[TriEmbed] Designing a LiFePO4 battery charger

Sun Mar 22 14:57:16 CDT 2020

Charlie,

It doesn't surprise me that Pete beat me to the punch on mentioning relays
after you said "one battery at a time". I expect Pete and I share similar
ideas. My though was connect each battery to the COM terminals of a DPDT
relay and use the Normally Closed (NC) contacts to wire the batteries in
series. When AC power is available for charging, you can use the relay's to
pull one battery at a time out of the chain, check its voltage and charge
if needed.

You would have the clack of mechanical relays but adding just two MOSFETs
should let you get the full expected mechanical life-cycle from the relays.
Include a strong N-type MOSFET at the bottom of your battery chain and in
the ground lead of your charger (keeping the charger isolated from the
circuit until that FET is enabled. These FETs allow you to ensure there is
no current flowing when you open or close the contacts of the relays.

I expect the life-span of the relays will be more than sufficient to
outlive the motors/gear train on the project. And using relays would make
the functioning of the charging circuit easier to follow. Additionally, you
can buy a off-the-shelf relay board for prototyping (I have used NCD boards
on multiple projects). Let me know if you have any questions.

https://store.ncd.io/product/4-channel-dpdt-signal-relay-controller-4-gpio-with-i2c-interface/

Good luck with your project,
Shane

On Sun, Mar 22, 2020 at 3:27 PM Pete Soper via TriEmbed <
triembed at triembed.org> wrote:

> 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> 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> 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
>>> _______________________________________________
>>> Triangle, NC Embedded Computing mailing list
>>>
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>>
>> --
>>
>> -------------------------------------------------------------------------------
>> Carl J. Nobile (Software Engineer)
>> carl.nobile at gmail.com
>>
>> -------------------------------------------------------------------------------
>>
>
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*Shane D Trent*
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