12v power source for my sat nav?

[somert8]

Hi all,I looked through past posts and couldnt find any answers to my problem which is as follows;

 

I have a `My guide` satnav which is great except for the battery life (integral),terrible,and was just wondering if I could just use a small 12v lead battery(house alarm type) with a female cigarette socket attached to the battery,and then I can plug in my car adaptor for my satnav and geocache for hours,yes I know it may be quite bulky and heavy but I can put it in a rucksack and travel for miles,or should I buy a Garmin etrex! and stop being a miser?

 

Any thoughts would be appreciated.

 

Thanks.