Buck boost converter for lipo 6s charge info request

Question

I need to dimension a Buck boost converter to charge two LiPo battery packs 6s 20ah starting from a variable input voltage. Both packs have their own PCB BMS board. This one: http://bestechpower.com/222v6spcmbmspcbforli-ionli-polymerbatterypack/PCB-D165.html

So I just need to charge them at 25.2 volts—being able to adjust the charging current automatically based on the input voltage. And I would like to charge them at 10 amps maximum.

I have a voltage input that may vary between 12 and 25 volts. Usually is fixed to 25 volts.

What I know is that Lipo batteries need constant current and constant voltage to be charged safely but I didn't find many Information about charging them or designing a charger for bigger packs than 2s.

My questions are:

1. Do I need to limit progressively the current during the charge? If yes, how do I do that?

2. Do I need a microcontroller to cut the power automatically when the battery voltage reach 25.2 volts?

3. Are there any control IC's that simplify the design of a 6s Lipo charger?

Answer 1

1. Do I need to limit the current progressively as the charge progresses? Yes. My understanding is that the battery will 'accept' lower and lower current as it nears the end of charge. You will need to be able to recognize that and then choose a value at which the battery is full. For example when charging current becomes 10% of original charging current then battery is full. For a faster charge, some people might choose a larger percentage as a cutoff.

2. Do I cutoff charging when voltage reaches 25.2 volts? As you can tell from the answer to number 1, no. To charge the lithium chemistry you start with a constant CURRENT until the battery reaches final voltage (25.2) then you charge at constant VOLTAGE allowing the current to slow down as the battery fills up, terminating charge as described in question 1.

3. Any IC to build charger around? Yes, lots. You do need to know how to build a charger around the IC but look here for an example. http://www.ti.com/product/bq24616. I am sending this via my phone so please excuse me if this link doesn't work. Just go to the TI.com website and search under power management and you will find around 200 to choose from.

Finally, the BMS that you are going to use could technically control the termination of the charging of the battery and it will in fact provide the all important balance charging that your battery will require, but most people recommend using it only as a backup to a proper battery charger and not to rely on the BMS as a battery charger.

Good luck.