# Building 3s 18650 battery pack

I got myself scrap laptop batteries for very cheap, only 2 cells out of 60~70 appear to be damaged (0v)

To charge 2 drained batteries, first I used a powerbank circuit that despite being less than a dollar does a very good and careful job at reviving low voltage individual pair of batteries, takes about an hour from 1.5ish voltage to 3.7 but it was taking too long so I put 220ohm resistor in series with whole 3s pack to a good 3s 12v battery, idea is to wait for batteries to hit 9v then take it from there, can anything go wrong with this method?

Then to balance the capacities for the pack, I will have to measure capacity of every pair or atleast 1 pair per pack which can also take very long, is there a quick reliable way to test battery capacity without fully charging and discharging batteries?

Regarding charging/protecting the pack, bms will cost me as much as batteries themselves, what if I use bms of working batteries instead, assigning each bms some extra pairs to charge and put all bms in parallel?

• I don't think there is any shortcut to measuring capacity. Given that the batteries are old, I would recommend that you measure the capacity of every cell. Actually, I would recommend that you dispose of all your batteries and buy new ones. But barring that, I would buy a battery analyzer or charger than can measure capacity and test each cell. Your question about BMS is totally unclear. I recommend you draw a diagram to illustrate what you have in mind. Feb 22, 2021 at 21:54
• With a good adjustable CV,CC power supply, you can measure ESR and dV/dt capacity and match within 1% , but if don’t have a balancer for 5 to 10% of load the next weakest cell will fail sooner than you’d like Feb 22, 2021 at 22:26
• Can you provide more info refer me to some useful links? I am not gonna use it rough tho, my load probably will never exceed 100w @tony
– asim
Feb 22, 2021 at 23:05
• I never found any links I just did it but 3S 20A BMS boards go for < \$3 Feb 22, 2021 at 23:09
• if balance error is 50mV and ESR is 50 mOhms vs 100m at 10A then balancer must dump 5W per cell affected, So SoC cutoff a, dV/dt and ESR are tradeoffs with useage Feb 22, 2021 at 23:15

First of all it's a nice idea to recycle batteries from old laptop battery packs from an environmental point of view. Depending on your planned usage I wouldn't expect too much from these batteries as they might not hold their full capacity anymore or might be damaged in another way (higher ESR, weak chemistry, ...).

To answer your question(s) more precisely it would be good if you could clarify some things:

1. how does your powerbank circuit look like?
2. what is this 220 Ohm resistor about and why should it speed up charging?
3. what exactly do you have in mind?

I don't think there is a shortcut to analyze the (remaining) capacity of your cells rather than fully discharing and then charging them with a suitable charger. The iMAX B6 Charger would be an example (there might be cheaper ones but this is a good quality charger used a lot from RC hobbyists).

Regarding the protection you must differentiate between protecting a battery pack or protecting a single cell. I would highly recommend doing both as there is the possibility that a defective cell will short and this is a problem if you put batteries in parallel because the whole pack will fail then rather then tripping the protection of the single defective cell.

• 1. My powerbank circuit is just a diy smartphone powerbank that you can use with your own batteries (from romoss) hence i am using it to charge individual cells at a time
– asim
Feb 22, 2021 at 23:14
• 2. I connected whole 3s laptop battery (3x in series) to 12v through a 220ohm resistor hence limiting current to 50mah max charging all 3 cells at the same time which is faster but is unbalanced charge just to revive batteries
– asim
Feb 22, 2021 at 23:17
• 3. To have a 12v battery that is both cheap, from recycled material and have good capacity just incase if i need to time travel i can take it with me
– asim
Feb 22, 2021 at 23:20
• You can do it like you said but I think you are doing more damage to the 18650 cells than actually reviving them for later use with the methods you described. It is for sure possible to build a functional battery pack out of recycled cells but this needs a little bit more effort than just connecting 3S cells to 12 V through 220 Ohm with a powerbank circuit??? I would recommend that you get more familiar with 18650 batteries and their properties in general. For ex. for proper operation you need to follow a predefined charging profile regarding voltage and current, namely CC-CV. Feb 22, 2021 at 23:42
• Powerbank circuit charges one 18650 or few in parallel but for it to recognize the battery and start proper charging, i am using resistor method to bulk revive 3 batteries at once (not fully charge) then i will use charger to actually charge them, then test them later, question was if reviving 3 batteries in series is a bad idea rather than reviving them one by one
– asim
Feb 23, 2021 at 0:01