Most but not all LA batteries will be damaged if taken below 12.5 volts for a long period, recharge as soon as you can.
The trickle charge value for a battery is dependent upon battery size, manufacturer, age and previous usage. The only good method is to adjust the trickle charge voltage so that the battery does not gain a charge, it just retains its charge, or even loses it very slowly. Recheck every 12 months of so and re-adjust if needed.
Charging above 13.6 volts will cause any LA battery to gas, even a sealed one! This also reduces total life expectancy of the battery.
When sizing a battery, assuming you want it to last a long time, buy one that 70% of the capacity will cover all your needs, then only charge to 13.4 volts (around 70% capacity), it will last a long, long time and never gas.
Battery gas can be dangerous to life and limb when it gets ignited by a stray spark or similar.....so its far better not to gas......humans also!!
1) "Wer nicht weiß, glaubt viel! "2) For those who understand, no explanation is necessary, for those who don't, no explanation is possible!