Where exactly? No, should not be any. Both batteries are pretty new and not much used. Maybe what you see is glue remains from the cover tape? Or that dark area left of the orange cable is in shadow. I had light coming from "all over" when I took the photo, from the circular lamp near by from my magnification lamp. It throws shadows here and there when taking macro photos.

I think that system is not good though. The blue cable should be get somehow other side of the circuit board. It is little too risky now if something nasty happens, like one drops the phone to water or smashes to the rocky ground or something and the 2nd battery will cause the 1st battery to short cut.

Edit:
status update:
The system has been running now 19 hours and the charge level is 49% says BatteryGraph. I had SleepAnalyser recording 8 hours and have used the device quite much today, couple of reboots. The capacity wise the hack seems to work and comparing the decrease-angles of BatteryGraph history, this now seem to give 2x effective usage time.

In the pic below, one can see little better how the original BL-5J battery looks from the top. There is no really other places to put that blue (-) cable. The red (+) cable has a good place on back of the connector, or even anywhere of the outer shell.

I wonder if somehow the blue (-) cable could be attached to the middle pin but same time able to insert the battery inside N900 battery hole and have it working. Maybe with thinner cable it can be done, but the cable has to tolerate that 1A.
Maybe replacing the edge "cap" (the black one in the pic) with something totally else, or modifying it to have a hole on top of the middle pin.

it is not that easy to do with original battery's, they have that hump on the '-' pol.
The battery's from 'christgift' are flat so they are easy'r to mod, they are the battery's im using.
Next will be the japod's or whatever battery that offers 1300mAh+

It (two original Nokia BL-5J batteries connected parallel under Mugen back-cover) took 43 hours to go "empty" (3.6V) from full.

I used the device moderately heavily in that time. For example 1 hour exercise with eCoach (GPS+bluetooth heartrate-monitor), 8 hours sleep with SleepAnalyser recording, two re-boots, upgrading couple of times new versions of applications, transfering files over Bluetooth and so on.

I re-charged the system using PC USB, so probably with 500 mh. Took 7 hours to fully charge (from 3.6 V to 4.146 V).

In the mean time I did another max-battery from two fresh 'christgift' ~1300 mAh batteries.

1) I unwrapped the tape from battery1.
2) I used thinner cables this time so I tried to solder cables straight to the (+) and (-) -connectors in that way, that female-connector still is able to be connected to N900 male connector. Used just some tin so it won't full up the connector. It succeeded; the cables are attached and yet the connector is able to connect N900's male-connector. (See pic)
3) I put the cables on the side and wrapped the cover tape back on, leaving cables underneath.
4) Checked that battery1 and battery2 had approximately same voltage (3.80 V and 3.72 V)
5) Then battery2 rotated 180 on top of the battery1. Taped them together tightly.
6) Soldered (+) cable to (+)-connector of battery2 and same thing to (-) cable.

It worked. Much easier, quicker and both protection chips were untouched and are in use in every direction.

I plan to change my battery-pack1 to this way also. Although it is working fine, I wanna make sure even if there is some catastrophe and the cables get shortcutted, each own protection circuit protect its own battery what ever direction we think of it.

This hack I can recommend if one is careful not to short-cut poles when making and both batteries are in the same voltage when connected to parallel; and same age + manufacturer + type + size.

that's another way to do it
that way is not hard to do since you don't have to take the battery's much apart.
BUT both battery's are connected AFTER the protected circuits and this can potentially cause trouble in the long run.

And yes there is a lot of space in the mugen cover.
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I will be doing a tutorial in the near future when i make another dual battery with the battery's 'oig.buy'.
This will be picture by picture and description along the way.

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Another battery tested and added to post #1

What kind of troubles there could be doing it this way?

I think, I will sleep better, because now each battery's protection circuit protects what it was design to do, each's own battery.

What I see the problem in the first version, was that if I drop the phone hardly and/or it gets really wet, the shot-cut caused by battery2 or by cables will shot-cut battery1 without any protection circuit in between the battery1 and the short-cut. If this would happen, it could set battery1 to "explode" to a fireball.

When i make my guide than you will see what i mean.
My battery's are directly connected together and then protected.
But you can expect a capacity drop because the circuits will always have small difference's, and charging with the phone's USB charging system can mean that they will not be charged all the way up.
also lets say one circuit goes dead, than you will only have orig capacity left.
I make battery's for everything from small flashlights to EV's and when i use protection it is not on all the cells if they are in parallel.
Do you think that the tesla (EV) has 6000+ circuits in it's battery.

give me some time and i have a guide for my battery.

A very requested battery has now been tested.
1500mAh battery from eastmaze

Again go to page #1 post #1 for results

Can see on track & trace that i should have the Japod battery for testing next week, the japod battery are also VERY requested.

Update: also tested the very cheap and very crappy battery from Dealextreme. Again page #1 post #1

What kind of charge parameters did you use to trigger the protection circuit?

Hopefully you write this to the review area of the DX page also as you are "a certified owner". They should remove it from the sell.

My charger goes to the limit at 4.3V until it puts 4.25V to the battery, and the eastmaze cuts off at 4.28V, all other battery's has not done this cut, so the circuit in the eastmaze battery is a very good circuit to protect the battery. All my battery's has never had any problem's with the charge voltage of 4.3V.

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Like EBAY DX does not listen much to complaints about products, so don't want to bother with that.
And every time i write a negative review at DX only about 10% gets posted....
The test on that battery was just to get the actual mAh on the battery and to show that there is a limit for how cheap the battery can be.
Thou the 'christgift' battery was cheap and still had only 50mAh less then orig. nokia battery.

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Update:
'oig.buy' and 'fashionuniononline' have closed shop so these battery's are only available from 'christgift', the 2 'sellers' was driven by the same person.