Changing power bank cells from Parallel to Series - Still rechargeable and usable at USB level?

Thread Starter

bbrown3112

Joined Dec 31, 1969
0
Hi guys,

I'm a total novice, but I'm working on a couple of little projects, and just wanted to be sure of something!

I've got a cheap little power bank that I've pulled apart. Unsurprisingly, there was 2x 3.7V IRC18650 cells inside, connected in parallel, which I can safely assume is put through a boost converter to get it to the USB 5V standard.

Here's the main thing I wanted to check - I've got something that needs 7V. Flipping one of the cells around, to put them into series would give me a workable 7.4V. So - two questions:

1. Could I reconnect the power banks integrated board to recharge the cells? The power bank charges at 5V, so I know it would be slow, but would there be any issues with charging the now 7.4V from the USB 5V?

2. The integrated board obviously has the boost converter to get it from 3.7V to USB 5V. What's the likelihood of the board being able to regulate the 7.4V down to USB 5V, so I can continue to use the on board USB ports? Or is that out of the question?

Thanks for your time - sorry if these are super bloody obvious!
 

ebp

Joined Feb 8, 2018
2,332
Lithium ion cells must not be charged in series without using a charge-balancing controller that individually monitors each cell to prevent overcharge. Overcharging LiI cells, as you probably know, can be disastrous.

It is possible to make a boost-buck circuit but it is more complex and expensive. Since the existing circuit was intended only to boost it won't be able to buck. The output voltage would be slightly less than the input voltage (by one "diode drop").
 
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