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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?

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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.

enter image description here

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.

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    \$\begingroup\$ Thank you.. So I just have to add a balance circuit (not just a protection one, right?) then use any li-ion charger supporting 4s batteries or a voltage-current power supply forgetting about balancing and protection, right? \$\endgroup\$ – frarugi87 Nov 21 '16 at 21:15
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    \$\begingroup\$ Yes. Balancing is required to ensure that all cells have the same voltage so the charging voltage divides equally between them, and the protection circuit guards against incorrect charger settings or running the battery too low. The charger then just has to limit current and voltage (open-circuit charger voltage must be set to 16.8V or lower to avoid tripping the over-voltage protection circuit). \$\endgroup\$ – Bruce Abbott Nov 21 '16 at 23:18
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    \$\begingroup\$ Sorry to bother you again. I have a couple more questions. What voltage and current should I set? If the balance voltage is 4.25V/cell, then for a 4S I should set 17V, right? Then the current can be 1A for instance (no need for a fast charge) but, then, the balancing current is 100mA, so should I lower the current when the balancing phase starts? And... What happens at the end of the charge? Should I manually disconnect the power supply or I can leave it attached? Thank you \$\endgroup\$ – frarugi87 Dec 6 '16 at 9:58
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    \$\begingroup\$ Nominal full charge voltage is 16.8V. You could go to 17V if needed for precise balance, but charging current won't drop to zero because at that voltage the balancer will be continuously bypassing every cell (assuming the balance circuit is accurate!). Current automatically reduces when getting close to full charge due to resistances in the circuit, so the balancer should be able to correct small imbalances. If imbalance is too large then it won't be able to hold all the cell voltages down and the protection circuit will cut in, disconnecting the charger. \$\endgroup\$ – Bruce Abbott Dec 6 '16 at 15:44
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    \$\begingroup\$ You should periodically check the cell voltages with a voltmeter, and charge at a low rate if there is significant imbalance (greater than +-0.05V at full charge). If you want the charge current to automatically taper off earlier then add a small amount of resistance (eg. 0.2 Ohm) in series. \$\endgroup\$ – Bruce Abbott Dec 6 '16 at 15:48
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Would it make sense to do the same setup as Bruce Abbot proposed (first regulator and then balancer/protection board), but to use a charging circuit like BC3770 instead of using a regulator?

The BC3770 would (as far as I understood) have sufficient power to charge all the 4 cells and it would be a lot "smarter" than just a regulator (trickle charge for deeply empty batteries, top up charge etc.)

Here is a link for the MC32BC3770 Datasheet

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    \$\begingroup\$ Welcome to Electrical Engineering Stack Exchange. Please take the tour at electronics.stackexchange.com/Tour to get the most out of this site. \$\endgroup\$ – SDsolar Aug 25 '17 at 20:27

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