Parallel BMS for 4 x 12v start battery in Parallel

Tommy L

Tommy L

100 kW
Joined
Dec 23, 2010
Messages
1,124
Location
Saint Augustine, Florida USA
Hello fellow EVers.......

I have a need to take some my old LiFePO4 batteries 4P4S 26650's A123 M1's 12.8v nominal and add a BMS board. These will be used for start batteries for a generator. I believe the incoming charge current will be low as the voltage of these batteries sit around 13.2v. My question is.... any good BMS's out there? or Bleed Circuits? I'm still BMS free on my Ebikes so I haven't kept up on this tech. Thank you!

I will have 4 x 4p4s boxes at 12.8v nominal. Can I put a BMS/PCM on each battery and run the outputs of the BMS in Parallel????? Yes parallel for this set up as it will be a start battery for a standby generator.

I've ordered 4 of these......

https://www.ebay.com/itm/12V-14-8V-...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
 
How much current does the startup need?

If it's anything like that needed by a car, you may not find an affordable BMS that can handle it, even with several in parallel.

But you could run the gate control line of the BMS output to a contactor driver, and use the contactor to perform the cutoff.

This all assumes your cells can handle the current (which I wouldn't guarantee, as most of the generic 18650 LiFePO4 stuff is barely 1C capable without a lot of sag, 2C if pushed).
 
parabellum said:
What is your plan for charging them, with BMS you linked?
Click to expand...

yes, from the generator after startup. If the bleed circuits can handle it, but I'm thinking that the voltage regulator will also limit the current from the generator charging system.
 
amberwolf said:
How much current does the startup need?

If it's anything like that needed by a car, you may not find an affordable BMS that can handle it, even with several in parallel.

This all assumes your cells can handle the current (which I wouldn't guarantee, as most of the generic 18650 LiFePO4 stuff is barely 1C capable without a lot of sag, 2C if pushed).
Click to expand...

1. even a 4 cyl car will draw 80amp initially to start motor rotation, then the amperage drops off drastically.

2. These 4p4s LiFePO4 were tested by me over the entire capacity pulling 5-7C without voltage sag. These are 9.2 ah at 12.8v nominal with 4 in parallel (36.8ah) ...... 184-257amps (2500 watts) easy without sag for the entire capacity. I've use 4 in parallel in my Honda Accord for the past 4 years. But I spec'd those years ago from Taiwan with a bleed circuit as my accord has a built in Low Voltage cut at 10.8v I didn't bother with a BMS.

I used these LiFePO4 batteries for my ebike builds 7 years ago........ I used 10 of these in series for 128v nominal and would maintain 128v for the entire capacity while most on the forum here were using RC batteries and having a voltage drop over the capacity used.
granted..... mine were heavier to carry :( I now use 100v of RC Batteries for my ebike. 10ah 4s multistars... 6 in series. I'm a high voltage guy.... ;)

I could technically just use bleed circuits to ensure that this battery will not charge above 3.6v per cell.
 
parabellum said:
What is your plan for charging them, with BMS you linked?
Click to expand...

yes...... so the question is ..... Can I run 4 BMS in parallel?
 
Main outputs in parallel, yes. And if they're separate, then the charging inputs in parallel.

I don't know if it would work to parallel the balance lines or not; I guess it depends on how the BMSs work and levels they switch on/off at. (if some switch on balancing before others, by just a tiny amount, then only those balance leads/resistors will be participating in balancing, and the others won't share current unless the cell voltage continues to climb, above the switching point of the as-yet-unengaged units. )
 
Tommy L said:
yes, from the generator after startup. If the bleed circuits can handle it, but I'm thinking that the voltage regulator will also limit the current from the generator charging system.
Click to expand...

I have not used those BMSs you linked but, usually you can parallel them for discharge.
I would keep all battery blocks individual, without P balance connection, just main leads connected, because paralleled balance circuits freak out normally.
If your generator has no drain, when off, like fancy computer for metrics, I would connect batteries directly and hang those BMSs like top balancers only, because you have no charge control on discharge Fets anyway but, in this case you should be sure BMSs balancing function is always awake and does not need current flow to get waked.
If you have something draining the battery when gen is not running, then discharge Fets should stay in line, the only question is: Can they take initial pulse, which can be many hundreds of amps, and not consider it as short circuit, tripping discharge?
 
I originally said 18650 cells....

they are in fact 26650 M1A's


So sorry for the misinformation. With all this new 18650 samsung stuff on the market as of late...... I wasnt thinking! :oops:
 
Tommy L said:
I originally said 18650 cells....

they are in fact 26650 M1A's
Click to expand...
That makes a lot more sense...I have yet to handle an 18650 LiFePO4 cell that could do what those 26650s could, regarding current flow vs voltage sag/heat. (all the ones i've run into were "generic" stuff, as have been the ones I've read of here on ES).

IIRC there *are* A123 18650s...but whenever people use those, they tend to say "A123" and not just "LiFePO4". ;)
 
amberwolf said:
Tommy L said:
I originally said 18650 cells....

they are in fact 26650 M1A's
Click to expand...
That makes a lot more sense...I have yet to handle an 18650 LiFePO4 cell that could do what those 26650s could, regarding current flow vs voltage sag/heat. (all the ones i've run into were "generic" stuff, as have been the ones I've read of here on ES).

IIRC there *are* A123 18650s...but whenever people use those, they tend to say "A123" and not just "LiFePO4". ;)
Click to expand...


oh yes..... my bad for sure........
 
In normal conditions, yes, any balancing circuit will.
Those A123 are beasts and do not go out of balance easily anyway.

But in your case, there are 2 scenarios:
1)Your GEN is holding charge at <V than needed to hold balancer triggered continuously or you can make that happen in some way (diodes etc) and its OK scenario

2) If your generator holds >14.4V, balance circuit will be bleeding full power on all cells, non stop, as long generator is on.
(at minimal setting)0.65Ax14.4V=9.36W heat you need to move away to not get your balancer toast and you can not beat the generator, it will just get the voltage on your battery where it wants to (unless you do something in between).
In this case, you may be more comfortable not desperately battling generator and get adjustable Voltage balancer or BMS, out of main discharge line, and making your cells less happy along the way.
 
Not much capacity above 3.5v but I heard lifepo4 can take higher voltage like Ping can go to 3.8v . But is bleed down to 3.6v and does not stay that high very long. As will self bleed down also. I like 3.5v for lifepo4. Can the alternator be turned down to 13.6v perfect for 3.4v per cell.
 
A rare word of caution from me...

I am sure you are fine with a genset - but the other day I was jumping a 5.8L V8 with a pair of 4Ah 4S Lipo packs - and we managed to create a considerable fireball.

Yea packs were out of balance
Yea packs were low
Yea its like a 500A draw
Yea yea yea...

But yea...
This was my first experience of instantaneous and violent eruption into flames :shock:

Once cell blew out
Light off the solvant
Big fire ball
Then nothing

So...
For super high discharge burst outputs... account for this possible eventuality.

...

I have thought very long and very hard as to how to implement a "good" car starter BMS.
I came up with the following:

1) Any kind of mosfet control is off the table. Is got to be a big 500A contactor.

2) Low Power has to be handled by a boot-strapping mechanism - so... the battery lays dormant... the switch closes the contactor... the contactor can blow open after a time delay on sub-LVC... where a heuristic is required to account for the highly dynamic loading which is SURE to sag the cells way below LVC dynamically... when the charge on the pack is well above LVC.

3) LiFe is the only way to go - Lipo just wont match up with existing infrastructure in an energy-density-efficient way

4) Balancing is very important so as to avoid "reversed cell fire-show" - at a minimum - lockout on a given imbalance percentage.

5) Nothing off the shelf will do it as they are not intended for extreme dynamic discharge handling

6) Dealing with protecting the cells for slow death (like lights or stereo left on) is the trivial case - pop the contactor. Dealing with preventing extreme failure, for using TINY packs, is difficult. Of Course you can just run a giant LiFe pack... but thats expensive and heavy. Where we are trying to go "hardcore" and run maybe... 8Ah of 4S.... and still be able to turn the car over reliably enough to use it as the standard - starter... its a tougher to solve problem.

7) I have, for maybe 8 years now, driven around with a set of 2AWG jumper cables in my car. These array out to 10AWG PP45's where I plug in anywhere from 1pcs 4S5AH to 3pcs 4S5Ah and jump anything from Hondas to big heavy duty trucks. I can say, that if the packs are topped off, and if you have at least 5Ah for a Honda and 10Ah for a big V8... and the C-Rating is somewhat reasonable (20C+)... that you can quite reliably and safely use packs like this for starting. (My epic fire fail was... "asked for and worked for")

We almost burned off Kimberly's hair... so I MEAN IT WHEN I SAY - STAND BACK - WHILE YOU ARE STARTING CARS WITH HOBBY PACKS :D

-methods
 
methods said:
A rare word of caution from me...

I am sure you are fine with a genset - but the other day I was jumping a 5.8L V8 with a pair of 4Ah 4S Lipo packs - and we managed to create a considerable fireball.

Yea packs were out of balance
Yea packs were low
Yea its like a 500A draw
Yea yea yea...

But yea...
This was my first experience of instantaneous and violent eruption into flames :shock:

Once cell blew out
Light off the solvant
Big fire ball
Then nothing

So...
For super high discharge burst outputs... account for this possible eventuality.

...

I have thought very long and very hard as to how to implement a "good" car starter BMS.
I came up with the following:

1) Any kind of mosfet control is off the table. Is got to be a big 500A contactor.

2) Low Power has to be handled by a boot-strapping mechanism - so... the battery lays dormant... the switch closes the contactor... the contactor can blow open after a time delay on sub-LVC... where a heuristic is required to account for the highly dynamic loading which is SURE to sag the cells way below LVC dynamically... when the charge on the pack is well above LVC.

3) LiFe is the only way to go - Lipo just wont match up with existing infrastructure in an energy-density-efficient way

4) Balancing is very important so as to avoid "reversed cell fire-show" - at a minimum - lockout on a given imbalance percentage.

5) Nothing off the shelf will do it as they are not intended for extreme dynamic discharge handling

6) Dealing with protecting the cells for slow death (like lights or stereo left on) is the trivial case - pop the contactor. Dealing with preventing extreme failure, for using TINY packs, is difficult. Of Course you can just run a giant LiFe pack... but thats expensive and heavy. Where we are trying to go "hardcore" and run maybe... 8Ah of 4S.... and still be able to turn the car over reliably enough to use it as the standard - starter... its a tougher to solve problem.

7) I have, for maybe 8 years now, driven around with a set of 2AWG jumper cables in my car. These array out to 10AWG PP45's where I plug in anywhere from 1pcs 4S5AH to 3pcs 4S5Ah and jump anything from Hondas to big heavy duty trucks. I can say, that if the packs are topped off, and if you have at least 5Ah for a Honda and 10Ah for a big V8... and the C-Rating is somewhat reasonable (20C+)... that you can quite reliably and safely use packs like this for starting. (My epic fire fail was... "asked for and worked for")

We almost burned off Kimberly's hair... so I MEAN IT WHEN I SAY - STAND BACK - WHILE YOU ARE STARTING CARS WITH HOBBY PACKS :D

-methods
Click to expand...

These are the A123 M1 26650 cells in a 4s4p 9.2ah @ 12.8v nominal pack with 4 packs in parallel. I've tested these packs and at around 5-7C there's no voltage drop whatsoever. Usually.... on a charge circuit the voltage regulator or the regulator itself will back off charge amperage when it senses the voltage of 14.2v which would be approx. 3.55v per cell group. The bleed circuit that I posted the link to above should take care of wondering cell groups with various IR's.

This generator is a propane 2 cylinder Honda motor. The specs call for a 250cca PB battery. Considering my Honda Accord has used basically the same LiFePO4 A123 26650 cells in a 4s4P 9.2ah x4 units in parallel for the past 4 years (but these were motorcycle start batteries I had spec'd and made in Taiwan and have bleed circuit only) These 4 packs do not have the bleed circuit. My Honda Accord has LVC built in to the car's computer. I'll probably be fine with just the bleed circuits..... but my original question was:

Can I parallel the original BMS that I put in the OP. Will the discharge and balance features work in Parallel?

METHODS...... I'm still using your 9C2810 and controller (Justin 7225?) that you beefed up so I can run it at 50amps! I purchased from you years ago! I'm running that controller at 98.4V HOC. Lipo's 4.1v max per cell using Meanwells in parallel for bulk charging.
6 x 4s.... 10AH multi stars. 24s total. :p
 
Tommy L said:
Can I parallel the original BMS that I put in the OP. Will the discharge and balance features work in Parallel?
Click to expand...

As I noted here:
https://endless-sphere.com/forums/viewtopic.php?p=1389948#p1389161
you shouldn't parallel the balance leads (of any BMS set), just the main charge and/or discharge leads.

The balancer channels will not work together; some will switch on before others do, so you won't get all of them draining cells, just the first one that turns on, for each individual balance wire.




And as noted here:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=94826&p=1389952#p1389009
it's possible the startup load will be too great for the BMS FETs; if so you can use a contactor to switch instead, controlled by the circuits that used to control the FETs' gates. Hopefully you won't have to, but the option is there if you need it. :)
 
999zip999 said:
No no pictures it didn't happen .. Meth where are the pics or video ?
What car we talking about ?
Click to expand...

I have pics of both.

5.4L V8 in an Orvis that had been sitting for months
No lead - just straight Lipo

Let me go download the pics from another site.....

4_20_2018_014.jpg


4_20_2018_009.jpg


I don't "do" video
Pics n prose are my my style

We knocked the pack on the ground
Covered it in a pile of dirt
Went to work

(From phone on the go)

-methods
 
Tommy L said:
METHODS...... I'm still using your 9C2810 and controller (Justin 7225?) that you beefed up so I can run it at 50amps! I purchased from you years ago! I'm running that controller at 98.4V HOC. Lipo's 4.1v max per cell using Meanwells in parallel for bulk charging.
6 x 4s.... 10AH multi stars. 24s total. :p
Click to expand...

Super glad to hear that !

I will let someone else answer your primary question.

-methods
 
Looks like Amberdog hooked you up with some good links to dig thru.

-methods
 
amberwolf said:
Tommy L said:
Can I parallel the original BMS that I put in the OP. Will the discharge and balance features work in Parallel?
Click to expand...

As I noted here:
https://endless-sphere.com/forums/viewtopic.php?p=1389948#p1389161
you shouldn't parallel the balance leads (of any BMS set), just the main charge and/or discharge leads.

The balancer channels will not work together; some will switch on before others do, so you won't get all of them draining cells, just the first one that turns on, for each individual balance wire.




And as noted here:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=94826&p=1389952#p1389009
it's possible the startup load will be too great for the BMS FETs; if so you can use a contactor to switch instead, controlled by the circuits that used to control the FETs' gates. Hopefully you won't have to, but the option is there if you need it. :)
Click to expand...


Thank you so much..... as thought..... I'll Parallel and use the other product for the separate bleed circuit! Perfect! Thank you! :)
 
methods said:
Tommy L said:
METHODS...... I'm still using your 9C2810 and controller (Justin 7225?) that you beefed up so I can run it at 50amps! I purchased from you years ago! I'm running that controller at 98.4V HOC. Lipo's 4.1v max per cell using Meanwells in parallel for bulk charging.
6 x 4s.... 10AH multi stars. 24s total. :p
Click to expand...

Super glad to hear that !

I will let someone else answer your primary question.

-methods
Click to expand...

:D :D :D :D :D :D :D
 
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