All diode type isolators have a voltage drop across the diodes. Good ones just have less voltage drop.
The voltage drop doesn't really matter until the battery approaches a full charge. Then, with the diode isolator, the battery will always be, in evldave's example, .7v below full charge. That's a significant amount.
The voltage drop though a wire is a little different. It varies depending on load, so as the battery approaches full charge, the amp flow gets less, and so does the voltage drop. So even if there is a voltage drop though a wire at high load, eventually, the battery will reach full charge voltage as the amp flow reduces and the voltage drop goes away.
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