A user recently reported that his Primer2 Li-Ion battery exploded while he was tryin to recharge it directly with an external charger.
IMPORTANT:
It is well-known that the recharge of a Li-Ion battery could be dangerous (could make the battery explode) in certains circumstances when the charging procedure is not followed scrupulously.
For this reason, the Primer2 was designed with a dedicated charger device (L6924D from ST) that guarantees that the exact procedure will be followed.
Only charge the Primer2 battery via the USB to avoid such problems.