Nokia BL-4C / BL-5C are 3.7V, 700-1000mAh (variants) Lithium-ion batteries, which are available in abundance and at very low cost ($4 - $7 a piece), since these are used in entry level Nokia phones, sold in most developing countries.

These batteries have 3 terminals, as explained here, for battery positive-terminal, ground, and a terminal (BSI) which I believe presents a fixed resistance value, that needs to be measured, to determine battery-type (chemistry, characteristics including capacity etc.)

So here are my questions:-

  1. Is anyone aware of any existing open-source hardware project, that has built battery-management circuit around such batteries ? (PS> In my searches so far, I've not come across any such project).

  2. How exactly, can one accurately measure the resistance offered by BSI terminal of the battery ? A very accurate and stable reference voltage would be required, I believe - but if the battery itself is the source of such reference voltage, it could vary with charge-status and condition of battery, so how could it serve as a good reference ?

  3. Is it necessary to have a temperature sensor in proximity/touching the battery ?

  4. What would be some low-cost battery-management IC's that are not too difficult to source or work with, for hobbyists, which can be used to charge such Li-ION batteries safely ?

Have found this question here related to 'resistance measurement', but the mechanisms described are far too cumbersome, expensive & require manual intervention, to be useful in automated battery-type identification. OTOH, it is possible that I've completely misunderstood the purpose of the BSI terminal.

As for whether "temp sensor" is needed, I think the answer is YES, based on this. However, the question then becomes, without access to proprietary battery characteristics information of this Nokia battery, what can one do with the temp-sensor ?

  • \$\begingroup\$ I would presume that the third terminal is the one connected to the built-in temp sensor of the battery. \$\endgroup\$
    – JimmyB
    Jul 30, 2012 at 15:07
  • \$\begingroup\$ Linear's LTC1733 is a very basic charger IC which will work with almost no external parts. - It's available ready-to-use on PCBs for US$ 2-4 from various online shops, too. Temperature-sensing is not mandatory; it is only a safeguard against over-charging the battery, e.g. with too high current. \$\endgroup\$
    – JimmyB
    Jul 30, 2012 at 15:11
  • \$\begingroup\$ @HannoBinder, thanks for commenting. Actually, some Nokia batteries have 4 terminals, where apart from +ve and GND, there is the BIS, there is the VTEMP (thermistor) whose resistance drops with increasing temp of the battery. Thanks for the tip-off on LTC1733. Will try to check it's data-sheet. \$\endgroup\$
    – bdutta74
    Jul 30, 2012 at 17:25
  • \$\begingroup\$ By the by, does anyone know what is the voltage threshold of the battery below which it is time to replace it? Mine shows 3 out of 3.7 V. \$\endgroup\$
    – Eugen
    Apr 7, 2013 at 16:34

1 Answer 1


3rd terminal:

The 3rd terminal on your battery is most likely to be an "on board" thermistor temperature sensor. Try this.
Determine -ve and +ve battery terminals.
Connect ohm meter between -ve and terminal that is not +ve.
Blow warm air over battery (NOT TOO HOT) and determine whether resistance varies with temperature.
Sensor could be comnnected to +ve rather than -ve but -ve connection probably most likely.


There are many ICs available for charging LiIon cxells. If you want to build your own Lithium Ion / LiPo charger for up to 500 mA charge rate then using the MCP83831 / MCP83832 charger IC is a very easy and economical way of doing so.
This is eg what Sparkfun use in the LilyPad Simple.
Data sheet here

It can literally be as simple as shown in the circuit diagram below.

enter image description here.

The resistor from Vss to Prog sets maximum charging current.
Several other options are available by selecting variants of the basic device. Unfortunately 3 different options are selected as a group (see datasheet page 21) providing less flexibility , but the device is still useful and well priced.
Options include cell voltage below which charger goes into "precondition" mode,
end point current termination level
and i_condition / i_charge ratio.

My main "complaint" with this IC is that the lowest voltage output level version is 4.2V and higher voltage (and very dangerous) versions are available.
Digikey sell 3 different versions (AC, AT, DC) with the AT mainly stopping charging sooner (longer life, lower capacity), while the DC variant will try to produce 'magic smoke' and 'vent with flame' if a very low voltage battery is charged.

Available in stock from Digikey for $US0.68/1 and $US0.42/100.

  • 1
    \$\begingroup\$ Thanks @Russell for taking time to answer. Isn't charging always at the lowest voltage output level (4.2V), an acceptable, safe method ? Sorry, could you elaborate on 'magic smoke' and 'vent with flame' -- not familiar with those expressions. \$\endgroup\$
    – bdutta74
    Jul 30, 2012 at 17:28
  • \$\begingroup\$ As per some of the sites which seem to have gathered reverse-engineered information about these seemingly proprietary batteries, there are 2 terminals, one which indicates battery-type/chemistry/characteristics and other which indicates battery-temp. The BL-4C and BL-5C, as per these sites, do not have on-board temp. sensor, but only the battery-type indication terminal. \$\endgroup\$
    – bdutta74
    Jul 30, 2012 at 17:30
  • 2
    \$\begingroup\$ @icarus74 - 4.2V is the highest you should run a LiIon cell at and there are advantages in running not over 4.1V and maybe 4.0V. 4.2FV os OK but must be temperature compensated so that Vout is always <= 4.2V. | Vent with flame" is the standard joking but serious term used re LiIon failures. A LiIon battery can self destruct semi explosively. "Magic smoke" is a standard EE term for destroying stuff with resultance smoke and/or flames. \$\endgroup\$
    – Russell McMahon
    Jul 30, 2012 at 18:10
  • \$\begingroup\$ Add a few diodes in series to lower the voltage to the pack. Only charging to 3.7v will increase the lifespan of the cell vs a cell that was always charged to 4.2v. \$\endgroup\$
    – david1024
    Jun 29, 2018 at 19:13

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