[TriEmbed] Designing a LiFePO4 battery charger

Charles West crwest at ncsu.edu
Fri Mar 20 22:24:35 CDT 2020


Hello all!

I hope the virus hasn't affected you guys too badly.  My little family's
been pretty much staying in our house for the last week and a half (since
our daughter's preschool closed), but we are doing OK overall.

The work on the sidewalk robot continues!  I'm in the middle of testing a
brushless motor controller/MCU combination to drive the four hub motors
that will be moving the Mk3 robot.  If all goes well, it will be built like
a tank and strong enough that I could ride on it if I wanted to.

The part I'm trying to figure out is battery charging/system protection.
The motors expect 36V, so I'm putting 3 4s LiFePO4 batteries in series to
provide it.  What I'm not really sure about is how to integrate a charger.
Each of the batteries (batteries
<https://www.amazon.com/gp/product/B07Q7FY8CC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1>)
is meant to substitute for a 12V lead-acid motorcycle? battery, with its
own built in cell balancer.  I'm hoping to charge them with power from a
24V DC regulator, potentially with a simple 2 terminal charging dock.

The issue I'm running into is that none of the charger ICs I'm looking at
can handle 12 cells in series (and they would probably require 40V or so if
they did).  I'm thinking that I should be able to have a seperate charger
IC for each battery, but I'm not entirely clear on how you would charge
them in parallel while having them connected in series.  I'm sure you can
do it, because my other charger does it for Lithium polymer, but I'm not
sure what the configuration would look like.

If I may ask, do you have any ideas?

Thanks,
Charlie
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