Measuring the voltage of a battery while not under a load will show it potential, but when you connect it up it can drop to a much lower voltage when the battery is going bad. measure the voltage while it is still connected. This will give you a better idea if it is working or not. Batteries do fail at times an they no longer can hold a charge. A Meter does not put a load on it, so it will seam fine. Then you install it an it goes dead right away.