Yes, that would be the idea, one could choose on daily base whether to use bigger battery or smaller battery and thinner phone.

But yes, I see the problem with uneven usage age of batteries this way.

Specific Instructions would be nice, how to make a connection from one battery's + and - pins to cables without breaking the battery case too much and still able to put it as a 1st battery to N900 battery slot.

I have currently two about same usage age original batteries, which are little too expensive to destroy by trying to experiment and figure out how to solder cables to one. The 2nd battery is easier, because I can find two pieces of metal which will fit to the actual connectors. Maybe even there is a net shop to be found which sells those male connectors.

side question,,,what is that on the left of the camera button ?

yes the connectors idea can be used to "spare" the battery's from being modified.
I will try to make an instructions guide in the start of this thread, but can't say when i will have it complete.

And and update:
USB charging works as it should, but damm it takes a long time compared to my dedicated charger...... Tested on my friend's N900

Thats an normal NOKIA 2mm DC plug i use for my dedicated charger and since my USB doesn't work.
--- sorry on the left side of the camera thats the IR "cover" ---

infra red ? what do we do with that ?

To try to keep this forum only about battery testing and MOD, you have to search other threads or make one your self.

The only answer i can give is you can use it for:
Data transfer
Operate your TV / DVD / Amplifier etc.. if it has remote control.

Everybody else please keep this thread clean.....

For the price, wouldn't buying a spare battery from one of the good suppliers be the way to go? Use the single battery on those days you don't need the extra power, and the 2 older batteries together for the extra power?

That'd prevent uneven wear that way for minimal cost. Get a desktop charger to charge whichever battery(ies) you aren't using.

I was going to say the same, me and my friend talked about it earlier, he all so wanted a BIG battery but only when there was a chance that he would be away for some time and not necessarily have access to power/charger, than he could all so just buy a mugen cover and i can make the 2500mAh battery for him.

Again: Mugen cover & 2x Battery's is only 17$ !!

Connecting a full battery in parallell with an empty battery will resukt in unsafe levels of current flowing from the full to the empty. It's fine as long as you connect together two batteries that are full, though.

Is this for zimon? for his idea of the dual battery connector?

Because i know this, it's basic charging 101, the greater the voltage potential of 2 sources like 2 battery's or a battery and charger the higher the amp draw will be, thats why my chargers all have current limiting that can be set at 0.1A intervals.
Charging or Discharging beyond the battery's limit can result in a dead battery or in worst case venting with or without fire.

Like my EV battery it gets hot if it is being pushed hard for an extensive amount of time.

Yes.

With PR1.3 update, it gets closer to 4.2 as a side effect. The reason why it doesn't seem to reach 4.20 ever, is that there's some mysterious extra resistance between the charger and the battery. Through staring at numbers from the bq27200 fuel gauge chip, I'm guessing it's 120-130 mOhm or so.

Also, the system load is behind this resistance path.

This causes a voltage drop between the charger and the battery, meaning the charger sees 4.2V but the battery sees something lower, depending on charge current and system load. The charge chip (bq24150) is programmed by bme to stop at 4.2V and 50mA (or was it 100mA? I forget..)

Independently of what the charging chip does, bme runs its own charge metering in software, probably using measurements mostly from the cellmo side of things, and will at some point signal full charge. Some time later the charge might reach full for real, at which point the charger stops but green light / fully charged is still indicated. The bq27200 fuel gauge chip also runs its own end-of-charge detection, and will jump to indicating 100%, usually before bme (which is what feeds the user interface) and before bq24150 charger.

Much later, when the system has run entirely off of battery for awhile, the voltage has dropped below the recharge threshold at approximately 4.1V (it varies alot), and the charger is switched on again. The User Interface displays battery full / green light all the time even if phone has been running entirely on battery and even when charging is restarted again.

When you switch off the phone battery charging gets switched off too..

So, it's kinda hard to actually know when the battery is fully charged, and kinda hard to remove it without using power..

Well except that putting it on charger overnight with the PR1.3 upgrade will pretty much have reached the equilibrium point of 4.2V minus voltage drop caused by 80mA minimum system load (100% idle) when on usb, or 4.19V or so.