@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
<[email protected] <mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>> 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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