Android GPS battery drain - fix?

When using my Android phone, with the GPS on, the battery drains rather quickly.
Found this idea on a geocaching forum.
Cheap - don’t worry about the no charger/leads bit use a small usb from PC/Laptop to charge.
Bluetooth doesn’t seem to drain the phone very much :slight_smile:
Its been on and running about 8hours and still half full battery.
You need to download Bluetooth GPS from Android Market and enable mock GPS provider.
Running APRSdroid at the moment G4OWG-10

Roger G4OWG

In reply to G4OWG:

GPS is always going to be power hungry. Our software is used by the chip makers to play with power optimisation and it’s surprising how much of the phone can be shutdown when you aren’t actually making a call but just sat there “sniffing” the network for incoming messages etc.

But with GPS, timing is everything. Simply maintaining lock of the spread C/A signal only gives +/-300m accuracy. If you can’t keep the time sync accurate you appear to jump about in position. So 1mS error will make your position jump back and forwards 300m. A 100nS error is 30m. If you try to shutdown the GPS and power it up you have to wait to regain code lock then fine tune your position. That can take a few seconds. The end result is a lot of the GPS electronics stays running all the time and that sucks the life out of the battery.

If you use an external GPS then from the phone’s view, it only talks over Bluetooth every sec. Bluetooth is 1600 slots/sec and max sleep time is 2secs without dropping into park mode. So for 1 position report from the external GPS the Bluetooth RX can sleep for 1596 slots then wake up, spend up to 4 slots regaining timing sync with the GPS, grab the position data into 2 slots and go back to sleep. The power saving is huge compared to having GPS on all the time. The apps CPU can wake up, chew on the data, decide you’ve not moved (or you GPS timing is good!) and sleep again in few hundred uS. Again not much current draw. It only really eats into the battery if it has to transmit your position.

The only reason non-smart phones had standby times of 10days was because they were hardly ever active. They also had a simple slow application/UI CPU and single GSM/CMDA core. Early smart phones like my E71 had a 380MHz ARM11 CPU with a few cores for GPS/Bluetooth/Wifi. Even with it all on there’s not much happening very fast! One of the modern Android phones uses an SoC we’ve worked on that features 1x dual core ARM Cortex A9 apps CPU at 1.2GHz, 2x 324MHz ARM1136 CPU for stuff, MPEG encode/decode audio/video core, GPU, 5x ARM7 CPUs for housekeeping, 600MHz 32bit DSP, USB, MMC/SD, GPS, Wifi, Bluetooth, GSM/3G/CDMA cores plus 7 band RF RX/TX, Wifi RX/TX, Bluetooth RX/TX. And it all fits in your pocket! But if you use it the battery dies quickly.

I remember back to my first analogue cell phone which had a standby time of about 20hrs. So considering what you get now in a smart phone it’s really impressive you get any battery life at all.


In reply to G4OWG:

This looks interesting and is cheap enough not to require approval from the central purchasing dept. My Nokia 5800 GPS similarly drains the battery rather too rapidly for comfort when relying on it for such things as self spotting, map storage, calling home, summoning rescue services etc. I may well invest in one. How sensitive do you find the receiver to be in practice?

Another advantage of an external unit such as this is that you can place it at the top of your rucksack so it can gets a better view of the sky than a phone would in a pocket against your body. However I wonder how well the inbuilt battery will stand up to sub-zero temperatures in such a location?

I have made myself (i.e. modified) an in car charger so that I can top up my mobile from a 12V battery (of which I tend to have one or more of with me)

Rick M0RCP

In reply to M0RCP:
Hi Rick
It’s better than my Etrex (old yellow one) but not as good as my Etrex Legend HTx.
I’d say its comparable with the Samsung Ace built in device.
The battery is 3.7v 700mAh Lion.Operating Temperature: -20° C to +60° C
Relative Humidity: 5% to 95%,non-condensing, according to manual :wink:

Roger G4OWG