batteries often show reasonable voltage but if a cell is weak, it will drop way down under load. Of course there can be a dirty connection or ground somewhere too. The only way to confirm it is not the battery is to put a voltage meter on it and leave it there while cranking. Then if it falls to like 12V it is OK but if it falls to much less than 11.5V it is weak or bad. If it holds 12.6 - 12.8V (that is not a typo) under no load, then put the meter between the starter and frame ground. If it falls more when cranking then it is a bad connection somewhere in between. Immediately after a 12V wet cell battery is charged it should red 13.2V to 13.4V. If it drops below 12.6V - 12.8V overnight it is bad. If the battery is more than 4 years old, or has been discharged for very long just replace it. They sulfate when in that state and it is nearly impossible to bring back. If someone wants to get rich, and they have no scruples, just sell a product that is guaranteed to bring a sulfated battery. The world will say they saw a product that will save them the cost of a battery.