Why is 8 the maximum number of lithium batteries in Parallel?

[isslandboy]

Hi All,

We are considering upgrading the FLA batteries to lithium, we can get 12, 16S 100AH for our system. We are being told we can only use 8 max without a master BMS, I have been given different explanations (one said it was the battery, other said it was the BMS) and have become very confused. It would be great if someone could explain this to me. FYI Our biggest load for the month of February was 75A. ( Our max charge rate would be 140A and the continuous output of the inverter is 140A, surge for 5 minutes ia 229A, and max surge for 1 second is 250A)

Thanks

 

[Ampster]

There is no limit for the number of cells in parallel... My Teslas have over fifty cells in parallel. Note I said cells. You described 16S batteries and if the manufacturer has a limit they would be best to answer that. My guess is that at a certain number of "batteries" in parallel, the individual BMSs in those batteries would see eddy currents and potentially trip a battery off line. Hence the suggestion for a master BMS.

 

[isslandboy]

Hi, thanks for the quick response. Doesn't Tesla use some kind of a contactor as a Master BMS? I will have to look up eddy currents as I have no clue what they are.

 

[Ampster]

Doesn't Tesla use some kind of a contactor as a Master BMS?

Tesla uses multiple cells in parallel then those groups are in series. They have only one BMS and the contactor is just that, a big relay but the BMS controls charging and motor functions like regen and acceleration. Eddy currents are currents that flow between parallel batteries. They are not as significant between parallel cells because parallel cell connections are very short.

 

[isslandboy]

OK, thanks, can I ask you what you think of the response I got from several manufacturers about this issue of 8 batteries in parallel being the max.

 

[chrisski]

Here is a link with some math that says you have to know what you’re doing when you parallel more than 2.

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

I only use FLA, so if I got to a point I needed more than 4 in parallel, I would really need to look at bigger 2 volt batteries and run those in series to get more amps Without going in parallel.

 

[isslandboy]

Hi, thanks for the link. The article focused on wiring which we were aware of. Our issue is master/slave BMS or stand alone BMS on each battery.

 

[Ampster]

OK, thanks, can I ask you what you think of the response I got from several manufacturers about this issue of 8 batteries in parallel being the max.

I am not going to out guess the wisdom of a battery manufacturer. You are talking about Lithium batteries for a pack of 57kWhs. I am going to ask why you dont want to build your own pack from cells and not have any constraints? That is an option instead of buying batteries at over $500 per kWh. I have 42 kWhrs that cost less than $5,000. How are you using them?

 

[isslandboy]

I am not going to out guess the wisdom of a battery manufacturer. You are talking about Lithium batteries for a pack of 57kWhs. I am going to ask why you dont want to build your own pack from cells and not have any constraints? That is an option instead of buying batteries at over $500 per kWh. I have 42 kWhrs that cost less than $5,000. How are you using them?

Hi,
That is basically what we are doing through my nephew that has contacts in China. But somewhere along the way we were told that when you have more than 8 battery packs in parallel you have to use a master/slave BMS, so we are just trying to establish if this is in fact correct. We have been through 2 sets of FLA, so we want to get it right and hopefully this will be the last time! I have a buddy that repurposed some used telecom batteries he has 12 of them running with no master, each one does it own thing and they stay in balance. We are using them in an off grid house in Panama, we can track the usage etc through the Schneider conext system, set charge parameters, limit output, lots of options. We honestly don't think there will be an issue as the discharge/charge rates are very low (not at all like a car for example), but as I said, we want to get it right. I really appreciate your input.

 

[Ampster]

Contacts in China?. Most people here not only have contacts in China but they have had good experience buying from one or two very reputable vendors. Read some of the threads.
You have not explained why you think the 48 volt batteries will be a better value than buying 64 LF280 cells and configuring them into a 4P16S pack with a very good BMS which talks to the Schneider? That would get you 57kW. There are several people on this forum who have had luck configuring them with the Schneider.
Or alternately why not buy the master BMS if that is what your nephew's contact suggests. You are searching for facts where there are only opinions. Your friend's used Telecom batteries are great until they end up like your previous FLA batteries. The facts are that paralleling 4 cells into a 4P16S pack has been shown to be reliable. If you want redundancy and the ability to service one segment, some people here prefer four 16S packs each with their own BMS.

 

[curiouscarbon]

Tesla uses multiple cells in parallel then those groups are in series. They have only one BMS and the contactor is just that, a big relay but the BMS controls charging and motor functions like regen and acceleration. Eddy currents are currents that flow between parallel batteries. They are not as significant between parallel cells because parallel cell connections are very short.

to support your explanation

 

[isslandboy]

Contacts in China?. Most people here not only have contacts in China but they have had good experience buying from one or two very reputable vendors. Read some of the threads.
You have not explained why you think the 48 volt batteries will be a better value than buying 64 LF280 cells and configuring them into a 4P16S pack with a very good BMS which talks to the Schneider. That would get you 57kW. There are several people on this forum who have had luck configuring them with the Schneider.
Or alternately why not buy the master BMS if that is what your nephew's contact suggests. You are searching for facts where there are only opinions. Your friend's used Telecom batteries are great until they end up like your previous FLA batteries.

Because we are totally off grid we want the ability to take packs offline if they happen to fail

 

[Steve_S]

I am running with four LFP packs in a bank and that is pretty much the end of where it is practical to use Stand-Alone BMS' because managing the Cutoffs and such becomes a major dance of compromises.

The reasons for a Master BMS System or Centralised BMS is so that it can coordinate the battery packs during charge & discharge, these are usually used with more advanced Inverter/Charger & SCC Systems which can interact using something like CanBus or ModBus. In such instances, charge rates can be managed (raised/lowered) or shut down prior to a forced BMS cutoff. Additionally the Low Volt/Temp cutoff process can also be managed and dealt with accordingly.

No "Value" Grade equipment is advanced enough to make use of such an interactive BMS Subsystem, this is mostly from Tier-2 and up level of equipment. Sometimes, an Intermediary System can used between an advanced BMS system and SCC/Inverter/Charger which can accept information from the BMS and use software to handle the needs using relays etc for the devices. This is a massive PITA.

ON EV Batteries !
Eeks I wish people would get a grip on this. They are NOT LFP and they are NOT Large Capacity cells !

18650 cell from Tesla Model S X P100 Capacity: 3400mAh.
Model 3 2170 Cell: Capacity: 4.8Ah / 17.3 Wh.

EV Cells are High-Density SMALL Cells which are Matched & Batched so that they are exactly identical. Many are set in Parallel and then into strings with fine management, each cell is also fused & protected. They operate quite differently because of their assembly and it just is not quite comparable in most ways.

Lead Acid in Parallel is yet another beast, I also run Heavy Rolls Surette Lead and the rules are indeed different and how the bank get's managed is also more involved. There are practical limitations and certain maintenance processes that have to be incorporated, sadly many skip certain essentials like battery rotations and whether or not to use Pulse Chargers & Balancing which comes into play with larger Lead Banks.

 

[isslandboy]

I am running with four LFP packs in a bank and that is pretty much the end of where it is practical to use Stand-Alone BMS' because managing the Cutoffs and such becomes a major dance of compromises.

The reasons for a Master BMS System or Centralised BMS is so that it can coordinate the battery packs during charge & discharge, these are usually used with more advanced Inverter/Charger & SCC Systems which can interact using something like CanBus or ModBus. In such instances, charge rates can be managed (raised/lowered) or shut down prior to a forced BMS cutoff. Additionally the Low Volt/Temp cutoff process can also be managed and dealt with accordingly.

No "Value" Grade equipment is advanced enough to make use of such an interactive BMS Subsystem, this is mostly from Tier-2 and up level of equipment. Sometimes, an Intermediary System can used between an advanced BMS system and SCC/Inverter/Charger which can accept information from the BMS and use software to handle the needs using relays etc for the devices. This is a massive PITA.

ON EV Batteries !
Eeks I wish people would get a grip on this. They are NOT LFP and they are NOT Large Capacity cells !

EV Cells are High-Density SMALL Cells which are Matched & Batched so that they are exactly identical. Many are set in Parallel and then into strings with fine management, each cell is also fused & protected. They operate quite differently because of their assembly and it just is not quite comparable in most ways.

Lead Acid in Parallel is yet another beast, I also run Heavy Rolls Surette Lead and the rules are indeed different and how the bank get's managed is also more involved. There are practical limitations and certain maintenance processes that have to be incorporated, sadly many skip certain essentials like battery rotations and whether or not to use Pulse Chargers & Balancing which comes into play with larger Lead Banks.

Thank you!

 

[Ampster]

Because we are totally off grid we want the ability to take packs offline if they happen to fail

That is exactly why you would want to configure four 16S packs with separate BMSs. The big advantage is if there is a failure it will be at the cell level and you only have to replace one or two cells instead of the entire 48 volt battery.
What @Steve_S said above is a good example of someone who has contacts in China and who has had real world experience.

 

[John Frum]

@isslandboy is there a specific master/slave bms you are thinking of?
The 8p max may be product specific.
Most here don't use a master/slave bms but that is probably because they are not that common.... yet.

 

[Steve_S]

IF you look here: http://liionbms.com/php/bms_options.php
You will see some Master/Slave & Distributed BMS which are available. They are out there BUT they are far from Cheap ! When you get into this level it is quite another game.

 

[Ampster]

They are out there BUT they are far from Cheap ! When you get into this level it is quite another game.

Perhaps one explanation for the high cost is that they have to manage 48 volts and pack level currents. Or they have a complex communication system to control the lower level BMSs. Anytime there are complex communication systems I would want system level support from a manufacturer. If I were off grid in Panama I would want to keep it as simple as the circumstances permit.

 

[JIK]

OK, thanks, can I ask you what you think of the response I got from several manufacturers about this issue of 8 batteries in parallel being the max.

Probably because of battery address combinations.. max address used

 

[time2roll]

Hi All,

We are considering upgrading the FLA batteries to lithium, we can get 12, 16S 100AH for our system. We are being told we can only use 8 max without a master BMS, I have been given different explanations (one said it was the battery, other said it was the BMS) and have become very confused. It would be great if someone could explain this to me. FYI Our biggest load for the month of February was 75A. ( Our max charge rate would be 140A and the continuous output of the inverter is 140A, surge for 5 minutes ia 229A, and max surge for 1 second is 250A)

Thanks

How many are expected to need? 10? 50? More?

I would start with looking for 200AH or larger. Possibly custom building from cells to as large as 1120AH (4x280 cells).
Certainly with four+ in parallel I would fuse each battery separately as it connects to the bus.

If a technical answer is not easy and clear..... I am not really buying it.