Aluminum is another good idea. For me i would have some trouble finding a metal shop but it's a good beginning. That would also make it light unlike pc7 or something else.

Photos of painted face plates can be found at

http://www.maemoapps.com/category/basics/

Posting this so somebody can stop me before I do something foolish.

Here is the plan.

I'm going to the electronics shop and find 3 conductive strips similar in width to the BP-5L battery contact.

Tape those three strips in a row and slip between where battery and power contacts meet inside.

Cut a slit in the battery cover, and run those through.

Secure a second BP-5L battery to the back, and make sure those strips contact the second battery.

My understanding is when connecting in Parallel you are doubling the capacity (amp hours) of the battery while maintaining the voltage of one of the individual batteries.

If nobody sees anything wrong with this idea, I'll press on and post a step-by-step with pictures.

The (first?) problem with doing that is the charging - LiIon batteries need to be charged in a very particular way, and also need to carefully avoid being discharged completely. The circuitry that does that in the devices is calibrated for a single battery. The results with a single battery may be damage to the battery or even battery failure (think "boom")

I think bizshop may be onto something here. I believe the Li-Polymer battery (BP-5L) is made up of a number of cells inside it. So, by connecting a second BP-5L in parallel, you could potentially double the capacity without doubling the voltage. As long as both batteries have identical state of charge/discharge, you could do what bizshop wants to do--the circuitry inside the N8x0 maybe wired for one battery, however, it won't care or see how many cells make up that battery unit (in this case, two batteries). I AM NOT AN ELECTRICAL ENGINEER: WHAT I DESCRIBE ABOVE MUST BE VERIFIED BY REAL ENGINEERS AND IT MAY RESULT IN DESTRUCTION OF SAID DEVICES AND I AM NOT RESPONSIBLE FOR ANY DAMAGES TO YOUR PROPERTY OR YOUR BODY OR YOUR SANITY.

I think the battery operation would be perfect, and would allow you even to 'hot swap' the external battery with another, since to the n800 the same amount of 'juice' would be powering the device when you take the external off.

What I'm trying to figure out now is a way to isolate the batteries for charging, given the problem megabyte405 warned of.

megabyte405 is 100% right, these batteries have very specific charging / discharging profiles and will explode. The charging circuit are designed with a predetermined number of cells and usually have a temperature sensing device which prevents them from over charging. If you stack these batteries, that sensor will not be able to detect the external battery's temp and it will overheat and both may end up popping! Do a little googling on Lion / LiPoly charging first.

As far as batteries in parallel, bishop is right about the extra amp hours, just don't do it with these batteries (lion / lipo cells).

This is just your (un)educated guess or you really know what you are saying (i.e. you are electrical engineer skilled in lithium polymer technology)?

My uneducated guess is that it may work. At most batteries will not explode but be charged slower or to half capacity because you are sharing the current and each battery takes less (not more).

In fact the chances are high that it will just work because this is how it works with battery in Tapwave Zodiac. There is one long battery inside made by sticking two separate li-pol batteries together. Google for 'tapwave zodiac battery' and maybe 'dissection' or 'replacement'. There are also known issues when one of those two batteries inside becomes dead sooner, it will still work but the capacity becomes worse.

See also this post http://www.emuboards.com/invision/in...dpost&p=294891 or this http://www.nerys.com/zodiacreview/zodbattery.html

So this idea is not as insane as it looks :-)

Still, opinion of someone skilled in li-pol battery technology in general or with charging procedure in our tablets in particular (igor?) would be useful.

First an intro to how Li-ion / Li-polymer batteries are charged:

First they are charged at a constant current (typically 0.2C to 0.7C where C = the capacity of the battery) until the terminal voltage (no load) of the battery = 4.2 volts per cell. Then the battery is charged at a constant voltage (4.2 vpc) until the charging current drops to ~10% of the initial charging current. A timer is incorporated to cut off the charge if it takes too long, and a temperature sensor is included to cut off charging if the battery temp > 45C (113F).

Now for the problems:

First, since the charger is designed for a specific number of cells, the charging current is calibrated accordingly. When charging the batteries act as a load, so assuming the batteries are at equal levels of discharge they will split the available current evenly. So each battery is being charged at half the rate it's designed to be. That also means the charge may take substantially longer, and the timer may kick in to terminate the charge. The combined effect is a dramatically reduced battery life due to incomplete or inappropriate charging.

In addition, the charger is designed to include reverse polarity protection. Even with a second battery in parallel the charging circuit should be able to protect both batteries from reverse polarity due to malfunction of the charger, but there is nothing in effect to prevent reverse polarity due to one battery having a higher charge than the other. You may think reverse polarity protection is trivial since that's actually how you charge the battery, but go back to where I said that the charger is current limited. With two batteries in parallel there is nothing to limit the current. The effect is the same as short circuiting the battery which, as we all know, can cause the battery to explode (obviously a worst-case-scenario, but a possibility nonetheless).

Long story short, without a complete redesign of the charging circuit, dual batteries in parallel might be possible, but I would highly recommend against it.


And, since everybody wants the opinion of an expert on the matter, here's my credentials:
Electronics Technician - US Navy
Electronics Maintenance Technician - Freescale Semiconductor

This post has been proof-read by my father:
Bachelors in Electrical Engineering
Electrical Engineer - Freescale Semiconductor
IEEE Member


For what it's worth!