The following is a thread for people who have some time to kill: http://talk.maemo.org/showthread.php?t=14139

The following is one interesting comment from the same thread: http://talk.maemo.org/showpost.php?p...&postcount=165

I now started building my selfmade charging device. The electric-circuit is finished (and works) but there are problems with the case... (This f*****g german electronic online shop has wrong descriptions of its parts and now the battery pack is some mm too big for the case and it has to be external.)

here a pic:



The good thing: with the snap-fastener I can connect every size of battery-holders.

I even could test it: 3:05 hours runtime with a completely empty internal battery, 4 external AA batteries, display and gps running. I noticed, that the internal battery doesn`t get charged - its power level is at about 0% all the time (read out with kcbatt).


Thats really bad efficiency. I`ll have to try the direct version. Just ordered two verry cheap BP-4Ls from Hongkong (just for the connectors to build a dummy for the internal battery leading out the cables to power it external).

In theory (I know there can be other loss or the device shuts down before the batteries are completely empty if the voltage drops to low) 3 AA batteries with about 2800 mAh should be able to run the device 1,8 times as long as the internal battery (1500 mAh). With the internal battery its runtime with display and GPS on is about 4 hours. So it should be 7,4 hours (connected directly without charging-loss)... I`ll give it a try.

I think the following are the formulas one can use to compare the energy a battery can hold: 4 external AA batteries of 2100 mAh can produce 4*1.22V*2100mAh/1000 = 10.3watt-hours while a 1500mAh Lithium battery can produce 3.7V*1500mAh/1000 = 5.5watt-hours. On the other hand the voltage of the alcaline batteries drops a lot so much of the energy can not be utulised.

Notice that these connectors are often very bad - one have to use additional support for the battery to make the electrical connection stable. And I have had a cheap battery that I was unable to use even with support.

Basic linear regulators are really bad for efficiency (since they basically feed all the extra voltage to ground...), you need step-down/step-up regulator.

For my experiments on the subject see this thread.

And a picture of one of the results: the frankencharger

edit: also this from DX might be a interesting to you

Ya know, you bring up an interesting concept. I've actually been working on a design for a bike powered battery charger. It's right now only for use with my bike, as it'd charge a bank of super capacitors that would then be used to power lights and other necessities. But in time it'll be modified so that I can prop the bike up on an improvised rear kickstand and use that to power or charge other items. And the capacitors *should* have enough power to give me 30 minutes of power for most devices with only 3-5 minutes of hard peddling. At least in theory. Reality may end up being a different story, but we'll see. ^_^

Mhm... Let`s hope I get them working. I think direct powering the device with a battery adapter is the only efficient, cheap and realizeable possibility for my needs. Every device that can be built or bought that gets connected to the charging-input will waste over 50% of the energy.

Does someone know, at which voltage of the internal battery the N810 shuts down?
And what do you think is the maximum voltage that the internal battery can do before the N810 gets demaged? A full charged BP-4L hat 4,1V... Max should be at least 4,2V - the end-of-charging voltage for Li-ion - or not?

Sorry for coming late here, but a few comments:

- the N8x0 (like any modern gadgets) have their own built-in regulator, so it would be ideal to avoid needing a second external regulator.

- I read the Nokia 2mm spec, as linked above by Fontus, allows up to 9.3V, so a 2-cell Li-Ion battery should work fine. However it appears that Nokia do not follow their own standards. Connecting one (7-8V) results in a "not charging" message on my N800 and on my Nokia phone
Note that this does not damage the tablet - it simply refuses to charge.

- But 5-6V in does work on my N800 and phone. A 4xAA NiMH pack can be directly connected to the 2mm Nokia plug.

- to enable hot-swapping internal batteries, you could add a capacitor inside the NIT across the battery terminals. e.g. 0.1F 5V. A software hack to make the screen stay off when battery cover is open would help.
Might need a small resistance in series (if lacking Ri) to limit current when the new battery is inserted.

- if you do want a cheap, light 5V regulator, see http://www.dealextreme.com/details.dx/sku.29208
(has micro-USB plug, so needs cut/solder the cable)

- you could add one or two 18650 cells in parallel with the cell of the standard battery, and attached to the back cover. I don't know how well the battery electronics would cope with re-charging, but it would be slow at best. You might need an external charger.

riderman, where do you get the <50% efficiency from?
I'd have thought 4xNiMH, or 2xLiIon+switchmode reg would do much better. Are you assuming the N8x0 uses a linear regulator?

That guy using a 7805 to convert 12V->5V made me shudder.

> Does someone know, at which voltage of the internal battery the N810 shuts down?

The battery has a safety cutoff (~3V?) but the tablet will shut down before that happens. You could just run the device flat, then measure the voltage. Why do you ask?

Do note that the spec has current limits that are pretty low at 7-8V. If you connect 2 Li-Ion batteries in series, they're able to provide several Amps of current, which exceeds the maximum specified in the charger spec. This is likely why you get "Not charging" message.

Battery low warning comes at 3.5V on internal battery, there are atleast 2 layers of sofware shutdown, with the first one happening shortly after battery low, and the second (backup?) shuts down the device at 3.2V..

today the china batteries arrived. Okay, they won`t win a price for their connectors... But it works.

I just opened one of them, removed the inner li-ion cell form the electronics, connected the cable to extern batteries with it and putted some plastic in for replacing the cell.

For security there is a schottky diode in the cable - this changes the voltage by -0,3V and secures it against inverted voltage.

Tomorrow I`ll test how long this will run the gps and the display.



Yes I know... I still need another battery holder


Because I did noth think about it myself before writing

riderman,
are you planning to use NiMH cells? You will not get so much runtime from the alkaline ones.
Some 18650 Li-Ion cells (e.g. from DX) would be even better.

I don't know: it's nice to have room in there for the 500A fuse.

Here is my on-the-road charging arrangement for the N900. You could change the connector to suit N8x0.
The small regulator is http://www.dealextreme.com/details.dx/sku.29208~r.60561251