3S7P 18650 Battery Packs

[Whoa]

I have built SEVERAL 3S7P 18650 battery packs, mostly for a pair of APC UPS's. I buy the battery boxes as a kit from Ali Express. The kits themselves are decent and the materials are pretty good. They can come with a 20A (or so) BMS. I put together around 30 of these about 12-18 months ago where 8 batteries fit into one of my UPS'. Roughly 2 months ago one of the UPS' master alarm went off indicating that there was an issue with the batteries. This tends to mean that the battery/s need replacing. My estimate on LiFePO4 batteries is that they should last far longer than about a year. In the last day or so the second one's alarm also went off.

I also built 2 battery packs for my 24v Dewalt power tools (6S2P, with a 6s BMS) and they work all right for about a month, but both packs are now not discharging at the 24v required for the power tool to operate correctly.

After some research on here it seems that a LiFePO4 battery pack should not just be charged using the internal mechanism of the UPS and rely on the BMS of the Battery Pack to maintain the battery pack. I do not understand why this is... The BMS is wired to each String of 7 cells and is supposed to balance the charge (and discharge) rate of each string. Obviously something is wrong so I would like someone to explain to me what I should actually be doing so that this does not fail (IMO) so quickly.

 

[Supervstech]

The voltage a ups uses is dangerously high for 3S LiNMC 18650’s…

Trusting the BMS to disconnect is very unwise in a house…

 

[Whoa]

I >VERY< much appreciate the feedback/response! I wasnt sure what anyone would say let alone actual feedback.

A) The APC UPS's Im using are ones that have very quality hardware involved. Charging and Discharging are very much within what is to be expected, meaning over a couple months of monitoring I never once saw any real fluctuation in voltage.
B) Im literally building a LiFEPO4 battery pack in a self contained plastic box so that they will load into the UPS normally.
C) With all of that said you didnt really address what my "issue" is. Why these batteries are failing?
D) Regarding safety I have tested for literally months prior to going to use them "Full Time" (roughly 2 years ago) and satisfactorily saw there was no need for concern with the hardware I am using.

With you concerns for safety set aside, do you have any suggestions as to what my problem/s might be?

 

[MisterSandals]

I have built SEVERAL 3S7P 18650 battery packs,

Just to understand this: did you put 3 cells in series and put 7 of these in parallel (aka 3S7P)?
This should require 7 BMSs. If you wired a 3S7P pack with 1 BMS, how did you do that?

The BMS is wired to each String of 7 cells and is supposed to balance the charge (and discharge) rate of each string.

I cannot picture how there is a string of 7 cells.

Have a pic or diagram?

 

[Whoa]

This post shows structurally what I built. https://diysolarforum.com/threads/3s7p-18650-in-nice-little-box.14432/

There's several Youtube videos on these battery packs. To answer your question, I believe the nomenclature is that the number preceding the S is the number of strings, and the number preceding the P are the number of cells in 1 string.

With that said, each cell in the string is wired Parallel and each string is wired in Series. The way the BMS works is that there is only "port" per string that you connect the BMS to. Im not sure how you would wire in 7 BMS's together into a 3S7P battery pack. Im pretty sure thats not the way a BMS is wired.

 

[MisterSandals]

I believe the nomenclature is that the number preceding the S is the number of strings, and the number preceding the P are the number of cells in 1 string.

I have never heard that. Have a link?

number preceding the S is the number in SERIES, and the number preceding the P are the number in PARALLEL

And, BMSs are specified as 4S or 8S or 16S for example. This is the number of cells on SERIES (not the number of "strings").

Additionally, the order of the S and P components specifies which operation is done "first".

 

[Whoa]

Well, Im by no means the author of the nomenclature. I am simply stating this as I understand it, but in pretty much every post I have ever read, this is the way the authors of those posts are describing their battery design/structure. Further, in other forums that sell components this is also how components are described.

number preceding the S is the number in SERIES, and the number preceding the P are the number in PARALLEL

Yes. In that one battery.

Additionally, the order of the S and P components specifies which operation is done "first".

Im not sure why it matters whether you put the P before the S...? If you are talking about a battery the values are the same whether the P is stated before the S, meaning if I said Im making a 7P3S battery I would visualize it the same as if you were to say 3S7P. You have 3 Series and each series contains 7 Cells in Parallel.

 

[Partimewages]

APC UPSs are usually FLA batteries. I'm assuming 12 volt?

 

[Whoa]

APC UPSs are usually FLA batteries. I'm assuming 12 volt?

Yes, well stated. If you were to purchase an APC UPS from APC which includes batteries, Im pretty sure that there's no option for LiFePO4 batteries, which is why I started this project 3 years ago. Why not? They should out perform FLA (FLA = Lead Acid batteries?) batteries in every way.

The Rack mount APC UPS's I have are 24v, but I have smaller deskside units that are 12v and Im having a HUGE problem making these batteries work with those...

 

[Partimewages]

Lifepo4 battery is 3.2 volts nominal. I think the working voltage is 3.0 to 3.4. I didn't follow the S and P configuration. Generally for 12 volt is 4S for 13.8 volts and 8S for 25.6 volts. FLA (Flooded Lead Acid) charging can accommodate Lifepo4. It appears to me you have a S and P problem and the batteries are failing because of over voltage.

 

[pvgirl]

A setup of only 3 batteries in series, will likely not be able to handle the voltages that a Lead acid based charger is putting out, and would be using to monitor and detect proper charging. Depending on how the batteries are wired in the UPS, there may be issues with the batteries getting out of balance.

Also a 20amp BMS is quite small for a UPS, where large draws are made off the batteries when providing power. A 3S battery with a 20amp BMS will only provide approximately 225watts DC, and about 190wats AC per battery depending on the UPS's efficiency converting DC to AC. If your load is close to or over the 190watts * the number of batteries, you could be overloading the BMS.

 

[Whoa]

Lifepo4 battery is 3.2 volts nominal.

I was going to "argue" that what you said is not accurate. That 18650 batteries are not 3.2v, and are in actuality 4v+ which is why you can make a 3S battery using them for a 12v pack, BUT this just may be a dynamic of the 18650 that is "abused". If an 18650 is properly used, it should not be used in a 3S configuration but a 4S (like all larger battery packs).

I didn't follow the S and P configuration. Generally for 12 volt is 4S for 13.8 volts and 8S for 25.6 volts. FLA (Flooded Lead Acid) charging can accommodate Lifepo4. It appears to me you have a S and P problem and the batteries are failing because of over voltage.

For the larger cells (25AH and larger) they are all 3.2v and battery packs are multiples of 4S. For 18650 batteries 3S is pretty standard for a 12v battery.

I am sitting here and as I was writing this I have to accommodate for the possibility that even tho you can "get away" with 18650's in a 3S configuration to make a 12v pack, maybe thats not the way they have been manufactured. Ok, well that may well be the answer... If you want to get the extended life that a LiFePO4 chemistry has to offer, dont use a 3S system. Well, thats a kick to the britches... I appreciate the discussion! I believe I have the answer I was looking for.

 

[Partimewages]

Better check your BMS settings to verify. BTW I never get more than 3-4 years out of my UPS batteries. I should build a UPS and stop buying batteries!

 

[Whoa]

A setup of only 3 batteries in series, will likely not be able to handle the voltages that a Lead acid based charger is putting out, and would be using to monitor and detect proper charging. Depending on how the batteries are wired in the UPS, there may be issues with the batteries getting out of balance.

Also a 20amp BMS is quite small for a UPS, where large draws are made off the batteries when providing power. A 3S battery with a 20amp BMS will only provide approximately 225watts DC, and about 190wats AC per battery depending on the UPS's efficiency converting DC to AC. If your load is close to or over the 190watts * the number of batteries, you could be overloading the BMS.

The wiring inside any UPS is pretty much set for 20amps (or less) per battery, meaning Im seeing 10ga wire inside the UPS. You can "get away with" 12ga, but 10ga is pretty standard for 20a. Im positive that overload is not the issue as these batteries were not overloaded. I am pretty sure (bordering on positive) that the voltages may have been incorrect for the long term usage I am looking for.

 

[Whoa]

Better check your BMS settings to verify.

I dont follow. Check my BMS settings for what? How much Amperage? Whether the BMS is 3S? In either of those cases its been validated on what it can do.

 

[Partimewages]

For voltage settings and charging parameters

 

[Whoa]

For voltage settings and charging parameters

Im very sure that the BMS's Im using are are charging and discharging correctly. The reason why my packs are not working correctly and failing is because Im setting up the system to abuse the components rather than use them the way they are supposed to be utilized.

 

[Supervstech]

Well, Im by no means the author of the nomenclature. I am simply stating this as I understand it, but in pretty much every post I have ever read, this is the way the authors of those posts are describing their battery design/structure. Further, in other forums that sell components this is also how components are described.

Yes. In that one battery.

Im not sure why it matters whether you put the P before the S...? If you are talking about a battery the values are the same whether the P is stated before the S, meaning if I said Im making a 7P3S battery I would visualize it the same as if you were to say 3S7P. You have 3 Series and each series contains 7 Cells in Parallel.

It matters in how the pack is built.

If you tie 7cells to each Pole, then tie 3 sets of each pack in series, you have a pack with a single bms protecting a 7P set of cells.

If you tie three cells in series, with its own bms, then do that 7 more times, you have a bank with the same voltage, but a bms protecting each individual cell.

See?

 

[Whoa]

It matters in how the pack is built.

If you tie 7cells to each Pole, then tie 3 sets of each pack in series, you have a pack with a single bms protecting a 7P set of cells.

If you tie three cells in series, with its own bms, then do that 7 more times, you have a bank with the same voltage, but a bms protecting each individual cell.

See?

I do see what you are saying. I guess my response is that manufacturer's of LiFePo4 battery packs that are composed of 18650 batteries are using 1 BMS for the whole pack. They are ALL doing that. None of them are using 7 BMS's for each cell of those series. I am using better BMS's than most manufacturer's, but still only 1 for the whole pack.

 

[Supervstech]

Yes.
They are all making 7p3s