No, it actually is trivial, if that's constructed like I think it is. Should have exposed wires you can solder to at every connection between two battery stacks. Actually, I think you can solder straight to the top of that battery contact visible there... What you need to do is find the connection in the center electrically (4 cells above, 4 cells below; should be the only internal connection on the same end as the outputs) and tap it. Now with the batteries all installed as indicated, the outputs will be +4 V and -4 V from there, but if you reverse the 4 cells on the - side, then they'll both be +4 V, and you can connect the "+" and "-" terminals together as +, and the center tap as -. Bingo! you've got a 4.8V/6V pack.

The only problem I can see is if the contacts don't contact right with the cells reversed -- if so, a dab of solder should build the contacts up enough to work.

OK, I can take it from the middle to avoid a step down step, that sounds good. Now, is there any ready made USB circuit board to give me the output? Or, I would have to gut out some of the USB extender battery? Maybe this is something within reach .......... I will see whether I can gut out a USB circuit board from my collections. I think an 8 cell AAs will last longer than a 4 cell AAs, that is why I am interested. Of course, if I can buy one that would be better :-)

bun

There are lots of DIY battery extenders at Instructables. I'm pretty sure several of them have handy diagrams. Basically, if you follow Benson's lead, all you need is a diode to prevent current flowing back from your tablet to the batteries. Or you could go to your local electronics store/Radio Shack and ask the friedly attendant what kind of diode you need (it depends on voltage and power throughput). and how you should solder it up (putting in a diode backwards kinda negates its purpose).

The 8 AA version will not only last longer, it might have actually enough oomph to fully charge a tablet's batteries: If you're using 2500 mAh rechargeables, you're providing a max of 5 amps to the charging plug, which it'll like.

Actually 7805 can supply 1A (with proper heatsink) which should be enough, as the charger that I use for my N810 (not 100% sure it's the one it came with, just some Nokia charger I had laying around) supplies only 890mA.

I'm also going on two long flight next month (there and back again and pondering how to best handle this, my first thought was to hook up a step-down transformer (aka buck-regulator) to my RC helis battery (11.4V 2100mAh) which should provide plenty of juice, OTOH charging that battery on the other would mean taking the special balancing LiPo charger and a 12V power supply with me so maybe it's not such a great idea afterall... (unless I take two battery packs, in which case there is no need to recharge them). There is also what security thinks of the battery packs as to keep weight down they're not in metal casings, just shrink sleeve. They do look a bit suspicious...

Then a quick calculation about using simple AA batteries; rechargeable ones are typically 1.2V and 2100mAh, which means 4 is either not enough or just barely enough voltage when they're full to the brim; a boost-regulator (step-up transformer) would be needed. Alkalines are 1.5V and thus can provide enough voltage to charge for a while even without the booster.

Maybe I need to go and check out what the shops have to offer before starting to build some contraption that will get me "special treatment" at every security check along the way...

ps. LiPo is just as dangerous as Li-Ion when it gets damaged. In fact anything with similar energy density is inherently dangerous.

The alkalines will only have higher voltage than the nimh for about 3 minutes or so...

There's a nice spec at forum.nokia on the different voltage and current windows the devices will accept. I don't remember exactly, but I think it had' 2mm charger plug' in the name. Was easy enough to find last time I needed it

Ended up getting official Nokia "emergency charger" (1 AA battery, this definitely has booster), need to test it to see if it is sufficient for my needs (and if not I'll take it apart to check a few things [like what ICs it uses...])

Edit: Also found the spec (http://sw.nokia.com/id/3378ff2b-4016...on_v1_2_en.pdf) plenty of good info (though I wonder how much smarts does the power supply really need and how much of the actual charging logic is built-in to the devices)

the 1aa emergency charger i bought from dealextreme (#3017295, aka "rainbamboo") did nothing for my n810. i'd be amazed if nokia makes one that works.

True enough it worked for only few minutes, time to take it apart and see if any of the electronics can be salvaged for a frankencharger (cue lightning and mad genius -laughter)...

I bought something similar from Dealextreme and get the same results. It's probably because of the lower tension (1.2v) of a rechargeable battery than the 1.5v of a regular AA battery.
I ended buying this and I'm now really happy.
Charge nicely my 770, even quicker than the original charger or than a regular USB port (claimed to deliver 1000mA, and the real figure must be close to this).
The included 18650 battery is rated as a 2200 mhA, but must be around 1800 mhA, and can nearly top up an empty tablet battery. As it's a 3.7v battery, it's like having a three AA pack.
And having a full size USB port is really handy to recharge all your gadgets. Can also be used with a Y cable to power your USB devices connected to your tablet.

Build quality is OK, and additional 18650s can be bought for a few $.

A must have for your tablet.

Found two interesting ICs:

First is a freescale one where google search with the part number shows up with some pages in chinese and then pages on freescale forum with people complaining that they can't find info on the part (and other people hinting that it's some "secret" OEM chip which would explain why there is no public info).

TEC103 (charge logic), as this device could also charge the AA battery inside (if it was of supported chemistry), the "problem" is that it won't work even the other way with "wrong" voltages (so 4AA was no-go, had to try though). 2AA worked fine and the datasheet also says it's ok (I guess the device itself was made 1AA for size reasons)

With two AA alkalines it was drawing about 700-800mA from the batteries (the connection terminal is small and I didn't want to solder anything to it at that point so the connection was less than perfect thus not exactly accurate figures here, saw over 1.2A for a moment as well but that might have been a misplaced probe).

Now test charging N810 with two NiMH batteries, seems to work fine. Picture: http://www.qaiku.com/home/rambo/show...e6b542f9b2f9b/

Ordered the thing agent770 suggested (thanks for the suggestion). They claim 1A charge current then it's twice the USB spec, and the official Nokia charger I have around is 890mA so 10% increase there as well.