Batteries are constantly going dead from vehicles with newer electronics or ones that are only being driven every few days... so naturally most people will assume that if they disconnect the battery from the vehicle that the drain on the battery will stop.
While speaking with our Engineering department and technical writer Dan Williams he was able to clear up the common assumptions that lead to battery pitfalls.
"There is a lot of misinformation out there. Just like the seemingly logical misconception that a 12 volt lead acid battery has a maximum voltage of 12 volts when it is fully charged.
For a quick overview, disconnecting the battery from the motorcycle (or vehicle) electrical system does make perfect sense to avoid the continuous drain of electrical / electronic parasitic loads. The amount of some vehicle parasitic current draw is significant, relative to the size of the battery. Let's say that the load is 1 amp, which we would consider to be pretty high. A large motorcycle battery would have a capacity of say 25 to 30 amp-hours. With a 1 amp draw, that battery will be totally dead in 25 to 30 hours. If the battery is used to start a high compression engine, then with the capacity below 50%, in this case 12 to 15 amp hours, there may not be enough energy to support multiple engine start 3-5 second bursts to attempt to start the engine. I would guess that 30% State Of Charge (SOC) would normally be enough to start a normal vehicle engine.
Let me just say that for a battery in reasonably good condition, there is a good correlation between the battery voltage measured at its terminals and the state of charge. So if we take 12.9V-11.4V = 1.5V Then that 1.5V represents the full 100% range of battery capacity, or stated as a ratio, then 0.015V on the battery correlates with 1% battery capacity. This is only true with no load and no charger connected.
A typical 12V lead acid battery has 6 nearly identical cells that are internally connected in series. The nominal cell voltage ranges from 2.15V to 1.9V. That means a typical 12V battery has terminal voltages that ranges from 12.9V (fully charged = 6 x 2.15) to 11.4V (fully discharged = 6 x 1.9V). With that assumption, and my previous statement about 50% capacity, I would think that if the battery voltage is less than 12.1V, than you may not be able to start the vehicle. I would give it a 50/50 chance.
A battery will support a higher voltage when it is connected to a charger. In fact, our Battery Tender battery chargers will take the charge voltage up to nearly 15 volts on a 12V lead acid battery, before dropping down to float levels of about 13.5 volts. When the charger is disconnected, it might take a few minutes for the battery voltage to settle at its fully charged voltage around 12.9 volts. You will see an immediate drop as soon as you disconnect the charger."
This is why it is imperative to constantly have a vehicle or a battery that sits on a proper smart charger!