter will only connect to one bank, so where should the fuses be?

 

[HRTKD]

Just to be clear, when a busbar is suggested, it looks something like this:



Both batteries would connect to the busbar on terminals 1 and 3, the inverter would connect on terminal 2. This leaves terminal 4 open for other loads or chargers. You would of course have two of these busbars. One for positive, one for negative. Don't forget your grounds.

The pictured busbar is from the Blue Sea Systems website.

 

[Devo1982]

Just to be clear, when a busbar is suggested, it looks something like this:

View attachment 256548

Both batteries would connect to the busbar on terminals 1 and 3, the inverter would connect on terminal 2. This leaves terminal 4 open for other loads or chargers. You would of course have two of these busbars. One for positive, one for negative. Don't forget your grounds.

The pictured busbar is from the Blue Sea Systems website.

Thank you.

And I could use a fuse like this between the battery and the busbar:

 

[Devo1982]

Just to be clear, when a busbar is suggested, it looks something like this:

View attachment 256548

Both batteries would connect to the busbar on terminals 1 and 3, the inverter would connect on terminal 2. This leaves terminal 4 open for other loads or chargers. You would of course have two of these busbars. One for positive, one for negative. Don't forget your grounds.

The pictured busbar is from the Blue Sea Systems website.

And I assume it's best to use wire between the battery and the busbar for flexibility? What size wire for my system please: 3000w inverter, 24v batteries.

 

[Rednecktek]

Battery-1 Positive -> Fuse -> + Bus Bar -> Inverter Pos
Battery-2 Positive -> Fuse -> + Bus Bar

Battery-1 Negative (P-) -> Shunt B- ( if applicable) -> Neg Bus Bar -> Inverter Neg
Battery-2 Negative (P-) -> shunt B- -> Neg Bus Bar

SCC Positive -> Fuse -> + Bus Bar
SCC Negative -> Neg Bus Bar

Wire size will be 1awg all the way around for anything below about 6ft distance. 150a fuses for each battery.

 

[HRTKD]

And I could use a fuse like this between the battery and the busbar:

My preference is to not use an MRBF fuse on the battery terminal. It can put stress on the cell/battery terminal. A Class T Fuse is recommended.

What size wire for my system please: 3000w inverter, 24v batteries.

To definitively answer that I would need to know the distance from the batteries to the inverter. Or, you could plug the numbers into a wire gauge calculator and get the answer:

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

 

[Rednecktek]

My preference is to not use an MRBF fuse on the battery terminal. It can put stress on the cell/battery terminal. A Class T Fuse is recommended.

Aren't the MRBF fuses designed to be bolted directly on the terminal?? I know the Class-T's need a holder to prevent bending, but the holder for a MRBF has it's own support.

Or am I missing something there?

 

[Devo1982]

Something like this?

 

[HRTKD]

Aren't the MRBF fuses designed to be bolted directly on the terminal?? I know the Class-T's need a holder to prevent bending, but the holder for a MRBF has it's own support.

Yes, but I wouldn't put it on a cell terminal. Battery terminal, not a big deal.

 

[Devo1982]

Yes, but I wouldn't put it on a cell terminal. Battery terminal, not a big deal.

Could I put one on the wire from the terminal to the busbar? I know it won't save the wire in that case but the wire will be very short.

 

[HRTKD]

Could I put one on the wire from the terminal to the busbar? I know it won't save the wire in that case but the wire will be very short.

Yes, that's how most of us do it. I haven't done a poll of course, but that seems to be the common way.

 

[Devo1982]

Just to be clear, when a busbar is suggested, it looks something like this:

View attachment 256548

Both batteries would connect to the busbar on terminals 1 and 3, the inverter would connect on terminal 2. This leaves terminal 4 open for other loads or chargers. You would of course have two of these busbars. One for positive, one for negative. Don't forget your grounds.

The pictured busbar is from the Blue Sea Systems website.

Last question (promise) what size wire for the grounds? I'm going to stick a grounding rod in the ground and connect the batteries and inverter to it separately.

 

[Rednecktek]

6awg is code.

 

[Devo1982]

6awg is code.

I know I said last question but I do just have one more Do I need a breaker between my busbar and aio inverter? If so, it can just be on the + wire, right? Guessing that should be 150a too.

 

[HRTKD]

I know I said last question but I do just have one more Do I need a breaker between my busbar and aio inverter? If so, it can just be on the + wire, right? Guessing that should be 150a too.

Yes, you need over current protection there also. Could be a breaker, could be a fuse. We don't often have to isolate the inverter, so a circuit breaker isn't as necessary in that position as it is in other circuits. A fuse is normally what you see in that position. High amperage circuit breakers are expensive. At least the good quality ones are. Fuses are less expensive.

Yes, put it on the positive cable.