If the bank will only accept 15a then it doesn't matter if the charger can do 40a or 16a, the battery will accept 15a. It won't hurt the batts if it happens the charger could do 40a in other circumstances.
The other limit is the power of the charger to keep raising the battery voltage with the charger at its max amps. Eg, a small amp charger will "stall out" at 15.x volts on a certain size battery where a bigger amp charger will not stall out till battery voltage reaches 16.x
You want a charger that will do as many amps as the battery will accept and also one that won't stall out at too low a voltage. If the charger is rated at 40a at 13.6v don't expect it to be able to do 40a at 15.6v. Read the specs carefully for how much power (watts) it has so it can do the amps you need at equalization voltages (near 16v)
Now the only thing is how big is the battery bank for how many of those amps have to be shared by each battery to see if each battery is getting near that 5% Mex likes. If not, then don't do so much of the bank at once, do half at a time.
