12v battery

My first winter with my Prius, I started to have 12 vdc battery issues as well. I blame that on the -40 temps, constant use of Max heat, auto headlights on, and constant use of electric rear window and mirror defrost.

It's important to consider how the Prius charges the 12 vdc battery. A conventional vehicle, like my 2007 FJ Cruiser, has electronic voltage regulation and a huge alternator. If necessary, it can pump +60 amps into the battery

The Prius has a constant voltage source for charging, so once the 12 vdc battery is depleted (Headlights on, Max heat and hi speed HVAC, electric defrost, etc), it's actually very difficult to get the battery up to proper charge. I suspect there are a few Prius cars out there with chronically undercharged 12 vdc batteries

I solved my battery problem by getting several VDC Electronics Battery Minder float chargers/conditioners at auction. This is the older Battery Minder model that only desulfates once float charge is achieved

I used the supplied VDC harness to hook up directly to the 12 vdc battery. I used to have a condo with heated underground parking, now I have a house with a heated attached garage. Either case, I plugged in the VDC overnight, or whenever the car sat for more than half a day.

The Prius PDI is pretty emphatic the car 12 vdc battery be fully charged with the Toyota automatic battery charger. There is nothing special about the Toyota branded charger, it's a 10 amp model made by Associated. Any good automatic battery charger set at 10 amp will work

Given you have stated in your avatar that you have a 2008 model, I'm wondering if the dealer followed the PDI? If not, this is fairly easy to fix yourself

You don't have to monkey around with crawling into the rear hatch to manaully release the catch. Pop the hood and hook the battery charger directly to the jump start connection point, as documented in the owner manual.

This should be a dealer issue though. If they appear incompetent or are unwilling to properly charge the battery, you must do so yourself. It's a very small battery, think motorcycle size, so it won't tolerate much abuse or discharge

You may still want to consider a battery tender. New, they are around $70 from VDC. Price a brand new battery, and you will find the battery tender is cheap insurance.

*12 Volt-1 Amp* Charger and Maintainer

Unlike regular battery chargers, the Battery Minder may be left on indefinitely. I use them on the diesel tractor and Ford plow truck at my hobby farm, they are always plugged in

As our Grumpy Old Moderator (tm) (c) suggested - btw I'm older than that young whipper snapper - that isn't bad fuel economy for the trip length and temps.

I used to have a 2000 GMC Sierra with Vortec 5.3. City driving at -40, I was lucky to get 6 MPG. First winter with the Prius, 28 MPG at the same temp. Once I made a winter front for the Prius, that increased to 38 MPG.

 

Yup,
There is NO 12v starter on a Prius. The Motor-Generator in the flywheel uses the 200v system to spin the engine up to speed to start the gas engine. As others have said, the 12V battery just boots the computers, then 12V is supplied by the DC to DC converter to run the low voltage systems and charge the battery.
Also, you don't need to open the rear hatch to jump the car, there are jumper cable tabs under the hood on the drivers side upper corner under a black cover. Check the manual.

 

And just to reiterate what Evan said, I have had NO problems with my 12V battery. This is Pearl's first winter, and I purchased a battery tender, "just in case". I haven't had to use it, so far. Do use the charging point under the hood. Clip the +ve there and the -ve to the bolt just above it on the firewall. Do so asap. The longer you leave a lead-acid battery discharged, the more damage it will suffer. Be aware it's not inexpensive! Somewhere around $300, depending on how greedy the dealer is.

The 12V battery is charged by a battery charger circuit built into the main inverter, that powers the "electric portion" of the hybrid system. It keeps the 12V system at 13.8VDC, which, at below freezing temps. is not enough voltage to fully charge the battery - you really want around 14 - 14.5V at really cold temps.

However, even saying this, your battery should not have died. Mine has not, and it's colder here than in Montana. Check all interior lights (the hatch light, for example, can be left on). BE SURE to exit the car from the drivers door, or at least open and close it after you shut the car off. If you do not the headlamps will stay on.

 

Parky, a young 12V battery would die only if it had been drained or otherwise abused, or if there were a fault in the charging system (which a dealer ought to be able to find very quickly). At this point your only option is to get a new battery and avoid killing it.

Always lock the car; that way if you leave a domelight on or the rear hatch ajar (which will drain the battery overnight) the car will give a long warning beep. Also I'd advise not using the headlight auto-off feature. It's too easy to get out of the car in the wrong sequence, which results in the headlights staying on and a dead battery.

 

Yes, we are absolutely sure

 

I've always had battery trouble in cold temps of -30 C and colder, no matter what the equipment. A lot of that has to do with the fact most batteries will become chronically underchaged at those temps, due to the extra demand

Once a lead acid battery becomes chronically undercharged, sulfation will occur on the plates. With sulfation, the battery will never become fully charged, will discharge easier, and the cycle repeats until the battery croaks

I'm pretty happy with my VDC Battery Minder units. The replacement battery on my diesel tractor at the hobby farm is just as expensive as a Prius 12 vdc battery. I've replaced the thing twice over the past 10 years, but this latest replacement I started using the Battery Minder on it

One thing I immediately noticed - with full time desulfation on the battery - is that the motor cranks *much* faster than before. If the VDC claim of at least twice the usual battery life proves true, the unit will more than have paid for itself

One thing about short trips: with conventional vehicles like my FJ, the electronic voltage regulator can really pump a lot of amps into the battery to try to keep it charged. A side effect of these fancy new high output alternators is that folks can get away with a marginal battery a lot longer

But nothing is free. The folks who try to get more life out of a marginal battery in a conventional car/truck end up cooking their expensive alternator instead

If I were you, I would remove the 12 vdc battery from the hatch, and hook it up to an automatic battery charger set at 10 amps. Don't put it back in until fully charged, this may take a few hours

Somewhere here I posted the official Toyota PDI on how to properly charge the 12 vdc battery

 

I'm wondering if that dealer is not following the PDI? It very clearly states a Prius must be delivered with a fully charged 12 vdc battery.

 

The VDC Electronics Battery Minder comes with a harness to connect directly to the battery. I hooked mine directly the the battery in the hatch. When I plug it in, I open the hatch, run the harness outside the car, slam the hatch shut, and plug it in

On my FJ Cruiser, I hooked directly to the battery and run the harness out the grille. It's very easy to hook up, and the VDC harness comes with a 5 amp inline fuse as a safety precaution

 

The problem with putting the leads on the battery is one of access. If the 12V battery is dead, it's a real pain to get at it to plug in a battery tender. The under-hood point is the same as across the battery, just with a 100 Amp fuse in-line and some cable. It can be accessed without worrying about things like leaving the hatch open, releasing an electric lock, etc. You can leave the hood open with no lights on.

The only thing you should have to worry about is whether your charger will damage any electronics in the car. A battery tender will not. Some other chargers might.

So it's a matter of convenience. Personally, I think the under hood point is most useful. Plus, as you're nowhere near the battery, you probably will have no chance of igniting the H2 it can generate, if there's a problem.

 

So it's still dead?

Right, it's a royal PITA to get the hatch open with a dead battery, especially in the cold. A motorcycle battery tender should work just fine, what is the model?

If I recall, the harness for my VDC battery minder is almost 2 ft. That is long enough to put the harness out the hatch with maybe 2 inches to spare