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Help Requested: Recommended BMS Size

pinchio

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Apr 13, 2020
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I am about to build a battery bank that will have a combined 560Ah total capacity @ 12V.

I have a Victron Multiplus 12V/3000w/120V inverter/charger. That particular charger can output 3000W.

My battery bank will consist of 2 x 12V 280Ah (DIY Prismatic cells).

Can you help me with recommending BMS specifications I should be looking for.
 
3000W/12V = 250A

If you build your batteries as separate 12V and use an overkillsolar on each one, that will get you 240A.
 
I have a Victron Multiplus 12V/3000w/120V inverter/charger. That particular charger can output 3000W.

My battery bank will consist of 2 x 12V 280Ah (DIY Prismatic cells).

The above configuration is where I plan to end up. I currently have the two 4s 280Ah batteries operational. I'm running an inverter that is only 1000 watts at this time. I have no intention of pulling the full 3000 watts from the inverter. 2500 watts sustained would be an upper boundary. I think my current battery/BMS setup will work.

Maybe I'm rationalizing my current setup into a box that isn't realistic, but my expectation is that when I fire up the air conditioner, the solar output will supply about 1000 watts, leaving the battery/BMS to take up the slack. The big loads like the air conditioner would be run mostly when the sun is out.
 
Do you recommend a particular BMS model or manufacturer? Got a link? Thanks
 
Is my understanding correct? If I have 2 batteries that are 12V 280Ah each and connected in parallel and each battery has a 100A BMS would the maximum continuous power output be capped to 12V x 100A (BMS) = 1200W?

If so then to get more power output I would need to increase the BMS AMP size in each battery?
 
Is my understanding correct? If I have 2 batteries that are 12V 280Ah each and connected in parallel and each battery has a 100A BMS would the maximum continuous power output be capped to 12V x 100A (BMS) = 1200W?

If so then to get more power output I would need to increase the BMS AMP size in each battery?
The bms is the limiting factor.
Each battery has a bms capable of delivering 100 amps.
2 of these batteries in parallel can deliver almost 200 amps.
I say almost because the current flow is rarely perfectly shared between the 2.
 
Is my understanding correct? If I have 2 batteries that are 12V 280Ah each and connected in parallel and each battery has a 100A BMS would the maximum continuous power output be capped to 12V x 100A (BMS) = 1200W?

If so then to get more power output I would need to increase the BMS AMP size in each battery?

No, my BMS are the Overkill Solar 120amp. So I have a total of 240 amps that I could pull out of the battery by an inverter. That's continuous. Supposedly the BMS can handle more surge amps than that, but only for a short period.
 
Another option - (its an expensive option). Batrium Watchmon4. This uses a contractor so I have the full amperage (OK its limited by the 400a Class T Fuse). I will be using in my MotorHome, and one of my design criteria was to not lose the ability to "Boost" the big diesel. It will also connect to my Victron CCGX on the CAN-Bus.

What are your other design criteria on your battery & BMS? Where are you installing it?
 
The above configuration is where I plan to end up. I currently have the two 4s 280Ah batteries operational. I'm running an inverter that is only 1000 watts at this time. I have no intention of pulling the full 3000 watts from the inverter. 2500 watts sustained would be an upper boundary. I think my current battery/BMS setup will work.

Maybe I'm rationalizing my current setup into a box that isn't realistic, but my expectation is that when I fire up the air conditioner, the solar output will supply about 1000 watts, leaving the battery/BMS to take up the slack. The big loads like the air conditioner would be run mostly when the sun is out.

With a multiplus 3K, a soft start is really a good idea. Your rationalization is at least partially correct, but the charge controller only adjusts so fast. You'll see a dip in voltage and a pull from the battery, but the solar will offset it.

The overkill overcurrent has a programmable delay. You could probably tweak that out a bit if you have to.

Is it a contactor or a contractor? I'm not familiar with that BMS.
LOL...

contactor
 
With a multiplus 3K, a soft start is really a good idea. Your rationalization is at least partially correct, but the charge controller only adjusts so fast. You'll see a dip in voltage and a pull from the battery, but the solar will offset it.

I've seen that in action when I fired up a space heater that used the inverter. The charge controller doesn't react as quickly as I thought it would, but it does get there within a few seconds.
 
Yep, but the worst of the surge is over in a few seconds anyway. I don't need the soft start with my Quattro, but I think I'm going to get one anyway just to avoid the hammer. The compressor surges to over 4kW.
 
To summarize then if I am ok with getting 2400W of continuous output from the 100A BMS in battery connected in parallel then I do not need to invest in a BMS larger than 100A. That is my take away. Of course if I want more watts output then I need to upsize the BMS.
 
To summarize then if I am ok with getting 2400W of continuous output from the 100A BMS in battery connected in parallel then I do not need to invest in a BMS larger than 100A. That is my take away. Of course if I want more watts output then I need to upsize the BMS.

My BMS is rated for 120 amps, not 100 amps. It is the Overkill Solar 4s 120 amp model.

 
My inverter is the Victron Multiplus 12v 3000W so in theory if I use a Daly 150Amp BMS in each of my 280A 12V batteries the maximum theoretical upper limit power output of the battery bank would be 300A ("2 x 150A BMS") x 12V = 3600W. Of course taking into consideration the efficiency loss of both BMS and Inverter I would still be well within limits to get the most Watts coming into the inverter. if I went with smaller BMS then I could find myself unable to fully supply close to the 3000W limit of the Inverter. So I'm thinking I should look for 150A BMSs and not 200A BMSs for my RV setup. Am I on the right track with my logic or am I missing something.
 
Do you KNOW that you are going to pull 3000 watts all the time? Even running the air conditioner, I doubt that I would come close to 3000 watts even a small percentage of the time. But my trailer isn't your trailer. You need to figure that out. Guessing at it could lead you to spending money that you don't need to. When it comes time to mortgage my left nut, I'm going to reanalyze my AC needs to see if I can get by with a 2000 watt inverter instead of the 3000 watt inverter.
 
More is never bad as long as you don't exceed the cells' current rating. If they are 1C, they can handle 280A.
 
Sorry to revive this old thread, but I have a similar question. I'm currently building a 4s 280ah battery and planning on building my RV system as I go. I'm curious which BMS I should get, and what size.

Currently I only have a 50w solar panel on the roof and a 100w suitcase, both with 10a pwm controllers. No inverter on the trailer (yet). Eventually I'd like to build a bank of 2 - 3 of these batteries, with about 900w solar and a 3000w inverter.

From reading above, it sounds like I should be able to get away with a 150a BMS, as long as I have 2 or more batteries, to run the inverter? Currently the biggest draw on the battery would be the landing gear, which have a 20a circuit breaker. Should I size the BMS for what I have now and upgrade down the road when I get the inverter, or should I just bite the bullet now and get what I will need for later?

Thanks in advance.
 
The bms should be sized to handle the current of the loads and the charge sources.
Usually your maximum load current will be larger than the current of your charge sources.
This is usually the inverter.
What is your maximum continuous load current in amps?
And what is your maximum charge current in amps?
 
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Because you are in a trailer with no current inverter, I would look at the Overkill 120a BMS. That will handle all current loads.

You could add a second battery and additional BMS, when you add the inverter.
 
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