[TriEmbed] Designing a LiFePO4 battery charger

Mon Mar 23 10:11:03 CDT 2020

Charlie,

But I do believe your strategy sound to isolate the batteries in series
with three FETs and pulling each battery negative terminal to GND with
another three FETs. I believe you would only need high-side gate drivers
for the three FETs connecting the batteries in series.

You might be able to charge the whole string at 4 * 14.6V and building your
own BMS style switchable load in parallel with each battery to limit
overcharging and let you balance the cells. I think you would only need one
GND referenced FET and three high-side FETs.

Shane

On Mon, Mar 23, 2020 at 10:48 AM Shane Trent <shanedtrent at gmail.com> wrote:

> 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*
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>
> 919-348-0061 | shanedtrent at gmail.com
>
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