As a QRP operator you donīt need to worry too much about what power supply you use at your home station. You can take what ever is at hand as long as it does not generate any noise. You will hardly need anything that is capable of delivering more than 3-4 Amp at 12 or 13,8 Volt.'
In my shack I have one delivering 3 Amp and one switched delivering 6 Amp. Both have served me well without any detectable noise, or rather, with the background noise I have, I canīt detect any. I have also tried a 4,5 Amp power supply from Mascot for laptops and printers. It didnīt work very well. It was obviously not stabilized. Voltage was dropping when transmitting. Thus, it can not be recommended as a power supply for a QRP transmitter.
For portable operations itīs quite another matter. I started out using lead acid batteries. They are cheap but heavy. Very heavy! The one at 4,5 Ah weights 1800 g and the one at 12 Ah weights 3400 g. Not something you want to carry too long on your back.
To have something more comfortable I started using NiMH batteries. Ten AA-size in a pack makes about 12 Volts and will weight about 330 g in total.
In the AA size you can get regular NiMH batteries with up to 2700 mAh, but they will self-discharge rather rapidly. If you, for example, are out on a two-week trek in the mountains itīs something to consider. Of the second generation or instant NiMH batteries with low self-discharge you can get batteries with up to 2500 mAh. Sanyo Eneloop batteries are supposed to be the best. With an ATS-4 tranceiver 2500 mAh will be enough for 8-9 SOTA or SMFF activations.
The disadvantage with this type of NiMH batteries is that they are rather expensive and you have to have a dedicated charger, which, if you want to be able to charge all your batteries overnight and within the limits of what the batteries are rated for, can be rather bulky.
I recently discovered self-contained 12 Volt Li-ion battery packs on www.ebay.co.uk that seems to be ideal for portable operations. There are different variants from 1800 mAh up to as much as 18000 mAh. They come both with and without a hard case. Protecting Li-ion batteries in a hard case is highly advisable as damage to the battery can create a short curcuit within a cell. In the worst case this can result in pack overheating, igniting and start a fire.
The one I got is rated 6800 mAh and is contained in a sturdy plastic case. The dimensions of the case is 108x65x25 mm. The battery itself weights only 225 g and the small charger 47 g. It charge the pack with 350 mA so it will take some time to charge a fully depleted pack. On the other hand you should never let a Li-ion battery become fully depleted as this will seriously shorten its life.
I have yet to put it to the test in the field, but so far it looks very promising.
Update May 2012:
After some practical experience I do not think that it is possible to get 6800 mAh out of one of these packs without causing serious damage to it. They are most likely widely over-rated. I have not performed any serious test, but letīs say that you are able to get perhaps one third of the rated power out of such a battery pack. Even so 6 to 7 SMFF- or SOTA-activations is no problem.
However, and even if that may be, the battery pack plus the small charger make a very handy combo compared to a set of AA MiMH-batteries and a charger of the usual design for such batteries.
For day trips into the bush or a weekend activating SOTA:s or SMFF:s using QRP it is clearly a better alternative compared to a set of AA NiMH-batteries. If you are thinking of a longer trip into the mountains you should likely consider a pack with much greater capacity or some other alternative.
Update September 2013
During this summer I have tried a 9800 mAh battery of the same type as the 6800 mAh I have used before. The weight - 436 g - is double of 6800 mAh. I have not used it until it was compleatly exhausted, but even so, my feeling is that it has also double of the power of the 6800.
Two lead acid variants
My battery packs for portable use.
From left: 9800 mAh, 6800 mAh, and pack of 10 Enelope batteries
On the left what you have to carry if you want to charge the Li-ion batteries overnight in the field. On the right what you have to carry if you want to charge your Enelope batteries overnight in the field. An easy choise, isnīt it!