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charging mobile battery with another battery

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

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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

Reply to
sam
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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.

Reply to
CWatters

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)

CWatters wrote:

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Reply to
Tube2IC

Be wary of some of the advice you're getting here...some of it is accurate, and some of it is rather confused.

A few facts: All rechargable batteries generate heat when charged. The faster you attempt to charge them, the hotter they get; all batteries are designed with a specific heat dissipation capability. If the battery cannot dissipate the heat fast enough, the battery will melt, ignite, or explode. So the answer to your first question is: Maybe you can charge the battery 3 times faster without destroying it...and maybe not.

webpa

Reply to
WEBPA

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

Reply to
CWatters

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

CWatters wrote:

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Reply to
Tube2IC

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!

Reply to
Tube2IC

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.

Reply to
CWatters

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