Increasing 48v battleborn battery bank with DIY batteries?

[Daley]

I am a complete novice at this so please forgive me if questions seem silly.

I have 4 x 12v 100ah battleborn batteries connected in series to give 48v.

I would like to increase the battery bank capacity.
Can I build another 48v bank and connect that in parallel to my existing bank? If so what would be a good and affordable route to take? Space is limited so building something smaller would be a bonus.

Many thanks

 

[GXMnow]

Yes, you could build another 16S LFP (LiFePo4) bank of cells, with their own BMS and have it connected parallel to your existing Battle Born batteries. But since the capacity and age as well as the connection resistance etc. are all not going to match, do not expect to get any more current, just more time. So whatever current your existing battery bank can safely handle, run the same current but for twice as long by adding a string of 16 100 amp hour LFP cells and a BMS.

 

[Daley]

Thanks for your help, much appreciated.

So I would need to buy 16 100 amp hour cells, a BMS (any recommendation's?), bus bars and some sort of housing, is that everything?

I am trying to increase my storage capacity, no problems with current, system works great.

I saw these 12v 100ah cells on alibaba, would four of these work?

I also saw these that appear to be a Chinese battle born equivalent.

The ready made one looks like it would work out cheaper.

 

[Craig]

Thanks for your help, much appreciated.

So I would need to buy 16 100 amp hour cells, a BMS (any recommendation's?), bus bars and some sort of housing, is that everything?

I am trying to increase my storage capacity, no problems with current, system works great.

I saw these 12v 100ah cells on alibaba, would four of these work?

I also saw these that appear to be a Chinese battle born equivalent.

The ready made one looks like it would work out cheaper.

You can get 100AH cells on alibaba for around $34.00 plus shipping. Much cheaper than the drop in replacement which I think you wont be happy with anyhow.

 

[Daley]

Hi,

Am I correct in thinking that those cells are 3.2v each and I will need 16 of them?

 

[Craig]

Hi,

Am I correct in thinking that those cells are 3.2v each and I will need 16 of them?

Yes you would

 

[GXMnow]

Thanks for your help, much appreciated.

So I would need to buy 16 100 amp hour cells, a BMS (any recommendation's?), bus bars and some sort of housing, is that everything?

I am trying to increase my storage capacity, no problems with current, system works great.

I saw these 12v 100ah cells on alibaba, would four of these work?

I also saw these that appear to be a Chinese battle born equivalent.

The ready made one looks like it would work out cheaper.

The first link is 3.2 volt cells, the 12 volt option is getting 4 cells. The second link is a pre packaged group of 4 cells like a battle born. Many pre built 12 units like that do not recommend wiring in series, because they can't balance between them. I would go with the 16 individual cells and a single 16S BMS to handle the balancing and protection of the new string.

 

[Keith Moreau]

I was thinking of doing the same thing eventually. I got 4 Lion Energy UT1300's to wire in series to make a 48v battery and connect to my Sol-Ark 12K. This was in a way a 'tester' system with about 5600 Wh capacity which has limited usefulness in a large house PV system. Later I was planning to build a large 16-32 LifePo with 280ah cells DIY with BMSs and run it in parallel. From my understanding the system would run ok, drawing equal amounts of current until the UT1300's voltage got too low and then their BMSs would kick in and shut them off but the DIY LifePO bank would continue. Am I missing something? Or is just a bad idea?

 

[GXMnow]

I do not recommend running any bank of batteries to the point where the BMS has to shut off. Have the inverter shut down before your batteries get that low, and have the charging stop before any battery get's too high. The BMS shut off should only happen if something goes wrong and it has to protect the system. If at all possible, you should use the same chemistry of batteries, so the charge voltage limit and the discharge cutoff limits should be the same. That way, with the banks in parallel, they may not all pull the same current, but they will stay close to the same state of charge and all reach the high and low limits at the same times.

Lithium cells work great in parallel due to how they charge and discharge. This does not work with nickle based batteries though.
Adding more batteries in parallel is great for getting more run time, but not so much for adding more peak current. If the batteries are not a perfect match, they will not evenly share the current. So let's take an extreme case. You have a 100 amp hour battery bank, and you parallel another 25 amp hour bank. This is a huge difference to explain what is going on. As long as you only pull as much current as the big bank can take, all is well. If the resistance of the wires to the 100 amp and 25 amp banks was the same, they may actually start as both trying to supply half the current. Obviously, the 25 amp bank will start to lose state of charge much faster, but as soon as the voltage of the cells falls even a few millivolts, the larger 100 amp hour bank will start to take more of the current. So the parallel connection self balances the state of charge. It is possible the smaller bank just might hit low voltage limit a little sooner than the large bank, but the difference will be small. Leave 1/10 of a volt above the BMS cutoff and it should all be good. On charging, the opposite will happen. The smaller bank will charge up faster, but as it's voltage tries to climb, the larger bank will again start to take more of the current due to the internal cell voltage difference across the wire resistance. Parallel banks will stay balanced within a few percent on state of charge as long as they are the same chemistry. Tesla has been using massively parallel banks since the first roadster. Using so many very small cells does pose some issues with sharing high currents, but as soon as the current drops off at all, the cells will quickly self balance again. For our solar storage, we actually use much lower steady currents than what an EV can demand in short peaks. A Chevy Bolt under hard acceleration exceeds 100 KW. Even at 96S, that is a lot of current. Nearly 300 amps. On a 48 volt system, that would still be 14 kilowatts.

 

[Keith Moreau]

@GXMmow: OK, so what I hear you saying is that running my 100Ah LifePo along with my 600Ah LifePo in parallel should be ok as the paralleled banks will 'self-correct' and act like a single battery to the inverter / charger. Yes? Thanks much for the great info.

 

[GXMnow]

As long as your max current draw does not overwhelm any imbalance in the system.

 

[j_t_l]

@GXMmow: OK, so what I hear you saying is that running my 100Ah LifePo along with my 600Ah LifePo in parallel should be ok as the paralleled banks will 'self-correct' and act like a single battery to the inverter / charger. Yes? Thanks much for the great info.

To add two things:

1) If there is a difference in the resistance of the batteries (which could be due to anything -- internal resistance, wiring, different connectors, etc), that will also cause imbalances. When the load (or charge) is removed, you can see the low-resistance battery will start charging the higher resistance battery. The amount of "balancing current" will depend on the how out-of-balance your system is and the load (or charge) current. (The "balance current" could get fairly large given the right conditions. I assume this is along the lines of what GXMnow means by the max current draw overwhelming the imbalance in the system).

2) If one of the two BMSes trip, you'll need to make sure your system handles that. If you run off of one battery for a while (either intentionally or by accident), the two batteries could get way out of sync and be at totally different voltages.


Due to these, I'd recommend considering a few things. Depending on your setup, you may consider these things and then not do them

• Use individual fuses for each battery
  • The BMS will probably shut down if you do something stupid (like accidentally connecting 2 batteries with a big voltage delta) -- but it's good to have fuses as a 2nd protection layer.
• Have some means of charging (or discharging...) individual batteries
  • If the batteries do get unbalanced, use the charger / load to get the voltage close (say 0.2v or less) before reconnecting in parallel.
• Use battery disconnect switches to select one battery, the other, or both for parallel operation
  • This can make maintenance easier when you want to mess with one battery

 

[GXMnow]

That is exactly what I am talking about. If you add more batteries to just get more run time and don't increase the current draw, you should be fine. The different resistances may cause one string to drop to a lower state of charge a little sooner, but as soon as the internal cell voltage drops, even a few millivolts, the other cells in parallel will start to see more of the current. If both strings can handle all of the current, it can never be a problem. If you try to pull over the limit of any one string, then the balance has to be closer for the system to be stable. The closer the match the better, but at reasonable currents, it is not as much of a concern. If a BMS ever does open, then yes, you need to be sure the balance is close before resetting the system. Just 1 volt difference in a 48 volt string could still mean hundreds of amps when the 2 strings are connected.

 

[curiouscarbon]

@GXMnow In a multi BMS system, do you prefer a setup that disconnects all BMS if even a single one goes into safety disconnect mode? This preserves inter pack voltage for easier reset but reduces availability. Thoughts?

 

[GXMnow]

I think I would keep them separate. If the shut down is cause by a single cell going out of safe voltage range, turn off that pack to protect it, but keep the other running. The problem being you might not notice it right away, so maybe an alarm would be a good idea. If the trip is due to over current, the other will trip out very quickly when the current jumps up, so that is not a real concern. As for reconnection, yes, you will need to balance before putting the bad pack back in service. This should be a given anyways, as most likely it is out of balance to have caused the shut down.