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LiFeP04 cells in parallel

Erik_S

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Joined
May 21, 2021
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6
Hi,

I purchased a Victron Multiplus 24v 3000w inverter, 16 LiFeP04 cells and 2 Overkill 24v 8s BMS's. My plan is to create 2 24v batteries and connect them in parallel at the inverter. I ran system over the weekend using 1 of the 24v batteries it was awesome being able to turn lights on in the middle of the night. Do I need to do anything special to have the 2 8 cell batteries and separate BMS's? I was talking to a friend an he said I should just use a single 24v 8s BMS's and put 2 cells in parallel then tie them in series for 24v. I think I would be better off using 2 BMS's in case of high charge current or load each BMS is rated at 100A. My well pump uses close to 50A alone.

So after all this typing my real question is can I just tie the batteries in parallel or do I need to use either a switch between batteries or some type of electronics.

Thanks

Erik
 
If you can afford the extra hardware you're better off with two batteries in parallel as you get 100% cell monitoring and redundancy.

And you're fine just sticking them in parallel. Just make sure both sets of leads to your common bus bar or stud are the same length.
 
Having two 8s Batteries (each with their own BMS) in parallel is fine, and I think most people here would say it is better than having cells parallel and one BMS.

I'm assuming all your cells are the same capacity, which makes it an even easier decision. If you were making batteries of different capacity it would introduce some other issues, but there are several here on the Forum who have made that work too.
 
Having two 8s Batteries (each with their own BMS) in parallel is fine, and I think most people here would say it is better than having cells parallel and one BMS.

I'm assuming all your cells are the same capacity, which makes it an even easier decision. If you were making batteries of different capacity it would introduce some other issues, but there are several here on the Forum who have made that work too.
They are all the same 200ah all are the same age. I thought I would be fine connecting the 2 8 cell banks in parallel. I ran system over the weekend and it was amazing, I brought the batteries home to top balance them and make an insulated case/ heater for them. Thanks for the quick response.
 
Really, unless the cells are Genuine Grade-A+ and matched, do not put cells in parallel.
The advantage of have 2x 24V/200AH packs in parallel are:
- You get 24V/400AH collectively from the Bank.
- Faul Tolerance / Fail Over, if one pack cuts off the other can keep going. *IF the packs are matched specs and capable of being a standalone pack.,
- They will divide All Loads & Charge Input, lessening the strain on cells & BMS.
- You can ADD more packs Later ! This is LFP and NOT Lead !
- It is BEST to keep packs close in AH Capacity, ie 200AH & 280AH not bad BUT there will be tricky spots at <10% SOC and above 80% with the smaller packs. Not a big deal easy to sort. IF packs are greater than 100AH apart then you will have issues.
- It is STRONGLY recommended to have a DC Disconnect Switch between (Charge Source & Inverter) and the Battery Bank.
--- Many use these from Blue Sea: https://www.bluesea.com/products/category/11/41/Manual_Battery_Switches/e-Series

Quick Summary to make it work well.
- Keep ALL Battery Cables (+) & (-) the same length ! from Battery Terminals to Common DC BusBars.
- Keep the wires together to reduce Electrical Noise & Magnetic Fields (This is DC).
- Keep wires runs as short as possible BUT never have Hard Sharp Turns/Twists, allow enough slack for maintenance etc.
- TIP, It is always better to use "Fine Copper Wire" cables versus Coarse wires, As Such Southwires Royal Excelene Welding Cable is a Perfect Fit.
UL Certified: http://www.industrial.southwire.com/en/tile/10/spec/70300/?country=CA
Non-UL Certified: http://www.industrial.southwire.com/en/tile/10/spec/70305/?country=CA

The General Reference for paralleling battery systems etc is here from Victron - Lithium Based starting page 17.

I am Offgrid, 24V/4kW system with a 5 Pack 30kWh LFP Bank + a Lead Bank with 22kWh. You would likely benefit from looking at the links in my Signature... My "About My System" pages have parts, lists and tech info invcluding AC Wiring and bridging etc...
BTW I use a 120VAC GrundFos SQ-5 Soft Start Deep Well Pump. 260' deep, pushing to 50 Gal Pressure Tank then 75' to house. It starts at 500W and ramps to 1100W by the time it reaches Cutoff @ 52PSI. Very Solar System Friendly ! SOFT-START RULES for all motors.

Good Luck, Hope it helps.
 
Using 1 bms for 2 24v lith banks is a bad idea. The main reason is the bms monitors each battery & balances them as needed. In that scenario you'd have 2 paralleled lith's in a string of 8 in series. If 1 of those lith's goes out of balance, the bms won't know which 1 of the paralleled 2 is out of balance & won't be able to balance correctly. Definitely use 2 separate BMS's.
 
Using 1 bms for 2 24v lith banks is a bad idea. The main reason is the bms monitors each battery & balances them as needed. In that scenario you'd have 2 paralleled lith's in a string of 8 in series. If 1 of those lith's goes out of balance, the bms won't know which 1 of the paralleled 2 is out of balance & won't be able to balance correctly. Definitely use 2 separate BMS's.

I believe this is the primary reason people use separate packs instead of parallel cells.

It is of course completely incorrect.

Two individual cells in parallel can not go out of balance with each other.
 
I believe this is the primary reason people use separate packs instead of parallel cells.

It is of course completely incorrect.

Two individual cells in parallel can not go out of balance with each other.
You're right but if 1 of those cells goes sour, it'll drag down the other & throw the whole pack out of balance.
 
You're right but if 1 of those cells goes sour, it'll drag down the other & throw the whole pack out of balance.
Have observed this myself and hence why I gave up very quickly in paralleled cells within a battery pack.
The fail-over fault tolerance gained by having 2 vs 1 battery packs is also a factor which is IMO preferable but hey, I'm just an off-gridder in the middle of nowhere and fault tolerance ranks high for me.
 
A couple of other points to consider…

Only join the two batteries when they are very close in volts and capacity- I.e.. when they are both full. The two batteries will try to balance the differences - you want that balance small.

Have each battery fused. (With a classT - for lithium’s). That way if there is ever a short in the system-,each battery has its own fuse.
 
I'm just an off-gridder in the middle of nowhere and fault tolerance ranks high for me.

Me too, reliable power is important to my family, so i use two completely independent systems. (both containing parallel pair lifepo4 packs)

There are advantages to multiple BMS, and advantages to parallel cells and single BMS.

Most off the shelf rack mount lifepo4 are multiple BMS. The BMS communication a
is the weak point of this configuration.

Where i live everyone is off grid (hundreds of properties). The DIY single BMS systems have proven to be the most reliable.

Yes cells can go bad, it is quite rare though.

In my experience with off grid - the inverter is most likely to fail.
 
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