Since ever I got a Nokia 5800 XM, I used to use it to the full extend as its a feature packed fone and what you wouldn't want to lose any time. The Phone used to give a decent working day with full GPRS and 2 days without. However the battery life was the only thing which limited me to its unlimited use. I travel a lot, so I thought I'd solve that too..
I started my thoughts of using a standby battery, but 5800's battery costs over 600RS and will only give me the same amount of charge I'm getting now and that too just once more. It was not worth the money, plus I have to switch off my phone to switch batteries which I dont like at all.
Then I thought of using ordinary china Li-ion cells but charging Li-ion's from an external source is too risky, so here comes our good old Ni-cd battery back.
The built in battery of 5800 is around 1350mA and this setup will give it a near full second battery life. If you need more, use NiMH batteries like of 2500mA which will give 3 or 4 times the original life, say 3-4 days with full GPS, wifi and Internet. But dont expect to get more than 3 days as these batteries are deep cycle batteries and are not designed to store charge for more than 3-4 days, but sometimes you'll get with good batteries upto a week.
I used 4 x 1.25V 850mA NiCd AA rechargable batteries to get a peak of around 6V and a nominal of 5V. Since these batteries are completely safe to charge off an adapter, anyone can use them without much knowledge. The best part is that one battery costs only 30Rs, and the whole kit cost me only about 130Rs to setup leaving me space for an upgrade to even 2800mA batteries if I wish to.
Another main point is that we dont need a sperate charger tp charge these batteries, our phone charger goes thru this pack and comes out to the phone. So everything will be done simultaneousely.
1. 4 x 1.2V Rechargeable AA batteries with 700-2800mA current (NiCd/NiMH)
2. 4 Battery holder with connecting wires
3. 1 or 2 feet wire pair preferably black in colour
4. Nokia Old charger to New pin converter wire, not connector
5. Wire Sleeves and Heat shrinking tubes
6. Blade or sharp splicing instrument
7. Insulation tape
Shown above is the battery case and converter.
Sleeve for wire.
First get the charger pin converter wire which you get for about 5Rs at any local mobile dealer and cut it at the middle. The one problem we are going to face here is that this converter will be having the old nokia charger connecter on one end. So if you dont have an old nokia charger, you will also have to buy a socket that matches ur new charger from an electronics shop and use that instead. You will have to show your new charger pin to the dealer and ask him for the socket matching it. It will be abit hard to get. Here I'll be explaining using the old charger because its easier to get from the cable we are cutting.
Now Splice the cut wires on both ends to get the copper conductors of the red and black wires.
Splice the 2 feet wire on one end and connect it to the Charger pin we got by matching colours. polarity is very important here. If you mess up the connections, you are messing up your phone.
Put a sleeve over one of the wires and connect the next one such that they do not touch each other. You may also use insulating tape.
Now Put two pieces of the heat shrinking tube on for both the ends but dont do anything now.
Splice the other end and connect it to the socket as well as the battery pack taking care for the polarity.
Again add a sleeve or insulation tape.
Connect the connect to battery case and put in the 4 batteries.
If possible check with a multimeter that you get around ~6V at the pin.
If all is well and confirmed to be ok, try connecting to your phone and check if its charging. If not, check all connections.
If everything is ok, heatshrink the tubes by heating it so that the joints are consealed and wont get loose.
Charge the batteries for 5-10Hrs on first use. Since the batteries are charged using a very high current, they'll charge quickly. For the same reason, never charge them overnight or unattended. They dont have any cut off circuit to cut the power when full like the phone. Over charging will reduce battery life drastically. Always keep them charged even if not in use.
I know how to make a fully automatic circuit with electronic cut off but for the ease of everyone, I did a simpler alternative with zero soldering and complexity as well as ease of construction.
WARNING : DO ALL THIS AT YOUR OWN RISK. I AM NOT TO BE HELD RESPONSIBLE IF YOU DO A MISTAKE AND MESS UP WITH YOUR PHONE OR STUFF. I HAVE TESTED THIS ON MY PHONE AND IS WORKING WELL.