Two 12 volt Lifepo4 in series VS. One 24 Volt?

[southeastflorida]

Im in the planning stages of a solar system and thinking of going with 24 volts system. what’s your opinion re Two 12 volt in series vs. one 24 volt (Lifepo4 of course). I like the idea of two batteries incase one goes out, I can still get a 12 volt battery easier (especially if its covered under warranty and I’m waiting for a replacement)? Do the BMS work together when two 12 volt are in series. Will one shut off early leaving one not fully charged? It would seem cheaper and easier in some respects to use two 12 volt in series, but this may be a flawed idea. I also like the idea of having separate charge controllers and inverters (modular to replace components should one break) maybe another flawed idea. Im a fan of Victron solar charge controllers and inverters. Several (or many) Victron charge controllers can work together to charge the battery bank-I like the redundancy if one goes bad-it would just reduce the amps of charge to the bank?).

 

[rodrick]

Match your battery voltage to your system voltage for the best results this way you avoid balance issues extra connections and other things that can cause problems

 

[southeastflorida]

Rodrick I appreciate your reply but was hoping to learn what the pitfalls or advantages are for example- BMS shutting off one battery when two 12 volt batteries are in series-does this happen or not? It would be nice to just get a big 24 or 48 volt battery bank but Im interested in being able to add to a 24 volt system in the future. It would be nice to have a modular system that has multiple $400 components instead of a system that has a $4000 single component? Thanks

 

[rodrick]

Rodrick I appreciate your reply but was hoping to learn what the pitfalls or advantages are for example- BMS shutting off one battery when two 12 volt batteries are in series-does this happen or not? It would be nice to just get a big 24 or 48 volt battery bank but Im interested in being able to add to a 24 volt system in the future. It would be nice to have a modular system that has multiple $400 components instead of a system that has a $4000 single component? Thanks

Yes one battery can shut down that’s what balance issues can cause, a 24 volt battery is about the same cost as 2 12 volt and on a 24 volt system you have to add 2 12 volt each time you expand your system, using 12 volt batteries your wiring costs go up with the large cables required to make the series connections and have a potential to have connection issues, less connections less potential for problems

also some of these batteries allow parallel or series connections but not both do your research

 

[MisterSandals]

BMS shutting off one battery when two 12 volt batteries are in series-does this happen or not?

The BMS is a cutoff switch. If one BMS cuts off, you have nothing. You can unhook the series connection and use a single 12V battery. The question is how useful 12V will be for your 24V system (inverter in particular).

 

[cs1234]

I have a bunch of 12v lifepo4 batteries I use in series at 24v, paralleled together as separate 24v batteries. They work fine.. if you don't push them to their limits on charge/discharge. I do have a battery balancer on them though, to help them stay balanced in series (voltage wise, not SOC).

I use them on a system where they have a constant 250watt load 24/7. They get recharged by solar every day and discharge at 250watts until they are empty.. rinse and repeat. They VERY RARELY fill all the way back up, because I don't have enough solar on them to do that often. They just discharge what they can until the voltage cut off point I have set on the inverter.

If I charge them more fully, they will have overcharge disconnect protection engaged periodically on single 12v batteries in the 24v series. However, the one that isn't disconnected has no problem continuing to charge with the 24v coming across the series connection. It's just disconcerting and not necessarily good for the life of the FETs in the disconnected battery to have to disconnect itself often.

Be gentle, no problem. Be aggressive, not great.

 

[TimE]

Two 24v batteries in parallel would be your best solution. Each battery with it's own fuse, isolator and Victron smart shunt. If a BMS falls over you still have a functioning system. Wire the batteries up to busbar to connect the loads. A simple and robust system

 

[southeastflorida]

Two 24v batteries in parallel would be your best solution. Each battery with it's own fuse, isolator and Victron smart shunt. If a BMS falls over you still have a functioning system. Wire the batteries up to busbar to connect the loads. A simple and robust system

Thank you for the reply. Your advice makes sense. The BMS with two 12 volt batteries in series would create potential problems that I should avoid. Have you had any good experience with 24 volt lifepo4 batteries that are reasonably priced ? I have been looking at REdodo, Timeusb, and Chins (trying to find a quality battery that won't break the bank). Will a battery still function if the BMS fails? I guess I don't understand BMS limitations all that well? thank you

" If a BMS falls over you still have a functioning system."

 

[southeastflorida]

I have a bunch of 12v lifepo4 batteries I use in series at 24v, paralleled together as separate 24v batteries. They work fine.. if you don't push them to their limits on charge/discharge. I do have a battery balancer on them though, to help them stay balanced in series (voltage wise, not SOC).

I use them on a system where they have a constant 250watt load 24/7. They get recharged by solar every day and discharge at 250watts until they are empty.. rinse and repeat. They VERY RARELY fill all the way back up, because I don't have enough solar on them to do that often. They just discharge what they can until the voltage cut off point I have set on the inverter.

If I charge them more fully, they will have overcharge disconnect protection engaged periodically on single 12v batteries in the 24v series. However, the one that isn't disconnected has no problem continuing to charge with the 24v coming across the series connection. It's just disconcerting and not necessarily good for the life of the FETs in the disconnected battery to have to disconnect itself often.

Be gentle, no problem. Be aggressive, not great.

I do like the ability of setting both overcharge and discharge limits to maximize the life of the expensive Lifepo4 batteries. I think the majority vote goes to ONE 24 volt instead of two 12 volt in series. Having the BMS of one battery shut off and blasting the remaining 12 volt battery, like you stated, isn't a good idea. If the batteries are in series and one BMS shuts off then It should block the charge to the remaining battery because the series is blocked by the BMS-Or do I not understand how a BMS functions (very possible lol). > +- +-< if one battery in a series flashlight is blocked or shut off then the power doesn't flow? Does a BMS allow the power to flow through to the next battery in series? thank you

 

[TimE]

I do like the ability of setting both overcharge and discharge limits to maximize the life of the expensive Lifepo4 batteries. I think the majority vote goes to ONE 24 volt instead of two 12 volt in series. Having the BMS of one battery shut off and blasting the remaining 12 volt battery, like you stated, isn't a good idea. If the batteries are in series and one BMS shuts off then It should block the charge to the remaining battery because the series is blocked by the BMS-Or do I not understand how a BMS functions (very possible lol). > +- +-< if one battery in a series flashlight is blocked or shut off then the power doesn't flow? Does a BMS allow the power to flow through to the next battery in series? thank you

I don't know the cheap drop in market well enough to advise you. I built my own 24v battery and I am just building my second.
The thing I want in a lithium battery is a good BMS and good cells. Building your own guarantees that.
I also want a BMS that has bluetooth that allows me to monitor what is happening at cell level. I can have all the information I need, and more, on my phone. I also want an active balancer as part of the BMS. I chose CATL 280Ah cells and a 200A JK BMS with 2A active balance. This gives me all the feature I need.
From thr brief research I have made into cheap drop in, I don't see these features

 

[southeastflorida]

I don't know the cheap drop in market well enough to advise you. I built my own 24v battery and I am just building my second.
The thing I want in a lithium battery is a good BMS and good cells. Building your own guarantees that.
I also want a BMS that has bluetooth that allows me to monitor what is happening at cell level. I can have all the information I need, and more, on my phone. I also want an active balancer as part of the BMS. I chose CATL 280Ah cells and a 200A JK BMS with 2A active balance. This gives me all the feature I need.
From thr brief research I have made into cheap drop in, I don't see these features

 

[southeastflorida]

Where do you get your components and Li cells? are they grade b or a ? I appreciate the info on what your using for components.
thanks

 

[JRH]

Rodrick I appreciate your reply but was hoping to learn what the pitfalls or advantages are for example- BMS shutting off one battery when two 12 volt batteries are in series-does this happen or not? It would be nice to just get a big 24 or 48 volt battery bank but Im interested in being able to add to a 24 volt system in the future. It would be nice to have a modular system that has multiple $400 components instead of a system that has a $4000 single component? Thanks

comment deleted…I solved the problem…
thx..J.

 

[TimE]

Where do you get your components and Li cells? are they grade b or a ? I appreciate the info on what your using for components.
thanks

Have a look at 18650 Battery Store. Grade A cells

 

[cs1234]

I do like the ability of setting both overcharge and discharge limits to maximize the life of the expensive Lifepo4 batteries. I think the majority vote goes to ONE 24 volt instead of two 12 volt in series. Having the BMS of one battery shut off and blasting the remaining 12 volt battery, like you stated, isn't a good idea. If the batteries are in series and one BMS shuts off then It should block the charge to the remaining battery because the series is blocked by the BMS-Or do I not understand how a BMS functions (very possible lol). > +- +-< if one battery in a series flashlight is blocked or shut off then the power doesn't flow? Does a BMS allow the power to flow through to the next battery in series? thank you

I don't know how the BMS actually deal with series voltage internally, or why they work in series at all. Somebody here undoubtedly knows the electrical aspects.

I do know, that the remaining 12v battery continues charging even when the other battery in the series has charging disabled. So, whatever magic the BMS does, it's still perfectly fine being the only battery standing when it comes to charging.

I honestly don't know what happens if one of the batteries disables OUTPUT though, effectively removing itself from the series. I haven't seen it occur. What I suppose happens is the series battery pack voltage drops to the voltage of the remaining battery. Being as there are chargers/inverters/other paralleled batteries in the stack, this might result in a large dump of current into the single remaining 12v battery hanging out in the reduced voltage series battery due to the much lower voltage. This would likely cause that 12v battery to disconnect also, or at least the breaker I have between each paralleled battery to trip, causing that entire series battery to drop out.

The lesson here, always keep high quality, high AIC breakers between each paralleled battery. Do this even for paralleled batteries that aren't in series. It's especially important in series batteries in parallel though, as they are more likely to drop out in an ugly fashion than non series batteries.

 

[jberger]

Why do you want 24V?
The market has already shifted to 48V, so if you are buying new and don't need to pull off 12V for a special purpose (RV levelers, etc) you'd be better off sticking with 48V.

There is no good reason to stack lower voltage batteries to get to the system voltage. You are increasing possible points of failure, management and price.
If you stack (2) 12V to make a 24V system and you have issues with one battery, your system will not work.

 

[southeastflorida]

Why do you want 24V?
The market has already shifted to 48V, so if you are buying new and don't need to pull off 12V for a special purpose (RV levelers, etc) you'd be better off sticking with 48V.

There is no good reason to stack lower voltage batteries to get to the system voltage. You are increasing possible points of failure, management and price.
If you stack (2) 12V to make a 24V system and you have issues with one battery, your system will not work.

im looking at options. I agree that more parts can equal more failures sometimes. Then again I don't want to put all my eggs in one basket. re the batteries I agree that putting batteries in series (that have their own individual BMS is a bad Idea as I am learning-thats why I asked the question). I will be using the solar to provide power for a one or two dedicated appliances ie a refrigerator, or a small window AC, fans. I like the idea of SEVERAL small systems with their own panels, solar charge controller, battery, inverter rather than one HUGE whole house system (for now). Why you ask? Well I would rather have say SEVERAL 1000 watt PV array , a victron solar charge controller , 24 volt 200 ah lifepo4 battery, 24 volt inverter to power some critical appliances. Id rather have several small systems than one Huge system that when one $4000 component goes bad Im dead in the water. multiple eggs with multiple baskets. I have several acres and space to do it so that's my idea. There's an old hippie in Hawaii on youtube called "projects in paradise 808". I watched some of his videos-he has 6 small systems and when one goes down he just plugs into one of his other systems. He's always got a back-up. I thought it was a little crazy at first but a lot of this stuff breaks and burns out so why not have a back-up? I know some stuff is better than others $$$$$. I can see the advantage of 48 system- more volts and less amps equals smaller wires etc.. But this solar game aint cheap so Im starting small and cutting my teeth on 24 volts. thanks for your input.

 

[southeastflorida]

Hey guy… I have one thing on my system I would change if I could…the voltage of the batteries …
I was told at first to buy 24v batteries by the sales guy… I decided not to listen to more experienced solar people and went with 12v batts …….

after living with the system running for about 8 months I realize I was wrong…should have went with 24V batts.. im in too deep to change batts now…
All victron gear- all battle borne batts , everything overbuilt and overbought…I love it all…everything is working great but it takes more effort , money and wire to do it with 12v batts wired.. series - parallel .. then you have the balancing issues and it’s drift over time.

even though I have learned how to balence them pretty good while connected it’s more of a fight and you must pay attention with more detail…..who needs to have another fight in life when it was avoidable AND easier And cheaper To do what I was told by some pretty smart people .

The voltage is the only thing I would change….

If I had known more back then I would have listened…sometimes it seems the most stubborn people are the newest people…( uhh, I was guilty of that big time) ..
listen to these forum guys on this topic .. they are totally correct…
J.

I agree and am learning how all this works. 12, 24, 48 volt. If we go with higher voltage we can have lower amps and smaller wires. Also MPPT charge controllers can handle twice the PV input voltage when going to voltage arrays. Backwoods solar.com has a great catalog that has explained a lot of this for me to begin to understand (verify for yourself). Im going to start small and learn along the way.

 

[MisterSandals]

two dedicated appliances ie a refrigerator, or a small window AC, fans.

You are going to need a fairly hefty system to run these.
Do you have any idea how many watt hours each will use each day?

You cannot realistically proceed without your daily energy consumption. Even if you break up your listed appliances, i think you are overestimating your solar production and underestimating your usage (this is my gut feel from what you've written).

I do like your thought process on building a system with many parallel and redundant pieces.