Extended use on Lead Acid Battery?

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Peter Mounsey - 30 May 2005 18:46 GMT

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.

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Stanley Reynolds - 30 May 2005 18:34 GMT

> 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
[quoted text clipped - 21 lines]
>
> 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.cfm/appnote_number/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.

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Jer - 30 May 2005 20:31 GMT

> 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
[quoted text clipped - 21 lines]
>
> TIA Pete.

I suppose if you're lucky, a terrorist will jump in here with the
details - they've been successfully building IEDs for quite a while now.

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email reply - I am not a 'ten'

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Stanley Reynolds - 30 May 2005 18:43 GMT

<snip>

> > Any ideas why I am getting so little use from this setup.

<snip>
Battery will self discharge over time, if possible you could add a solar
cell to keep the battery charged. But I think most of the watts are wasted
heating up your charger.

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Frater Mus - 02 Jun 2005 02:55 GMT

>> > Any ideas why I am getting so little use from this setup.
><snip>
> Battery will self discharge over time, if possible you could add a solar
> cell to keep the battery charged.

That's what I was thinking. A few years back there was a company that
made solarcell battery covers for cellphones, such that the phone
could live in standby indefinitely (although calls would drain it).

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John Henderson - 30 May 2005 22:22 GMT

> 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
[quoted text clipped - 9 lines]
>
> 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

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