Why a Prius goes flat

Every Prius has two batteries. The high-voltage hybrid battery moves the car and is covered by Toyota's hybrid battery warranty for 10 years or 150,000 miles on current models. The small 12-volt battery powers the computers, lights and the start-up sequence. It is smaller than a conventional car battery, so a dome light left on, a long stretch parked on the street or an aging battery in a cold Chicago week will drain it. Symptoms: nothing on the dash, a rapid clicking, or the car powers up but never shows READY.

Step by step

1. Park the booster vehicle close, but not touching. Both cars off, in Park, parking brakes on. Turn off lights and accessories on the Prius.
2. Open the Prius hood. Lift the fuse box cover and find the jump-starting terminal with its red cap. On 2023 and newer models, use the positive terminal on the 12-volt battery, which sits in the engine bay.
3. Red cable: clamp to the Prius jump terminal first, then to the positive post of the booster battery.
4. Black cable: clamp to the negative post of the booster battery, then to a solid, unpainted metal point on the Prius such as a bracket or engine mount, away from the battery and moving parts.
5. Start the booster car and let it run for about five minutes at a slightly raised idle.
6. In the Prius, press the brake and the power button. Wait for READY on the display. If nothing happens, wait another few minutes and try again.
7. Remove the cables in reverse order: black from the Prius, black from the booster, red from the booster, red from the Prius. Refit the cap and the cover.
8. Drive for 30 minutes or more. A hybrid charges its 12-volt battery from the hybrid system, but only while READY.

Things not to do

• Do not jump another car from a Prius. The hybrid's 12-volt system is not built to supply a conventional starter, and the current can damage it.
• Do not connect the last black clamp to the battery or the jump terminal. Use bare metal, so any spark is away from the battery.
• Do not reverse the cables. The cap and markings are there for a reason; reversed polarity can take out fuses and modules.
• Do not keep cranking. A Prius does not crank; if READY does not appear after a couple of tries, the 12-volt battery may be past saving.

If it keeps happening

A 12-volt battery that needs a jump more than once is near the end of its life, typically after four to six years. We can test it, replace it with the correct AGM battery for your Prius and check the charging system. Schedule a battery check. Signs of a weak hybrid battery are different: lower fuel economy, the engine running more than usual, and a charge gauge that swings quickly; those call for a hybrid system inspection.