Hi,
The right way to do this is to use a power supply (the power supply) to power both the trickle charging AND the lasers.
In other words, the lasers main power is the power supply, and only use the batteries if the power goes out from the power supply.
Whenever the power supply is on, the batteries are getting a trickle charge. The lasers do not use any battery power at this time, only when the main power goes down, all other times they use the main power itself.
This preserves the life of the batteries.
If you have to run the laser for long periods without the power supply power you may have to use a different method to charge the batteries as they might take too long to charge back up on a low trickle charge like C/40.