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Hi Nick and you got it in one.
The USI-160 will not allow you to jump start from the auxiliary battery if you have a failed/faulty cranking battery.
This has been done to protect the auxiliary battery from being damaged.
With just “10ish” volts in your cranking battery, I think you will find you have dropped a cell.
While you could use a single jumper lead to try to start the motor, normally an auxiliary battery will not be able to start the motor by itself.
With a failed/faulty cranking battery, it usually requires being started form another “Donor” vehicle, with it’s motor running, so you can force raise the voltage of the stuffed cranking battery enough to allow you to start the motor.
The JUMP START feature will work with a low cranking battery, any battery above 10v, but the program is designed to disallow any attempt at starting the motor from the auxiliary battery, if the cranking battery is below 10v.
There has been a change to the USI-160’s program where it use to do the same for a flat/faulty auxiliary battery, but experience has now shown that once the motor is started, from a good cranking battery, it really does not matter what state the auxiliary battery is in.
So in the latest version, it now connects the auxiliary battery once the alternator is producing a high enough voltage to activate the USI-160 now battery what the voltage level is at the auxiliary battery.
This change also means you can use the USI-160 ( when set to SHEARED mode, thats with the switch positioned way from the LED ) ) to run accessories from your cranking, even if you have removed the auxiliary battery, and the USI-160 will still protect the cranking battery so it can not be discharged below a safe level to be able to start the motor.
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