Brian:
You may want to check with a meter or scope to make sure that the current through the input diode is indeed completely off when you intend it to be. An easy way to check is to measure voltage drop across the series feed resistor. If there is some residual diode current then the opto-coupler may not be fully off when you intend it to be.

Also make sure that the opto-coupler output transistor is fully turning off when you cut the drive to the input diode. Check this by measuring across the collector to emitter of the output of the coupler. Sometimes these cheap couplers can have some leakage in the circuit and it may be necessary to put a 470K or 1M resistor from the opto-coupler output base to the emitter to bleed off any leakage. These base resistors can degrade the current transfer ratio of the coupler and require slightly higher drive of the input diode to compensate.

For both of the above cases, if the opto-coupler is not fully off, the gate of your IRF840 MOSFET may not be able to pull fully to the +V_BAT rail. The data sheet for this FET shows that the 0.85 ohm R_{DS (on)} is not achieved unless the V_GS is at 10V or more. Without the FET fully enhanced the drain source resistance could be higher than you expect.

Michael Karas