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When you check the battery voltage, I expect you are checking the voltage across the terminals.

When you get the 11V indication at the dash/ ignition switch, the voltage is dependent on which ground ( -ve) point you use. Corrosion where the battery negative cable attaches to the engine is common ( the exhaust manifold drain dumps water over them !). To check if it's a ground issue, run a (long) wire directly from the -ve battery terminal to your multimeter; I used to keep one on board "just in case"!

When you check the battery voltage, I expect you are checking the voltage across the terminals.

When you get the 11V indication at the dash/ ignition switch, the voltage is dependent on which ground ( -ve) point you use. Corrosion where the battery negative cable attaches to the engine is common ( the exhaust manifold drain dumps water over them !). To check if it's a ground issue, run a (long) wire directly from the -ve battery terminal to your multimeter; I used to keep one on board "just in case"!