Question for electronics boffins with intrest in GPS

I have a Fortuna Clipon BT GPS unit with a 1200mA Lithium-Polymer
battery which lasts for 8.5 hours, give or take a bit.

I also have a Pocket PC with a 1350mAh battery which, however long
it lasts is insufficient for long walks or day long cycle rides in the country,
especially with bluetooth+GPS continuously on.

given the above specs and the fact that the PPC needs 5V at 2A, how
long will I get out of a 12V 7Ah sealed lead acid battery or a 17Ah unit ?

reason is, I can get a 12V 7Ah battery for £8.80 ($17) which I can lob in the
rucksack or attatch to my bike whereas spare PPC batteries cost upwards
of £25 and I'd have the hassle of swapping them out every hour or so.

I've looked for some simple formulae which will allow me to work it out
but I can't seem to find the right starting point.

all/any help, tips and links welcomed.



RT

[news] wrote:
2A sounds rather high for the PPC, are you sure it really draws that
much current continuously? But I'll assume it's correct.

How will you be converting the 12 VDC of the battery into the 5 VDC that
the PPC needs?

If you use a linear regulator, then getting 2A at 5V will require 2A at
12V and your 7 Amp-hr battery will only last for 7 Amp-hr/2Amp = 3.5 hours.

But switching regulators are much more efficient. Ideally you could get
2A @ 5V with only 0.83A @ 12V (power = voltage x amps, so this would be
with no loss of power or energy). Real switching regulators aren't
quite that efficient, so let'c say you need 1 A instead of 0.83. Then
your 7 A-hr battery will last for 7 A-hr / 1 A = 7 hrs.

Peter wrote:
country,

blimey, thanks for the quick response :-)

under the PPC battery there's a load of specs and that's where I got 5V/2A from.


I have a 12V to 5V dual plug power lead that I use in car for the PPC and GPS
and I've no idea if it's a switching regulator or not. add a plastic battery box
and
a cigar socket and that's all I need.


ah, I didn't realise that 2Ah into 7Ah was all I had to work out, is that it ?
seems obvious now you mention it.


that's much better, 7 hours is a days cycling when a long lunch is factored in
12 hours would be even better but then the weight of the inexpensive batteries
I can get comes into the equation.


RT

[news] wrote:


Without knowing the voltage of the battery pack, it's not going to give you
ideal calculations.



That 2A figure is just the manufacturer's specifications. You'll find it
doesn't take much more than an amp if only less that that. (My iPaq only takes
an amp anyway).


You'll very likely have to use a 12v adaptor with your PDA, and possibly with
the BT GPS receiver, but you would need to actually measure the current
consumption at the 12v side before being able to tell the battery life on one
charge.

I've done it with a smaller (4A/Hr) battery, and a hand-held GPS reciever
(that would be lighter with power consumption than BT), and it lasted me all
day (nearly 12 hours) with the iPaq screen on, and the backlight at full
brightness.


It probably won't help with accurate numbers if you're starting at the device
power consumption. Not if you *need* to use it with a 12v auto style adaptor
anyway. There are innefficiencies there that you have to account for.

That's why I mentioned the need to measure the current use at the 12v side,
WITH the adaptors in place.


If you don't have a current meter, mentioning the type of PDA and BT
receiver, someone else with the instruments (and hopefully the same/similar
units) can give you a better idea of what the power use is.

From there, it's easy to predict the battery life.
--

G'day John

Not wanting to hijack the thread, I just have one question and it seems
you might be able to help??

I've just bought an iPaq HX4700 to use for in-car Sat Nav....still
trying to decide if I should buy CF/SD/BT GPS having never seen either
of them 'in the flesh'...which one has lower usage (in reality, as specs
aren't always a true indication). Ideally I'd like to be able to use
the GPS in my notebook at times too, but that's not essential as I have
a Garmin eTrex.

Where I am at the moment, I can see pros and cons for each type and
finding it hard to make a decision based on lack of experience with
PPC/GPS use. Any suggestions would be appreciated.

Thanks in advance
Bronwyn ;-)



John Tserkezis wrote: