Parallel Battery Limit

[AndyRonLI]

Folks,
Trying to understand why there would be any limit to the number of batteries in parallel If each battery had dedicated connections to a properly sized buss bar.
Ampertime's published limits are 4S4P, I understand the series limit as all current flows through all batteries. But the parallel limit isnt quite as obvious. Is this the limit if the batteries are connected in parallel only using battery interconnection? Then the terminals of the battery at the load/charge end of the string will certainly see a lot of current. But if they are all connected to a big buss bar, why would there be a limit?
I would like to put six 200AH ampertimes in parallel. Each battery would have individual cabling (same length) to a set of 1000 amp bus bars.
This exceeds the published 4P limit.

Thanks
ANdy

 

[FilterGuy]

The parallel limitation is not clear to me either. I have seen statements about sudden surges when one or more of the batteries shut off leaving the remaining battery(s) with a momentary huge overload. I kinda understand that, but that does not sound as big of a deal as shutting off a short-circuit current. Hopefully, someone can provide the answer. (I have wondered about this but never had a situation where it was a limit for my design, so I never really dug in to understand it)

The series limit is due to the BMS needing to be able to switch the full system voltage, not just the battery voltage. When the FETs of one of the BMSs turn off, the full system voltage appears across the FET. For example, if two 12V batteries are used to create a 24V system. Then the FETs in each BMS need to be able to handle the full 24V.

 

[Koldsimer]

I always thought those paralelling recomendations were for non bus bar applications. Mostly due to max amp ratings for the max cable size they suggest for direct paralell apps.

 

[FilterGuy]

I always thought those paralelling recomendations were for non bus bar applications. Mostly due to max amp ratings for the max cable size they suggest for direct paralell apps.

You might be right, but if I was a marking person in the battery company I would make dang sure people understood they could do the bus-bar method (Why unnecessarily limit the sales?) However, I have never seen that so it makes me think there is something else going on.

 

[MisterSandals]

I have seen statements about sudden surges when one or more of the batteries shut off leaving the remaining battery(s) with a momentary huge overload.

I always related this to the discussions about FETs failing in the closed state when over amped. Regardless of why the first battery shuts off, the remaining load is distributed across the remaining battery(s). Having subsequent battery(s) fail because of over amp condition, would significantly increase the chance of cascading shut offs.

I would imagine that if the initial battery shut off for other reasons, like low temperature, there would be fewer ramifications for the other battery(s).

I am wondering why the remaining battery(s) would have only a momentary huge load? Does it matter why the first battery shut off?

 

[FilterGuy]

I am wondering why the remaining battery(s) would have only a momentary huge load? Does it matter why the first battery shut off?

As I said, I am not sure I follow the engineering behind the statement.

 

[FilterGuy]

It seems unlikely to me that a battery company would limit the parallel use of the battery for fear of a cascading overload. The battery has a current limit. If the current limit is exceeded the BMS shuts off. A system designer needs to understand the limits and design accordingly.

If a cascading overload could somehow damage the battery then yes, the manufacturer will probably want to limit the number of parallel batteries...but how would a cascading overload damage the battery? (We are back to the original question)

 

[AndyRonLI]

So the batteries I am currently using are the ampere time 200ah they are rated at 100 amp continuous discharge.
I suppose I could protect each battery with a fuse, though that seems like overkill.
If I were pulling 400 amps off of a 4S system and one dropped out I would be pulling 133 amps off of each battery.
One would think the BMS and Inverter could manage that without damaging the batteries, though the load might suffer a rude shutdown.
And as 4S is allowable, I have to think ampere time would be ok with that,

If I were pulling 600 amps off of a 6s system and one dropped out I would be pulling 120 amps off of each battery.
That should be less stressful, and less likely to cause an issue.

Fundamentally at max total current, one could argue that the more batteries in parallel, the smaller the potential overload if one battery fails.

I could also argue that a 6S system pulling 500 amps could fail a battery and carry on with no problem. Thats 6000 VA DC! @ 12Volts

I am planning on using the 12V multiplus-II 2X 120V in my RV, 500 amps seems to be sufficient for 5500 W Max Power, the spec sheet is not completely clear on how long that surge could really last. At max continuous AC output of 3000VA, that is about 4000VA DC input, Thats less than 60 amps per battery if all 6 are supplying power.

I did send an email to ampere-time, but I think I have talked myself into seeing a 6S System as safe if I use a bus bar.
Thanks for all the input
Andy

 

[RCinFLA]

There is not a limit to number of parallel batteries, but there are practical limits that you must be cautious of. Each parallel battery should have its own BMS for best safety.

I run ten 48v battery strings in parallel. Have been for over 20 years.

Parallel batteries will not be perfectly matched for their current contribution for charging and discharging,. You have to be careful if some BMS's drops a parallel battery off-line that charging or discharging current does not exceed a safe limit for any of the parallel batteries left on-line. If you exceed current limits set by any of the BMS's they can cascade off-line as fewer batteries are remaining online to take over the total load or charge current.

Do not assume you can draw twice the current for two parallel batteries. Best situation is any single battery can take your maximum load or charge current without detriment. The more parallel batteries you have the more you can push the maximum currents and still feel no single battery will exceed safe current rate.

Other big thing is if you have a battery failure, like an internal short or high leakage, the other parallel batteries can feed it current making the situation worse. BMS should help manage this by disconnecting defective battery. You should have a 'last line of defense' fuse on each parallel battery.

You should have each BMS give an audible warning indication if it goes off-line.

I use Anderson 350 connectors on each parallel rack. This is good for doing battery maintenance work and testing as you can easily take one of the parallel strings off line and check things out without influence of other parallel batteries.

Some rack batteries have inter-communication such that if one BMS drops out it signals all BMS's to shut down. This is safest but can be a pain. Whether this is right to do depends on how many parallel batteries you have. The more parallel batteries you have, the less you want one BMS to shut everything down.

Ideal situation is if you have customizable system monitoring to put out a warning if current imbalance gets greater than a set amount. This will help catching an issue before it becomes a big problem.

 

[Watts Happening]

Exact same question I have. I bought (24) 200ah batteries and I’ll be wiring them up in a 48v system. I suppose we’ll find out just how realistic that limit is. I can’t think of a reason parallel should matter.

That said, I don’t expect to even have the theoretical possibility to draw enough current to matter if a bank dropped offline.

 

[RCinFLA]

For self contained 12v LFP you have to be careful of the BMS voltage rating when connecting batteries in series stacking. The warning of no more than four series connected 12v LFP means the BMS has 80v to 100v breakdown voltage series switch MOSFET's.

Higher breakdown voltage MOSFET's cost more money so some manufacturers use lower voltage rated MOSFET's in their internal BMS.

Some cheap 12v LFP self-contained batteries BMS's use lower voltage breakdown MOSFET's. I have seen some with 30v breakdown MOSFET's in their BMS. If battery spec does not specifically state their maximum series stacking, assume they cannot be series stacked.

This also applies to using two 8s BMS's for a 16s cell series stack. Many 8s BMS's use 40v rated MOSFET's. For example, JK active balancing BMS uses 100v MOSFET's for 20s version and 40v MOSFET's for 8s version.

The warning about no more than two in parallel is a manufacturer judgement call to minimize warranty exposure. If you have two in parallel and run an RV air conditioner with high battery current draw and one of the parallel battery's BMS drops out, the remaining parallel battery will likely get degradingly high discharge current draw.

The most common issue with self-contained 12v LFP batteries is users don't give enough balancing time causing cells to become imbalanced. They then call supplier wanting warranty replacement because they don't get the battery capacity they had several months earlier. Running high %C(A) discharge current accelerates rate the cells become imbalanced. Heat from BMS can cause one or two of the four cells to see a higher temperature causing accelerated rate of imbalance.

 

[rodrick]

View attachment 135650
Exact same question I have. I bought (24) 200ah batteries and I’ll be wiring them up in a 48v system. I suppose we’ll find out just how realistic that limit is. I can’t think of a reason parallel should matter.

That said, I don’t expect to even have the theoretical possibility to draw enough current to matter if a bank dropped offline.

The worst case scenario you loose a bms you start building 16s packs with a new bms

 

[Jex7Elm12]

So the batteries I am currently using are the ampere time 200ah they are rated at 100 amp continuous discharge.
I suppose I could protect each battery with a fuse, though that seems like overkill.
If I were pulling 400 amps off of a 4S system and one dropped out I would be pulling 133 amps off of each battery.
One would think the BMS and Inverter could manage that without damaging the batteries, though the load might suffer a rude shutdown.
And as 4S is allowable, I have to think ampere time would be ok with that,

If I were pulling 600 amps off of a 6s system and one dropped out I would be pulling 120 amps off of each battery.
That should be less stressful, and less likely to cause an issue.

Fundamentally at max total current, one could argue that the more batteries in parallel, the smaller the potential overload if one battery fails.

I could also argue that a 6S system pulling 500 amps could fail a battery and carry on with no problem. Thats 6000 VA DC! @ 12Volts

I am planning on using the 12V multiplus-II 2X 120V in my RV, 500 amps seems to be sufficient for 5500 W Max Power, the spec sheet is not completely clear on how long that surge could really last. At max continuous AC output of 3000VA, that is about 4000VA DC input, Thats less than 60 amps per battery if all 6 are supplying power.

I did send an email to ampere-time, but I think I have talked myself into seeing a 6S System as safe if I use a bus

Andy, What about charging, is it possible to charge e.g. a 120 Ah whole package?

 

[pollenface]

Does it have something to do with reverse polarity protection?

(just stabbin in the dark)