there is something I have taken the time to see. Over time people will build really low coils which is fine and dandy. But when you build a coil and it takes a longer ramp up time that when you know your pushing your batteries more then they should. For instance voltage is great and all but the sad thing about it is when the voltage is low the amp can cause more damage. For instance a high voltage 240 compared to 120 the amp with hurt less. This is from experience and touching things I shouldn't. So when you place a battery on a load far out of it's expectations you have a ramp up time and your internally killing that battery. But over time each battery doesn't maintain it's chemical structure over time it degrades. so by that thought process every battery is different even within it's same brand.