i need to know please how to know how much capacity a lead acid battery or any battery still have ? is there any way to build a device or something for that ?
No, not in general, capacity depends on future current drain and temperature, and you can't predict
the future. The best capacity estimates are done by battery monitoring systems that record data on
every charge/discharge cycle and thus learn common usage patterns.
The most reliable estimate for an old wet-type lead-acid battery used to be to measure the electrolyte density
with a hydrometer.
If lead-acid batteries are abused the capacity drops like a stone - never over-discharge a lead-acid
battery or leave one in a low-charge state, or store without regular top-up charging. Manufacturer's
data is always the best source of information for a particular battery model (capacity v. discharge rate,
capacity v. cycles).