8V can mean shot, or severely depleted, or under a high loading for it's present capacity.
The conditions of the voltage test mean a great deal.
Take a brand new known good battery and put a wrench between the posts. While you watch it glow and before it melts away, measure the volts across the terminals. Won't be near 12v, and almost certainly much lower than 8V. That wrench would present very high loading to any battery. And, the battery cannot sustain expected or desired voltage while supplying such a high current load.
This is an extreme, of course. It is possible to bring the voltage of a new, good battery to a very low depletion level. Without a load it will self recover to something close to 2V per cell. It may still not be able to deliver much current after self recovery, and the voltage will fall toward zero as more is demanded from it. On the other hand, if a cell or two has died, the battery/cell assembly voltage can be below 2V/cell. The remaining cells can even deliver full current load power. But, the sum of what the battery's team of cells can deliver is diminished by 1/6 for each cell that has died.
The standard wet cell MC starting battery can sustain about 200 amps drain and still maintain a 10V post terminal voltage (for a period of time). As the battery ages, such loads will make the battery voltage lower and lower. Fortunately, our bikes usually take only about 25 Amps during electric start. But eventually, even that current load will lower the output voltage during the drain below 10V, even if all the cells are doing their best.
Remember, voltage by itself is pretty much useless. Everything around you has a voltage potential, most of it static. It's not until current and resistance enter the picture that useful work can be produced.
