1 BMS or 2?

[zipsonic]

I'm building an 8 cell (3.2v 100ah) 12v pack as a "drop-in" replacement for my current lead acid setup. My intention is to make 2 sets of 4, then run them in parallel. Based on mobile-solarpower.com, i'm looking at this BMS:

https://overkillsolar.com/product/bms-120a-4s-lifepo4/

It's designed for 4 cells. However, they have this diagram that shows an 8 cell 12v system. Something doesn't seem right about this. I'm guessing I'd have to prebalance the pairs? Also it would seem that you'd only know that a specific pair failed, not an individual cell, which doesn't seem too bad for troubleshooting.

So I guess I'm looking for recommendations. Would it be better to run 2 BMS's or is the single ok? Drawbacks? Is there another system that is recommended?

Thanks all, learning more day by day....

 

[Ampster]

That diagram is known as 2P4S and only requires one BMS because the pairs of cells automatically balance themselves. A two BMS configuration is known as 4S2P.
The pros and cons of each are many. I prefer simplicity of one BMS but others like having redundancy of two separate packs. You can always try one BMS for starters and change your mind later.

 

[HRTKD]

I started out on the 2p4s route, mainly due to space limitations in my trailer. I allowed myself to be convinced that 4s2p was the better way to go and I searched harder for available space.

I ordered two 4s BMS devices from @OverkillSolar this morning. I wasn't planning to spend the extra money, but I would rather implement a system that is more on the right way of doing things than it is on the less expensive way to do it.

 

[Ampster]

I'm not sure there is a right way. Space limitations can cut either way. What are some of your other reasons to prefer two packs instead of one?

 

[HRTKD]

4s2p provides a finer granularity of control and reporting. The BMS can "see" each battery and act directly upon it. If you have a cell that is out of spec in a 2p4s configuration, the other cell in the parallel pair could mask the problem.

Lugging around a 4s battery (46 lbs) is less likely to give you a hernia than a 2p4s battery (93 lbs).

One other plus for 4s2p is that I can get a 120 amp BMS for each battery, which in theory gives me 240 amps of discharge. There are certainly a large variety of BMS devices, some that handle more than 240 amps with a single unit. So this reason is easily argued.

There is some redundancy with 4s2p. If one BMS fails, you still have the other 4s battery operational.

 

[Xerxes]

If your batteries are new, matched, and balanced the only definable gain with two BMS's is redundancy of system. That is if something happens too one cell or BMS you can still limp along with "half a battery" if you absolutely can not be down at all and having at least half a pack has huge value (while you work on the other pack to get it running). Also preferred if batteries are not matched-new-balanced or if you batteries need to be portable so you only have to lift half the stinking heavy battery.

Downside is the simple engineering concept of keep it simple stupid, the more parts you have the greater chance one will fail.

If you are a belt with suspenders type of guy that then adds duct tape, superglue, and No. 9 wire..... to keep your pants up then you will always want the two BMS system with two separate-able battery packs.

With new, matched and balanced cells, ........splitting the pack with two BMS's is safety on top of safety on top of safety and really does not get you any statistical definable value for safety of system and battery protection.

Think about it for a minute-O'l Musk himself puts 60 or 70 or 80 or possibly hundreds of batteries in parallel to make one large cell that is then series stacked for "THE-BATTERY" and protected by one BMS in his cars. Electric bikes typically have 4 to 10 batteries in parallel to make one cell depending upon Amp-Hour desire.

This comes down to a preference thing if cells are balanced, new, and matched.

 

[Steve_S]

Think about it for a minute-O'l Musk himself puts 60 or 70 or 80 or possibly hundreds of batteries in parallel to make one large cell that is then series stacked for "THE-BATTERY" and protected by one BMS in his cars. Electric bikes typically have 4 to 10 batteries in parallel to make one cell depending upon Amp-Hour desire.

This comes down to a preference thing if cells are balanced, new, and matched.

Apples to Potatoes !
18650 / 21700 NMC do NOT hold 200AH in each cell. The format demands fused (welded) connections to the cells.
Each cell is FUSED ! for protection and disconnect to prevent thermals.
Take a look at a Tesla BMS and then forget anything considered "simple" they are far from it.

A mosFET based BMS like those sold by Overkill are cheap insurance and NOT a great expense at what $100 USD +/-.

Understanding that large LFP calls (big AHrs compared to individual NMC 18650 mAh cells) are NOT the same and behave differently not only due to capacity but the chemistry as well. Larger cells will also wander (voltage differentials) a lot more simply because of the capacities within. This happens with cells that are perfectly matched for Voltage & Impedance, regardless of maker.

 

[zipsonic]

I don't feel so bad asking the question now. Lots of opinions on the matter.

After reading the replys, I believe 2 BMS is the way to go. If 1 BMS dies, you still have another "battery". You can "see" each individual cell. Smaller 30lb batteries (3.3kg * 4 * 2.2 + BMS & Connections). Easier maintenance, take one set out at a time, but still have power. And $100 is pretty cheap insurance.

Downsides: extra configuration, more wiring, more cost.

2 BMS it is! Thanks for the help!

 

[HRTKD]

You came around a lot faster than I did.

I can be rather stubborn at times.

 

[learning all the time]

This seems to be a newer thread so FANTASTIC! I am in the process of putting together a 560AH 8 cell system with 2 BMS for redundancy just as HRTKD explained. I am also using OverKill BMS. I have some problems. 1) Beautifying the system to keep it clean but I have some ideas 2) Are you guys using some sort of battery post to connect battery a to b? 3) Heres the stupid question and its probably asthetics bothering me but I believe the bms black cable from battery "A" goes to the blue cable neg battery "B" port and the positives should be linked to where the red cables from the bms's on both batteries correct? Wish I had a drawing to explain better. 4) Last, if this is correct with having the two bms's at the same negative terminal cause any issues?

 

[HRTKD]

Go to this page and look at the two battery/two BMS image: https://overkillsolar.com/product/bms-120a-4s-lifepo4/

 

[learning all the time]

Go to this page and look at the two battery/two BMS image: https://overkillsolar.com/product/bms-120a-4s-lifepo4/

Excellent! I knew I saw this somewhere thank you. My hookup is correct except I am losing almost 1v across one of the BMS modules. I will contact overkill for advice.

 

[ZebraPainter]

I don't feel so bad asking the question now. Lots of opinions on the matter.

After reading the replys, I believe 2 BMS is the way to go. If 1 BMS dies, you still have another "battery". You can "see" each individual cell. Smaller 30lb batteries (3.3kg * 4 * 2.2 + BMS & Connections). Easier maintenance, take one set out at a time, but still have power. And $100 is pretty cheap insurance.

Downsides: extra configuration, more wiring, more cost.

2 BMS it is! Thanks for the help!

I've been looking for exactly this conversation in this forum. I also like the fact that it is recent. I'm getting eight 280Ah cells at the end of September and plan to build a 12V system. This issue has been driving me nuts. It just drove me nuts that Overkill showed both options (1 vs 2) but doesn't say squat about it. I'm leaning towards two for the simple reason of more current capacity. How would I run a 3000W inverter on just one BMS? This thread is good.

 

[HRTKD]

It just drove me nuts that Overkill showed both options (1 vs 2) but doesn't say squat about it.

What are you looking for him to say? The BMS is relatively agnostic about how you setup your cells. It has no idea if you're in a 1p4s or 5p4s configuration. The volts it monitors are all the same, either way.

 

[Steve_S]

I've been looking for exactly this conversation in this forum. I also like the fact that it is recent. I'm getting eight 280Ah cells at the end of September and plan to build a 12V system. This issue has been driving me nuts. It just drove me nuts that Overkill showed both options (1 vs 2) but doesn't say squat about it. I'm leaning towards two for the simple reason of more current capacity. How would I run a 3000W inverter on just one BMS? This thread is good.

From your description, you have TWO options.
1) Single 12V Pack using 8 cells. 2 Cells in parallel within the pack, using a single 4S BMS
- only 1/2 the cells would be managed as an "average".
- If the pack shuts down for any reason it's down. No Fail-Over.
2) Two 12V packs using 4 cells. Each cell is independent and manageable with a 4S BMS
- Each cell is manageable and monitored.
- If one pack shuts down, the other will carry the load, while both are operating, they will share the load & charge as well (less strain on packs)

KEEP IN MIND WEIGHT ! Each 280AH cell weight 5kgs / 11 Lbs Something to consider when thinking of moving these, or placing them (such as in a mobile application)

 

[ZebraPainter]

From your description, you have TWO options.
1) Single 12V Pack using 8 cells. 2 Cells in parallel within the pack, using a single 4S BMS
- only 1/2 the cells would be managed as an "average".
- If the pack shuts down for any reason it's down. No Fail-Over.
2) Two 12V packs using 4 cells. Each cell is independent and manageable with a 4S BMS
- Each cell is manageable and monitored.
- If one pack shuts down, the other will carry the load, while both are operating, they will share the load & charge as well (less strain on packs)

KEEP IN MIND WEIGHT ! Each 280AH cell weight 5kgs / 11 Lbs Something to consider when thinking of moving these, or placing them (such as in a mobile application)

Thanks. Its what I was thinking too. I'm not too concerned about the extra money to buy the second BMS, I'm more concerned about safety and running on the edge of a single BMS. Thanks for confirming my thoughts.

 

[Mex Rider]

I'm configuring two 4S packs/12 volts/100 ah with two BMS (all Overkill) If I connect a 3000 watt inverter to power an 1800 watt continuous, 4500 watt startup table saw, will the startups adversely affect the life of my cells. The Sigineer inverter will take a surge up to 9000 watts but I am interested in longevity for my cells....and BMS. All my other loads could be satisfied by a Victron or Samlex 2000 watt inverter. Thoughts please

 

[learning all the time]

From your description, you have TWO options.
1) Single 12V Pack using 8 cells. 2 Cells in parallel within the pack, using a single 4S BMS
- only 1/2 the cells would be managed as an "average".
- If the pack shuts down for any reason it's down. No Fail-Over.
2) Two 12V packs using 4 cells. Each cell is independent and manageable with a 4S BMS
- Each cell is manageable and monitored.
- If one pack shuts down, the other will carry the load, while both are operating, they will share the load & charge as well (less strain on packs)

KEEP IN MIND WEIGHT ! Each 280AH cell weight 5kgs / 11 Lbs Something to consider when thinking of moving these, or placing them (such as in a mobile application)

Steve you are so right! My coach is only 25ft and the battery compartment under the stairs is small. Long story short, batteries are on original battery brackets wrapped in plastic for weatherproofing. Not the design I envisioned at all but works great. Currently in full bore testing.

 

[learning all the time]

Thanks. Its what I was thinking too. I'm not too concerned about the extra money to buy the second BMS, I'm more concerned about safety and running on the edge of a single BMS. Thanks for confirming my thoughts.

Zebra, I am running the two BMS system. I was confused on a few things.
1) Parallel design. Normally you do positive to positive and negative to negative of the two 12v systems. To me originally I thought one black BMS cable should go to the same negative as the blue BMS cable of the other battery. This leaves the second black BMS cable as a battery cable. It seems as Overkill wants you to connect your power between the two BMS black cables for power distribution.
2) Programming the BMS. Overkill recommends not changing any setting as it could really mess up the BMS and in turn the battery. I have been testing at 10 amps for about 14 hours now and both BMS read battery at 0% but my victron shunt says I have 2 days left. My motorhome dummy button says I am at 100% charge so I need to get these three in agreement or figure out which is the more correct device.
3) I have not used the solar panels on them yet. I have 500w (300W brand new) plugged in and ready to go. I currently have them covered so when the sun comes I will remove the cardboard and see how the batteries charge with my cable configuration.

 

[learning all the time]

I'm configuring two 4S packs/12 volts/100 ah with two BMS (all Overkill) If I connect a 3000 watt inverter to power an 1800 watt continuous, 4500 watt startup table saw, will the startups adversely affect the life of my cells. The Sigineer inverter will take a surge up to 9000 watts but I am interested in longevity for my cells....and BMS. All my other loads could be satisfied by a Victron or Samlex 2000 watt inverter. Thoughts please

Would a soft start work for you to take some intitial torque out? I just added one to my RV AC for emergency use with my solar system. I hope to test it this weekend as I have my new batteries in.