Charging individual 3.7V Li-Ion batteries that are connected in series to from 7.4v supply

Thread Starter

Tapish

Joined May 20, 2021
2
Hope you all are staying safe and healthy.

The Pandemic lockdown has forced us to work with the only components we have at home. I am working on a battery-operated esp32 pico board having other sensors hooked up to it that need 5v of supply. So I connected two rechargeable 3.7v Li-Ion batteries in series and attached them to LM1117t for 5v regulated supply. I am planning to charge these batteries with a solar panel interfaced with an mcp73831 charge controller (I have two units with me). I have checked the charger setup and sure enough, it charges a 3.7v Li-Ion battery up to 4.2v. The problem is, how to charge these batteries individually when they are connected in series to form input for LM1117t regulator? :
View attachment BatterySelector.jpg
lm1117t can withstand up to 10v input. In order to extend battery life, the esp32 goes to deep sleep and wakes up every hour for a 30-second operation before going to deep sleep again. I have bits, op-amps and two more 3.7 batteries with me right now, so Any ideas, suggestions and/or modifications would be of great help to me, Thank you.:)
 
Last edited:

KeithWalker

Joined Jul 10, 2017
1,789
I am planning to charge these batteries with a solar panel interfaced with a mcp73831 charge controller (I have two units with me).
Do you mean that you have two solar panels and charge controllers or one solar panel?
If you have two panels that are not interconnected, you can connect a panel and charger to each cell while they are connected in series.
If you only have one panel, it will be more difficult to find a solution.
 

Thread Starter

Tapish

Joined May 20, 2021
2
Do you mean that you have two solar panels and charge controllers or one solar panel?
If you have two panels that are not interconnected, you can connect a panel and charger to each cell while they are connected in series.
If you only have one panel, it will be more difficult to find a solution.
Thank you for replying, I do have 2 solar panels, and individually charging the cells was the idea, but one of the objectives of the project is for it to have a small form factor, so I am exploring this solution.
BatterySelector.jpeg
 

Irving

Joined Jan 30, 2016
1,573
Why do you want to charge them individually, rather than as a pair?

An alternative approach would be to put the cells in parallel and use a boost converter to go from 3.7 to 5v. This simplifies the charging greatly.
 

dl324

Joined Mar 30, 2015
12,873
Welcome to AAC!

Why not use one battery and use a step up regulator?
clipimage.jpg
clipimage.jpg
You can change the output voltage by changing the ratio of R1 and R2.

MC34063A switching regulators can be salvaged from many inexpensive battery operated devices.
 

KeithWalker

Joined Jul 10, 2017
1,789
The example in the mcp73831 charge controller data sheet is a little vague on the supply input but VSS is a common terminal for both input supply negative and battery negative so your proposed scheme will not work for two chargers connected in series.
It will work if you separate the two panels and connect VSS on one charger to VBAT on the other.

Panels.jpg
 

Irving

Joined Jan 30, 2016
1,573
Mcp73831 Max Vin is 6v so no joy in stacking them on one panel, two panels and stacked as @KeithWalker has shown is about the only solution.

Having said that, these charger chips are for USB charging, they won't be great from a solar cell as they can't track the panels MPPT point. Charging is going to be a bit hit n miss with them. What's the spec on the panels?
 
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