12V Battery questions from a prospective buyer

Hi Boji,
I am picking up my Gen III in a couple of weeks, so the answers below are what I have gleaned while lurking.

1) What is its purpose?
I believe the 12v batteru powers the low voltage electronics - radio, lights along with all the computers that reside in the Prius.

2) What does it power exactly in the current gen Prius?
I don't know.

3) Also, since there's no conventional alternator in the Prius, how is it recharged?
The 12v battery is recharged from the main high-voltage battery. I tend to think of the 12v battery as a mammoth capacitor that helps reduce ripple from the main supply.

Cheers
Warwick

 

One common situation is when you get up in the morning or are leaving work after many hours of freezing rain or snow/ice. There are times when your car looks like it's been in an ice cube tray in the freezer. In these cases, there's not much you can do but start the car and let it idle while you scrape the snow and ice off the windows. It sucks, but it's a fact of life. I left my '90 Toyota pickup at the Manchester, NH airport for a week and when I got back the entire truck was encased in about an inch and a half of solid ice.

 

1. Its purpose is to power conventional automotive electricals/electronics. Plus, when the vehicle is switched off it is maintaining power to ECUs (for learnt adaption), SKS (smart key system), trip computer/audio system/clock to retain memory.

2. 12V battery powers everything except MG1/2 and A/C compressor.

3. DC-DC converter in the inverter unit. Converts 201V (from hybrid battery) to around 14V to keep the 12V battery charged.

 

One important point not listed: When the Prius is powered off, the high voltage battery is completely disconnected. Some sort of power source is needed to power up the computers and reconnect the HV battery. The 12V battery serves this purpose. Think of it as booting up the Prius.

Tom

 

If the 12v battery is drained, you will not be able to turn on the Prius. You need a "jump" -- but really only enough to boot up the Prius as once it's "READY" the HV battery and DC-DC inverter is supplying 14v to the electrical system. (And charging the 12v battery.)

Because there is not high draw starter like on a normal call, the 12v battery is much smaller than on a normal car ... and those little jump start kits would very easily supply enough power to the car to become "READY."

Also, if the HV (high voltage) battery is completely drained (After sitting for years, I suppose) you are SOL and the car would need to be towed to a dealer. There is no way to jump start a prius in that condition. I assume the MFD (multi-function diaplay) would show a "MAIN BATTERY FAILURE" message and the car would not be able to become "READY."

The manual says you need to drive the car at least every six months (if I recall the right time interval) to make sure the HV battery stays charged. Luckily NiMH batteries loose their charge pretty slowly, especially compared to LiION batteries.

 

Anyone know how many amps are required to jump start?

Would a 7amp sealed lead acid battery have enough amps to boot the Prius to "Ready?"

Also would a low amp trickle charge from a solar cell keep the battery up enough without hurting anything?

 

There's a measurement on this forum (graph of amps vs time) that shows what is used to get to "ready". My measurements (DVM, not graphical) showed about a 50 A 1/2 sec. pulse, one for the brake booster and one for the electronics.

A 7 a-hr gel cell battery -might- be able to do it, but they aren't designed for that high a current. I recommend a portable "booster", sold in auto parts stores. I think the best type are the ones that plug into the cig. lighter socket. That would ensure you -don't- have to remember to charge it from time to time. Note that when the car is off the cig. lighter socket and the accessory socket are both shut off. You will still have to connect the device to the boost terminal under the hood. It's covered by a flit-open red plastic cover. Use a bolt above the fuse box for the -ve lead.

 

I've read specific instructions somewhere. As I remember, the Prius does not have enough 12V to run the other car's starter -- which makes sense, since the Prius itself doesn't have a starter -- so the key idea is that you need to let the Prius charge their battery for a while, then disconnect and let them start off of their own battery only.

So you're not "jump starting" them so much as charging their battery.

 

A good thing to know also:

The 12V battery is required to open the hatch (it's electrical). Therefore, to jump-start the car, you need to use a positive terminal in the fuse box under the hood as you can't access the battery itself (unless you're highly motivated to empty the trunk from inside the car to access the manual lock of the hatch).

Also, the only mechanical part that can be engaged/disengaged on a Prius is the park pawl. It requires 12V to be moved (anyway you need to boot the car to make the shifter do anything). So: no 12V, no Neutral...

A Prius can easily be boosted from another car (as long as you follow the instructions and don’t short/overload the 12V circuit in some unusual way) but Toyota does not recommend boosting another car from a Prius. The reason it that the DC-DC converter (in the inverter box) is not made to supply a current peak such as the one that is required when you try to start another car engine from the Prius 12V circuit. As the Prius 12V battery is quite small, the DC-DC converter would do most of the job. You could still use the Prius though to slowly recharge the other car battery before disconnecting the jump cables and trying to start its engine; but that would require time.

If you do some searches on PriusChat, you will easily find recommendations for portable 12V emergency chargers. It seems that a small one can do the job (supply enough current to boot the computer and have the high voltage battery back online to charge the 12V battery).

 

Yep, sure does.

 

In the unlikely event of a tow truck without jumper cables, yes, he is going to skid the front tires rather than roll them.

 

And probably risk breaking the parking pawl in the CVT as well.

 

Unfortunately, I had to be towed several time with my 2005 Prius due to 12v battery problems. Never had a problem with a quick recharge to get in onto the tow truck. On a side note, I notice that with my 2010 Prius III, my "ODO", miles, and clock lights/icons do not shut-off when I turn the vehicle off but do not lock the doors! Being hyper-sensitive about battery "drainage" I wondering if this is a unique problem or can someone explain why those particular lights/icons stay on until the vehicle is locked...thanks

 

Quite normal. The odometer and clock will stay on for about 30 seconds and then turn off automatically. It's a sort of a Euro thing that TMC have adapted (my wife's Golf GTI does exactly the same thing).

 

You can't tow the Prius with the front wheels on the ground, even in Neutral. (It's ok to pull it onto a flat bad, though.)

12v runs from the boot of the car where the 12v battery is to the fuse box under the hood. In that box is a terminal specifically for connecting a jumper cable too... Plug another battery or car into that terminal, push the start button and you are good to go. If the HV battery is dead, it will still power up the electronics so you can disengage the parking pin and go into neutral.

 

It's so you can check your mileage, say at the mechanic. without having to turn on the car. Most cars with electronic odometers show it for a minute or two after cutting power.