Removing 12V Positive. Do I need to isolate it from ground?

Good point uart. It would be interesting to see what a DVM would show if one was to disconnect the positive terminal from the battery while the brake capacitor is still connected to the system. Maybe Patrick Wong will know that answer.

When I swapped the 12 volt in our Prius, I didn't worry about the presets. But I did unplug the brake capacitor and isolated it from the system before I disconnected the negative cable at the frame of the car and then proceeded to disconnect the positive terminal from the battery.

In the end I had NO problems whatsoever. It did take two pushes of the power button to get back into ready mode. Several folks have posted about that.

Of course, I did lose the radio presets and needed to initialize the auto up window plus the two trip odometers read zero. But no other problems, no crazy lights/codes etc.

 

This is only a theory, but I would imagine the brake capacitors would be protected from discharging back to the battery by a blocking diode.
If this were not the case the capacitors would be discharged by other vehicle loads "in case of battery and inverter failure" or even trying to recharge the failed battery. This would leave nothing to operate the brakes. So I would say the answer to your question is this should not be a problem. I believe there also would be a warning in the handbook.

John (Britprius)

 

There have been several recommendations on here to insulate the positive lead after removal to prevent accidental contact with ground.

 

The capacitors on the DC to DC converter would be discharged almost certainly by a resister across any smoothing capacitor fitted. Normal procedure in such circuits.
This capacitor would not need to be large in value as the battery would act as a large capacitor anyway, and would mainly form the function of RFI suppression. This circuitry is likely also to be protected by diodes.
A circuit diagram of the converter or even the whole inverter would be interesting to see, but I do not think Toyota are going to release this any time soon.
Metering from the positive terminal to ground may not be conclusive, but I doubt any voltage will show otherwise the radio presets ect would not be lost when disconnecting the battery as the capacitors would act as a battery to hold them.

John (Britprius)

 

Following the procedure of disconnecting the negative lead first for safety, would give ample time for any charge that may exist to dissipate before the positive lead is disconnected. However there is no harm in being safe.
I have just searched the Prius circuit diagrams and can find no trace of the brake capacitors in the braking or power distribution sections.

John (Britprius)

 

The level of charge that could be discharged by grounding the 12+ lead should be commensurate with the charge transferred when reconnecting again. In typical inverters with largish electrolytics (like I use here for power) one has to steel themselves for a healthy POP when reconnecting live power leads (and which many locally were happy to pay me to do, as it is equivalent to death in their minds). I don't hear any reports of a significant spark at reconnection time, so the smoothing caps on the bus can't add up to much.

 

I found this circuit diagram in the 2010 Repair Manual.

 

Thanks, unfortunately it is only a basic block diagram so it does not really show the detail. It does show a capacitor at it's output, but no value or type.

John (Britprius)