Question: Prius Battery Information 101

You won't "run down the batteries" by driving the car unless there is a major failure within the car or you run out of gas and try to keep driving. The hybrid battery is protected by the car's computer from depleting its charge to less than 40%.

Actually, both batteries are needed to start the car. The 12V battery first powers up the electronics, including the systems that control the hybrid components. Once the electronics are ready, then one of the electric motors (powered by the hybrid battery) spins up the engine before it actually lights.

Therefore, you can use jumper cables to start the car. You can connect them directly to the 12V battery, but because that battery is tucked away inside the trunk, you'd probably find it easier to use the jump-start terminals under the hood. See your manual for more on that.

If the 12V battery is weak, you can further drain it by leaving the car in IGN-ON mode (two pushes of the power button without your foot on the brake). If you want to listen to the radio, better is to have it in Accessory mode (one push of the power button without your foot on the brake), though you can't run the AC. If the 12V battery strength is marginal to begin with, however, even that may be enough to drain it to a level where a jump start is required. If the car is in Ready mode, no worries about either battery. The car will light the ICE as needed to keep the hybrid battery charged.

But then you're burning fuel unnecessarily. You might as well go shopping with your lady.

 

The Prius 12V battery is fine for most purposes, but like any car, if you use accessories with the car turned off or in one of the accesory modes you will use up the power in the 12V battery. As long as the car is "on" (in other words in "ready" mode), and as long as the 12V battery isn't old, damaged, or defective, and as long as the car isn't in need of repair, you won't drain your 12V battery.

Both. The High Voltage (HV) battery is disconnected from the car when the car is not is "ready" mode. The 12V battery is used to connect the HV battery, and then the HV battery is used to run the electric motors and to start the Internal Combustion Engine (ICE) when needed.

Yes you can. The 12V battery is in the back on the opposite side from the fuel door. It can be difficult to access if the 12V battery is drained, so additional jump points are located in the front under the hood. Check you owners manual to identify the proper location under the hood if you need to do this.

If you typically carry a spare 12V battery in your cars, then I suppose you'd want to in the Prius as well. I have never done this in any of my cars, and see no extra reason to do so in the Prius.

I've never tested how much time it would take for the radio to drain the 12V battery, but keep in mind that the AC compressor runs off the HV battery. This means that if the car is not in "ready" mode, the AC will not work. Since you're asking for opinions I suggest leaving the car in "ready" mode if you are going to be sitting in it with the radio on for a while. This will give you access to the AC, and it will keep both batteries charged as needed. When the car is in "ready" mode, there is an DC/DC convertor that provides a bit more than 13V from the HV battery for the radio and other accessories to run off of. Therefore the vehicle doesn't need to "sense the 12V" to keep it charged. The 13V is typically enough to operate the accessories and keep the 12V battery charge topped off as needed.

 

Related question - I know the ICE will start to charge the Traction Battery, but will it start to charge the 12V batter if the car is in 'READY?'

 

I have a related question too....

I've noticed in my 08 that I can drive to work.... The battery is green, all but the top line filled. (Does the top line ever get green?) I turn off the car. When I return to the car to drive home (11 hrs later) the battery is all blue with two or three lines at the top blank. Is this normal? It seems like the battery is discharging itself rather quickly.....

 

When the Prius is READY, the DC to DC converter within the inverter is functioning. The job of that system is to convert 202+VDC from the traction battery to 13.8VDC to power the DC bus. The 12V auxiliary battery is connected to the DC bus, so it will be charging whenever the Prius is READY.

When the traction battery SOC drops down to two pink bars, then the ICE will turn on to keep that battery charged, as long as you are not in N gear.

I frequently see eight green bars on my 2004, but my daily commute includes descending from my home at 1,100 ft elevation, to my office at sealevel.

It sounds like when you left the car, the SOC gauge showed seven green bars; then when you next start the car the gauge shows five or six blue bars. The traction battery has to use some power to start the Prius ICE before you can see the battery SOC, so it is not too surprising that the SOC reads lower. If you don't see any warning lights and do not notice any driveability issues, I would not worry.

 

Tina,

Yes, the top line of the battery can get green. My wife has driven her Prii for almost 7 months and she has had a full green battery maybe four to five times while driving in-town. It is rare, but, it does happen occasionally. I have had my car less than a month and have yet to get a full green battery.

If you shut off your car with a green battery, it should be green when you return. Are you leaving something on when you leave the car? Interior dome lights, hatchback light, personal map/reading lights in the overhead console? Push the map reading light buttons to ensure they are both in the off position. Check the position of the slide switches on the dome lights. They are three-position (Door, On, and Off). The normal position is "Door" so the dome lights come on when you enter or exit the car. The hatchback light has a rocker switch on the left side of the rear hatchback wall. Look in your Owners manual to see the correct switch position for lighting the hatchback area when lifting the hatchback gate.

A good habit to get into that turns off all of Prii electrical systems (except the SKS system) as well as locking the car is to push the little black locking button on the door handle after exiting the car. If all is okay with the car (no key FOB left inside the car), you will get one beep and a flash of the lights when the car locks and turns off all systems.

Hope this helps !!!

 

Hmmm.... I'm not intentionally leaving anything on.... No FOBs left inside.... And I normally lock the car with the little black button.

I'll double check the lights... but I thought the lights, accessory outlets, etc. were run off the 12v system... not the hybrid system. Does the battery on the MFD energy screen show total battery or just hybrid??

 

Would you carry a spare 12V battery for any other car? I doubt it.

Both batteries are used to start the car: the 12V powers computers and various fluid pumps, and the traction battery cranks the engine. If only the 12V battery is drained (example: you left the interior lights on or the hatch open overnight) then a simple jump using the jump points under the hood get you going again. Of course the 12V battery may be damaged by being drained, same as in any car. If you drained the traction battery, say for example by running out of gas and then running the car under it dies completely, then you'll need a tow to a Toyota dealer.

If you want to use the accessories for more than a very few minutes, just leave the car Ready and in Park. The engine will start just as it always does to keep both batteries charged. It will do this until you shut it OFF or run out of gas. *Don't* leave it in Neutral, because the batteries won't get charged. Neutral is only for getting towed through a car wash. And of course don't leave it in Ready if the car is in a completely closed space with no exhaust ventilation.

 

The battery on the MFD is showing you the SOC (state of charge) of the HV (hybrid) Battery.

 

I carry a little booster battery in my Prius, even though in other vehicles I just carried battery cables. Because :

a) I don't have to involve my Prius to help with someone else's dead battery. I have only used the booster twice in the past year, both times on someone else's car.

b) I don't want to trust anyone else to make the connections on my car if I ever need to jump the 12v battery.

 

You are receiving some misleading information here.

First of all, there is some overlap in the colored bars battery display.

7 green bars can indicate anywhere from 64% to 77.5%
6 blue bars can indicate anywhere from 55% to 66.5%

So if you happen to be between 64% and 66.5% charged with 7 green bars when you shut down, you very well might still be at 66.5% but with blue bars when you start the car later.

See the chart here:

http://priuschat.com/forums/fuel-economy/37529-tips-battery-charging.html#post486414

You are correct that the accessories and such run off the 12V system. When the car is off, the High Voltage (HV) battery (a.k.a. Traction battery, a.k.a hybrid battery) is disconnected from the car. The accessories can't drain it. When you turn the car back on it is reconnected.

When you first turn the car on, you have access to the Energy screen before the ICE starts up, and before the car starts to move. I think the car only draws a few amps for the few seconds it takes to get to the point where the Energy screen can be display. Therefore, unless you shut the car off with the battery charged just barely enough to get green bars, I doubt that the power drain from starting the car is causing the display to drop to the blue bars.

The battery on the MFD energy screen shows only the HV battery.

While it is possible that the HV battery may discharge a bit when left disconnected for extended periods of time, you are more likely seeing the effects of the overlap in the display.

 

Be aware as well that the traction battery temperature affects its' voltage. When you are about to shut down, it's probably warm from use. When you start the car it's probably at ambient temperature. Usually colder than when you shut it off. This will change the voltage. The SOC meter isn't accurate enough to account for this and may read differently. Nothing to worry about.

As stated in other posts, the traction battery is completely disconnected (both sides) when you power off the car.

Do be careful to shut off the headlamps when you park and wait for someone. If you leave the switch on (as many do) you -must- open and close the drivers door to shut the headlamps off. If you don't, you could end up with a dead 12V battery. The two headlamps draw from 5-10 Amps on low beam, so you've got about 4 hours on a new fully charged battery, but most of us don't have a new fully charged battery. Assume 2 hours max.

The 12V system runs at 13.8 V, so even if your battery is new, and you've been in "ready" for hours, it may not be fully charged.

 

Some of the loss of juice in the traction battery can be due to all of the pumping and switching and whatnot the car does after it's powered down. In addition, there are some systems that stay continually powered, such as the Smart Key System (SKS) receiver and the security system.

 

But the 12V battery is used for these mundane tasks, not the hybrid battery. Right? The hybrid battery is disconnected unless in "ready" mode. This seems to be a point people are getting mixed up on.

 

Correct, but then, as soon as the car goes to Ready, the traction battery is tapped to recharge the 12V battery. That plus the coarse granularity of the display and voltage as a function of temperature probably accounts for most "why-did-the-display-drop-overnight" observations.

 

I will say this because yesterday (April 23, 2008) I officially ran out of gas on the road.. (intentionally by the way) .. and the battery went dead on me about 6ft from the gas stations' pump ... with that said .. my battery went dead .. no voltage and .. let me tell you put your prius on neutral and push it .. man that car really is lighter than it seems .. So .. yes you can run your battery dry .. and well theres only one hun ..... (Oh by the way ... it will never go completely off because even though my prius wouldnt move it still started the engine so I suppose it has a minimum cap .. I could be wrong... Oh and I drove 10 miles at 50mpg on half a battery.

 

Why?

 

Perhaps I'm an ignorant moron, but I've never understood this Prius owner fascination with intentionally running out of gas, and making the car go as far as possible on the battery alone. Possible desire to void the battery warranty, and cook the in-tank electric fuel pump in the process, is there any *purpose* to this?

It's not like you see Honda Civic or FJ Cruiser owners bragging about how they ran the tank dry. Every vehicle I have ever owned, half a tank means fill up. One quarter tank I start getting nervous

 

I hope this is not off topic since it has to do with the battery. My battery failed to start the car after six days without being driven. The smart function was on. The next time I took the car to the dealer for my 20K mile service (2005 Prius Service Pack 7) service tech told me this was normal due to the system kept on drawing a small amount of juice and suggested that I should disconnect the battery when I wouldn't be driving for a week or so. Disconnecting the battery in the back of the car is a real pain so I was not a fan of that.. When I had the 25K miles service I commented the information to a different service tech and he said that the problem existed I didn't deactivate the Smart function. He turned out to be correct as I found out the next time the car sat idle for over a week with the Smart function off. I eventually found the sentence in the manual that said essentially the same thing. Has anyone else received conflicting responses from the service techs?