3 -18650 in parallel for USB battery bank

Hello all.

I wish to use 3 - 18650 in parallel in order to create a portable rechargeable bank for charging my iPhone on the go via USB. I know about general BMS however unsure about the best board for the job of charging and discharging. Perhaps a single 18650 would do the job?
If so it would simplify things as there is a board on eBay that I known off that can achieve this purpose and the example show only one cell.

If you wish to use 3 - 18650, I think a single 18650, by definition, fails to do that job.

You can most certainly use 3 separate single-18650 power banks.

It is one approach to have three seperate single chargers like the ones being sold. This would mean that I would have to manually remove the 3 cels from the charger and put them in parallel in the power bank with a step up board for 5v USB voltage.

Or I think I could use a 3S BMS in circuit and use a step down power converter to take it back down from 12v to 5 volts.

Whatever you do, I believe the recommended limit for paralleling lithiums is right at 3 cells. Series as much as you can. Or look at some inexpensive banks that use prismatic cells (pouches) that are higher capacity.

Agreed. When using/charging multiple 18650 in a project it is best to use them in series with protection/balanced chargers and use boost/buck converters accordingly based on applications. Thanks

Having an iPhone, and trying to make a powerbank that you can buy for <$20 somehow doesn’t go together.

Having an iPhone, and trying to make a powerbank that you can buy for <$20 somehow doesn’t go together.

Well, he wasn’t going to make an iPhone

Buy a battery bank from a reputable brand. Li-ion batteries are explosives, you don't want them exploding in your pocket. If you're not careful, they can - and will - set your house or your pants on fire.


The main problem with charging multiple Li cells is that, when they are configured in series, unless employing some form of voltage control over each cell, there is no way of preventing one or more of the cells being charged to too high a voltage. By using a BMS, each cell is monitored by an active controller, and once optimum voltage is reached, the BMS will shunt current around any fully charged cell.

Conversely, when cells are mounted in parallel all cells sit at an identical voltage. If it's voltage which is the problem with Li charging then, 3 li in parallel are all exposed to the identical voltage and hence may be considered as a single cell of triple the capacity.

Provided all cells are at an identical voltage before they are connected in parallel, no intercell current will flow and hence may be safely connected together.

From then on, a single cell charger, with cell state-of-charge control, should safely charge the bank of three cells.

I await to be corrected !

Parallel looks swell- on paper.

BMS also looks swell on paper - but some say it stands for "Battery Murdering System"

First time I've heard of it being called that. Better warn them Tesla folks!