Alright, I promised an update...
The ideal diodes arrived and they work nicely! Under full load, I'm only seeing a ~.2v drop, but (because of
course there's a but) I'm still getting low-power alerts from the OS throughout much of the discharge cycle. The OE pack contains 10 NiMH cells in series, which should measure from ~1.5vdc per cell charged to 900mv discharged (15v - 9v overall), and I'd surmised that a Li-Ion replacement outputting 12.6v - 10.5v should fall nicely within that range. I think the problem lies in the fact that NiMH cells spend most of their discharge cycle at ~1.2v, and only drop off rapidly toward .9v at the very end of their useful life. Even with the ideal diodes, under load my Li-Ion pack is measuring in the elevens throughout much of its discharge cycle, which the power manager is interpreting as a mostly discharged battery.
That "but" notwithstanding though, the pack works quite well! E.g., I ran the 280c for a few hours this afternoon with the added draw of an Etherdock and my wifi-module. I'm not really interested in trying to boost the output voltage with more internal electronics since it'd just waste energy, but I wonder if there might be a potential software solution? I suspect the acceptable voltage ranges are flashed onto the ROM or Power Manager (can anyone confirm?), and this passage from the Power Manager Developer Note is particularly interesting:
- Reading the Status of the Battery and the Battery Charger 6: The Power Manager monitors the voltage level of the internal battery and warns the user when the voltage drops below a threshold value stored in parameter RAM. If the voltage continues to drop and falls below another, lower value stored in parameter RAM, the Power Manager puts the computer into the sleep state. The Power Manager provides a function that allows you to read the state of charge of the battery and the status of the battery charger.
Infuriatingly, neither that Developer Note, nor the Duo's, indicate WHAT those values are, or WHERE they originate from, but I'd bet an extension could override them. There is, for instance, an existing "Type III Battery" extension for Duo type III NiMH packs that ostensibly does something similar to make the OS / Power Manager play nicely with those higher-capacity batteries. I pulled that extension into ResEdit and HEX dumped it, but haven't made heads or tails of the instructions yet.
I'd welcome assistance from someone with more experience in that arena than I have. I think all this hypothetical extension would need to do is update the value of "batteryLow" in PRAM to a slightly lower value.
Anyway, that annoying but mostly cosmetic wrinkle aside, here's an updated, accurate circuit diagram and BOM. I've also attached the STLs for any who want to 3D print the enclosure I designed. I printed mine using ABS (Octave's Gray) on my MakerBot 2X with 2 shells at 20% infill.
Bill of Materials - PowerBook Duo 280c Battery
Name
Qty
P/N
MFG
Vendor
Unit Cost
Total Cost
URL / Notes
Apple OEM Duo Battery
1
M1499
Apple (Other?)
eBay
$15.00
$15.00
Needed for ID chip, thermistor, & contact pads. Wild guess on the price
Samsung 3500mAh Li-Ion
3
INR18650-35E
Samsung
Various
$8.04
$24.12
https://ebay.com/p/2155089627
Li-Ion CC/CV Charger
1
DD23CRTA
Eletechsup
AliExpress
$4.45
$4.45
https://aliexpress.com/item/32993044945.html
Li-Ion 3S BMS (10A)
1
HX-3S-01
Generic
Amazon
$2.33
$2.33
https://amazon.com/gp/product/B07T2P5XHS/ref=ppx_yo_dt_b_asin_title_o00_s01
Ideal Diode
2
YH11040A-2
???
AliExpress
$3.90
$7.80
https://aliexpress.com/item/1005001367556002.html
3D Printed Enclosure
1
N/A
Sutekh
N/A
--
$0.00
See attached Top / Bottom STL files
#4 x 3/4in Flat Head Screws
5
N/A
Various
Various
$0.06
$0.30
4 to hold the case together, one to secure it to the sliding lock
Misc 18 AWG wire, solder, etc.
N/A
N/A
Various
Various
$1.00
$1.00
I'm not going to itemize ever little thing, but you get the idea…
Total
$40.00
View attachment Duo 280c Battery Top.stl
View attachment Duo 280c Battery Bottom.stl