# charging mobile battery with another battery

Discussion in 'Misc Electronics' started by sam, Dec 8, 2004.

1. ### samGuest

Hi folks,

I'm just after some advice/thoughts. A friend and I have been thinking
and talking about renewable energy and such. We're thinking that we
could use an exercise bike to charge some sort of battery, and then use
that battery to charge our mobile phones.

Firstly, is it feasible to charge the phone directly from the bike
(using an alternator and glue circuitry of course)? I'm not sure if
these calculations are right, so please critique as necessary Let's
say that the phone charger provides 3.7V at .337A and takes three hours
to charge. So that should mean that a full charge of the phone is
about 0.003 kWh. Assuming we can generate 0.0001 kWh per minute (we
got that figure from http://www.scienceshareware.com/bike_gen_alt.htm)
that would mean that about 30 minutes of pedalling should charge the
phone. Is all that possible?

OR (and this is what we're thinking instead since it is more useful
anyway): what about if we charged a battery and then used that battery
to charge the phone? How would we go about doing that? What sort of
battery would we use to charge a phone? I assume any (rechargeable)
battery would do, as long as we could bring down the voltage and
provide the correct current. Do phone chargers (nokia 3310, or the
3xxx series, specifically) have any fancy electronics in them?

If this is all crazy stupid let me know It's just some
experimentation we'd like to do; nothing serious and we certainly don't
want to power our homes from an exercise bike

cheers
sam

2. ### CWattersGuest

Set it up so that the bike charges a 12V battery (lead acid from a car, or
Nicad etc). Then you can recharge almost anything using chargers intended
for use in cars (eg Laptop charger, mobile phone charger etc.). Not as
efficient perhaps but it makes the electronics easier.

3. ### Tube2ICGuest

Sure you can charge one battery from another. You just need to ensure
the following:
1. Charging battery should be at higher potential than the battery
being charged. i.e. if you are charging a 3V Battery, the charging
battery should be at least a few volts higher. This will allow the
charging current to flow from the charging battery to the battery under
charge.
2. Beware of Battery chemistry. Some batteries drop their potential as
they discharge. If the potential drops below that of the battery under
charge, then the charging will reverse. [This can be prevented by using
a diaode in series. You will need to compensate for the forward drop of
the diode which is usually 0.7V for a silicon diode]
3. It is probably best if you use NiMH high capacity cells to build a
6V or 7.5V battery and construct a harness to plug it into your cell
phone's charger jack (Nokia's are 5.2V to 5.6V and it would probably be
OK to feed it 6VDC)

4. ### CWattersGuest

Hi Tube..

Do you know for sure that current limiting is built into the phone these
days? Othewise it's not a good idea to feed it with a voltage source.

Colin

5. ### Tube2ICGuest

What I am saying here is to hook the external battery of 6V to the
cellphone's charger port. The Cellphone charger port (on Nokia) is 5.4V
to 5.6V depending on model. [(6-5.6)/6]*100 = 6.6% That should be well
within the 10% tolerance usually designed into the charger ports. Your
car charger probably varies about as much.

Tube2ic

6. ### Tube2ICGuest

All valid points if a direct charge of the battery is being planned.
Since the Cellphone's charging circuitry is used to control the rate of
charge etc. there is no danger of expolding batteries. The cell phone
battery will charge as designed by its manufacturer.
What must be ensured is that the specifications of the charger port are
not exceeded. i.e. Do not connect a 12V car battery to a 5.4V charger
port!

7. ### CWattersGuest

Yes that's ok if the phone is designed to use a charger that is a pure
_voltage source_ (eg the battery charging current limiting is built into the
phone). Perhaps the charger spec is 5.5V nominal but with a built in 300mA
current limit. eg when connected to a Li battery the voltage falls and it
switches into current limiting mode? If the charge current limiting circuit
is built into the charger like this it isn't present when you connect the 6V
battery in it's place.