Car battery dead (I think) can't start. How to jumpstart from the trunk?

There shouldn't be any unpainted chassis element back there, so just hook jumper cable positive to battery positive and jumper cable ground to battery ground. READY the car. That should start the ICE and trickle charge of the 12V. Then remove the jumper cable ground before the jumper cable positive.

 

AirBoss is right assuming you can get the hatch open and take all the floor panels and boxes out. Just be absolutely sure plus to plus and negative to negative.

 

In my experience the OEM battery has pretty indistinct identification of plus and minus terminals. Get a good light in there, and as a cross check: the RED cap is on the positive terminal.

Wouldn't sweat the lack of a ground point. If you're struggling with that, just connect directly to the negative post. I know it's frowned on, but seriously: if you blow across the top of the battery directly before connecting, ANY hydrogen gas will be gone. The battery is vented too. Blow again just before disconnecting.

The sequence is:

1. Connect to good battery positive terminal
2. Connect to dead battery positive terminal
3. Connect to good battery negative terminal
4. Connect to dead car ground (or directly to negative terminal)

Once your car is running, disconnect in the opposite order.

I would check the battery first with a digital multmeter, it may or may not be the battery. If you've got say 12.5 volts or better, it's likely not the battery.

 

Ready is push Power with the brake on. If the computers feel like it, they will start the engine, it is never something you decide.

Off topic: Most cars use the engine as a part of the electrical system. (The alternator produces electricity) There is no particular reason to use the engine on a Prius as a negative connection point for the jumper cables as there is no electricity generated by the engine and not much consumed by it.

 

Ready is the on or started state. You could use the body for negative but directly to the battery is arguably better.

 

clamp it to the body ground if you can. if not, you'll have to go with the battery post. you can try starting the car right away, but if it doesn't work, wait awhile and try again. it depends on the state of your battery.

 

Why not jump using the under-hood terminals?

 

One worry about using both terminals is the spark may ignite Hydrogen gas escaping from the battery. The 12 volt battery in the first 3 Generations of Prius (2001 - 2015) use externally vented batteries to outside the car. Externally vented batteries are rarer, but Toyota uses them in the Prius as the battery is in the passenger compartment. (instead of under the hood)

 

Why are you not recharging this discharged battery? You'll have to drive or leave the car in READY mode for a long time, if you want the car to recharge this battery. Easier to let a smart AGM compatible battery charger do this for you.

Two good smart AGM compatible chargers.
Schumacher, $47, Amazon LINK
Ctek, $68, Amazon LINK

Christmas is coming up. Buy your Prius a gift.

 

We use the Ctek 7300 along with LED battery state indicator/charge pigtails on each vehicle and power equipment battery. Love it. Couldn't do without it.

 

There are maintainers and chargers. Some maintainers will only operate if the battery is already charged to a minimum voltage. Below that it will do nothing. Those are designed for long term storage and can be left connected for months. Some chargers can't be left on for too long or they will fry the battery by overcharging. Some units will do both. Some add a de-sultfication cycle which helps restore a completely discharged battery. So if buying, be sure of what you are getting. They range from $28 to hundreds.

For this reason I have a maintainer, a charger and a load tester as well as a voltage meter.

Be very careful you get the polarity right or you can spend $1k plus repairing the damage.

But on the Prius, where the traction battery charges the 12v, do you disconnect the battery before hooking up a charger? I'd think that, once started with the ICE running, you'd just go drive somewhere for a good while and charge the traction battery so it could charge the 12v.

 

Hydrogen off-gassing is primarily an issue in a closed area at high charge rates when the battery is over 14v. Even then lots of RVs and older trucks have the battery in the passenger compartment where people smoke and relays arc. Some of us have a small UPS like a Tripplite near their computer system that has a lead acid battery that can off-gas. Of course having a vent is best practice when charging and maintaining a safe charging rate using a quality charger is even more important.

 

Now that you have had this experience and could not get to the recommended jump start terminals under the hood, you should consider getting a portable jump start box that can even charge your phone in an emergency and has spark proof technology and reverse polarity protection built in (sometimes called "Smart Cable").

The smaller lithium based units are best as they hold their charge for long periods. They are available everywhere but I saw a good deal at Costco.

Lithium Jump Starter And Portable Power Bank

 

You got the car running, but the battery is still "running on empty". You should et it checked, or do your own checks, then go from there: charge it and/or replace it.

For maintenance:

1. Charger (smart charger, 4 amp range)
2. Digital Multimeter (for doing a simple assessment of at-rest voltage)
3. Digital Load Tester (solar BA5 is one, gives more in-depth assessment, shows actual Cold Cranking Amps)

For emergency:

1. Jumper cables (cheapest, least convenient)
2. Jump pack

For battery swaps, not necessity, but saves settings:

Memory saver of some sort, to prevent settings being lost when battery is disconnected. A good method is a cable that connects between jump pack and the car's OBD port. Supplies power, and has indicator light(s) to confirm connection.

 

IMO, everyone should have a home battery charger with jump capability. That plus an extension cord would easily reach the front of the car and the hood jump points. A few minutes will get enough voltage into the 12v and have the car "ON". BTDT

Jump packs are also a good, nifty idea.