LiFePO4 Cell Balancing Hardware

[aaron_c]

I'm about to get eight of the Xuba (or whatever the replacement supplier is) 280 AH cells from the group buy thread. Hooray for that, I'll save some money!

I'm trying to pick a decent BMS (and maybe a balancer?) for this setup. This unnamed BMS from Electric Car Parts Company has been mentioned and looks good, but they mention using it alongside a QNBBM balancer (I assume this is what they're talking about?). However I can't tell how necessary the balancer is. The unnamed BMS has some ability to balance cells... it says it's balance current is 35 mA which...seems like not much?

Just trying to figure out how I should be going about this, given that I will tend to draw the batteries down to maybe 20-30% SOC before charging them again, and I will likely only ever charge them up to 90% SOC, particularly since I'll be mostly charging them with a generator in the foreseeable future (I hope to add solar, but I suspect that it'll only power all my stuff in the summer, if then).

Any thoughts are welcome, including suggestions on alternate BMS. BMSi? BMSs? BMSi.

 

[snoobler]

The QNBBM balancer ACTIVELY balances cells at higher currents by moving capacity from the high cells to the low.

Passive BMS balancing activates resistors to bleed off high capacity cells.

I have purchased those active balancers from ECPC - their single cell units for lithium as well as their units for 12V batteries. Very happy with the performance.

 

[aaron_c]

I've read that active balancing is really more helpful if you'll be topping up your batteries instead of charging to 90%. So would I be ok with the 35 mA of balancing that the electrical car parts company BMS offers if I just wanted to do "passive" balancing?

 

[aaron_c]

OK....just saw that for large cells passive balancing may not be enough (just because the current offered for passive balancing by the BMS is often sized for a smaller cell), and many people find it needs active balancing. So if the consensus is that that's the smart thing to do then maybe I'll get the QNBBM one.

 

[snoobler]

If you've "manually" top balanced your batteries, and they are reasonably well matched (within 5% of capacity of each other), top balancing only via passive and during charge only is generally sufficient. It can even be sufficient if cells were not top balanced.

Even at 90%, you're getting into the top leg of the charge curve, so it's going to be effective. The purpose of the top balance is to ensure that the weakest cell only limits the total discharge capacity of the battery. Its doesn't really matter if it's charged at the top at 90% or 100%, it's important that it's top balanced at YOUR top.

Passive balancing during charging-only above a set voltage is typically best.

The active balancers are dumb. They see a difference, they balance. Doesn't matter if it's charge, discharge, top or bottom. IIRC, they do have a voltage below which they shut down, If you really need to wring out the most capacity of your battery, they do provide some benefit there.

I'm using them because my Li-NMC bank is scary, and I want SOMETHING working all the time to keep them level even though I've tested all 420 cells and found them to be very very close. I've also made all of my parallel groups essentially the exact same capacity, so there's little need for actual balancing. But it's a cheap insurance policy for me.

The active balancer isn't a BMS. You can always add it later if you determine that you need it.

 

[Daham]

If you've "manually" top balanced your batteries, and they are reasonably well matched (within 5% of capacity of each other), top balancing only via passive and during charge only is generally sufficient. It can even be sufficient if cells were not top balanced.

Even at 90%, you're getting into the top leg of the charge curve, so it's going to be effective. The purpose of the top balance is to ensure that the weakest cell only limits the total discharge capacity of the battery. Its doesn't really matter if it's charged at the top at 90% or 100%, it's important that it's top balanced at YOUR top.

Passive balancing during charging-only above a set voltage is typically best.

The active balancers are dumb. They see a difference, they balance. Doesn't matter if it's charge, discharge, top or bottom. IIRC, they do have a voltage below which they shut down, If you really need to wring out the most capacity of your battery, they do provide some benefit there.

I'm using them because my Li-NMC bank is scary, and I want SOMETHING working all the time to keep them level even though I've tested all 420 cells and found them to be very very close. I've also made all of my parallel groups essentially the exact same capacity, so there's little need for actual balancing. But it's a cheap insurance policy for me.

The active balancer isn't a BMS. You can always add it later if you determine that you need it.

This the assurance that I am waiting to hear. Thank you sir.

 

[Steve_S]

That BMS is a rebranded DALY BMS. They also sell Chargery rebranded as "Choice BMS". The Active Balancer you show there is a QNBBM which is made by DeliGreen, I have one in each on my 24V packs.

Battery Blancers/Equalizers - Changsha Deligreen Power Co., Ltd. - page 1.

BMS Choices are Far & Wide. It all depends on what you need, want and would like to have it do. Price Points are subject to all of that as well.

 

[aaron_c]

So what BMSs (or BMS/ battery balancer pair) should I look into if I want

A) Something that'll give me 200 A of continuous discharge current capacity for my 24 V 280 AH LiFePO4 battery bank?
B) Something that'll keep said bank reasonably balanced. I don't care if it's balanced actively or passively--I'm fine checking out the capacity on my battery cells and seeing if I need to replace any of them, so the whole "different internal resistance" thing shouldn't be a major issue--but I do need something with enough balancing capacity that my 280 AH cells won't just become more and more unbalanced as I use and charge them.

Thanks yall.

 

[Steve_S]

200A continuous @ 24V (4800W ? that's much) if FET based you want 300A unit. not an easy find
A BMS that uses Relays/Contactors can handle amps according to the relay capacity.
You really don't want to manually balance, tried it... much arghhh long story it's ShunBin related.
You can add an Active Balancer along side the BMS.
Monitoring / Management capability like Bluetooth ?

Depends exactly what you'd like and if things like having a relay or two is not in the plan... They don't suit all purposes. As this is for a "fixed" type installation it likely isn't an issue. It would be better to know what you'd like to make suggestions. I'm a Chargery guy but they don't necessarily suit everyone

I've been Thrash Testing my LFP battery bank for over a week, the cells will wander when used hard and some can really drift. I'm going to have to re-top several cells and level up packs again as a result of my bashing on them... and I have gear to help with that...

If you want a Chargery, I have a couple of spare BMS8T-300's and assorted bits if needed, even energy saver relays etc.
product link: 2S-8S 300A BMS (chargerystore.com)

 

[aaron_c]

Thanks @Steve_S

Hmm...

-I don't need bluetooth monitoring. Just need to be able to plug something in.
-Hah, I don't even understand the purpose of the relay. Is that the thing that would let me shut down the inverter/charger if the BMS shuts down so that way I can plug the inverter/charger directly into the battery without going through the BMS (and thus I can exceed the BMS' current limits briefly with the inverter)? Er...if that's it (and if that's a real thing) then no, I don't need one. The inverter will pull a max of 175 A or something like that (it's a 2200 w inverter), while the BMS should have a 200 A max continuous capacity.

And since you're a chargery guy... I've heard chargery has a pretty decent balancing current. But do you think it's enough to deal with 8S 280 AH cells? If it can do that then it definitely seems like an ideal solution to me.

 

[Steve_S]

Relays/Contactors are what disconnects the power from the battery & the inverter / SCC. they are hardwired between the battery & equipment, not something you plug in/out. Chargery does Passive Balancing (burns off hi volts from hi cells, and it's only 1.2A which is peanuts and pretty much useless for 280's. That's why I have QNBBM Active Balancers on all my packs, which transfer Hi volts to Lo volt cells..

 

[aaron_c]

Relays/Contactors are what disconnects the power from the battery & the inverter / SCC. they are hardwired between the battery & equipment, not something you plug in/out.

Is it any different from a disconnect switch? I'll have a big bluesea 300A disconnect switch between the battery and the positive buss bar.

 

[Steve_S]

I'll rephrase.
The BMS controls the Relay or Contactor.
If the battery cells reach Low or High Volt cutoff point, the BMS will trigger the relay to open. Same if Over/Under temp limits.

With FET based BMS', this is done internally using solid state circuitry.

The physical disconnect switch like the BlueSea is a good way to ensure the battery is not feeding out any power. There are things to do with LFP when storing them, but that's a long topic too, and it's been talked about elsewhere.

 

[aaron_c]

The BMS controls the Relay or Contactor.
If the battery cells reach Low or High Volt cutoff point, the BMS will trigger the relay to open. Same if Over/Under temp limits.

With FET based BMS', this is done internally using solid state circuitry.

Oh, ok. So FET vs relays/contacts are just two ways that BMS' cut off the battery from the rest of the system when certain conditions are met. And probably it doesn't matter to me except that it might be hard to find certain sizes of the FET-based one.

The chargery you mentioned looks good and seems like the price is right. I'm curious what the downside might be? One question that came up in particular is what happens when the battery reaches the low-termperature threshold. I'm mostly worried that, being in a trailer that will be in areas that flirt with freezing temperatures in the winter, the trailer might get cold and then a BMS might prevent the use of the batteries to power the fan on my furnace--thus preventing me from heating the trailer (and the batteries) to a temperature where they could be charged.

Maybe this isn't the hugest problem in the world. Maybe I could fix this by just using my hands to warm the temperature sensor until the trailer got warm enough from the heater. But...that seems a bit problematic and best-avoided if possible.

 

[aaron_c]

@Steve_S I know it's been a bit--been dealing with getting booted out of the campground I was going to spend winter in because of a lockdown--but now I'm back to the project, and I've got a follow up question:

Any reason not to use the Chargery 5008B Balance Charger instead of the QNBBM active balancer? As far as I can see the Chargery balances both by charging some cells and by discharging others. I like that just fine. And it's cheaper than the QNBBM one.

 

[Steve_S]

I don't know anything about the Balance Charger. Jason never even proposed it to me when I decided to look for an Active Balancer and ended up with the QNBBM's... The new "P" Series BMS is supposed to be coming out with Active Balancing and more but I dunno that much about it. The QNBBM's do their job but lack of interactive interface is a strike and DeliGreen support SUX the Big One ! (That Vendor is now officially on my Shyte Flogger List).

 

[aaron_c]

Thanks Steve, I guess I'll ask Jason about the Balance Charger. Hopefully he'll get back more quickly than he did last time (it took a month)!

 

[aaron_c]

OK, so I'm sure this conversation has been had before but...I just watched the Prowse Video where he says "don't use an active balancer" unless and then he lists a few situations where it might matter, including if the cells are mismatched (because you didn't buy the highest grade of cells) and if you use cells in temperature extremes. I think those apply to me--I got cells in a group buy thing on this forum and my RV may fluctuate between, say, 35 degrees F (almost freezing) and 90 degrees F (32 degrees C).

However, I've got a 280 AH battery that will be charged with, at most, 100 amps. So I won't be pushing the limit on charge current.

So I guess my question is simple: Should I just assume that because I didn't get top-end cells, that I need an active balancer?

Thanks yall.

 

[Just John]

I'm about to get eight of the Xuba (or whatever the replacement supplier is) 280 AH cells from the group buy thread. Hooray for that, I'll save some money!

I'm trying to pick a decent BMS (and maybe a balancer?) for this setup. This unnamed BMS from Electric Car Parts Company has been mentioned and looks good, but they mention using it alongside a QNBBM balancer (I assume this is what they're talking about?). However I can't tell how necessary the balancer is. The unnamed BMS has some ability to balance cells... it says it's balance current is 35 mA which...seems like not much?

Just trying to figure out how I should be going about this, given that I will tend to draw the batteries down to maybe 20-30% SOC before charging them again, and I will likely only ever charge them up to 90% SOC, particularly since I'll be mostly charging them with a generator in the foreseeable future (I hope to add solar, but I suspect that it'll only power all my stuff in the summer, if then).

Any thoughts are welcome, including suggestions on alternate BMS. BMSi? BMSs? BMSi.

The BMS you linked to looks like a Daly, just FYI.

 

[Just John]

OK, so I'm sure this conversation has been had before but...I just watched the Prowse Video where he says "don't use an active balancer" unless and then he lists a few situations where it might matter, including if the cells are mismatched (because you didn't buy the highest grade of cells) and if you use cells in temperature extremes. I think those apply to me--I got cells in a group buy thing on this forum and my RV may fluctuate between, say, 35 degrees F (almost freezing) and 90 degrees F (32 degrees C).

However, I've got a 280 AH battery that will be charged with, at most, 100 amps. So I won't be pushing the limit on charge current.

So I guess my question is simple: Should I just assume that because I didn't get top-end cells, that I need an active balancer?

Thanks yall.

My advice would be to try it with just a good top balance. You can add an active balancer later if you find it necessary.