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Open Roads Forum  >  Tech Issues

 > Not the cells that fail it is the battery Management System!

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pianotuna

Regina, SK, Canada

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Posted: 04/07/22 09:39pm Link  |  Print  |  Notify Moderator

Hi all,

"The Weak Link - The BMS

We now have a few years of experience with lithium-ion batteries, and what is becoming clear is that while the LiFePO4 cells hold up very well, that is not the case with the Battery Management System (BMS). Overall the number of prematurely failed batteries is small, but with 10,000+ batteries sold it is clear that in 99% of cases it is the BMS that fails, turning the battery into an expensive piece of gender-neutral-cave decoration!

While we very much advocate for using batteries with a build-in BMS (without one the battery would be unsafe and likely fail very quickly!), manufacturers struggle to make their BMS as bullet-proof as it should and needs to be. Surge currents due to the input capacitors of large inverters, motors, and air-conditioners can and at times will kill the BMS, rendering the battery useless.

At least one well-known battery manufacturer is now enforcing their warranty conditions to the letter, and that requires the use of an external current limiter when their batteries are used with large inverters (“large” being defined as 3,500 Watt and up). This leads to the ironical situation where the BMS is there to protect the battery, and now a current limiter gets connected to protect the BMS. What will be next to protect the current limiter…

Seeing how the BMS has become the weak link, manufacturers should really work (hard) on hardening that. Nothing is 100% bomb-proof, but there certainly is room for improvement! Another solution could be to acknowledge that the BMS is the weak link and make it so it can be replaced without too much effort, for example a gasketed lid on the battery that is removable with a few screws, and a BMS that has connectors and bolted lugs, so a repair shop can swap the board. It makes no sense to throw away a battery where 90% of the cost is in the cells, and 10% in the BMS, just because the BMS failed."

From:

https://www.solacity.com/how-to-keep-lif........9CxovYxjyEIFqUJWPCi-QPpPdNm8p41WLt3yzQWA


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

wa8yxm

Davison Michigan (East of Flint)

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Posted: 04/08/22 04:41am Link  |  Print  |  Notify Moderator

There is a Pod cast (Amateurlogic.tv) a few episodes ago Tommy Martin built his own LiFePo4 battery set.. He bought cells (Without BMS) and a BMS. and built a case to put 'em in.

Why am I pointing this out?

The BMS can be replaced on Tommy's design.. Easily in fact.


Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times


folivier

Southeast Louisiana

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Posted: 04/08/22 06:50am Link  |  Print  |  Notify Moderator

SOK batteries have a removable lid to access the BMS and cells making them replaceable.

time2roll

Southern California

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Posted: 04/08/22 07:53am Link  |  Print  |  Notify Moderator

Connecting an inverter seems to be the real issue. Use a pre-charge resistor when connecting to avoid the large inrush and the lug damaging spark. This is a good idea with any battery.


2001 F150 SuperCrew
2006 Keystone Springdale 249FWBHLS
675w Solar pictures back up

StirCrazy

Kamloops, BC, Canada

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Posted: 04/08/22 08:32am Link  |  Print  |  Notify Moderator

wa8yxm wrote:

There is a Pod cast (Amateurlogic.tv) a few episodes ago Tommy Martin built his own LiFePo4 battery set.. He bought cells (Without BMS) and a BMS. and built a case to put 'em in.

Why am I pointing this out?

The BMS can be replaced on Tommy's design.. Easily in fact.


there are lots of companies out there now that are using cases that can be unscrewed or opened with out dammaging it. just funny that solar city would have this rant, but all there 12V setups that are afordable are sealed cases. if your going to rant like that you should be "walking the talk" and changing out your product lines for ones that are easily replacable.

Steve


2014 F350 6.7 Platinum
2016 Cougar 330RBK
1991 Slumberqueen WS100

3 tons

NV.

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Posted: 04/08/22 10:40am Link  |  Print  |  Notify Moderator

The post-internet era paradox we now reside in is that never in the history of humanity have humans had soooo much access to information (including mal-information…) than today - this post-modern trend comes with both it’s upside and a downside - recall the old adage, “they couldn’t say it if it wasn’t true!!”, thus, one’s own discernment (aka, critical thinking!!…) is a requirement!… Does anyone else sense that nowadays the sky seems to always be falling!! Nor am I suggesting that this is not prudent information that one should not consider, because it is…

Yet upon closer review the stated problem (while seemingly vindicating the Li cells), IS ‘limited to’ larger inverters (i.e. 3500w and upwards…) where a potential high ‘sudden-surge’ current draw (larger-inverter capacitor, and motor related) may adversely impact the BMS (though I’m sure not all inverters are equivalent?…). Though left unmentioned, it would also seem that this particular malady might depend upon the number of batteries in one’s battery bank (e.g. undersized?)…However, I’d also note that this is not a ‘one size fits all’ concern - those who fit this particular 3500w and above inverter/motor description should arrive at ‘their own informed’ conclusions and respond accordingly - In my view this is an issue of limited impact, and for some folks, BB’s suggesting of a current limiter only makes perfect sense to me…JMO

3 tons

ScottG

Bothell Wa.

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Posted: 04/08/22 10:49am Link  |  Print  |  Notify Moderator

This is very helpful Don, thanks for posting. I have been trying to decide whether to buy bats with built in BMS' or buy 4 cells and an external BMS. I have only seen a couple of assembled batterys that can be easily opened and they are much more expensive and have no other attributes - you're just paying for the case.
I think this makes the decision for me.
Another benefit of stand alone BMS' is they tend to more often have better temperature protection and blue tooth. I can also over-size the BMS to head off trouble.

S Davis

Western WA

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Posted: 04/08/22 11:43am Link  |  Print  |  Notify Moderator

ScottG wrote:

This is very helpful Don, thanks for posting. I have been trying to decide whether to buy bats with built in BMS' or buy 4 cells and an external BMS. I have only seen a couple of assembled batterys that can be easily opened and they are much more expensive and have no other attributes - you're just paying for the case.
I think this makes the decision for me.
Another benefit of stand alone BMS' is they tend to more often have better temperature protection and blue tooth. I can also over-size the BMS to head off trouble.


That is where I ended up to, I have put together four 12 volt 280ah batteries, I am using two for a 3000 watt inverter on my work truck each has a 250amp BMS. I think I have about $1100.00 into each battery.

Battery bank

pianotuna

Regina, SK, Canada

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Posted: 04/08/22 11:48am Link  |  Print  |  Notify Moderator

3tons,

I can say my first inverter was 2500 watts. I powered it with 875 amp-hours of marine batteries because I did not want problems with high demand. It was able to run the roof air.

The current inverter/charger is 3000 watts (but really that is VA). Again I powered with 556 amp-hours of AGM reconditioned telcom jars. I love my load (not voltage) support!

In neither case was overloading the bank a consideration.

If there were an affordable Li formulation that does cold and surge too, I'd love to have one. So far the only cold unit is Li Titinate--which has a reputation for low amps of draw--so I'd need six or seven to meet my needs @ $1800 EACH.

I've seen more than a few LiFePo4 that are 100 amp-hours in size but the BMS limits to 50 amps.

Now my tires have timed out, and rubber is through the roof, so since the telcom jars are still ok, (load test) I won't be able to afford the SiO2 that I want (which can do 400 amps from a 100 amp battery, continuous).

Covid has not helped--2020 income dropped by 75% and 2021 it is less than 50% of 2019. 2022 is 1/3 gone and it appears I might do about the same as 2021.

Off tomorrow on a ten day work trip.

Itinerant1

Itinerant

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Posted: 04/08/22 11:53am Link  |  Print  |  Notify Moderator

With more and more folks switching over to lfp, with economy priced dropin batteries and thinking that they are some sort of miracle battery are just undersizing their system then tasking with high demand loads.

The 13th of this month will be 6 full years of everyday fulltime use and over 1,900 partial cycles of 35-45% discharges daily and lately 45-55% dod using the 15k air conditioner off of solar/ batteries.

This isn't your basic dropin battery setup and do have a couple of 400a solenoids that assist with the protection of the system.
[image]


12v 500ah, 20 cells_ 4s5p (GBS LFMP battery system). 8 CTI 160 watt panels (1,280 watts)2s4p,Panels mounted flat. Magnum PT100 SCC, Magnum 3012 hybrid inverter, ME-ARC 50. Installed 4/2016 been on 24/7/365, daily 35-45% DOD 2,100+ partial cycles.

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