[TriEmbed] Designing a LiFePO4 battery charger

Sun Mar 22 19:20:53 CDT 2020

@Pete
I'm glad you guys are OK.  The crux of the issue is that I haven't seen any
LiFEPO4 charger ICs that handle more than 7 cells in series.  Each of my
three batteries have 4 cells in them, so I think I need to have one charger
for each battery which charges the 4 cells of the battery in series.

I only get 1-2 hrs of working time a day right now due to my daughter's
pre-school shutting down, so it will be a little while before I get a
functional schematic out.  However, I was tentatively thinking of using the
following components:
NMosfets (used for all):
https://www.mouser.com/ProductDetail/ON-Semiconductor/NVMFS5C612NLAFT1G?qs=%2Fha2pyFadugLwIkhTqkZTKk6f9YgRBDTRk6R4I0d7T5BfE4p4JIMyYRpkPiujU25
High side NMosfet driver (for NMosfets which are not connected to ground):
https://www.mouser.com/ProductDetail/Analog-Devices-Linear-Technology/LTC7003EMSEPBF?qs=sGAEpiMZZMve4%2FbfQkoj%252BKKapMsNT2INsri6aFIMPoQ%3D
Single battery charger (would use 3 of these with a 24V supply):
https://www.mouser.com/ProductDetail/Texas-Instruments/BQ24630RGER?qs=sGAEpiMZZMsZtvfwwjgKgY9Zc%252BP5Y9S3

Same schematic with auto-annotation:
https://drive.google.com/file/d/19qjaiqOR5wqI34htY-u9LFqNepMjd0cY/view?usp=sharing

Apologies if I wasn't clear.  I've annotated the schematic.  WIth the
updated one M1, M3, M5 would be on and M2. M4 off during "normal"
operation, resulting in ~36V from the 3 ~12v cells.  During charging, M1,
M3, M5 would be off and M2, M4 would be on.  The idea is that M1, M3, M5
control charge moving from the high side of a battery to the low side of
the next one, so disabling them isolate the cells from each other.  The
charger ICs seem to expect the low end of the batteries to be connected to
ground, so M2, M4 being on makes that happen.

Given that during series ("normal") operation, M1, M3, M5 have an
approximately 0 voltage difference between source and drain (and M5
requires a gate voltage relative to ground higher than the 24V the charger
is getting), I think a gate driver with a charge pump is needed to drive
the those 3 mosfets.  Does all of that make sense?

Relays would be great in terms of functionality but I'm a little worried
about reliability.  The hub motors I'm using are brushless & direct drive
with a lower mechanical load than they were designed for, so they might
last a fairly long time.  It would certainly simplify the design though.

Thanks,
Charlie

On Sun, Mar 22, 2020 at 3:57 PM Shane Trent via TriEmbed <
triembed at triembed.org> wrote:

> 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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>
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