On Mon, 03 Dec 2012 11:34:17 -0000, Granity
wrote:

Lithium batteries have a history of catching fire, indeed they brought
down a 747 in Dubai recently when a cargo container of them caught fire.

It's mainly due to charging the battery faster than it should be,
causing minute lithium dendrites to form on carbon anodes which in turn
cause short circuits which heating the battery up beyond it's safe
level. So make sure it's the right charger for the unit when you use
it.

Lithium batteries (and nicad and niMH) are all more critical with their
charging requirements.
Many devices such as mobile phones containing lithium batteries now have
the charge control circuitry built into the device rather than the
charger, so they can be run from relatively simple PSU's without risk of
damage.

The OP shouldn't have any of these problems since his gardening equipment
will have been bought complete with batteries and chargers.

I've never heard of lead acid batteries catching fire, as such,
exploding due to blocked vents happens occasionally though or melting
when someone drops a spanner across the terminals.

Or charging in a confined space where the liberated hydrogen can build up
to dangerous levels.