[TriEmbed] Designing a LiFePO4 battery charger
Pete Soper
pete at soper.us
Mon Mar 23 10:12:47 CDT 2020
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.pdfYou 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.ShaneOn 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
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)
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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