BMS help please

Steve_S said:

4P8S ? Are you putting 4 cells in parallel with 8 in Series for a 24V Battery ?

IF SO, then an 8S BMS is what you need, 1 BMS Lead per 4 cells as the 4 cells are "ghosted" and you will only see the "average" between the 4 cells.
I posted the image below in another thread for a 2P2S 24V.
4P cells are a bit much and ill-advised IMO when you can have 4 independent packs that provide backup / fault tolerance and finer control over the assets. Batteries are an asset as they are not cheap.

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Nova22 said:

Hi Steve,
Thanks for the reply,I'm also from Canada but I'm here in the hot Philippines trying to build my Off Grid dream. To clarify you are correct it is a 24v system and feeling like I'm a little out of my league, I've learned just enough to be dangerous I believe is the expression.

I bought 32 90AH 3.2V batteries and 1 8S Daly BMS thinking I can just do a system of 4P8S but seems like your recommendation would be to get another 3 BMS in order to have each 8S packs more protected which makes sense to me.

By having my 4 cells independent as you mentioned if one fails the other 3 would not be affected and my system will still function? This means I would consider my system 8S4P correct? and now the question that shows how little I know, how will I know if my BMS's are working properly and if everything is running smoothly without a bluetooth? My Daly 3.2V LiFePo4 8S 24V 80A unit has no monitor or anyway to know whats going on or if somethings wrong

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Steve_S said:

For 24V, you would use 8 cells in series which in your case would be 24V/90AH each pack. You would then parallel the packs into a complete bank, so 4 packs of 24V/90AH for a total of 24V/360AH gross. Each pack, of course, should have an 8S BMS and it's own fuse.

The BMS decision depends on your usage and how many amps the system will pull from the batteries.
Example: 100A off 24V system which amounts to 2400 Watts, 4000 Watts translates into 166A @ 24V
(Watts ÷ Volts = Amps. Amps X Volts = Watts) to keep it simple.
There are many types of BMS', the most common are FET Based units such as Daly, ANT BMS, DYKB BMS, and other "Smart BMS' " which people use here with good results. As a general rule with FET based units, it is best to oversize them by 25% of what you expect your max amp draw to be, so you never take them to the edge of their specified max amps.

I will not recommend a BMS to you, as it is somewhat subjective, I do use Chargery BMS8T-300's for my packs using Deltec 500A/50mv Shunts and external 200A Energy Saver Contactors (relays), which is a different beast and dis NOT FET Based... Approximate cost is about $150 USD (I bought shunt & relays separate and my costs are different due to what I have been doing).
Link to BMS8T for info: https://chargery.com/BMS8T.asp
UN-OFFICIAL Chargery BMS8T Draft 4.1 Owners Manual.
https://diysolarforum.com/attachments/chargery_bms8t_owners-manual_v4-1-pdf.15938/

With the BMS', they should all have matched settings (better if you can modify the configs yourself) so no single BMS acts differently than the others. The next MOST CRITICAL is ensuring the battery bank is properly wired and configured so that the Discharge Load and Charge Capacity is as balanced as possible. Refer to this document for extended details. https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

The TRICKY PART !
Now assume you setup for 200A max draw capacity which on a 90AH pack is over 2C rate (which is not necessarily what the cells can handle) as standard LFP cells usually max out at 1C or in this case 90A on a 90AH battery. VERY IMPORTANT TO NOT EXCEED CELL SPECS ! But with packs properly balanced in the pack they should load share that out. The lurking GOTCHA (like the ookpik lurking in the woods) is to setup the "packs" to be able to operate as a single battery should the others shut-off for some reason. This is harder (near impossible) to do with smaller capacity cells such as 90AH.

Keep in mind that a BMS is the Last Ditch Safety System and the BMS should NOT be responsible for stopping charge or discharge but rather that is the job of the SCC ( Solar Charge Controller) and the Inver/Charger System, which should shutoff just before the BMS forces it. This also requires that the system be Voltage Calibrated as well so everything reads properly and acts accordingly (see link in my signature related to that).

Hope that helps,
Good Luck
Steve

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tictag said:

Whilst I believe it would be possible to get away with just one 8S BMS (i.e. parallel 4-cells), this is an awful lot of energy and personally I'd go with @Steve_S's suggestion and have one BMS for each 24V pack - then at least you know every single cell is being monitored.

I've not seen any 'oversight' systems to centrally monitor and control multiple BMS's so, unless you're going to do something fancy, I'd just go with a FET-based 8S BMS for each 24V pack. At 90AH pulling a max of 1C you're only looking at 90A for each BMS, though you'll probably want to add at least 50% to that. If you want to pull maximum load i.e. 90A x 4 packs = 360A (that's 8,640W!!), you'll obviously need to spec your cable and components appropriately. Maybe something like this?

View attachment 17621

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Nova22 said:

Got to be honest with you the more I get into this the more I feel like I'm getting in over my head but I'm invested in it now and need to figure this out. Where I'm at there's only one person who will be able to fix it if it breaks and that's me so I better start studying. Watching Will always made it seem so easy, lol.

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Steve_S said:

you will only see the "average" between the 4 cells.

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tictag said:

@Nova22, you're most welcome.

You linked me to a BMS with multiple options, which rating did you go for?

You don't have to pull 90A, all I was suggesting that if you wanted to pull the maximum discharge current you'd need a 90A +50% BMS. Just be aware that if, for example, you bought the 80A variant I would encourage you to de-rate it by 50%, so 40A max, just to be on the safe side. This means that whilst your battery would be capable of pulling 1C (i.e. 90A for a 90AH battery) you would be limiting it by design to 40A. Which again is fine if that's all you need.

If you do de-rate by design, you would also use lower rated fuses and cable.

I know that this might be a pain, but as you're probably going to want to buy additional BMS's anyway, I would recommend buying four of their new SmartBMS models, at least 90A + 50%. I know, I know, binning something you've already bought hurts but I think doing it right, doing it once is a relevant maxim here.

Your SCC in the above diagram would just connect to the +ive & -ive busbars, as would everything else. Imho, it's best to use busbars for easier wiring and maintenance. Other opinions on this are available.

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Steve_S said:

For 24V, you would use 8 cells in series which in your case would be 24V/90AH each pack. You would then parallel the packs into a complete bank, so 4 packs of 24V/90AH for a total of 24V/360AH gross. Each pack, of course, should have an 8S BMS and it's own fuse.

The BMS decision depends on your usage and how many amps the system will pull from the batteries.
Example: 100A off 24V system which amounts to 2400 Watts, 4000 Watts translates into 166A @ 24V
(Watts ÷ Volts = Amps. Amps X Volts = Watts) to keep it simple.
There are many types of BMS', the most common are FET Based units such as Daly, ANT BMS, DYKB BMS, and other "Smart BMS' " which people use here with good results. As a general rule with FET based units, it is best to oversize them by 25% of what you expect your max amp draw to be, so you never take them to the edge of their specified max amps.

I will not recommend a BMS to you, as it is somewhat subjective, I do use Chargery BMS8T-300's for my packs using Deltec 500A/50mv Shunts and external 200A Energy Saver Contactors (relays), which is a different beast and dis NOT FET Based... Approximate cost is about $150 USD (I bought shunt & relays separate and my costs are different due to what I have been doing).
Link to BMS8T for info: https://chargery.com/BMS8T.asp
UN-OFFICIAL Chargery BMS8T Draft 4.1 Owners Manual.
https://diysolarforum.com/attachments/chargery_bms8t_owners-manual_v4-1-pdf.15938/

With the BMS', they should all have matched settings (better if you can modify the configs yourself) so no single BMS acts differently than the others. The next MOST CRITICAL is ensuring the battery bank is properly wired and configured so that the Discharge Load and Charge Capacity is as balanced as possible. Refer to this document for extended details. https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

The TRICKY PART !
Now assume you setup for 200A max draw capacity which on a 90AH pack is over 2C rate (which is not necessarily what the cells can handle) as standard LFP cells usually max out at 1C or in this case 90A on a 90AH battery. VERY IMPORTANT TO NOT EXCEED CELL SPECS ! But with packs properly balanced in the pack they should load share that out. The lurking GOTCHA (like the ookpik lurking in the woods) is to setup the "packs" to be able to operate as a single battery should the others shut-off for some reason. This is harder (near impossible) to do with smaller capacity cells such as 90AH.

Keep in mind that a BMS is the Last Ditch Safety System and the BMS should NOT be responsible for stopping charge or discharge but rather that is the job of the SCC ( Solar Charge Controller) and the Inver/Charger System, which should shutoff just before the BMS forces it. This also requires that the system be Voltage Calibrated as well so everything reads properly and acts accordingly (see link in my signature related to that).

Hope that helps,
Good Luck
Steve

Click to expand...