24v system with 16 cells

[guidecca]

Putting two BMS's on my (16) Eve 2.3v 280aH cells and need some advice. Can one connect the positive output of one BMS to the negative output of the second BMS? Thanks.

View attachment 156106

 

[fafrd]

Putting two BMS's on my (16) Eve 2.3v 280aH cells and need some advice. Can one connect the positive output of one BMS to the negative output of the second BMS? Thanks.

View attachment 156106

Sure, that’s making a 48V battery from 2 24V batteries.

The two BMSes will keep each of their 8 cells balanced but there is nothing to keep the 2 24V batteries balanced.

A 48V / 16S BMS would keep all 16 cells balanced (though you could just add a 16S active balancer to your 2x8S battery and achieve the same thing…

 

[guidecca]

48v will not work with my 2200W inverter/charger that is rated for 12v/24v. Not sure which configuration to use now.

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[TomC4306]

Let me make sure I understand.
You wish to build two 24 volt batteries each with its own BMS.
And the title says 24 volt system, so I'll assume you wish to connect these two 24 volt batteries in parallel.
Most bms's do not connect to battery positive except with a balance cable. Most bmss are a switch on the negative between battery negative and system negative.
So tell me again about negatives and positives and what you want to do.

 

[Horsefly]

I may be missing something, but I think you just want to build two 24V batteries, each with 8 cells and a BMS. Then you put the two batteries in parallel.

 

[sunshine_eggo]

Putting two BMS's on my (16) Eve 2.3v 280aH cells and need some advice. Can one connect the positive output of one BMS to the negative output of the second BMS? Thanks.

View attachment 156106

What you have depicted are two 12V batteries in series for 24V:




You would be better served by 2X separate 8S batteries, each with its own BMS, and the two batteries in parallel

 

[guidecca]

I need enough power to run a mini-split A/C. I had two steel boxes built, one box for each battery, because the batteries will be in my van. The configuration is somewhat convoluted but the goal is to create a 24v battery to power the air conditioner.

 

[fafrd]

48v will not work with my 2200W inverter/charger that is rated for 12v/24v. Not sure which configuration to use now.

​

You said ‘connect positive of one BMS to negative of the other’ - that’s a series connection of 2 24V batteries creating a 48V battery.

If you want a 24V battery, you want a parallel connection - this means the two BMS + are connected together and the two BMS - are connected together.

parallel connection will give you a 560Ah 24V battery and you will no need to worry about imbalance developing between the 2 8S / 24V packs (self-balancing).

This configuration should work fine but may be slightly dangerous if you run at very high current exceeding the current limit of a single BMS.

Current will be shared between the two packs / BMSes, but if you are pulling currebt exceeding the limit of a single BMS, it’s possible for only one BMS to shut down driving all the current through the second BMS.

But as long as your maximum current draw (or charge) is below the limits of a single BMS, parallel configuration should work fine for you.

 

[fafrd]

I need enough power to run a mini-split A/C. I had two steel boxes built, one box for each battery, because the batteries will be in my van. The configuration is somewhat convoluted but the goal is to create a 24v battery to power the air conditioner.

Tell us the maximum current ratings of your BMS and the maximum current draw you expect and we can tell you if there is any possible issue with a parallel configuration.

Maximum current draw = maximum AC power being delivered / inverter efficiency.

 

[sunshine_eggo]

I need enough power to run a mini-split A/C. I had two steel boxes built, one box for each battery, because the batteries will be in my van. The configuration is somewhat convoluted but the goal is to create a 24v battery to power the air conditioner.

Unlikely that you'll achieve that goal. Two 12V BMS in series means they must pass the full current through each BMS. If the BMS is rated for 100A, you only get 100A.

You could simply wire the cells in each box in series, 8S, use an 8S BMS for each box and put the boxes in parallel. In this way, you could get to mostly add the two BMS ratings, i.e., if the BMS is rated for 100A, two in parallel would give you 200A (assuming perfect sharing).

Are you powering a mini-split? If not, have you factored in the surge current of the compressor.

Bottom line, your battery bank configuration is:

1) too complicated.
2) not optimized.
3) pretty much the opposite of best practices.

Now is the time to do it right. You don't want to re-do it.

 

[guidecca]

Surge current of the SENL-09CD/X mini split unit is under 25 amps. Maximum circuit ampacity of the mini split is 20A. The 24v Samlex inverter specifications are here: https://samlexamerica.com/products/2200-watt-pure-sine-inverter-charger-evo-2224/. The BMS is an Overkill 100A 8S 24v BMS. I don't expect current draw to exceed 100A.

 

[sunshine_eggo]

Surge current of the SENL-09CD/X mini split unit is under 25 amps. Maximum circuit ampacity of the mini split is 20A. The 24v Samlex inverter specifications are here: https://samlexamerica.com/products/2200-watt-pure-sine-inverter-charger-evo-2224/. The BMS is an Overkill 100A 8S 24v BMS. I don't expect current draw to exceed 100A.

RLA of 6A looks pretty good.

Unfortunately, the image you provided shows two 4S batteries and 4S BMS in series for a single 8S battery.

If you have 8S BMSs, then you can't use them as you've shown. 8S means the cells must be in series, so the good news is you're already there, you just need to fix the diagram.



Sloppy, but I hope you get the idea. Ignore your interconnects and use only the red lines I've added. BOTH batteries must be wired like this with the two batteries in parallel.

Again, you can't wire your 8S BMS as you depicted. An 8S BMS is for 8 cells in series, not 4 groups of two cells in parallel in series 2P4S). Cells in parallel with each other will read exactly the same voltage and can't be monitored separately.

 

[fafrd]

Surge current of the SENL-09CD/X mini split unit is under 25 amps. Maximum circuit ampacity of the mini split is 20A. The 24v Samlex inverter specifications are here: https://samlexamerica.com/products/2200-watt-pure-sine-inverter-charger-evo-2224/. The BMS is an Overkill 100A 8S 24v BMS. I don't expect current draw to exceed 100A.

Your mini-split consumes 782W cooling and 925W heating.

That’s AC and since your inverter has 90% efficiency:

782WAC consumes 869WDC and 925WAC consumes1028WDC.

This is very modest consumption and we’ll within the capability of your inverter (assuming no other significant loads are being powered at the same time.

869WDC = 34A @ 25.6VDC and 1028WDC = 40.15A @ 25.6VDC.

Your 100A should be able to handle this current without issue (so 2 in parallel should be fine).

All of this, again, is assuming there are no other significant loads being powered at the same time.

 

[guidecca]

Very significant progress in understanding and work being accomplished due to your posts. Thanks for this help!

 

[guidecca]

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