JBD BMS battery 650AH capacity limit

After buying 4 pcs of the JBD 7-20S 200A BMS I found when setting them up that they will not accept battery capacities over 650AH. I confirmed this with their customer support. I did not see it documented anywhere and post this to inform others. I have a 720AH 4P16S battery and cannot get a accurate SOC readout because of this limitation.

I am using the uart interface and the PC application.

What battery configuration are you running in which puts you over that limit?

Samsonite801 said:

What battery configuration are you running in which puts you over that limit?

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4P16S with 180AH prismatics.

There may be a programming limit that won't allow you to set the Ah capacity and cycle values beyond 650. As a programmer, that's an odd number to have as a limit. 1024 maybe.

Just because the capacity and cycle values cannot be set beyond 650 doesn't mean you can't use the BMS. As long as you don't charger or discharge more than 200 amps, it's no big deal.

There is something wrong with your numbers that I don't understand. You have four BMS, but a 4s16s battery which would use only ONE of those BMS. If you have four BMS, then create 16s4p (four batteries). Problem solved and all the numbers work.

HRTKD said:

There may be a programming limit that won't allow you to set the Ah capacity and cycle values beyond 650. As a programmer, that's an odd number to have as a limit. 1024 maybe.

Just because the capacity and cycle values cannot be set beyond 650 doesn't mean you can't use the BMS. As long as you don't charger or discharge more than 200 amps, it's no big deal.

There is something wrong with your numbers that I don't understand. You have four BMS, but a 4s16s battery which would use only ONE of those BMS. If you have four BMS, then create 16s4p (four batteries). Problem solved and all the numbers work.

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Yes, it is a limit in the firmware.

I realize I can use the BMS, in fact I am using it. The problem is that you don't get a accurate SOC reading as I stated. The SOC readout is one of the best features of the JBD bms, plus something you need to know for obvious reasons.

The numbers are correct. JBD expressly forbids using BMS units in parallel fashion. It is also discouraged in various other documents. The reason is spike currents that can be huge when one BMS disconnects or reconnects as one example. That is why I went with a single BMS.

Sparky_SC said:

The numbers are correct. JBD expressly forbids using BMS units in parallel fashion. It is also discouraged in various other documents. The reason is spike currents that can be huge when one BMS disconnects or reconnects as one example. That is why I went with a single BMS.

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My understanding is that they're just talking about using multiple BMS in parallel on the same cells. There should be no issue using 4 separate BMS in parallel each with their own 16s sets of cells - which is what I believe @HRTKD was referring to.

In fact this is addressed in the Overkill (rebranded JBD BMS) manual's FAQ;

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The key phrase here being "complete assemblies".

Sparky_SC said:

JBD expressly forbids using BMS units in parallel fashion. It is also discouraged in various other documents. The reason is spike currents that can be huge when one BMS disconnects or reconnects as one example. That is why I went with a single BMS.

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I think you're wrong there. Please provide a link to those documents. There is the Orion one that has been roundly disparaged here on the forum.

Symbioquine said:

There should be no issue using 4 separate BMS in parallel each with their own 16s sets of cells - which is what I believe @HRTKD was referring to.

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And - in case it's not obvious - in such a configuration the Ah value would be set to the Ah rating of a single cell (180 Ah) on each of the BMS because each BMS is responsible for a single 16s battery.

If you wanted to monitor the battery bank as a whole, you could combine the data from each BMS in software or run all your loads/charging through a (presumably large) shunt.

HRTKD said:

I think you're wrong there. Please provide a link to those documents. There is the Orion one that has been roundly disparaged here on the forum.

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If a BMS disconnects for any reason and later reconnects that battery will not be at the same voltage SOC as the others and a large current can flow. As long as none never disconnect, no problem. Naturally, there is no communications between multiple BMS so one knows what the other is doing (connected or disconnected)

Another reason is that you would be using 4 BMS units, where one could do the job, that is 4X the cost. The SOC displays for EACH BMS would have to be added to get the total array SOC, not exactly convenient.

Here is a cut/paste from the JBD spec sheet, I confirmed this with their tech support.

● Parallel use of battery packs is not supported (battery packs are directly connected in parallel, and there is a problem of large current discharge from high-voltage battery packs to low-voltage battery packs).

Symbioquine said:

And - in case it's not obvious - in such a configuration the Ah value would be set to the Ah rating of a single cell (180 Ah) on each of the BMS because each BMS is responsible for a single 16s battery.

If you wanted to monitor the battery bank as a whole, you could combine the data from each BMS in software or run all your loads/charging through a (presumably large) shunt.

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Kind of defeats the purpose of a smart BMS with integral SOC readout doesn't it? This is a 200A BMS and they also have a 300A version, use of a large battery shouldn't be unexpected. The whole point is why the 650AH limit? Its just a software change, comes down to poor engineering decisions.

By the way, it appears the limit used to be 256AH a while back. https://diysolarforum.com/threads/jbd-bms-cant-set-capacity-to-over-256ah.30513/

Sparky_SC said:

If a BMS disconnects for any reason and later reconnects that battery will not be at the same voltage SOC as the others and a large current can flow. As long as none never disconnect, no problem. Naturally, there is no communications between multiple BMS so one knows what the other is doing (connected or disconnected)

Another reason is that you would be using 4 BMS units, where one could do the job, that is 4X the cost. The SOC displays for EACH BMS would have to be added to get the total array SOC, not exactly convenient.

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He already has the four BMS according to his post. It's a sunk cost.

If the BMS is disconnecting, then there is a physical problem or there is a configuration problem. A charging device that blindly charges to 14.6 volts is not one that I would have in my system.

My Victron BMV-712 is where I go to get the state of charge. It allows me to set a combined Ah value based on my two distinct batteries. Very convenient.

I have heard parallel BMSs on a single stack of cells is not well recommended.
However making four separate batteries each with a separate BMS all tied to a common bus in parallel is not an issue.

Sparky_SC said:

Yes, it is a limit in the firmware.

I realize I can use the BMS, in fact I am using it. The problem is that you don't get a accurate SOC reading as I stated. The SOC readout is one of the best features of the JBD bms, plus something you need to know for obvious reasons.

The numbers are correct. JBD expressly forbids using BMS units in parallel fashion. It is also discouraged in various other documents. The reason is spike currents that can be huge when one BMS disconnects or reconnects as one example. That is why I went with a single BMS.

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Can you just run 16s4p, and add in like a Victron Smart Shunt on the main DC bus to read master SoC? I'd rather have 4 separate banks paralleled anyways, easier to maintain, and break off banks for maintenance when needed in my opinion...

Sparky_SC said:

If a BMS disconnects for any reason and later reconnects that battery will not be at the same voltage SOC as the others and a large current can flow.

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Let's explore this a bit. Keep it simple and assume a 4s2p battery bank.
Assume the batteries are slightly out of sync, #1 at 99% stage of charge, #2 at 96% stage of charge. At this high state of charge, I suspect that the charging source is not in bulk mode and would therefore not be charging with high volts (> 14) or high amps (> ~10). The BMS on #1 decides its had enough once it has reached a 100% state of charge and turns off the charge circuit. Battery/BMS #2 now gets the full float voltage/amperage, which really isn't that much. So there isn't really a huge spike and battery #2 eventually achieves 100% state of charge and the two batteries are essentially in sync.

Many of us are sizing our components in such a way that they're OK operating with a single battery active in the system. The most my PV will charge at is 100 amps, which is well within the 120 amps of a single BMS/battery.

If your batteries are so far out of sync with regard to state of charge in the first place, then something is wrong in your system. After a year in place, my two batteries, as well as the cells, are almost identical in their voltage and state of charge.

I just installed a Muller JBD 12V BMS and I have a 912Ah system and I hit the same capacity setting ceiling. It is actually a 16 bit integer variable. It goes from 0 - 65535 with a 0 placeholder at the end so the actual limit is 655350 MAh. You actually can't put anything in the last digit other than 0.

garyrayrussell said:

I just installed a Muller JBD 12V BMS and I have a 912Ah system and I hit the same capacity setting ceiling.

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As mentioned earlier that should not affect the functioning of the BMS. It may report your battery capacity on the conservative side. I would rather have my BMS telling me I have less capacity left than the other way around and in the next minute have the battery be dead.

Just stop building batteries "wrong". lol