Thanks Solarman, that's what i was led to believe. our particular problem is when we leave the US, our unit does not get stored at one particular location, but storage near where we end up. Therefore, we are unable to easily source one person to maintain any lithium battery if we were to purchase. As to solar charging, we either use a cover for the unit, or would locate under cover storage, rendering our roof solar useless. I don't fancy leaving a portable panel just to charge. Looks like lead acid is our only real choice, and as we hope to return to our unit in November after a year home, we will see how they have faired (and our choice of cover) just by having them disconnected.