Measure manual charger voltage disconnected. Can the charger put out more than 5 amps? Is it a ten amp charger? Good.
Go to hardware store. Buy old fashioned ceramic base for screw in light bulb.
Buy 10 feet of #14 wire
Next go to NAPA parts store
Ask counterman for FIFTY WATT 12 VOLT BULB FOR DROP LIGHT.
Return home
Connect NEGATIVE charger clip direct to battery.
Cut new wire in two.
Strip all 4 ends 1"
Turn lamp base over
Connect one wire to one screw terminal
Connect 2nd wire to other terminal
Connect battery charger to one or the other wire stripped ends
You have one unoccupied wire end left
Connect it to the battery positive terminal.
Screw in the 12 volt light bulb into the base
Plug in the charger and set it on HIGH. Even if it has a 50 amp boost circuit set the charger on HIGH.
The light bulb is an AMPERAGE REGULATOR. Absolutely vital.
Congratulations. You know have a battery equalizing circuit far superior to anything on the market.
Allow voltage to slowly climb to 16.0 then shut off charger and let the battery soak it up for a half hour.
The almost 5 amp rate works perfect for a 100 amp hour battery.
No true battery engineer would dare challenge this methedology.
I just hope you have a 10 amp charger.
Bigger is better. This would work perfect with a 200 amp manual charger.
The charger CAN NOT BE A SMART CHARGER. They are too stupid to do this.