[TriEmbed] Designing a LiFePO4 battery charger

Mon Mar 23 20:46:31 CDT 2020

@Shane Trent <shanedtrent at gmail.com>

I was thinking of using 3 floating battery chargers (it's just 3
batteries), but I'm finding it somewhat complicated by the fact that the
batteries are wired in series.  I'm concerned that the 24V power supply
will not be able to charge the last two cells because of their high
voltage.  It only works if the chargers use the battery ground but the
supply voltage is somehow raised to X higher than the output of the
previous cell.  That's why I'm hoping to more or less break the serial
connection before trying to charge them.  I'm a little leery of trying to
charge multiple cells in parallel, even if they had charge balancing.
Everything I've read seems to indicate that you should be very careful to
pay individual attention to each cell (I'm somewhat nervous about the 4
cell series charging, but it seems to be what they are designed for).

@Pete
Yes.  Each battery has its own charging circuit that connects to each of
its terminals.  The FETs are meant to basically switch it from a serial
battery setup to one where each battery is connected to ground on the -
side and the charger on the positive side.

I would love to have my robots last that long.  It seems pretty unlikely
though.  The batteries would certainly give out before then, even given the
more durable chemistry.  I'm mostly trying to limit what can go wrong to
try to make it as reliable as is practical.

@All
I'm wondering if it might be better to add a dedicated microcontroller to
the design and remove the specialized charger circuits.  A few tutorials
I've read (such as this one: https://www.microfarad.de/li-charger/) seem to
indicate that you can do the normal constant current/constant voltage
algorithms charging batteries just with relatively unfiltered FET PWM
(perhaps just a cap on the output).  If so, it would allow me to
drastically reduce the cost of the charger/#number of different components
and allow programmable change of the charging characteristics.  Does that
sound practical to you guys or do you think that some sort of LC low pass
filtering would be needed?

Thanks,
Charlie

On Mon, Mar 23, 2020 at 11:12 AM Pete Soper <pete at soper.us> wrote:

> Also, if you're thinking of lifetimes past ten years, at least look into
> flash memory retention specs. If I read them right, to prevent the charge
> going away will require rewriting much like DRAM, but with N year vs X
> millisecind timing.
> Pete
>
> -------- Original message --------
> From: Shane Trent <shanedtrent at gmail.com>
> Date: 3/23/20 10:49 AM (GMT-05:00)
> To: Pete Soper <pete at soper.us>
> Cc: Charles West <crwest at ncsu.edu>, TriEmbed Discussion <
> triembed at triembed.org>
> Subject: Re: [TriEmbed] Designing a LiFePO4 battery charger
>
> Charlie,
>
> What about using four, floating 14.6V chargers that are powered when AC is
> available? You could leave the batteries in series and eliminate the extra
> isolation FETs (or relays). And it would charge 4x faster than charging
> them serially.
>
> About relays, If you use FETs with relays to eliminate current for open
> and close events, the relays should last almost forever. The datasheet
> below the DPDT relays used for NCD's 5A model (actually rated for 8A).
> Looking at the "Electrical Endurance" chart, if you limit the open/close
> current to less than 1 amp, you could expect 10^6 operations. That would
> allow cycling the relays every 10 minutes for 19 years.
> https://media.ncd.io/20181002093211/RT2_1014-735742.pdf
>
> You could also use a FET H-bridge for each battery to swap the batteries
> out one at a time for charging. But as you pointed out, gate drivers would
> be needed for 15 of the 16 FETs. I am starting to like the idea of the
> faster charging of four floating chargers.
>
> Shane
>
>
>
>
>
> On Sun, Mar 22, 2020 at 9:57 PM Pete Soper <pete at soper.us> wrote:
>
>> So some other circuitry connects the individual batteries to the
>> charger(s) some how?
>>
>> These relays are cycled what, a few times a day at most? I think
>> depending on part selection they could last a very very long time if
>> genuine parts from a vendor with a reputation to protect. Look for
>> automotive ratings.
>>
>> I shouldn't have used the term "Rube Goldberg". That was with my
>> application in mind, not yours. As Shane pointed out it could be straight
>> forward.
>>
>> -Pete
>>
>> This <http://triembed.org/images/relays-schematic.pdf> is a Rube
>> Goldberg circuit. :-)
>> On 3/22/20 8:20 PM, Charles West wrote:
>>
>> @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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>>>
>>> --
>>> [image: photo]
>>> *Shane D Trent*
>>> Patent Agent
>>>
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>>>
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>
> --
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> *Shane D Trent*
> Patent Agent
>
> 919-348-0061 | shanedtrent at gmail.com
>
> ShaneTrent.com | Skype: skype:shane.trent1
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