I’m about to start building a portable power pack, and if succesful, probably a second one.
I have so many questions, until the last couple of days I knew nothing about batteries and now my brain hurts.
My plan is 7S7P using 3400mAh 18650’s.
On the unit I will have 4 x 45w usbC, 2 x 18w QC3 usb, and one 12v car fag lighter outlet.
Use a DC/DC converter to drop voltage down to 12 from approx 24 and a 7S BMS and either a 7S or 10S charger.
What has spurned me on here are the numerous power cuts we get. Now we have a 10Kw woodburner, so we have heat and something to cook on, but I really hate having candles burning everywhere. So my thinking is I can have this for running some LED lighting and other bits and bobs in the house/garden when required, but also use it for field charging when out and about. Plus I like to tinker and learn new things. I have the usb outlets and battery spot welder on order. My mate has 900 or so 18650 batteries already collected, tested and sorted into their different ratings, so a BMS and charger are all that is required apart from cables etc, maybe a cooling fan and a thermistor to switch it on and off when needed.
Does this sound do-able? any hints or tips to share before I start bodging it together? Does my maths stack up?
Why not tap out at 3S for the USB points? yes it may marginally unbalance the cells but I do this on my re-used EV car module and a balance charge at the end of the day sorts it out. The USB power draw will be far less than the main power used for recharging your LiPo’s
Instead of a BMS why not bring out a conventional balance plug and charge it on an ISDT (or similiar) charger, this will handle the balancing for you and give you sight of the individual cell voltages for sanity checking.
You lost me at that first sentence, which is exactly why I am here asking questions 
Everything I have read so far says that a BMS is a must unless you want to go up in a puff of smoke and destroy your house? To be honest I don’t evem know what an ISDT is? Back to google
I’ll send you some pics of my pack which should explain better.
For ISDT read Hobby LiPo charger
https://www.google.com/search?q=isdt+k2+air&sca_esv=595122014&sxsrf=AM9HkKnePaGdnKM-os2wCj6CXAGkCy_2Ng%3A1704215235138&ei=w0KUZdGNCPa0hbIPmvWF4AU&oq=idt+air2+charger&gs_lp=Egxnd3Mtd2l6LXNlcnAiEGlkdCBhaXIyIGNoYXJnZXIqAggBMgsQABiABBiiBBiwAzILEAAYgAQYogQYsAMyCxAAGIkFGKIEGLADMgsQABiABBiiBBiwAzILEAAYgAQYogQYsANI_BdQAFgAcAB4AJABAJgBM6ABM6oBATG4AQHIAQDiAwQYASBBiAYBkAYF&sclient=gws-wiz-serp
This allows you to charge the pack as if it was an ordinary lipo battery with XT60 connector and a balance plug
The advantage of using a BMS is that it handles the cell balancing, maximum and minimum charge and discharge limits and you only need to connect two wires, positive and negative.
All good but I prefer to see exactly what the cell voltages are, thus confirming the BMS is working as it should, because unfortunately some of these Chinese BMS are not reliable unless bought from a proffesional source.
I take it this wont handle the balance of each individual 18650 cell, but rather the row of seven?
How long will it stay at 12v when running a pump or small air compressor etc? I thought it would be better and easier just to have the 25 or so volts. Plus everything I have read so far says that although 3s and 4s will work, they’re not ideal and that going down the 7s route will give less headache in the future.
Or did you mean take a seperate tap (wire) out at 3s to supply the usb? The outlests I have bought work up to 32v and regulate themselves down to 12v. I got them for that reason, less messing about.
BUT, I am of course here to learn. There are so many conflicting things on the tinterweb!!!
Yes, exactly, but just my suggestion.
Feel free to do what you prefer, nothing wrong in your approach 
would there be any advantage/disadvantage over either of the approaches?
Less cost and greater efficiency without the 24v-12v DC/DC unit
But would still have the full current and watt capacity of the whole pack?
Yep…
let me try to find time to send you a circuit of my pack today
Built using a recovered Lithium pack from an electric vehicle, originaly 12S1P but I re-configured to 6S2P (It’s actually a Samsung pack used on Range Rover Hybrid)
I’ve still to add my USB outlets but they will connect straight to the 3S output giving a voltage IN range of 9.6v to 12.6v Typically on a flying day 12v to 12.6v.
It has a link cable for the balancing.
No rush, still waiting on bits to arrive and batteries to be collected. Then there’s more research to do and a box to make or buy!!!
The parts have started to arrive 
First off is the spot welder and tape, battery holders and the usb/12v outlets
Already had a play with the welder and I’m fairly impressed so far!!! Melts holes clean through two pieces of nickel tape on setting 8!!! Setting 4.5 is good for sticking just two layers of tape together which proved impossible to pull apart.
Welder 1 - Finger 0
Then my mate turned up with the all important batteries 
I thought he was bringing some of the 900 or so that he has already tested and sorted, but no 
24 lenovo, 4 Toshiba and 2 Dell. 6 cells in each and each pack is rated between 55Wh and 57Wh and around 11volts. So I’m guessing they are wired in 3S2P?
His reasoning for not giving me already prepared ones is that
“I want you to learn and feel the pain of taking them apart, then cleaning them up, charging them and testing, followed by grading them”
What a bastard, eh?!!! Still, they were free and I haven’t had to try collecting them myself. Although I now have offers rolling in from 4 different Freecycle groups. I’ll probably collect them anyway. I’ve started learning you can never have too many batteries
Now it’s time to start breaking these mofo’s apart, I’ve already done one to see how easy it is, I could be here a wee while!!!
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As I started splitting another pack down I managed to have a wee short. Nothing to be concerned about but it brought my attention back to safety and storage. So out to the shed to get an old metal tool no longer in use.
Now I can safely keep them all seperated from shorts, and then once finished for the day I can put them outside on the concrete patio and go to sleep
This mornings job before I go out for a flight.
Start sorting the cells by voltage. Left hand pile for 2 to 3 volts, right hand pile for anything above 3 volts. And a scrap pile for anything below 2 volts. I might be able to salvage some of the scrap pile with the charger I’m waiting on arriving? But for now I’ll stick to working on the over 3 volt pile as they’ll probably be better conditioned cells.
Well, I now have 300 charged and graded cells waiting to be welded. Just another 150 or so to do. Way more than I need for the initial project. Just got to finish my power supply this weekend and then make a start on spot welding the batteries.
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After doing a lot more research I’ve had to go back to the beginning. Plus a lot of these cells had already been graded by someone else. Some of the capacities written on them were ridiculous. I’m not sure you get a 7270mAh, 18650 cell 
So, start off by putting in charger. Anything above 4 volts kept to the side for further testing.
Anything below four volts put in a box to look at later.
Anything at 4.2 volts and showing zero IR put to the side for discharge. My tester wont show IR once the cell is fully charged, this is to avoid overcharging. They need to be discharged to 4 volts and then start the charge process again to get IR resistance reading
Quite a few showed up as Ni-Mh, binned.
Quite a few showed ERROR, then I found out I put them upside down, what a twat
Some showed FULL at less than 4 volts, binned
Anything over 150mOhm put to the side for further testing.
The ones I am planning on using (have the most of) are Sanyo R1122. Not a great make as far as I have read, but it means I don’t have to mix and match between manufacturers, possibly slightly different chemistry, charge/discharge current etc. I haven’t yet been able to find the official IR so not sure yet how high or low to discard. The vast majority so far, are between 70 and 100mOhm. Does it really matter that much?
