Help with a DIY battery bank

[krell313]

Hello all, I am considering making a large (2200 A/hr) 24v lifepo4 bank but I am having a problem finding compatible BMS monitoring to connect to my existing system.

What I have now is 2 outback inverters and 3 charge controllers with a large FLA battery bank that will need to be replaced in the next few years. This system has voltage set points to close relays to signal alarms and start the generator. Voltage set points are pretty much useless for lifepo4 systems.

I was originally thinking of going with 280 A/hr cells in a 8p8s bank but fusing each cell for safely seems a bit complicated. I still worry if a cell fails it could be a big mess if the fuse fails to open. This setup would allow me to use a single Orion BMS with configurable outputs for battery alarms and automatic generator start.

A better option, I think, would be 8, 8s banks using something like an Overkill BMS but these do not have any way to activate a relay. Eight Orion BMS’ would be quite expensive.

Since this installation is off-grid and sometimes left unattended for weeks, my existing monitor/control system works well but I will have to figure out a way to get a new BMS to connect to my system.

I do worry about the 8 cells in parallel so I am looking for any suggestions on how to proceed.

 

[Supervstech]

55kWh bank is a LOT on a 24V bank.
Are you considering an all in one inverter? I love my outback 24v system i got free, but dang, i would sure go 48v if i could...

 

[krell313]

55kWh bank is a LOT on a 24V bank.
Are you considering an all in one inverter? I love my outback 24v system i got free, but dang, i would sure go 48v if i could...

I don't want to replace my inverters and very expensive lightning protection to handle the higher panel voltage. I did think of that though.

 

[740GLE]

So 64 280ah cells at say $90 is what $5700? but replacing an inverter and some lightning protection is too costly??

Btw check out this thread for some other 8p8s build wisdom

64 eve lLF280k version3 cells in 8P8S configuation 24-volt build December 28th, 2023

64 cells 280Ah version 3 cells: only charging two cells this day: March 10th, 2024 March 10th, 2024 >>> I splurged and scored a new (open box) .25-inch x 4-inch x 24-inch tinned copper busbar with predrilled holes for $67.51 including shipping and tax...(worth well over 100 dollars) I already...

diysolarforum.com

Personally I’d run 2p8s banks, with 4 200a jk BMS into a 500a smart shunt and Lynx power in. You can set up a relay from the shunt to start a generator based on SOC.

 

[krell313]

So 64 280ah cells at say $90 is what $5700? but replacing an inverter and some lightning protection is too costly??

Btw check out this thread for some other 8p8s build wisdom

64 eve lLF280k version3 cells in 8P8S configuation 24-volt build December 28th, 2023

64 cells 280Ah version 3 cells: only charging two cells this day: March 10th, 2024 March 10th, 2024 >>> I splurged and scored a new (open box) .25-inch x 4-inch x 24-inch tinned copper busbar with predrilled holes for $67.51 including shipping and tax...(worth well over 100 dollars) I already...

diysolarforum.com

Personally I’d run 2p8s banks, with 4 200a jk BMS into a 500a smart shunt and Lynx power in. You can set up a relay from the shunt to start a generator based on SOC.

 

[krell313]

Thanks for the link, 8p is too much. As for going to 48 volts, I know that would be better but the 24 volt system I have runs more than just 2 inverters. I also have a pressure pump and other devices connected to it. I do like the 2p8s approach but I would need more than 1 output for gen start and battery soc alarms.

 

[740GLE]

An external relay operated by a shunt can control multiple contacts/devices at once.

Do you need multiple things operated at different SOC values?

 

[offgridandy]

Hi, I'm on a similar journey, also being a great fan of Outback. I have at present a 24v system (1000ah) and have an VX3024 inverter and 3 Outback FM80 solar CC. So from the inverter plus the CC I have 4 relays, voltage controlled. Could you not use these CC relays for extra switching.

I have lived with FLA for 25 yrs but the LiPo set up is new to me. But I am advised that if you do not care about charging to the very top voltage. you can use voltage set points to switch off charging before overcharge (PV) takes place and similarly at low V setting to trigger a gen start.

My upgrade is going the whole hog upgrading to 48V. Pics of the modules below. 50Kwh . The battery before I dismantled it was 400VDC.

 

[740GLE]

Also one thing to keep in mind 2200ah of LiFePo is about the same as ~3000ah due to usability limitation of FLA and efficiency gains of LiFePo while charging.

It’d be best to plan for a robust system one for one swap, but you maybe way over building the new bank compared to your current use.

How often do you currently dip low into the SOC range of the current FLA bank? Is it sized to ride out 4 cloudy days?

 

[krell313]

Also one thing to keep in mind 2200ah of LiFePo is about the same as ~3000ah due to usability limitation of FLA and efficiency gains of LiFePo while charging.

It’d be best to plan for a robust system one for one swap, but you maybe way over building the new bank compared to your current use.

How often do you currently dip low into the SOC range of the current FLA bank? Is it sized to ride out 4 cloudy days?

Right now I have about 1700A/hr of FLA batteries and this will get me through maybe 2 cloudy days if I'm conservative.

An external relay operated by a shunt can control multiple contacts/devices at once.

Do you need multiple things operated at different SOC values?

Currently I would need 2 SOC controlled relays plus one dedicated for the generator.

Hi, I'm on a similar journey, also being a great fan of Outback. I have at present a 24v system (1000ah) and have an VX3024 inverter and 3 Outback FM80 solar CC. So from the inverter plus the CC I have 4 relays, voltage controlled. Could you not use these CC relays for extra switching.

Using voltage set points does not work well with lithium batteries because of their relatively flat discharge curve.

What I need is a lower cost BMS, since I will probably need four, or some other device that I can have turn on relays at different SOC's.

 

[MisterSandals]

What I need is a lower cost BMS, since I will probably need four, or some other device that I can have turn on relays at different SOC's.

I think we all start with this thought. As soon as one tries to manage their battery(s) it becomes VERY apparent that connectivity like bluetooth and user friendly apps are a necessity.

If you're building 2P8S or 4P8S batteries, you'll rely on a top quality BMS even more. You'll see.

 

[krell313]

I think we all start with this thought. As soon as one tries to manage their battery(s) it becomes VERY apparent that connectivity like bluetooth and user friendly apps are a necessity.

If you're building 2P8S or 4P8S batteries, you'll rely on a top quality BMS even more. You'll see.

You are right that bluetooth and user friendly apps are needed but I can not control relays with those apps. I was thinking of making a device that could read a CAN buss or some other serial output that would give me access to SOC and close an appropriate relay but that would require a lot of work on my part. I was hoping that there are some ready-made solutions.

 

[MisterSandals]

or some other serial output that would give me access to SOC and close an appropriate relay but that would require a lot of work on my part. I was hoping that there are some ready-made solutions.

Victron BMV712 does all this (shunt battery monitoring with programable relays).

 

[krell313]

I did look at this but it only has one relay. I guess I could get three if I could use a single shunt but I don't see how to do that in the documentation.

 

[MisterSandals]

I did look at this but it only has one relay. I guess I could get three if I could use a single shunt but I don't see how to do that in the documentation.

The relay is for a single generator start?
You will have your batteries connected in parallel so could install a shunt/BMV there instead of on each battery.

Or am i misunderstanding your system and needs?

I have 2x 48V batteries connected in parallel and monitor at the battery bank level. I suspect you want to start your generator when the bank is low and not just a single battery (however that may occur)?

EDIT: Oops, i missed this:
>I know that would be better but the 24 volt system I have runs more than just 2 inverters. I also have a pressure pump and other devices connected to it.

 

[offgridandy]

Using voltage set points does not work well with lithium batteries because of their relatively flat discharge curve.

I'm using CATL 3.7v lithium Ion NMC cells and it seems the charge discharge curve is different. With NMC it should be possible to use V set points.

 

[krell313]

The relay is for a single generator start?
You will have your batteries connected in parallel so could install a shunt/BMV there instead of on each battery.

Or am i misunderstanding your system and needs?

I have 2x 48V batteries connected in parallel and monitor at the battery bank level. I suspect you want to start your generator when the bank is low and not just a single battery (however that may occur)?

As it stands now, I think I will have 4x 24v (2p8s) batteries in parallel. I need one relay for starting a generator and 2 more relays for activating SOC operated devices.

 

[offgridandy]

This Dutch company make bespoke BMS systems for car builders. May be of help.http://www.wdrautomatisering.nl/