Nokia battery

[arhi]

Hi,
I have few BL-5C batteries (from some nokia phones) that I would like to use in some of mine projects. I'm not very good on analog electronics, especially on the "how to charge the battery" part...

The idea is to create a system that will use 1 (or more if needed) nokia BL-5C. The system will use the external + battery power (on/off the base station) and while on external power, it should charge the battery. I'm using PIC16Fxxx or PIC18Fxxx (not yet decided but might go with 18F4550 or 16F877A + 18F4550 combo + some 12F508 and 16F628 for some daughterboards) and the PIC should be aware of "battery status". (for e.g. if the battery is low - it should move to base station and recharge).

I was thinking about ICL7673 or similar "automatic battery back-up switch" to run circuit on both external and battery power, but I'm stuck not knowing:
- what is the middle pin on BL-5C
- how to charge BL-5C
- how to read "remaining power" in BL-5C

I believe a datashit of BL-5C should explain all this but after 3 days of googling I just found whole bunch of "purchase charger", "this and that phone datasheet" etc .. no technical details on the battery itself

Thanks in advance

 

[arhi]

I have just found this:
**broken link removed**

1 Gnd
2 BSI Battery Size Indicator (typically a 47K resistor)
3 Vbatt

Now, I believe BSI is resistor to GND that changes according to the battery capacity. What I need to get now is how to charge the battery (if anyone knows - the Voltage and the Amperage - it is 3.7V battery)

 

[audioguru]

There are many sites that talk about how to charge a lithium battery.
Usually you use a lithium battery charger IC that regulates the charging voltage and current accurately to prevent the battery from catching on fire.
If the battery has been discharged to a voltage too low then the battery charger IC senses it and reduces the beginning charging current so the battery does not catch on fire. The charge continues until the battery reduces the charging current to a certain amount then the charging IC disconnects the charging to prevent a fire.

If you make your own charging circuit then the battery will probably catch on fire. The fire is extremely hot and water makes it burn hotter.

 

[picasm]

Some batches of BL-5C batteries have been found to be at risk of potentially
over heating whilst being charged.
Check the web site for the affected battery IDs.
**broken link removed**

If some can overheat even when using the proper charger, your risk may be far greater with a home-made one!

 

[arhi]

I'm aware of the overheating problem and will add some temp monitor to the circuit (there will be over 5 temp monitors on the circuit anyhow)...

The main problem is still how to determine "how much power left in battery" .. the BSI seams like fixed resistor to the ground defining the max capacity of the battery and does not change in relation to the remaining power. so it looks like i have a bigger problem then I anticipated

- determine the remaining power (looks like the only way to do this is to measure the voltage on the battery)
- charge the battery (looks like I will have to use ready made charger)

more and more I believe I will use the NiCd batteries

 

[audioguru]

The voltage of a lithium battery drops as it discharges. You could use the voltage when the battery has a load to roughly determine how much charge remains. You need to find out the minimum voltage then make a circuit to detect it then turn off the load.

Ni-Cad batteries are toxic and are obsolete. China still makes them.
Ni-MH batteries are much bettter. The voltage drops a little as they discharge. It would be difficult to accurately calculate how much charge remains.

 

[arhi]

Ni-Cad batteries are toxic and are obsolete.

I was thinking about NiMh (I can only purchase NiMh and lead batt's here)... thanks for the explanation tho .. not sure how to proceed .. trying to find a way to make lithium ion bat's charge .. as there I can easily detect "alert - we are shutting down in 2min" and on the other hand NiMh are easier to charge, cheaper to get but hard to accurately detect the "oh my oh my we are shutting down" ..
ATTM I'm thinking about NiMh + LiIo bat packs, where LiIo would be "backup" - so when NiMh shut down, I switch to LiIo and run for the station ... as LiIo does not have "memory problem" I can manually recharge them from time to time ... will see .. still trying to find proper solution