BMS, other Q's: 36 battery 48 volt AGM system (!)

[OahuSolar]

First solar project. Budget sensitive.

I will build a 4x9 36 battery 48V system out of 86Ah 12V AGM UPS batteries. Great shape but used. Won't last forever. Not deep cycle but should be ok for a starter battery setup. Price was right.

Does a BMS make sense at all? Can I skip it for this first install since the budget is low, anyway? Shoestring budget..

The system has 4/0 copper with corresponding copper lugs for interconnects in series. I'll use copper bus bar for parallel sides.

Looking to possibly add an Outback VFX3648, 310W panels sourced locally thru Victron charge controllers, when budget frees up.

But for now, focus is on the batteries and adding VFX3648 or similar to maintain batteries and start using.

All batteries measure about the same.

Safety first:

* What fuses/breakers are needed for the 4x9 48V battery array?

* Will the Outback battery input need external overcurrent protection?

* Any other safety considerations?

* Any other considerations for a smooth startup? E.g. how not to blow the Outback capacitors, how to initially connect and charge the batteries for ideal conditions, etc

* I may add an isolation xformer to clean up inductive loads. Will this help prevent the Outback from tripping? e.g. washer, fridge kick-on, power tools, etc

I am slow and methodical. These batteries are capable of serious discharge (UPS batteries - 37kWh!!) so I will consider all free advice as preliminary before getting a PE involved to confirm the design.

I am a beginner with solar and large battery arrays but have a good amount of related experience and knowledge.. just not enough to stamp an electrical construction document.

Thanks,
Aloha

 

[OahuSolar]

Victron's wiring doc says I should limit strings to 3-4 (versus 9 as planned) due to difficulty in balancing.

Makes me wonder if there's a solution that uses 9 in series (108V), but standard seems to be 48V..

Is there a way to reliably balance a 48V system in 4x9 w/ 12V AGM Lead Acid batteries? Should I plan to instead build three small systems at 4x3 (48V, 12kWh)?

 

[OahuSolar]

No answers so I'll journal a bit.

After reading the Victron wiring doc, I'll start with a (16) battery setup in 4x4 at 48V. Total is 16.5kWh. Targeting 20% drawdown means I am aiming for about 3.3kWh. Enough for internet and lights overnight.

These batteries charge very fast. So, a standard 6000W cluster of (20) 300W panels is the working plan.

I am a fan of KISS. Therefore I may get:
* (20) 300W panels
* Victron charge controller
* Victron inverter
* $250 48V/25A charger (we have generators)
* Fuses/breakers
* Possibly a interlock on the primary service panel to manually switch to UPS.

As it's my first attempt at desiginging/building a solar system from scratch, less is better.

Open to any input. I don't have the funds for a Sol-Ark all in one, I don't trust Growatt or any of the cheaper options...

 

[Quattrohead]

48V is your only option with most inverters, no 108v that will go bang.
I would start with 4s2p and see how they go, then maybe add another 4s2p into the mix.
You can and should long term use a balancing system, such as this - https://www.electriccarpartscompany.com/Lithium-Lead-Acid-Battery-Balancers-Equalizers-12V
or this - https://watts247.com/product/eq-48-...-batteries-or-any-x-3-2v-batteries-in-series/

 

[OahuSolar]

48V is your only option with most inverters, no 108v that will go bang.
I would start with 4s2p and see how they go, then maybe add another 4s2p into the mix.
You can and should long term use a balancing system, such as this - https://www.electriccarpartscompany.com/Lithium-Lead-Acid-Battery-Balancers-Equalizers-12V
or this - https://watts247.com/product/eq-48-...-batteries-or-any-x-3-2v-batteries-in-series/

Great, thanks Quattrohead.

Looks like one will work for a single string of (4) in series. So, would I get two of these for a 4s2p setup? And if so, how do you balance between the two 48V strings?

 

[Quattrohead]

The watts247 diagram shows the balancer doing 2 batteries in parallel as an option so go with that. There is no real way to balance between the sets, they will kind of do it but if 1 battery/cell starts to get away from you it will upset the whole thing.
So match your batteries as best you can.

 

[OahuSolar]

The watts247 diagram shows the balancer doing 2 batteries in parallel as an option so go with that. There is no real way to balance between the sets, they will kind of do it but if 1 battery/cell starts to get away from you it will upset the whole thing.
So match your batteries as best you can.

Back of napkin calcs suggest a need for humble expectations for kWh using these batteries at max 20% drawdown.

I will power low watt LED's (about 50W), about 200W in networking hw, and a few 50W chargers. That would already be ~5kWh over 24 hrs.

To be useful, I'll need all (36) of these 86Ah 12V AGM lead acid batteries.

Can we go against the grain and push all (36) into service via meticulous application of interconnects, balancers, bus bars, fuses, and possibly disconnects?

We can start with four in parallel and use our 12V charger and inverter. But this is likely just a waste of time that will confirm the calc.

Expectation is 3-4 yrs of service anyway. Replace with LiFePO4 anyway.

Alternative plan is build various small battery banks, and just use one charge controller and inverter per bank. Deal with the limitations... (e.g. separate feeders from solar to home for each low kWh battery bank).

Napkin calcs:

(36) * 86Ah * 12V = 37,152Wh
20% of 37kWh = 7.4kWh
7.4kWh / 24 hours = 309 W per hour over 24 hrs

Is my estimate correct? All (36) 12V AGM 86Ah batteries, will only be good for about (on avg) 310W per hour during a 24 hour period, on battery targeting 20% drawdown. Will need 12,000W of panels *just to charge* all (36) on a sunny day. Is this about right? And if my appliances draw 450W per hour, that using all (36) in a single bank, even if somehow installed and balanced correctly, will exceed 20% drawdown (!) ?

And, it seems that making a (36) battery bank on a shoestring is infeasible anyway?

Maybe I should just make a small 4-8 battery bank, and "kill it" with 50% daily drawdown... kind of a shame but might learn more that way? The batteries can handle this at first but supposedly will die quickly under such conditions.

As I am budget constrained these batteries could wind up being awesome or potentially a waste... they could do 3-4 days with no sun once in a while if I deploy them with a conservative load.

 

[Quattrohead]

To start I would rig it all up and beat the snot out of the batteries and see how it goes real world.