You'll already have seen how I'd been experimenting with battery swapping by putting the N8's BL-4D cell into the Nokia 808 as a spare for the BV-4D that the 808 comes with? I suggested at the time that the reverse might be a good idea, i.e. putting the higher capacity, higher (nominal) voltage BV-4D into the N8, as an ultimate battery upgrade, perhaps replacing an ailing, 2 year old cell with one that has more juice than the original did when new. Here, I show how to do exactly that and report on any stats and caveats I notice along the way.

The BL-4D in the N8 and the upgrade

I've swapped cells a couple of times over the last two years, partly just because I can and was practising my disassembly skills - not that the N8 presents any kind of challenge. Charging my N8 to the very brim and monitoring the voltage (and 'remaining capacity') with Phonetinfo, I noted that the battery voltage topped out at 4.179V. As you'll have noted from previous articles of mine (e.g. here), once under serious load from the processor and screen, this peak voltage quickly drops until the battery gets near its nominal voltage (3.7V in this case), whereupon it doesn't change much until the battery starts to run low.

The BV-4D is one of the new breed of Nokia batteries offering higher capacity within the same form factor (you may remember my noting the higher rated Bl-5K as well?) and means that it should make for an excellent mid-life upgrade for a beloved N8 (now starting to come out of the two year warranty period*). But what of the higher nominal voltage and higher capacity - would this present issues for the N8's circuitry? This is what I set out to try.

You can source a BV-4D from various sellers now (e.g. here).

* it goes without saying that it's unwise to try anything like this while still in warranty - so check with your local Nokia Care Point if you're uncertain whether you're still within the official Nokia warranty period.

Swapping the batteries over

The only tool you'll need is a Torx T4 screwdriver, as shown below. Again, these are easy and inexpensive to find online. Undo the small screws either side of the bottom end cap by half a dozen turns - don't take them all the way out unless you fancy losing them or fiddling around later trying to get them in again!

Once loosened on both sides, ease off the end cap, which will come away completely:

Underneath the phone, where the end cap was, you'll see this long plastic clip, hinged on the left. Note the right hand U-shaped plastic lugs:

Pull gently up on the most exposed lug and the clip will detach and let you swing it up, as shown below. If it comes away altogether, don't panic, it's easy enough to prod back into place.

Note the blue transparent tab of plastic that's now exposed. Pull it gently and out will come the BP-4L battery that has been in your N8 since day one. The tab is sticky, adheres to the battery and is only there for easy removal of the cell:

Peel the transparent plastic off the old battery and put it in roughly the same position on the new BV-4D cell. Note that, as shown below, my 808's BV-4D didn't have a traditional Nokia hologram. This was a little disconcerting, but as the 808 came direct from Nokia I've no reason to doubt that it's a genuine Nokia battery. Odd, though!

Replace the new battery in the N8, fold down the clip, slide the end cap on again (gently - make sure you line up the little slide slots with the centre line of the screws) and, while holding the cap firmly in place, re-tighten the T4 screws. Job done.

Power up the N8 as normal and try it out with the BV-4D upgrade in situ:

With the cell in place, I ran the N8 through a complete charge - discharge - charge cycle, running it right down to 1% and back up to 100%. Of note was that at the top of the cycle, the battery still reported ending up at 4.179V peak voltage, by the way, indicating that this figure is either set in stone in the OS/firmware, or a natural characteristic of Li-Ion cells in this package.

Each time the BV-4D was charged in the N8, Phonetinfo reported a 'nominal capacity' of 1252mAh, rather than the expected 1320mAh, which was a little odd, with the 'remaining capacity' maxing out each time at about 1207mAh. (Note that the BV-4D has been in daily use in my 808 for four months, mind you, so I'd be expecting it to be running at around 90% of its original capacity.)

Postscript: Back in the 808 and swapping warning

Interestingly, putting the BV-4D back in the Nokia 808 PureView again after the test and booting up Phonetinfo revealed a 'nominal capacity' of 1320mAh again, so there are clearly some differences in the charging circuits (in the N8 and 808) and how they talk to the battery and what gets communicated.

When charging in the 808, battery voltage was quickly reported as high as 4.336V, so a good 160 millivolts higher than the same cell when charging a few minutes earlier in the N8. In fact, I was worried that the voltage would rise further and I suspected that using the N8 to stuff the BV-4D full to the brim and then transplanting it straight into the 808 and plugging the latter's charger in was a recipe for disaster. I was worried that the cell might be in full-on charge mode rather than trickle charging, as you'd expect with the battery already full up. See the disclaimer below, etc. - we've all seen the YouTube videos where an unregulated Li-Ion battery gets exploded!

Therefore, I loaded up the Nokia 808 for an hour or two of hard work (playing a movie!), to give it some normal usage and take it below the trickle charging threshold, and then tried charging to the brim again with the voltage again rising as high as 4.336V but I kept a watchful eye on the temperature of the battery and all was well as the 'remaining capacity' edged up to... 1290mAh. Again, fascinating how the numbers are different on each phone - partly differences in electronics, but also partly I suspect, that the numbers are being reported differently.

As a rule then, I'd say avoid swapping a fully charged battery from one device to another and then immediately charging the latter! - best to let each device build up to a 'last 10%' trickle charge at its own pace, etc. and generally manage the last bit of its own preferred charging curve.

In summary

I expected to find little difference in charging parameters (voltage, capacity change) between the N8/BL-4D and 808/BV-4D, but although the two pairs of components are largely compatible, there are minor differences in peak charging voltage and the reporting of internal battery stats. I still think the BL-4D makes a great 'spare' for the 808 and that the BV-4D makes a possible 'upgrade' for the N8, but I'd welcome more data points - if you've experimented with this as well, comments welcome on how the BV-4D performs in your N8. Any issues? Any charging oddities?

PS. Nokia E7 owners (which also uses the BL-4D) might also want to think about the upgrade if they're skilled with micro tools (the E7 takes a good ten minutes to get into, I'm reliably informed).

PPS. One side observation of hours of watching Phonetinfo displays while charging phones has been that the reported 'Battery charge level', the parameter which is used by Nokia Battery Monitor and the top status bar graphic, is pretty hopeless, rising to 100% far too early (I suspect it's voltage-based), at one point even showing 100% when the battery was (being charged and) only half full. If you've ever wondered why the on-screen battery guage is so unreliable on Nokias in years past, this is one major factor. Phonetinfo reports this 'charge level' but also, more helpfully reports the 'remaining capacity' in both mAh and percentage (of nominal capacity), presumably directly querying the battery circuitry, and this seems many times more accurate.

PPPS. The usual disclaimer applies. Don't mess with taking devices apart and juggling batteries and chargers unless you know what you're doing and are aware of the explosive characteristics of Li-Ion cells. All About Symbian takes no responsibility if you damage a phone or burn your house down in a freak charger fire....(!)