2008 Prius won't start, not sure if it's a dead battery

You need to recharge the battery. No need to drive it around, just turn it on with one foot on the brake and leave it like that for at least 3 hours. The engine will kick on and off as necessary.

 

Lcruz, welcome.

The 12v battery can be checked by placing the MFD in Maintenance Mode. Here's a video:

Report back with the voltages.

I believe the battery needs to be charged overnight. Driving the car for a little bit doesn't do any good.

Maybe something was left on that discharged the battery. Maybe the battery wasn't fully charged when it was installed. The replacement battery I bought for my 2006 from the dealer didn't last 6 weeks. After going through the start up procedure 3 times I was able to drive to the dealer.

Is your battery under warranty?

I recommend hooking up a BatteryMinder to keep the 12v charged up.

 

Yes, as mentioned above you must try and recharge the battery with either a charger or put it on a trickle charger. Just running it unless you are driving somewhere for a couple of hours may not be enough. Also realize that when you completely drain a battery that there is a chance of no return where one of the cells is destroyed. Sometimes that can happen with one deep discharge. Sounds like you have done this numerous times.

 

On the Gen2 equipped with smart key, we have a button under the steering column to turn off the SKS (smart key system) on trips like that. This helps save the battery from discharging. The Gen2 re-charges the 12v battery quite slowly, I am told, so a short move to the driveway won't help much I am afraid. You would need a decent 60-min trip or so to really get the charge back (or use a charger or as per USNAVY method above). So you are correct to focus in 12v batt as cause of problem. Some of us go with Optima Yellow Top 12v batt which is a little better at deep discharges.

 

When the 12V battery is discharged sufficiently so that you need a jumpstart, you need to fully-charge the battery. This usually requires an overnight charge, whether the Prius is left READY for that period or you hook up a charger to the battery.

 

As other's have stated, the focus is on your battery. Get yourself a good smart charger for your battery. The Prius 12 volt is a very small battery. It does not take much to kill it. Once you start jump starting, it is only a matter of time before you need a new battery.

The best suggestion is this. If you don't already have a "smart charger" such as the "battery minder" mentioned above or any of the new modern type battery charges that will charge a AGM type battery, you should obtain one. After that it is a good idea to top off the battery regularly at least once a month. This will make the battery last longer than just relying on the charging system of the Prius.

Also as mentioned, if your car has the SKS system on it, you should disable the SKS when leaving the car parked for an extended period of time such as two weeks. The parasitic drain on the battery due to the SKS system and others systems on the Prius will drain the battery if the car is NOT driven regularly.

You will probably find that in the end you will need to buy a new battery, but for now charge the battery up fully with a "smart charger". After that, measure the voltage of the battery with a good volt meter. With NO load on the battery, the voltage of the battery should read a minimum of 12.7 volts after setting over night. If the voltage is lower than 12.7, the battery is getting weak.

Refer to chart below for more information concerning voltage levels of the Prius Battery. Remember it is a AGM type battery.

Ron (dorunron)