DIY Portable Power Bank design question

kino0924

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Aug 20, 2021
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Hi,

I am trying to build my first DIY power bank for camping but not sure if my design is correct.

I am trying to be budget as possible since this is my first project.

This is the cell that Im planning to use
This is 13s 48v Panasonic cells and I am planning to parallel connect 4 of them.

I am planning to use this with 48v to 12v 30A Step Down converter so I can place 3 cigarette connector and few USB charging ports.


Now, here is tricky parts that I couldn't find answer.

1. Since I am planning to use 4 of 13s modules, do I need 4 BMS and connect them parallel?
Or Can I connect each terminal in parallel with single BMS?
(I am planning to use Daly 13s BMS)

2. To charge these cells, I am planning to use used Dell laptop chargers (19.5V).
If I connect charger directly to BMS, will it charge? or do I need to put 48V source?


I know my design is not well optimized, This is due to I dont have any equipments yet and Im trying to stay away from spot welding.

Bottom line is, I want to utilize 4x of 13s panasonic cells to make 12v power bank

I am open to any suggestions so please help me!


Thank you so much
 

chrisski

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1. Since I am planning to use 4 of 13s modules, do I need 4 BMS and connect them parallel?
Or Can I connect each terminal in parallel with single BMS?
(I am planning to use Daly 13s BMS)
I don’t see how you’d get around 4 BMSs unless you were to disassemble the cells and place them 4P and then 13 of those i series. I’m not sure of the chemistry since this is the info:

Nominal voltage - 46.8v (3.6v per cell)
Fully charged voltage - 54.6v (4.2v per cell)
Fully discharged voltage - 36.4v (2.8v per cell)
Max continuous charge - 5a (1.25a per cell)
Max continuous discharge - 100a (25a per cell)

This is a high C discharge cell, so I expect it is LTO, but I’m not sure.
2. To charge these cells, I am planning to use used Dell laptop chargers (19.5V).
If I connect charger directly to BMS, will it charge? or do I need to put 48V source?

19.5 volts would not cut charging since it will need somewhere around 54+ volts to charge.

=================

300 wh at 24 volts comes out to about 14 ah. QC protocol is supposed to be 1 amp at 24 volts, so if it takes an hour to charge each device, you may get 14 hours of charging off this.

When I’ve measured the amps, I’ve gotten less than 1. Anywhere between .3 and .6 amps, so you may get 20 to 30 hours of charging off that pack.

==================

From the same place, I got 8 of the 25 ah lithium iron phosphate cells. To assemble it, was about $230 just for the battery. BMS, copper bar, and hardware to build the case took up about 1/3 the total cost. The cells were the rest Of the cost. That battery pack will have about 500 wh.

A charging device of about the size you said, 130 wh would have been perfect. I just could not find lithium iron phosphate cells that I could get to 24 volts and have BMS. I could have put a bunch of LTO cells together for a charger, but with the high discharge rate and assembly, that was not what I wanted to do when I started, but may have been a better idea for a USB charging power source for camping.
 

kino0924

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From the same place, I got 8 of the 25 ah lithium iron phosphate cells. To assemble it, was about $230 just for the battery. BMS, copper bar, and hardware to build the case took up about 1/3 the total cost. The cells were the rest Of the cost. That battery pack will have about 500 wh.

A charging device of about the size you said, 130 wh would have been perfect. I just could not find lithium iron phosphate cells that I could get to 24 volts and have BMS. I could have put a bunch of LTO cells together for a charger, but with the high discharge rate and assembly, that was not what I wanted to do when I started, but may have been a better idea for a USB charging power source for camping.

I guess you are referring this item

So make it as 8s configuration as 24v and use 24v to 12v Step Down Converter right?

Will I be able to charge these cells with 19.5V dell charger?


Thanks
 

kino0924

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Aug 20, 2021
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With 8 cells, if your ultimate goal is 12V, why not make a 2P4S battery and add a 4S BMS?

Thats good question.
Its simply I do not want the power bank to drop below 12v

Please correct me if Im wrong
I believe LifePO4 cell's Fully discharged voltage is 2.5V which means that it can be produce 10V with 2P4S isnt it?

Or, can I program BMS to cut off power if each cell produces below 3V?


Thanks,
 

MisterSandals

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Its simply I do not want the power bank to drop below 12v
I don't think anybody here is letting their battery drop below 13V on a regular basis. All my batteries live between 12.9V and 13.8V.

There is very little power below 13V so not much reason to make that part of the working range.
Sounds like you are looking to make sure your 12V stuff gets 12V. Well a 12.8V LiFePO4 battery is just the thing!

LiFePO4 SoC chart.png
 

kino0924

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I don't think anybody here is letting their battery drop below 13V on a regular basis. All my batteries live between 12.9V and 13.8V.

There is very little power below 13V so not much reason to make that part of the working range.
Sounds like you are looking to make sure your 12V stuff gets 12V. Well a 12.8V LiFePO4 battery is just the thing!

View attachment 61041

Thank you so much!
I will go with 2p4s configuration (or find 50ah cell so I can just have 4s configuration)
 

MisterSandals

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I will go with 2p4s configuration (or find 50ah cell so I can just have 4s configuration)
I have 2P4S and 4S, they both work great.

If you're going to top balance 8 cells at once, make sure you get enough bus bars! (14 bus bars)

Here is a pic of my 8 cells being top balanced. Charger is powered by my 4S bank.

 

kino0924

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Last question,
Does any BMS support for me to set to use only between 10% ~ 90% of capacity?

Thanks,
 

MisterSandals

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Does any BMS support for me to set to use only between 10% ~ 90% of capacity?
The BMS is a safety cutoff. I would only use it sparingly and not as a day to day voltage (charge) regulator.
You can set it to disconnect (low voltage disconnect) around 12V if you like. I'm not sure i would set it much inside the LiFePO4 battery spec/limits (3.65Vpc and 14.5V battery).

Your charger should manage high voltage for battery. I like a nice conservative 13.8V, that gives me almost full capacity and my cells behave nicely at that level too.
 
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