Duo Li-ion Battery

[sutekh]

I see a few threads where this has been discussed, but not any that seem to have resulted in a fully-functional Li-ion pack. Now that my 280c is re-capped and functional, the next step is to make it portable! It's OE NiMh is long dead, so an upgrade is in order. I want the pack to charge and operate properly in-situ (no janky discharge only, remove to recharge arrangement), have as much capacity as possible within the limited space available, and be safe to operate.

Given those parameters, I first considered Lipo, which I have quite a bit of experience with professionally, but their lack of internal regulation and penchant for expansion make me nervous--even if they'd be the most volumetrically efficient. LiFePO would be my preferred option from a stability standpoint, but the cell voltages don't align very nicely with the original output of the 10-cell NiMH pack. Using that chemistry would require regulation in and out wasting energy / creating unnecessary heat, so it's off the island. That leaves the tried-and-true 18650--specifically 3 x Samsung 3,500mAh cells. That'll provide over 200% more capacity than the original battery, which should provide a very satisfactory runtime in conjunction with the installed SCSI2SD   

Choosing cells is really the easy part though. Aligning the voltages and addressing Lithium batteries' specific sensitivities is the trick. If it weren't for the need to add some additional electronics for regulation and charging, I might have been able to stuff 6 cells into the available space, but c'est la vie... I'm measuring 20v coming from the +/- leads of the notebook when plugged in, which I'm sure drops to a suitable level for the NiMH under the load of charging, but an unregulated circuit certainly won't serve for a Lithium battery. Some form of variable input, regulated 3S Li-ion charger is needed. Also necessary is a BMS (battery management system) to keep the cells balanced and disable the input / output if any cell gets too low, too high, or the current demands exceed reasonable thresholds.

Here's what I have mocked up so far. The little PCB on the left is a 3S Li-ion charger that accepts an input voltage up to 24vdc. The 3S BMS is on the right.



Another challenge will be steering current through the charger when charging, but bypassing it when discharging. I'm planning to address that with a pair of P-channel FETs vs. diodes (much lower forward voltage drop). I'll post updates with success / failures as it starts to come together...

 

[cheesestraws]

I will be watching this project with great interest!

 

[worldofJCK]

At the risk of hijacking your thread (which looks awesome by the way), I am an extreme amateur who has been trying for a long time with no success to get my Duo to run off of a custom battery...

Lacking your technical know-how, I have simply replaced the original cells with 1700 mAh NiMH cells and fit the whole thing back together. The laptop detects the battery (software is 7.5.3 rev 2), but does not seem to want to run off of it, ever. I get a charging icon, but it dies the second I unplug it. Multimeter shows 12 volts (proper voltage) on the contacts and it is making contact with the board.  
 

Can you think of an obvious problem with this setup that would be causing this? Sorry to bother you and I can’t wait to see how yours turns out! 

 

[sutekh]

Interesting... I can't tell for sure from your pic, but did you retain the little TO-92 packaged (looks like a transistor but with 2 legs) ID chip when you rebuilt it? I guess it wouldn't be recognized if you hadn't. My experience with a duo running on battery is thus far non-existent, but maybe a subject matter expert will chime in.

I will say that my initial attempt to run the 280c from my bench power supply were unsuccessful. I didn't try very hard and chalked it up to a bad connection with the ID chip, but maybe there's more to this than I thought. My assumption has been that as long as it sees something in the range of 13.5 - 9.5v on the outer 2 pads, and gets a reading from the thermister and ID chip from the middle 3, it should work.

More testing required...

 

[sutekh]

Made a bit of progress on the battery front yesterday and today. I wired up the Li-Ion cells, BMS, and charger, and was able to get it to charge from the Duo. I still need to do more thermal testing and wire in the FETs (they just arrived today!) before trying to go both directions.

Try as I might though, I just couldn't get the larger diameter 18650s into the original battery enclosure as originally hoped so a custom case was ultimately necessary. I drew up and printed the bottom half pictured below, and it looks like version 1 is going to work (that's somewhat rare!).

The top half is printing now, and I'll report back once the remaining wiring is completed and tested. Fingers crossed...
 

 

[Byrd]

That looks like a magnificent battery case, interested to see how you go with charging and longevity.

 

[TimHD]

Likewise very interested in this project. Hope you manage to get it working.

Are those small boards custom, relatively easy to source or originals (retained)?

 

[sutekh]

Are those small boards custom, relatively easy to source or originals (retained)?

They're both modern and readily / inexpensively available:

• https://www.ebay.com/itm/Li-Ion_Charger/173381402988
  
• https://www.ebay.com/itm/3S-BMS/133439050821
  

Assuming this ends up working, I'll summarize the entire BOM.

 

[Johanncerecke]

This is Genius. Following with interest, would be very interested to do the same with my Duo 280c.

 

[sutekh]

Well, shame on me for making bad assumptions about the sophistication of the charging circuitry in the Duo. It appears to be software controlled and current limited. Unless it detects a valid battery in the voltage range of a depleted NiMH pack, it doesn't engage. Probably also has some other fancy algorithms for bulk / float charging and "learning" pack characteristics. The Duo 280c service manual actually details much of this and provides the function calls to query the charge status had I bothered to look. I'm by no means giving up though! I just have to trick the charger into thinking it's looking at a discharged NiMH...

 

[sutekh]

Some progress this weekend, if measured. I ordered an external charger off the auction site for bench testing and reverse engineering (I really don't want to wreck my Duo), only to discover when it wouldn't charge that there are apparently two versions of said unit. Come to discover that P/N M7778, which is what I ordered, will only charge Type I batteries :/  M1812 is what you apparently need for Type II & III (mine is type II). All of this is outlined in a service bulletin, which I of course didn't find until after running into this problem, found here: https://www.macrepaircentral.us/powerbook-200-series-duo-210-duo-230-250-270c/battery-recharger.html

Thankfully, someone had the updated version listed inexpensively, and so after another untenable delay, my testing will continue. I hate waiting...

 

[blusnowkitty]

Now this is interesting - My Powerbook 150 has a surprisingly good battery for being a 25-year-old NiMh, but I've got several really cool PC laptops with dead NiMh batteries that I'd like to revive at some point.

 

[sutekh]

With my new PB 180c distraction out of the way (because it's 100% functional!), I had a chance to get back to this project. My M1812 charger arrived and I've been able to test a lot of different scenarios. As I surmised, it's current regulated, and must see the following before engaging a charge cycle:

• A valid response from the ID chip
  
• A reading from the thermister  (haven't tested where this cuts off due to heat, but it can't be open)
  
• A voltage from the battery within a range that would require charging. Haven't tested below 9v, but it seems to cut out around 13.8v, which is perfect as the charger I'm using needs an input of at least ~1v above the fully-charged pack voltage of 12.6v.
  

All of those conditions met, it starts feeding ~600ma to the pack.

Point of clarification from above, the FET steering circuit in my schematic is flawed. Using a FET as a cheap "ideal diode" (very little leakage or voltage drop) in 3rd quadrant more works great with a single transistor, but with 2 of them in an "or" arrangement as I depicted, even after the current reverses, the gate of the previously active FET remains energized across their common grounds and current can flow both directions. I should have caught that  For now, I've just simplified the circuit with a pair of 1N5401 diodes and accepted the 0.7v loss, although I'm going to revisit with a pair of actual ideal diodes because a charged pack (12.6v) minus 0.7v is low enough that the Duo thinks the battery is dangerously discharged and pops up warnings.

Anyway, I've buried the lead here. The pack is working! Charging and discharging as expected, both in the duo and external charger. Here are a few pics of the finished article. The little li-ion charge board I used has an SMD LED indicating charge status that I removed and routed to be externally visible through the small hole where the release button was on the original pack. I didn't attempt to re-create the delicate sliding lock rail geometry of the original (that wouldn't have 3D printed well), and instead am locking the pack to the retained external slide lock portion of the original pack via a screw once it's slid into the locked position. It's secure and cosmetically adequate (if not perfect).

I have one more OE Type-II Duo battery, and when I rebuild that one I'm going to use 6x 14300 cells (vs. the 3x 18650 used here), which I believe should fit easily within the original case and provide similar capacity. Before I get to that though, I'm going to build a similar battery for my PB 180c. That should be much easier, as it only has +/- and its 7.5v input is right next door to a 2S Li-Ion's 7.4v  No more cord tethers!!!
 

 

[cheesestraws]

Very nice!

 

[Johanncerecke]

this is genius. Beyond my level of expertise or understanding, but should you find time to create a tutorial, I would certainly have a go at trying to make my own.

 

[sutekh]

this is genius. Beyond my level of expertise or understanding, but should you find time to create a tutorial, I would certainly have a go at trying to make my own.

Thanks! The above posts provide most of the bullet points, but I'll add a bill of materials. I also forgot the two most important photos!

Look Ma, no wires!



Plugged in and charging...

 

[Papichulo]

wow nice. wish i could do my duo 280c/2300c. whats weird is the 1994 battery still holds a charge for 10 seconds!

 

[mdeverhart]

@sutekh awesome work! Can you provide an update when you figure out the ideal diode changes?

 

[sutekh]

Certainly. I purchased a lot of 10 on AliExpress and they're currently in the international shipping black hole. Once they arrive I should have enough for 2ea Duo 280c and PB 180c batteries + a few spares.