Extended use on Lead Acid Battery?

Archived from groups: alt.cellular.gsm (More info?)

"Peter Mounsey" <mounseyp@msn.com> wrote in message
news:yjIme.2667$Mi4.2577@newsfe2-win.ntli.net...
> I need to keep a cellphone running for months without using a mains
charger.
> Would prefer to use a 12 volt 17 Ah battery to maintain a Nokia 3310 or
> similar.
> Tried using car charger but it still managed to run the battery down in 3
> weeks.
>
> Can only assume that the charger circuitry is very inefficient as a simple
> calculation where
> the phones internal battery lasts for 5 days and is 700mAh at 4 Volts
would
> be
> 0.7 * 4 = 2.8 Watt hours
>
> and the Lead Acid holds
>
> 17 x 12 = 68 Watt hours
>
> so the cellphone should run for (68 / 2.8) * 5 = 121 days.
>
> Any ideas why I am getting so little use from this setup.
>
> Anyone know of a car charger which is more efficient than the unbranded
one
> I have,
> or a cellphone / charger combo which is better?
>
> TIA Pete.
>
>
A 6 volt golf cart battery would work better and could be connected to the
phone charger jack directly, add a small fuse for protection. You could use
a switching regulator to convert the 12V to 3.6 the phone needs they make
such for computers but it may not work at so low an amp draw. Look here:
http://www.maxim-ic.com/appnotes.c [...] umber/2031
http://www.semtech.com/pdf/sc4517a.pdf (need a pdf reader for this one)
You would need to replace the cell phone battery not keep it charged for the
longest life.

Archived from groups: alt.cellular.gsm (More info?)

Peter Mounsey wrote:

> I need to keep a cellphone running for months without using a
> mains charger. Would prefer to use a 12 volt 17 Ah battery to
> maintain a Nokia 3310 or similar.
> Tried using car charger but it still managed to run the
> battery down in 3 weeks.

You need a much bigger battery, or some means of keeping it
charged. Even a batteryless dedicated GSM device, designed to
run on low-voltage DC will likely exhaust this battery in
around 5 weeks. My Wavecom Fastrack 1206b modem is rated at 12
mA at 12V in idle mode, and 140 mA when transmitting. Assuming
an average of 20 mA, that's around 35 days if the full 17 Ah is
actually available.

Also keep in mind that a lead-acid battery will have a very
short total life if it spends much time at less than full
charge.

> Can only assume that the charger circuitry is very inefficient
> as a simple calculation where the phones internal battery
> lasts for 5 days and is 700mAh at 4 Volts would be
>
> 0.7 * 4 = 2.8 Watt hours
>
> and the Lead Acid holds
>
> 17 x 12 = 68 Watt hours
>
> so the cellphone should run for (68 / 2.8) * 5 = 121 days.
>
> Any ideas why I am getting so little use from this setup.

The charger will be dropping the 12V to around 5V by passing the
required 5V current and converting the other 7V to heat. If 30
mA at 5V (150 mW) is being fed to the phone by the charger,
then 30 mA (360 mW, plus more for other losses in the charger)
is being consumed on the 12V side. In the phone, the internal
battery charging circuitry will be busy consuming power too, so
requiring a 30 mA feed to support a 6 mA receiver load is not
unreasonable.

> Anyone know of a car charger which is more efficient than the
> unbranded one I have, or a cellphone / charger combo which is
> better?

This isn't going to solve your problem, if you need months from
a 12V 17 Ah battery.

John