I've been reading about charging protection for lithium coin cells, and
I think we might want to swap out the BAT54C for something with as
little reverse leakage as possible.
I thought a few uA wouldn't really matter, especially if only during
on/active time, and didn't worry about it, but I guess even that causes
the cells to die early and only last a few months instead of years.
So I think you should not use the BAT54C from the original BOM and use
maybe BAV170 instead:
https://www.digikey.com/en/products/detail/diodes-incorporated/BAV170-7-F/822819
or any of these:
https://www.digikey.com/short/q9bfjch5
This was my other pick:
https://www.digikey.com/en/products/detail/central-semiconductor-corp/CMPD6001C-TR-PBFREE/5325257
Lowest reverse leakage but a little higher forward voltage drop and also
more expensive.
Probably neither difference actually matters much at the voltages and
currents involved.
Maybe the BAT54C is actually fine as long as the computer isn't turned
on for long periods in 90F heat.
The reverse leakage graph for the BAT54C at 2v (difference between 5v
from the computer and 3v from the battery) at 25C (77F) is only 0.3uA,
but could go up around 5uA for 3v (say the batts are almost dead at 2v,
so 5-2) at 40C (104F).
So... maybe not really a problem 99% of the time. But still.
I'm ordering some for myself and if you mail me your address I'll mail
you some so you don't have to make a whole digikey order and shipping
just for that.
No change to the PCB other than the silkscreen. The new part drops in
the same place.
Sorry about that.
--
bkw
On 9/20/23 14:19, Ken St. Cyr wrote:
It turned out nice and looks pretty solid (at least in the pictures). I
ordered boards and parts the other day, so I'm just waiting for those to
arrive now. I'll report back and share my experience after getting one
together -
//Ken
------------------------------------------------------------------------
*From:* M100 <m100-boun...@lists.bitchin100.com> on behalf of Brian K.
White <bw.al...@gmail.com>
*Sent:* Tuesday, September 19, 2023 10:18 PM
*To:* m100@lists.bitchin100.com <m100@lists.bitchin100.com>
*Subject:* Re: [M100] rexcpm battery
I have repaired my REXCPM and now the github readme includes pics that
show a good way to mount a qwiic connector and where to connect bodge
wires if you want to take on the challenge of such small loose wires
going to such small places. All 4 solder points are at least a
reasonable chip leg, no soldering directly to delicate traces. So the
wires are reasonably robust in the end. You can install & remove the
rexcpm module without straining the wires. I will probably further
secure them with glue sometime later but haven't yet.
I don't think it's worth it just to get the connector if your original
pins are still intact, even though the connector is more convenient in
the end, but since my original pins rotated and broke their traces
anyway, I had to do a bodge wire repair anyway. And mounting the
connector seems to be solid enough by super-glueing it to the top of a
chip, and by using one of the black versions of the connector which I
think is a material that the glue can stick to easier. The natural
colored ones look like they might be something no glue will stick to.
The black ones feel like they have some glass filler, and can be scuffed
to a rough texture. Seems to be pretty solid so far though I'm not
testing-to-destruction just to find out. :)
single https://www.sparkfun.com/products/14417
<https://www.sparkfun.com/products/14417>
10 pack https://www.adafruit.com/product/4208
<https://www.adafruit.com/product/4208>
My rexcpm is running and working so I can resume testing battery life.
I think it's safe to order pcbs if you want now. The dupont breakout
cable is a good solution for keeping the REXCPM side of things simple
and stock, and adding the single wire for gnd is easy, reasonably safe
against blunders, and leaves the REXCPM still compatible with it's
original bus board and cable. So the bus board can keep the qwiic
connector.
I have tested the 102/200 board now too. good to go. I don't have a
pcbway or gerbers up for that yet but will shortly.
--
bkw
On 9/19/23 04:29, Brian K. White wrote:
My boards came in and at least the model 100 one works fine.
There's room, but no good way to solder the qwiic connector to the
REXCPM, so the most practical way to handle the REXCPM side is just
use the pre-made adapter cable that has female dupont sockets, and
stick them right onto the original pins.
For the GND wire, you can treat it as optional and leave that wire
unconnected, or add a single solid-core wire to the big cap for the
gnd connection. If you use 23 gauge solid core wire, or maybe a size
or so thicker, the wire is stiff enough to hold it's shape and thick
enough for a female dupont socket to stick onto it and not fall off. I
used 23 gauge wire from some solid core ethernet cable and that was
about perfect. Other common sources, thermostat wire, doorbell wire.
Take a 25mm piece of wire, strip 3mm on one end and bend it sharp 90
degrees, strip 6mm on the other end, poke the wire in between the cap
and the 3 pins and solder the short bent end to the cap.
Then the 4 female dupont wires go like this:
black (GND) -> wire on cap
red (/WR) -> closest pin to cap
blue (RAM) -> middle pin
yellow (RAM_RST) -> furthest pin from cap
It turns out you have to be very careful not to strain the original
pins on the REXCPM.. They rotate pretty easily and break the via free
inside the pcb, and then the traces break.
At one point I was stuffing the gnd wire down from the top in between
the pin and the cap, and that put sideways strain on the pin.
My REXCPM doesn't work now unless I hold the pins a certain way to
cause them to make contact.
I have to desolder a few parts to figure out where two of the traces
go so I can find where to run bodge wires. Fixing the original traces
right where they broke at the base of the pins is probably too fragile
now that the vias are moving like that.
Maybe I'll just remove the pins, fix the existing traces where they
broke, and solder flexible wires instead of pins where the pins used
to go, and secure the solder joints with glue or epoxy. Hopefully the
combination of the glue and the flexible wire, and being super careful
from now on, will be enough to keep the traces from breaking again.
So I think the battery board is fine, and the cable connection at
least for model 100 is practical enough (I'd prefer a matching
polarized connector instead of 4 individual wires, but there is just
no good way to add that tiny connector that won't be too delicate and
just break in no time) but my rexcpm is out of commission and I can't
do any further real testing to get real current and voltages etc right
now.
I did have it fully hooked up and running briefly before I twisted the
pins.
I can't test the 102/200 board right now, and the pre-made
qwiic-dupont cable is only 150mm, not long enough to reach all the way
from the board to the rexcpm. You could add short male-female dupont
extensions/jumpers, but that's kind of annoying.
For the 102/200 board, really maybe for both boards, maybe it's better
to just go down to 2 or 3 cells and full size generic 4-pin headers
instead of the qwiic connectors.
It does work fine for the 100 version though since there is a pre-made
cable that is long enough all in one piece.
I've updated the github with bom and pcbway links, and gerber zip for
the model 100 version.
I say it's safe to order the 100 version at least.
The 102/200 version I haven't worked out how best to connect to the
REXCPM yet.
https://github.com/bkw777/REXCPM_UPS <https://github.com/bkw777/REXCPM_UPS>
--
bkw
