If it has been dead flat in the past and allowed to stay dead flat for more than a few hours, the
battery may be so damaged it is unable to hold a charge.
As a first test, disconnect the
battery and charge it by using a decent three-stage charger running through a full bulk and absorption cycle. Disconnect the charger and let the
battery sit for at least 10 hours. Then measure the voltage with a digital voltmeter. More than 12.5V indicates that the
battery has absorbed a reasonable charge. Leave it sitting for another day and measure again. The voltage should not have dropped significantly. Less than 12.4V indicates a faulty
battery
Connect a load drawing around 5 amps - a low beam headlight is good.
Measure the voltage (with the load still connected) after 5 minutes and then every 30 minutes after that. Stop the test once the voltage gets to say 11.8V.
Let us know the results and there will be some bright spark who can interpret the results for you.
Of course there are many ways to test the system, but often the real problem is the
battery gets seriously damaged by incorrect charging and over-discharging, freezing of discharged batteries, overcharging by using faulty or poorly-designed chargers or even lack of maintenance by not keeping electrolyte levels up enough, or by overfilling. The obvious cure is to go to a
battery dealer who will be more than happy to pronounce the old one beyond help and will happilu sell you a replacement. You will be happy and the dealer will be happy - and he will be even happier when you return in 2 years for another because the original problem is still there.