I assume you are talking about a "spotlight" with a typical 6V lead-acid. if the charger you are talking about is just a typical "wall-wart" transformer, then the voltage rating doesn't really matter all that much if you time your charging accordingly.
What you want to do, is look at the capacity of the lead-acid cell, and the current output of the 9V wall-wart, and figure a reasonable charge time for the configuration. The wall-wart is not really going to be able to exceed the current rating, and it will probably maintain that current rating continuously when charging a 6V lead-acid. Take the capacity of the lead-acid battery, and divide by the current output of the charger. Multiply that figure by about 1.5 for losses to heat during charging and charge for that amount of time when the battery is dead (units should be in hours). Use less charging time when you figure the battery isn't completely depleted.
Just for reference, most 6V spotlights contain a 6V 4AH battery. If you need help figuring out charging time, just let me know what the mA output of the charger you have is and I'll help give you an approximate charge time.
sealed-Lead-acid batteries are pretty robust and aren't going to explode if over-charged a bit, it just wears them out faster.
Eric
