I have a problem with LI-ION technology.
I'm planning to use four 18650 cells in series, and I want to build a circuit for charging them. The final application is a sort of small UPS; I should keep the 18650 batteries charged and, when the main power goes off, switch to the 18650 batteries; when the power comes back the power switches back to the main adapter and the batteries can be charged again.
As far as I know, the four cells should always be balanced, but I haven't found any IC producer (TI, Analog, Maxim, Linear, Microchip) that offers a charger with cell balancing.
So, here are my questions:
- Is there any IC capable of charging four li-ion cells in series keeping them balanced?
- I found some cheap balanced charger on ebay (just search for BC-4S15D). Will this fit my needs (so charge and balance four 18650 batteries and keep them charged until needed)?
EDIT: Another idea came to my mind: could I use "floating" grounds to charge each battery separately? I mean, one single battery assemble is
- one 18650 battery
- one 5V li-ion charger (e.g. one based on a TP4056)
- one step down converter (e.g. this)
Then these assemblies are attached in series, and all the positives (only positives) of the step down converters are attached to a 24V supply.
I mean something like this
Since the max voltage on the "last ground" is 12.6V, the 24V input has enough room to generate the 5V voltage.
Do you think this design can have some problems?