Battery Discharging when lights are on

I would start by checking your 12v battery and connections. Make sure the ground is tight on the body and battery post.

Also, check the health of your 12v battery.

 

Which battery is it that "starts dropping sharply" ??

Either way, it sounds to me like you have a short in the headlights somewhere.
Could be a bulb, corroded socket or (if HID) a ballast.
Sure sounds like something is drawing excessive current.

 

The first place to check is the negative battery lead where it joins the bodywork below the rear light. This is a known source of trouble either being loose or corroded. Remove clean refit and tighten.

John (Britprius)

 

If that is true, I think you should be REALLY concerned, maybe to the point of not using the headlights at all until you find out what the problem IS and get it fixed.

 

The voltage drop could be caused by a number of things.
In order of likelihood (in my opinion):

• Bad battery.
• Poor connections.
• Faulty component.
• Short circuit.

 

Problem sometimes goes away after hitting bump so it’s probably bad connection somewhere. But it can be in inverter, hybrid ecu, battery ecu, hv-battery, 12V battery, wiring, or some other place.

If you connect techstream to car and problem is present it should be possible to narrow down problem. Or if there is fault codes present (there can be even if fault lights haven’t been on.

-First check your 12V battery Toyota Prius: How to Check the 12v Auxiliary Battery Health - PriusDIY.com
-If it ok then check its ground cable as suggested by Britprius.
-If its ok then check that there isn’t codes (you’ll need hybrid specific scanner (techstream is best and cheap)).
-If there isn’t any code then you’ll have to diagnose problem with techstream connected to car while problem is present.