[TriEmbed] Designing a LiFePO4 battery charger

Sun Mar 22 14:27:12 CDT 2020

Hi Charlie! Jenny, Emily and I are well and happy.

Your circuit made me chuckle, 'cause when I was thinking of your earlier 
posting I was going to share the scheme I intended to use for charging a 
capacitor with a string of microbial fuel cells by switching then 
between parallel and series connections. But that was at silly low 
currents were analog multiplexer chips would work. But the prototype was 
using relays. If you're interested I could dig up the schematic. 
Definitely the Rube Goldberg approach with relays, though, but your "one 
battery at a time" requirement would make it simpler. :-)

Your schematic implies wanting to just charge one battery at a time, but 
I can't see your circuit working past an initial point. But I think it's 
in the right direction. (Nit: your schematic symbols are for some kind 
of very generic FET transistor and I'm sure you'd be using high current 
ones with body diodes, right? Bigger nit: if there were part numbers we 
could more easily reason about the wiring).

So numbering the transistors from left to right as Q1-5, then with Q2 
and Q3 off but the others on, that's "normal mode", right? With Q1-3 off 
but Q4 and 5 on a lower voltage could charge the third battery. But I 
don't see how you go beyond there with this circuit.

Or am I misunderstanding this? At a minimum you'd have to arrange for 
your single-battery charging voltage to reach the positive sides of the 
first two batteries, right? So maybe have Q6 and Q7 between the right 
side supply and the "positive side" of Q1 and Q3, using the Q3 and Q5 to 
disconnect paths as needed and then perhaps a  Q8 and Q9 to select 
between running the system to conduct the higher "all in series" battery 
voltage to the load and the lower, charger voltage to the one of three 
batteries. That is, a SPDT switch above the rightmost net going upwards 
in your schematic.

Alternatively, figure out how laptop batteries are handled. They seem to 
be always one big series connection, but maybe the extra connection pins 
we see are for this same approach? I have no clue about that.

-Pete

On 3/22/20 1:09 PM, Charles West via TriEmbed wrote:
> Hey Carl!
>
> I'm glad to hear that you are doing well.  The 12v batteries have 
> built in balancers/protection.  It's isolation for charger that I'm 
> trying to figure out.  I think I have a potential solution 
> (https://drive.google.com/file/d/1JxSStAuKn-OMZUCreYQjGUVy5fR2ADpU/view?usp=sharing) 
> with the NMOSFETs between each battery needing a high side driver.  
> The idea is that when the batteries are operating normally, you turn 
> on the between battery mosfets and disable the to ground mosfets, then 
> inverse for charging.
>
> Does that make sense to you guys?
>
> Thanks,
> Charlie
>
> On Sat, Mar 21, 2020 at 4:42 PM Carl Nobile <carl.nobile at gmail.com 
> <mailto:carl.nobile at gmail.com>> wrote:
>
>     Hey Charley,
>
>     We're doing ok, I'm working from home 100% of the time now.
>
>     This may not be the exact answer to your issue but it may help.
>     Banggood has a lot of LiIon battery protection boards. You may be
>     able to use one of these, it would make the actual charger a bit
>     simpler.
>
>     https://www.banggood.com/search/liion-battery-protection.html?from=nav
>
>     ~Carl
>
>
>     On Sat, Mar 21, 2020 at 8:17 AM Charles West via TriEmbed
>     <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
>
>         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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>     -- 
>     -------------------------------------------------------------------------------
>     Carl J. Nobile (Software Engineer)
>     carl.nobile at gmail.com <mailto:carl.nobile at gmail.com>
>     -------------------------------------------------------------------------------
>
>
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