[TriEmbed] Designing a LiFePO4 battery charger
Pete Soper
pete at soper.us
Sun Mar 22 20:57:50 CDT 2020
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 <mailto: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 <mailto: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 <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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