Battery

Where did you find a battery that still has removable caps? Most batteries in the last 20 years have been sealed to prevent contamination of the acid from using a hydrometer that was used in the older non-maintenance-free batteries.

The only way for the water to boil out of the battery is if it is overcharging. Use a digital voltmeter to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is considerably higher, the voltage regulator is defective. If it is within that range, the battery may just be old. As they age, the lead flakes off the plates so there is less material to absorb the current entering from the generator. That makes them get hotter than normal which helps the water in the acid boil off.

Most of the batteries sold now do have removable caps. They have moved from "maintenance free" to "low maintenance". They do consume water but at a very slow pace. If you let the water get low enough to expose any plates, then it's too late and the battery is history.