I have heard innumerable ideas on recharging a GEM car deep cycle gel cell…,
Some say let it go down to 50%; other 60%; some 80%…
my experience with seep cycle batteries ( mainly on boats ) is let them go way down, then recharge, so they don’t form a “shallow” limit…
The only thing you really need to worry about is to not over/under charge the battery (temp compensated) as stated by Deka:
“If the right pressure (voltage) is used for the temperature, a battery will accept charge at its ideal rate. If too much pressure is used, charge will be forced through the battery faster than it can be stored. Reactions other than the charging reaction occur to transport this current through the battery—mainly gassing. Hydrogen and oxygen are given off faster than the recombination reaction. This raises the pressure until the pressure relief valve opens. The gas lost cannot be replaced. Any VRLA battery will dry out and fail prematurely if it experiences excessive overcharge.”
"In many respects, undercharging is as harmful as overcharging. Keeping a battery in an undercharged condition allows the positive grids to corrode and the plates to shed, dramatically shortening life. Also, an undercharged battery must work harder than a fully charged battery, which contributes to short life as well. An undercharged battery has a greatly reduced capacity. It may easily be inadvertently over-discharged and eventually damaged.
As well do not go past 10.5v (open circuit 11.8v) which is 100% discharged.
Oh and here’s Deka’s answer to your question sorry.
“Do VRLA batteries have a “memory” like ni-cad batteries? One of the major disadvantages of nickel-cadmium (ni-cad) batteries is that after shallow discharge cycles, the unused portions of the electrodes “remember” the previous cycles and are unable to sustain the required discharge voltage beyond the depth of the previous cycles. The capacity is lost and can only be restored by slowly discharging completely (generally outside the application), and properly recharging. VRLA batteries do not exhibit this “use it” or “lose it” capacity robbing effect known as memory.”
draining your gels past 50% lowers their life span . The best thing you can do for a 2005 and newer gem is to plug it in after every use . i did a lithium upgrade on a 2 year old gem that had 10,000 miles on it with original batteries . rental plugged in every night .