When Should I Change my External Battery?

Change your batteries when they stop performing as you want them to. If you are running them at a low wattage ie under 15 watts then they could last a year or more. If you are draining them heavily at 50 watts or more then they could well be underperforming after 6 months. Your charging regime will play a big part in this. Make sure your external cells have perfectly intact wraps on them If they have any of the metal showing under the wrap or the insulator on top please either re wrap or buy new cells 

If you have any doubts about your charging routine just ask. Batteries get warm when they charge but if they are red hot then its time to look into it