# Supplemental Cell Phone Power Source

I

#### Ignoramus31975

Jan 1, 1970
0
I have a Motorola V557 cell phone.

My problem is that its battery and charging system is very lousy,
batteries take a long time to change and discharge quickly if I use it
with my Linux laptop as a cell modem (pppd).

Note that its AC wall wart charger is rated for 5VDC, 700 mA.

So, what I was thinking of doing is as follows.

1. Buy an extra AC charger (cost about $5). Cut off its wall wart part and throw it to garbage. Keep the cell phone plug. 2. Make a auxiliary power source, using - DC plug (which can be called receptacle) from the AC charger for output, - four AA battery holder, - linear regulator to achieve 5 VDC output. Since every charged battery holds about 1.62 volts, four would give me about 6.48 volts fully charged, and as long as they are over 1.25 volts each, I could still be getting 5 volts DC as output. I think that four AA rechargeable batteries could give my cell phone a decent run time, and there is plenty of nice four AA battery chargers available at minimal cost (I have one at home anyway). I could use LM2825 regulator with 5V output option. See http://www.national.com/pf/LM/LM2825.html This one is highly energy efficient. I could also use it with 9v batteries for smaller foot print. It is, however, very expensive, so maybe I will opt with a lower efficiency linear regulator. Any thoughts? i E #### ehsjr Jan 1, 1970 0 Ignoramus31975 said: I have a Motorola V557 cell phone. My problem is that its battery and charging system is very lousy, batteries take a long time to change and discharge quickly if I use it with my Linux laptop as a cell modem (pppd). Note that its AC wall wart charger is rated for 5VDC, 700 mA. So, what I was thinking of doing is as follows. 1. Buy an extra AC charger (cost about$5). Cut off its wall wart part
and throw it to garbage. Keep the cell phone plug.

2. Make a auxiliary power source, using

- DC plug (which can be called receptacle) from the AC charger for output,
- four AA battery holder,
- linear regulator to achieve 5 VDC output.

Since every charged battery holds about 1.62 volts, four would give me
about 6.48 volts fully charged, and as long as they are over 1.25
volts each, I could still be getting 5 volts DC as output.

I think that four AA rechargeable batteries could give my cell phone a
decent run time, and there is plenty of nice four AA battery chargers
available at minimal cost (I have one at home anyway).

I could use LM2825 regulator with 5V output option. See

http://www.national.com/pf/LM/LM2825.html

This one is highly energy efficient. I could also use it with 9v
batteries for smaller foot print. It is, however, very expensive, so
maybe I will opt with a lower efficiency linear regulator.

Any thoughts?

i

What kind of chemistry gives 1.62 volts per cell? If you're
talking NiMh or NiCd, it's ~1.43 per cell immediately after
charging - and that drops rapidly to 1.2 volts per cell,
nominal.

If you don't mind the size/weight, go for a 6V gel cell and
a diode or 2 in series to drop the voltage.

The regulator chip you specified won't work in either
case. According to the datasheet, it needs at least 2
volts headroom - so at least 7 volts in for 5 volts out.
I suspect, but do not *know*, that you don't need a
regulated 5V DC. If that is true, then you could use
4 NiCd or NiMh cells with no regulator chip.

Ed

J

#### jasen

Jan 1, 1970
0
Since every charged battery holds about 1.62 volts, four would give me
about 6.48 volts fully charged, and as long as they are over 1.25
volts each, I could still be getting 5 volts DC as output.

what sort of battery are these?

P

#### Paul Hovnanian P.E.

Jan 1, 1970
0
Ignoramus31975 said:
I have a Motorola V557 cell phone.

My problem is that its battery and charging system is very lousy,
batteries take a long time to change and discharge quickly if I use it
with my Linux laptop as a cell modem (pppd).

It sounds like you are charging the phone over the USB connection. If
so, your laptop USB port doesn't do a great job because it doesn't have
sufficient capacity to rapid charge the phone, so the USB power
handshaking between the phone and the port steps the current draw down.

If this is the case, you might speed charging up by putting a powered
USB hub between the laptop and phone.

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