J820 on AAA batteries?

I got an 820 off of eBay recently, the seller claimed the battery was still in its wrapper and had never been used -- but it didn't work, and on opening it up, I found two of the four cells were ruptured.

It got me thinking, though: what if I put in a 6-AAA battery holder and wired it up, could I run the machine on AAAs? I guess 6x1.5=9 is a little high if I used alkalines, but I seem to remember that NiCADs run at 1.2V (and maintain this voltage for more of the discharge cycle).

dont touch nicd's, they are poor. nimh is the same 1.2V and somewhat better but it is far behind li-ion. 6 nimh's will give a poorer performance than those 4 liions did (iirc that was 3.2Ah, right?). what i'd do is get new li-ion cells, 2600 mAh, they'd make a 5.2 Ah@7.2V battery for your jornada 820

Edited by cmonex 2007-03-29 5:22 AM

3.2Ah sounds right. I googled the Sony part number for the Li-ion cells that were in the battery pack and came up with a circa-1991 Sony catalog. Where does one find new Li-ion cells? I'm also not sure the charging circuitry is OK, so I was thinking the AAA's could be easily swapped out for a new set while charging the first set in an external AAA charger.

i usually order the cells from http://batteryspace.com

Thanks for the link. Have you re-celled a J820? It has 4 cells, and the wiring is a little strange: there's a common connection for all four but then two other connections each connecting to two cells. It looks to me that it might be set up to charge two pairs of two in parallel but then supply power in a series configuration (7.2V).

Two cells were ruptured but the other pair looks OK; I'm tempted to split the pair (which are in parallel) and put one of them in to replace the ruptured pair; it seems I'd recover the ability to run them in series. On the other hand, I'm now more suspicious about the charging circuits for the ruptured pair. If all four had been ruptured, it could have been heat (the J820 was recovered from a storage facility that might have gotten hot).

gjcoram: no i never tried with a 820.. are they 18650? i assume so but just to make sure..

If you do buy new cells, do NOT replace just the two visibly bad ones. Replace them all. Li-Ion batteries degrade very fast, even if they're sitting unopened in a box. If you mix two new cells with two old cells, you'll end up with a unit that eather only charges to the (Massivly reduced) capacity of the two old cells, or kills them by trying to charge to the capacity of the two new cells.

No, they're not 18650. There are four Sony US103463. It's a box-shaped battery. I Googled the part number, came up with a Sony catalog from 1991, a link for J820 linux, and a bunch of Japanese links. Periodically, I see eBay listings for J820/J928 batteries, but the picture is nothing like my battery; I think there's a glitch in someone's (Amazon's?) database that says they use the same battery.

Edited by gjcoram 2007-04-19 12:07 PM

Jax -
Thanks for the reminder. I can't find replacements for the original cells, though, so I'd have to replace everything.

I keep reading about LiIons degrading fast, but I still have the original batteries in my first J820 that I got back in 2000. I haven't tested the run-time recently, but they go for more than a week of intermittent use, checking e-mail.

Which reminds me of another hack I'd been thinking of: can I power it off the phone line? I have a vague recollection that old answering machines used to run off the phone line, it's got a decent voltage on it, would Ma Bell really notice a few missing mA?

Li-Ions degrade really quickly if they're hot or kept at full charge. A battery charged to 100% and stored inside of a very hot laptop can be effectivly dead in less than 2 years, even if it's never used once. Higher temps can shorten their life to about 9 months, again without actually being used. The optimal storage conditions for Li-Ion is at 40% charge and around freezing. That's water freezing, not li-ion goo freezing.

A phone line has a surprisingly high voltage, which goes even higher while ringing, but it's a very low current. Furthermore, it would be rather difficult to make use of what little power it has.
To start with you would need to regulate/step down down the ~48V DC to what the unit wants. And you would have to do this without making the phone company think you've lifted the reciever.
After that you would also need to make the converter operate on the ~88v AC that is applied to the line to make the phone ring, and the converter would have to switch modes without interuption. Otherwise it would go up in smoke the first time someone called you.
Once you've done all this, you might be left without enough current to actually run the device. That unit probably draws close to a half an amp. Maybe as little as a quarter of an amp if you dim the screen and stuff. That's still quite a bit to try to get out of the phone system. And it can't draw so much that it prevents the phone from working.
In the end it would be easier to just find an outlet.

no, i dont think 0 celsius (water freezing) is good for storing the li-ions. more like 15 C.

I found some docs around the web, one gives a schematic for drawing up to 150mA (@ 5V) while the phone is off-hook. Another says the FCC rules say you have to provide a 5MOhm impedance if you're not communicating; it provides a nice schematic for using the 10uA (48V/5MOhm) trickle charge to charge a 1.5F (yes, Farad, not uF) capacitor and provide a 5V output -- but only 10mA peak output current. The schematics all handle the voltage conversion and have bridge rectifiers to handle the ac. But the current draw of the J820 is probably more than 150mA. Ah, well.

0 degrees C is perfectly fine for Li-Ions. A Li-Ion should never be frozen, but what most people don't know is that the Li-Ion goo doesn't freeze until around -40 degrees. 0 degrees is not freezing it.
At 40% charge and in the area of 0 degrees, a Li-Ion cell will only degrade by 2% of it's capacity per year.
For comparison, a fully charged battery inside of a hot laptop will drop by roughly 30% to 50% per year. That's just from existing. If the laptop is regularly running off of this battery, especially if it's draining it all the way, it could be effectivly dead in a year.

Jax: ok, battery experts recommended 15 C, and not 0

you're totally right about laptop batteries, mine has just been killed - the joy of long term lending your laptop to your family. i did tell them NOT to do that to the battery, i realized too late that they did not listen.

gjcoram: don't dream on, 150mA and 5V is no good for the jornada, it wants 7.2V and who knows how much mA (i'd guess 300+ )

Edited by cmonex 2007-04-19 4:20 PM

I am willing to bet one could easily get a AA/AAA battery pack to work inside the Jornada- Easiest done making a 12v pack (20cells, 5 laterally 4 long) with a VREG and attaching it via the 12v adaptor plug :-D I am willing to be one could even mill it (CNC) and make it look real nice.