We had to do this in our glider for an old radio that required a 14v system..

The original owners solution was a 12V SLA in series with a 2V SLA. Yes, 2V sealed lead acid cells do exist. We've used these to build 14V packs:

http://www.power-sonic.com/ps_psg_series.php#2volt
http://www.sonnenschein.org/A500.htm

This is most effective when both packs have a similar Ampere-hour capacity. Otherwise, the lower capacity pack will be the weak point in the setup.

We got tired of messing with the 14V and switched to 12V LiFePO4 batteries which have a very gradual voltage drop as they're discharged. The result is longer operating time before the LiFePO4 drops below the radios minimum voltage. At least, this is compared to our old SLA setup.

On Friday, June 5, 2015 at 6:24:53 PM UTC-4, wrote:
We have an XCOM radio in one of our gliders.
How do other build a 14v battery?

r/Jim