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Combine (relay?) multiple cellphone batteries for one large powerbank Answered

I rarely trade my previous primary mobile devices when upgrading; as a result, my cache of obsolete phones with perfectly useable batteries is a dozen, give or take. I'm sure not many, if any, tinkerers, modders, hobbyists, etc., would recommend such a project, as these batteries can be unforgivably volatile if used any way other than their manufacturer's suggest, and sometimes even then. So let's assume we've all been warned; no need to reply "don't...". that being said, has anyone here ever attempted anything like this, or know, or heard of someone who has? if so, links would be much appreciated.

fyi, i ask because i'm considering trying to mod an old rc car into a wifi/app controlled car, and repurposing as much of my neglected junk as i can to do this, would be soo satisfying.



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1 year ago

For a bank you want batteries with even capacities.
Otherwise you get a troubles with the charging and discharging.
course you could get rid of the attached electronics on the cell and
implement you own circuits to safely charge and discharge them as a


Reply 1 year ago

Thats where the individual charging- and protection PCBs shine: They switch off any battery in the pack which is too low on voltage.
But as you have them in parallel, the voltage on all batteries will be the same (except the minute differences in the protection-PCB-losses) thus all will dicharge with the same Voltage-drop per time and will finish at the same voltage almost all the same time. No matter what capacity they have...
Basically, the load gets spread out automatically: If a bat has more capacity, the voltage dowsnt want to drop so fast as with a bat with low cap. True. But as the low cap bat is discharging, the high cap bat has a few microvolts more voltage than the rests and will have to deliver the brunt of current until the voltage is equalised once again... This happens in microvolts and below... Ohms law at work.
Trust me, thats used in one of our current projects and products.


1 year ago

Generally, there are 2 types of such batteries: Foilpacks and Hardpacks.
Foil: https://www.welectron.com/media/image/product/1079...
Hard: https://images-na.ssl-images-amazon.com/images/I/4...
If you arent making a good enclosure, dont bother with the foilpacks. They puncture easy and pose a bigger risk in terms of accidents by fire.
From here on forward, if i name Batteries, i mean the hardpacks.

Now, with your batteries, first thing i would check is their capacity remaining. I used in the past a very cheap mAh-Meter from Ali: https://www.aliexpress.com/item/32960593121.html
Be sure you discharge (for the test) with a reasonable current, close to what you will use in the end.
Of course, fist they need to be charged before we can measure anything.
Use one of the cheap LiIon-Charging-Boards with protection i linked
further down...
Now, see if your capacity of each battery is still usable. Discard all dead or low-cap batteries.

Those Batteries normally (I never have seen one without) have protection-circuitry built in already. This means: Switch off at undervoltage, normally switch off for overvoltage, normally switch off at short. That said, this is no guarantee and i would double up with the protection (More on that later on).

As you want those batteries all in parallel (For one BIG 3.7V battery) each cell needs its own charge- and protection-circuit.
Now luckily, those things come super cheap and plentifull in either without protection (https://www.aliexpress.com/item/33036481329.html) or with protection (Over & undervoltage, short) (https://www.aliexpress.com/item/32984862014.html)
As mentioned before, i higly recommend going with the protected ones as they arent really expensive either (About 3$ for 10 pcs delivered).

Now, you need to wire them... Normally, there are the following connectors on the Terminals of the battery:
You need to find the GND and BAT. Measure with a voltmeter for 3.2-4V of voltage. Mark them and also mark the polarity (+ and -)

Now you need to connect them...
I made a short gfx to show how it will need to be done: See Attachment.

Now, you have one big LiIon-Battery with the combined capacity of all your old batteries.

If you ant to have a 5V USB-Out as common in Powerbanks, you simply need a LiIon to 5V Booster like https://www.aliexpress.com/item/32806931939.html where you connect the 3.7V of your Pack to the VIN+ and the GND from your pack to VIN-.

Bam. Simple as that. Now do yourself a favor and design a nice 3D-printed enclosure for the thing and have it really tidy ;)