Electronic – Multi-cell li-ion charger

batteriesbattery-chargingcharginglithium ion

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:

  1. Is there any IC capable of charging four li-ion cells in series keeping them balanced?
  2. 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)?

Thank you

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

schematic

simulate this circuit – Schematic created using CircuitLab

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?

Best Answer

Your idea of grounding the step-down regulator to the cell it is charging is quite clever, but it won't work properly because its supply current also flows through all the cells below it. This will cause the lower 3 cells to get varying amounts of uncontrolled charge current that their associated chargers cannot get rid of, which will eventually cause them to become unbalanced and possibly overcharged (which is potentially very dangerous).

To do it properly you need 4 fully isolated power supplies, or use a single 4 cell charger and wire a balancer onto the battery. Cheap 4 cell balancer boards are readily available on eBay and elsewhere. These often also have over-charge and over-discharge protection.

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If you use a protection board then the charger can just be a regulator with adjustable current and voltage, like this.

enter image description here

A charger built with these components may be more efficient, more reliable, and possibly even cheaper than a bunch of step-down regulators and TP4056 chargers.