I happen to have a free supply of Dell laptop batteries (Li-Ion) which are in a good state.
At first I made my own packs 3S2P, which are fine for 10W with the KX3.
Recently I made a 4S2P pack, but at full charge they are 16V, so too much for the KX3.
I added this automatic voltage reducer by Phil AD5X : http://www.arrl.org/files/file/QST/This%20Month%20in%20QST/April2015/SALAS%20QS4.pdf
Here a picture of my pack in use for SOTA, I did 4 activations in a row at 15W (SSB and CW) before the voltage went under the 12.8 V limit for 15W.
There is a 20A car fuse inside the pack, and I have a seperate balancing input connector for charging the pack.
73 - Luc ON7DQ/KF0CR