4s16p x2 or 4s32p - Lifepo4 26650 cells also.... 2 bms? or 1 bms?

Sween · February 2019

I'm curious, what is the best method to make my portable solar generator battery strictly using LifePo4 26650 (3300mah) cells?
I'd like to stick with 4s due to the 12v nature of my projects Except, 4s32p is too long of a pack for my case.
Here's my question...

Should I make TWO 4s16p packs and connect them parallel side by side?? this would make the pack fit....
AND
If I make Two 4s16p should I use two BMS boards (one for each?) or just use one bms?

Below is a photo of my cells how I want them in the box.

also I am open to any and all suggestions as well!

THANKS!!!

Image: https://us.v-cdn.net/6024911/uploads/editor/h5/2turh4gq6auo.jpg

mcgivor · February 2019

Two 16 in parallel would still be 32 parallel, so a single BMS would be all that's needed.

mike95490 · February 2019

First, what's your method for connecting all the cells together
Spring terminals ?
Soldered wires ?
Spot weld tabbing ?

How do you intent to dissipate heat from the middle cells in the pack ?
Mineral oil bath ?
mineral oil bath in flames ?

Ampster · February 2019

mcgivor said:

Two 16 in parallel would still be 32 parallel, so a single BMS would be all that's needed.

I understood the OP to say two 4S16P in which I believe two BMSs would be needed as @mike95490 said. Perhaps the fire he was referring to was the BMS wires catching fire when trying to use one BMS. Otherwise LifePO4 are less susceptable to fire than other chemistries.
The picture looks like two 4S16P, but if he flipped alternate rows he would have 32 P consisting of two rows of 16P.

Sween · February 2019

mike95490 said:

First, what's your method for connecting all the cells together
Spring terminals ?
Soldered wires ?
Spot weld tabbing ?

How do you intent to dissipate heat from the middle cells in the pack ?
Mineral oil bath ?
mineral oil bath in flames ?

Im going to Spot weld nickle tabs, and I'm going to have 12v fans on/In the case for ventilation. It should be good.
I was just wondering if having two 4s16p with each their own BMS would be redundant versus one 4s32p and one BMS... Im still a little confused on this.

mcgivor · February 2019

@Sween

With the layout topography is it necessary to have a gap between the cells as illustrated in the image or can they be grouped closer together? Having two rows of cells oriented the same way to create a cell block would be the simplest, as opposed to alternating, this way would be the easiest to create a bank. When I previously posted I was thinking this way, there would be no need to have two BMS's as the battery would be a 4S 32P.

Sween · February 2019

Thanks Mcgivor, I could push the cells closer together for sure but I wanted the gap for ventilation and a wire channel. Thanks for clarifying the bms issue for me. One BMS it will be for the 4s32p...

mcgivor · February 2019

Depending on how the battery bank is used heating may, or may not be an issue, a rapid charge/discharge cycle may increase temperature, operating well below this threshold is not an issue, in fact my particular battery temperatures are significantly lower than ambient, even with 39°C or 102°F ambient as it was today, 3 seperate measurements were within 3 degrees of 30°C. It must be stated overnight ambient drops to <20°C, so some cooling effect. Couldn't say this for lead acid in the same circumstances, they would retain the heat, adding to it the next day, probably due to inefficiencies and sheer bulk of lead.

karrak · February 2019

This arrangement would allow you to have just one block and share the current evenly between the cells.

The blue are negative and the red are positive bus bar. If the gauge of the wire of the links between the bus bars and the cells is carefully chosen they will act as fuses to protect against any individual cell going short circuit.

It is important to use the zigzag arrangement of the connections between the busbars to balance out the current going to the individual cells.

Image: https://us.v-cdn.net/6024911/uploads/editor/o1/hvewybzv3g9x.png

There are plastic cell holders like these that will help keep the cells apart and avoid the risk of shorting between the cells.

Have you sourced a BMS yet? can you give more information on your application? how are you going to charge the battery?, What are the loads? are the other electronics going in the same box? What brand are the LFP (LiFePO4) cells?

Simon

Sween · February 2019

karrak said:

This arrangement would allow you to have just one block and share the current evenly between the cells.

The blue are negative and the red are positive bus bar. If the gauge of the wire of the links between the bus bars and the cells is carefully chosen they will act as fuses to protect against any individual cell going short circuit.

It is important to use the zigzag arrangement of the connections between the busbars to balance out the current going to the individual cells.
There are plastic cell holders like these that will help keep the cells apart and avoid the risk of shorting between the cells.

Have you sourced a BMS yet? can you give more information on your application? how are you going to charge the battery?, What are the loads? are the other electronics going in the same box?
Simon

I have 2 - 4s BMS pcb's left... I'm currently waiting on my pure nickle stripping. (should be in Monday). The Batteries will charge via solar (eventually) and through an AC 10a lifepo4 charger. I'm going to include a 2-3000w inverter for ac application, fans connected to a disc thermostat, 12v DC ports (lighter), some USB ports, some leds, etc etc... In all honestly I am doing this to learn about solar and off grid battery applications, and to have some fun during the colder months of the year when I'm stuck inside.

IM open to any advice, as this is my first build besides a few 4s4p lifepo4 packs.

also Im still rather confused on how to combine the two 4s16p packs in parallel.... do I just connect negative to negative positive to positive on the end cells???
thanks so much everyone...

mcgivor · February 2019

The layout in the image is how I envisioned it, all the positive and negative attach to their respective busbar to provide lower resistance as well as even current distribution, the bottom is the mirror image. The are not two packs to combine, it will be a single pack, the jumpers one on top, two below are the series connection, think in terms of a cell block, the 32 cells in parallel each with its own separate positive and negative busbar.

@karrak, very nice diagram Simon.

Sween · February 2019

Also,, (sorry), the cells are Full River 3300Mah 3.2v

Do you think bus bars are worth the effort vs Spot welding tabs?
Honestly I have a spot welder So, I was planning on using that.

karrak · February 2019

Without some sort of links between the paralleled cells the battery is basically 32 separate 12 volts batteries wired up in parallel. You need the busbars to make sure that all the paralleled cells in each block of 32 cells are all at the same voltage otherwise you would require 32 BMSs to make sure that all cells remain in their safe operating voltage range.

Can your spot welder weld the nickle tabs onto a copper busbar?

Can you make the nickle strips into fuses?

I did a Google search on Fullriver LFP cells and came up with these links

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=2ahUKEwj_mt69-rDgAhXVXn0KHaIcCY0QFjAEegQIBhAB&url=https%3A%2F%2Fendless-sphere.com%2Fforums%2Fviewtopic.php%3Ft%3D85645&usg=AOvVaw1iPcuXHStm6w8DIKkci86r

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=2ahUKEwj_mt69-rDgAhXVXn0KHaIcCY0QFjAFegQICRAB&url=https%3A%2F%2Fsecondlifestorage.com%2Ft-Testing-new-Fullriver-LiFePO4-26650-cells-for-capacity-before-building&usg=AOvVaw3OVfLDPJr7mEy3OfoMaQQd

The Endless Spheres forum is a excellent source of information for the sort of battery you are trying to build and spot welding. The Second life storage forum also has some good information.

Simon

karrak · February 2019

Sween said:

I have 2 - 4s BMS pcb's left... I'm currently waiting on my pure nickle stripping. (should be in Monday). The Batteries will charge via solar (eventually) and through an AC 10a lifepo4 charger. I'm going to include a 2-3000w inverter for ac application, fans connected to a disc thermostat, 12v DC ports (lighter), some USB ports, some leds, etc etc... In all honestly I am doing this to learn about solar and off grid battery applications, and to have some fun during the colder months of the year when I'm stuck inside.

IM open to any advice, as this is my first build besides a few 4s4p lifepo4 packs.

also Im still rather confused on how to combine the two 4s16p packs in parallel.... do I just connect negative to negative positive to positive on the end cells???
thanks so much everyone...

OK, so the battery will have a capacity of around ~100Ah. For long life I would limit the charging current to ~0.5C (50A) and the continuous discharge current to 1C (100A, ~1300W). Are you aware of the size of the busbars, cable and connectors you will need for a 2kW or 3kW inverter?

High temperatures are the biggest killers of any battery. The charger, inverter and other electronics will generate far more heat than the battery so I would try to separate the battery from the electronics. It is unlikely that the battery will require any cooling if it is operated at the charge and discharge rates I have mentioned but the BMS should monitor the battery temperature.

Can you give some more details on the BMS you were planning on using.

Simon

karrak · February 2019

Here is another variation on wiring the cells which doesn't need any heavy duty cable between the banks of cells. Top is the top of the battery looking from the top, bottom is the bottom of the battery looking from the top.

Image: https://us.v-cdn.net/6024911/uploads/editor/q5/ziyu7nqjvlkg.png

Sween · February 2019

https://www.ebay.com/itm/4S-14-8V-30A-BMS-Protection-PCB-Cell-Board-Li-ion-Lithium-18650-Battery-Balance/232964067302?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D55991%26meid%3Ddd623381fe4f40cdaae0de8a2bc575af%26pid%3D100008%26rk%3D6%26rkt%3D12%26sd%3D192039991627%26itm%3D232964067302&_trksid=p2047675.c100008.m2219

^^^ This is the BMS I was going to use.... nothing fancy, over/under volt, short circuit, 30a

Sween · February 2019

karrak said:

Here is another variation on wiring the cells which doesn't need any heavy duty cable between the banks of cells. Top is the top of the battery looking from the top, bottom is the bottom of the battery looking from the top.

This is similar to what I envisioned minus the bus bars...

If I do decide to use the busbars is it possible to use steel or aluminum? nickle dosen't spot weld very nicely to copper Ive heard... thanks again everyone

mcgivor · February 2019

Without the busbars there would be 32 separate batteries, each needing it's own BMS. Using copper as a busbar material building a fishbone with the nickel strip screwed to it before the welding process may solve the welding to copper. With lithium always start with parrallel then series, unlike lead acid where it's the opposite series then parrallel. There are pre made strips available which could save time such as these https://bangteng.en.alibaba.com/product/60678614714-807438859/26650_Battery_Copper_Nickel_Plate_Welding_Copper_Nickel_Strip_for_Battery.html they have the busbar included, just some thoughts

karrak · February 2019

Sween said:

https://www.ebay.com/itm/4S-14-8V-30A-BMS-Protection-PCB-Cell-Board-Li-ion-Lithium-18650-Battery-Balance/232964067302?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D55991%26meid%3Ddd623381fe4f40cdaae0de8a2bc575af%26pid%3D100008%26rk%3D6%26rkt%3D12%26sd%3D192039991627%26itm%3D232964067302&_trksid=p2047675.c100008.m2219

^^^ This is the BMS I was going to use.... nothing fancy, over/under volt, short circuit, 30a

The thing that worries me about these cheap BMS units is that they do not output any data to the outside world so you have no idea if they are working properly or not and you can't see your battery status without getting the multimeter out.

This particular BMS is also set up with an over voltage of ~4.1-4.2V which is much to high for LFP (LiFePO4) batteries. 3.6-3.7V is more appropriate. The current output of only 30A will also limit the power output to only ~360W.

IMO a better option is are these BMSs which have a serial output to either a bluetooth module that can link up with an Android phone app or a serial to USB module that can be plugged into a computer.

How do you intend to check and balance all the cells as part of the battery commissioning process?

Simon

Sween · February 2019

karrak said:

Sween said:

https://www.ebay.com/itm/4S-14-8V-30A-BMS-Protection-PCB-Cell-Board-Li-ion-Lithium-18650-Battery-Balance/232964067302?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D55991%26meid%3Ddd623381fe4f40cdaae0de8a2bc575af%26pid%3D100008%26rk%3D6%26rkt%3D12%26sd%3D192039991627%26itm%3D232964067302&_trksid=p2047675.c100008.m2219

^^^ This is the BMS I was going to use.... nothing fancy, over/under volt, short circuit, 30a

The thing that worries me about these cheap BMS units is that they do not output any data to the outside world so you have no idea if they are working properly or not and you can't see your battery status without getting the multimeter out.

This particular BMS is also set up with an over voltage of ~4.1-4.2V which is much to high for LFP (LiFePO4) batteries. 3.6-3.7V is more appropriate. The current output of only 30A will also limit the power output to only ~360W.

IMO a better option is are these BMSs which have a serial output to either a bluetooth module that can link up with an Android phone app or a serial to USB module that can be plugged into a computer.

How do you intend to check and balance all the cells as part of the battery commissioning process?

Simon

Point well taken... Im going to invest in a BMS similar to this one... Thanks.

Sween · February 2019

basically I am at a stand still right now because the cheap wand/pen on my sunko 709A spot welder has melted. (I assume I used it far too fast for the unit). That and 25 feet of nickle strip was not enough for both 4s16p's

But I have one 4s16p pack assembled!!! (it's reading 13.07v, which at full cap. it should be 12.8v-13.2v (3.2v-3.3vx4)

I am going to order a new BMS that's rated for lifepo4 for li-on (with the Bluetooth module) (Thanks Simon)

Basically, I am going to build Two 4s16p packs, get them both to full charge (around 13.2v), then I am going to connect the two packs in parallel via 6 Gauge copper wire.

Image: https://us.v-cdn.net/6024911/uploads/editor/u3/hd21ilr1kwoe.jpg

While I am waiting for parts for delivery, any further advice or discussion is totally welcomed...

like.... "What would you do differently"?

Thanks everyone!!!

Ampster · February 2019

I understand the limitations of your case means that you can only fit 16 cells in a column. That does not mean you have to go to two 4S16P packs with two BMSs. A simpler solution that would only require one BMS would be two columns with positive connectors up, then two columns with positive connectors down and so fort to give you a 4S32P single pack with one BMS.

Sween · February 2019

Ampster said:

I understand the limitations of your case means that you can only fit 16 cells in a column. That does not mean you have to go to two 4S16P packs with two BMSs. A simpler solution that would only require one BMS would be two columns with positive connectors up, then two columns with positive connectors down and so fort to give you a 4S32P single pack with one BMS.

Understood. I will keep this in mind for future packs. This is my first pack over 4p So this is how I envisioned it.
However, even assembled as is , I would only need one BMS, as Mcgivor stated... because after connecting the two 16p packs the positive chain and negative chain combine forming 32p shared amperage across all cells. the BMS can then trickle charge each parallel positive bank (4 in total) (man are those balance wires small....) until even voltage is acquired across the 4 32p cells.

Correct me if I'm wrong though. Like I said, I am very new to manually assembling packs.

Sween · February 2019

Oh my bad, I also forgot to introduce myself. My name is James Sweeney. I'm a 35 year old Cyber Security Major in College that loves to tinker with electronics. Eventually I plan to take an old Porsche and convert it to electric drive. This is what has me interested in batteries and solar energy right now. I am learning so please forgive my intelligence on the matter. That being said, thank you all so much for the help and input thus far. I will keep you all updated on this "Solar Generator" project here.

mcgivor · February 2019

Normally before any series connection the cells, or cell blocks should be connected in parallel to ballance their capacity either top or bottom or mid, by changing or dischargeing, the cell ballance provided by the BMS is usually designed to adjust minor variations of a pre ballanced bank. As most BMS's use top ballancing it's probably betting to top ballance, here is a link to a site which has some very useful information, http://liionbms.com/php/cell_prebalancing.php mostly the content is regarding use in EV but the information still pertains to smaller scale.

jonr · February 2019

I ordered this BMS. I like that it has serial output, full source code and schematic. All easy to understand.

Search ebay for "18v 5s 20a lipo li-ion charge discharge protection circuit balancing bms".

Ampster · February 2019

https://forum.solar-electric.com/discussion/comment/395656#Comment_395656

Are you using LiPO cells or are the charge and discharge curves of your cells similar to LiPO? If not I hope the BMS is programmable so you can set the right parameters for you pack chemistry. I have used those kind of BMSs on electric bike packs where the pack might cost me several hundred dollars. For my stationary pack that would probably cost several thousand to replace I use a BMS that is fully programmable and offers pack quality reporting and bells and whistles. It is simply a risk management decision and I am risk adverse when it comes to losing at $3000 pack, so that is my feedback.

jonr · February 2019

It contains an Arduino compatible processor with simple source code. Changing the program changes all of the chip (BQ76920) behavior. Max and min voltages, when to balance, etc.

4s16p x2 or 4s32p - Lifepo4 26650 cells also.... 2 bms? or 1 bms?

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