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5x4S VS 4S5P
Howdy all,

In the last year I decided I wanted to learn more about electronics, so I started tinkering and it's gotten out of hand.  I've been tinkering for a while now and have reached a point where my ability to find data is quickly outpaced by my ideas, and I lack sufficient contacts in my area to bounce these questions off of, so here I am.

Now for some background data about the project itself:

I have cobbled together a portable Raspberry Pi / Arduino lab.  I would give schematics, but I'm still learning to make them well, and also I am an adult with adult things to do.

In short, the lab is powered by a 90W (20V, 4.5A) power supply from an old laptop, that goes through a buck taking it down to 16.8V (6A max, more likely 5A max due to power supply and buck power loss), that goes in and through my batteries, that goes into a master power switch, that goes into a voltage/current/power/energy meter, that is then distributed to three more bucks converting the power to 5.2V (for the Raspberry Pi), 9.5V (for the Arduino), and 11V (for the HDMI controller board for the salvages laptop screen).

That's the setup in a nutshell, now back to the batteries.

Like many people here, I've been learning about and salvaging 18650s.  These range in manufacturer, chemistry, model, manufacture date, and so on.  What I have mostly heard or learned is that 18650 batteries should only be put into packs with batteries that are very very similar (same model, and preferably the same batch).  That's tough to do when you are getting between 4 and 9 cells per thing you scavenge.  So now I have a random collection of about 64 good 18650 cells, but no more than 9 that meet that same model/same batch rule, so I made some wild assumptions that justified my beliefs and made things.

What I have at present is five separate 4S packs each with it's own 4S 16.8V 5A 18650 Charger PCB BMS Protection Board.  All five of these packs are connected in parallel and inline with the power supply.  There's more to it than that (the distribution board has a relay and a diode to stop the batteries from powering the buck backwards, a small volt meter to see the incoming power voltage from the power supply, a button and a battery power indicator to see current charge level of the conglomeration of packs, and so on...) but this isn't about my circuit design, so moving on.

These are the specs of the five 4Ss I have in this lab at the moment:

Sanyo UR18650FM - LCO (ICR)
Red Body \ Cyan Ring \ 6 Spokes
Voltage (per cell): 3.7V nom (4.2V max, 3V cut off)
Capacity (per cell): 2600 mAh (2.6 Ah) 9.62 Wh
Voltage (4S): 14.8V nom (16.8V max, 12V cut off)
Capacity (4S): 2600 mAh (2.6 Ah) 38.48 Wh

Sanyo UR18650FM - LCO (ICR)
Red Body \ Cyan Ring \ 6 Spokes
Voltage (per cell): 3.7V nom (4.2V max, 3V cut off)
Capacity (per cell): 2600 mAh (2.6 Ah) 9.62 Wh
Voltage (4S): 14.8V nom (16.8V max, 12V cut off)
Capacity (4S): 2600 mAh (2.6 Ah) 38.48 Wh

Samsung ICR18650-26D - ICR
Pink Body \ White Ring \ 6 Spokes
Voltage (per cell): 3.7V nom (4.2V max, 2.75V cut off)
Capacity (per cell): 2600 mAh (2.6 Ah) 9.62 Wh
Voltage (4S): 14.8V nom (16.8V max, 11V cut off)
Capacity (4S): 2600 mAh (2.6 Ah) 38.48 Wh

Sony US18650GR (G5) - LCO (ICR)
Green Body \ Black Ring \ 3 Spokes
Voltage (per cell): 3.7V nom (4.2V max, 2.75V cut off)
Capacity (per cell): 2200 mAh (2.2 Ah) 8.14 Wh
Voltage (4S): 14.8V nom (16.8V max, 11V cut off)
Capacity (4S): 2200 mAh (2.2 Ah) 32.56 Wh

LG S3 1865 - ICR
Blue Body \ White Ring \ 4 Spokes
Voltage (per cell): 3.6V nom (4.2V max, 3V cut off)
Capacity (per cell): 2200 mAh (2.2 Ah) 7.92 Wh
Voltage (4S): 14.4V nom (16.8V max, 12V cut off)
Capacity (4S): 2200 mAh (2.2 Ah) 31.68 Wh

I've made these packs using a few methods (battery trays, magnetic nickle tabs, etc...) that I thought would make them easy to disassemble if needed.  Mostly with the idea that if a cell dies, I want to be able to replace it without having to completely destroy a whole bunch of work, and also because I couldn't figure out a safe way to solder the batteries, and my experimental capacitor based spot welder just wasn't strong enough.

Now you have so very much data, so here are my actual questions:

1: Is this safe?  Why or why not?

2: With each 4S having it's own BMS, won't each 4S stay more or less level with the others?

3: Even though three of the packs have a capacity of 2600mAh, and two have a capacity of 2200mAh, because each pack is on it's own BMS and they are all 16.8V max, shouldn't each pack give or take power when needed?

4: Is this safe?  Why not?

5: Is this more or less efficient than a single 4S5P pack?

6: Seriously, my wife is concerned...  something about the house burning down...  I didn't really listen because I was trying to solder.

7: Assuming the answer to 2 is "yes", if one of the packs starts to weaken, and I leave the lab unattended for a few weeks, won't the other four packs just keep feeding the weak pack to keep it up?

8: Would this configuration actually charge faster than trying to charge a whole 4S5P since each of the BMSs have access to a share of the available 5 or so amps available?

9: If I replace one of the packs with a newly built pack that is only at like 50% charge, assuming that the existing four are mostly charged and the power supply was plugged in as well, would the newly installed pack actually get charge from the four existing packs as well as the wall power supply all at the same time?

10: again...  is this safe?

11: How do I figure out what the maximum safe current draw is for this system?  Would it be based on the weakest packs stats, and average of all of the packs, or the total of all five?

12: Am I right in saying that the packs theoretically add up to about 12.2 Ah of capacity, or around 175 Wh of energy (minus some due to cells not being new)?

Your feedback is greatly appreciated
I answer from my perspective. Note that building high current packs where you push a cell to its limit IS NOT the same as a big pack where you hardly stress the cells.

1. Which one is safe?
2. Yes potentially yes. Since they all balance with each other and the outer/total voltage is same since they are in parallel.
3. Yes they should. The higher capacity cells will be stressed a bit more due to higher capacity.
4. Save is a word... Wink If it is safe depends on so much more. Like current you drain, temperature and so forth.
5. Its more work, higher cost. Not sure if that is your answer your looking for but I would not build that small packs.
6. Dont have the cells in your house. that is RULE number 1 for everyone building that are new to it. It is as simple as that. Keep all your lithium based stuff that you work with a safe distance from your house... I have 29000 cells... I have them a good distance from my house.
7. Correct. Thats what happen.
8. No it wont. You loose some more energy having multiple BMS so potentially it will charge some mS slower Wink
9. Yes it will. And potentially you will damage the cells or even start a fire. You never short packs or cells together in parallel that arent in the same voltage range.
10. No. You need to slowly sync the packs in a matter of decent current. You can use resistor in between or a normal 12V bulb as example.
11. It is based on the weakest pack yes. If you have 4 cells in parallel and you test all your cells with max current of 1A your max load is then 5A. If you test at 0.5A then its 2.5A max... You can potentially drain more but to play it safe dont stress any cells above the tested current nor their recommended max from the datasheet. The lower number that comes first is what you use.
12. correct.
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I hate to admit it, but one of the other reasons I went with 5 4S packs instead of a 4S5P pack was that I had purchased these BMS boards and wasn't sure if I could use them with more than 4 cells. Do you have any insight into this? Can I use these with a 4S2P, 4S3P, 4S4P, and/or 4S5P?
If I was doing what you were doing I would use a parallel board and connect everything together(including balance connectors) so they stay balance when charging/discharging, it would be just one big battery. This way you only need one bms to monitor everything. With the parallel board, one battery won't quit before the others. Do you have a picture of the distribution board?

The bms you have can be use on any 4s battery pack, even a 4s1000p, but it will only be able to handle 6 amps, it would just be underpowered for larger packs. And its a myth that you have to use certain 18650 on your battery packs, as long as the battery checks good when you test them, use them. My packs are made with sony,sanyo,lg,generic,etc from 1200 mah to 2600 mah, if it checks good it goes in the battery pack. Heres a picture of one of my packs 3s 94ah, notice all the colors. Stayed in perfect balance. If you were making a pack for a racing bike, then yes the 18650 need to more demanding, but for what you plan to do, you can mix them together. A large pack is always better then a small one, the chance of one cell going bad is near zero, In 4 years with 8 or 9 packs that I build, never had to replace a cell. Finding and replacing one bad cell is so much work that it is usually better to just rebuild a new pack. The cells you used are excellent mah, they make a very good 4s5p.


For more safety use some sort of overvoltage alarm, like the tenergy lipo checker or isdt bc-8s. Something that will sound a loud alarm if a battery is getting above 4.20 volt. All my lithium packs have a loud alarm. I don't trust the bms to always stop the charge because I had some that went charging past the cutoff point. I like the isdt bc-8s, it has nice screen on it. You need to monitor the battery balance everytime, its easier to do on a large pack then on a bunch of smaller packs. 


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