In many forums. one has to get to the fact vs opinion. A review of the previous post have some validity, but ... I'll try to help.
The switch MUST be on the Positive lead and as close as possible to the battery. If not, you still have a HOT battery that can cause a short, .... Granted the circuit would be open, but if you touch ANY positive wire inside to ground, you WILL have a complete circuit and KABOOM!
I would disconnect the battery whenever doing external charging so that the charger can get proper load measurements it needs from just the battery alone.
Battery Charging is a long story - You can read the following FAQ from a website that explains it in detail, but it has a lot of extras too. ZRD FAQ - Charging, Remote Charging, Battery Requirements
Personally, I ensure charge is initiated when the battery bank gets near 12.5volts and certainly by the 12.3-12.0 volt range to ensure long life.
Most standard cells are 2.2Volts when they are fully charged (times 6 = 13.2Vdc). Charging Voltage has to be 1.0 Volt above battery voltage to work. This is why the typical single stage automotive type alternator is set to output at 14.2-14.4 Vdc. This will enable the battery to reach the 13.2Vdc to complete a charge. Read the FAQ above for additional info on 3stage charging. For a single automotive type battery, it is not necessary, but if you have a large expensive battery bank it is imperative.
Multiple charging sources are great, but only use 1 at a time to charge a battery/ bank. Ensure your solar is turned off/disconnected or it will confuse everything if you are not using the solar exclusively.
BTW, one of the biggest killers of batteries is leaving them almost permanently on a charger. Consider it eating one teaspoon at a time, but never stopping.
I hope this helps/clears some thing up. Ask away if you need additional info.