How long should 12V battery last in accessory mode?

My sincere advice: Never use ACC mode. Never use ANY electricity in a Prius unless you are in Ready.

The 12 volt battery is very small. Draining it is easy and means jump starting your car. Reversing the cables while jump starting costs $4000 USD. None of us have $4000 to spend. So stay in Ready.

If for some reason you feel your life is not possible without ACC, buy a little jump start box or cables that are polarity compensating.

Amazon.com: Schumacher IP-125 Instant Power Jump Starter With 12 Amp Hour Battery: Automotive

Amazon.com: Juice Performance BC0880 SafeGuard 16' 6 Gauge Booster Cable: Automotive

 

I will second what Jimbo said. Just leave it in "Ready Mode". Chances are the 12 volt is original which is not helping matters, As stated above, the 12 volt is very small and it does not take much to drain it.

Best of luck to you and "Welcome to Prius Chat".

 

Jump starting and driving is not gonna restore a seriously depleted battery either. You should hook a 3~4 amp smart charger, let it run a COMPLETE cycle, and continue to monitor with a digital voltmeter.

 

There are oddities in the Prius emulation of a car. <G> Besides the HV battery doing all the charging of the 12 volt battery, so you want to be in Ready, there is also the fact that no charging of the HV battery can happen in N, so you want to avoid N if at all possible. This may also not be how you learned to drive.
Treat B like L in a normal automatic, only use it to slow your car on long steep downhills (400 meter vertical drop or more. Whistler is almost 700m)

 

Yes, she'll be fine if the car goes into Ready. As noted, in Ready the traction battery charges the 12V battery, and when the traction battery drops to its low set point the engine will come on for a short time to run it back up again.

 

If she leaves the car in Ready for an hour so the 12 volt battery charges, sure. If she turns it back off before that, all bets are off.

(The $4000 USD liability to doing it wrong may be the reason they are not allowed to jump start it)

Out of curiosity, which Victoria? I have been assuming BC in Canada.

 

do you have to use accessories on the ferry?

 

I remember several great Victoria BC vacations via the Black Ball ferry from Port Angeles! Mostly I just walked on, but going the other way you would want a car. (Or maybe you use the BC ferry to Tsawwassen)

 

There's nothing intrinsically different about a Prius 12 volt battery, comparing to other cars. The only thing: it's a bit smaller. Any car in accessory mode is inexorably running down the battery, it's just a matter of how fast. And I'd wholeheartedly recommend you look into smart chargers, multimeters and just read up a bit. Jump starting and a drive does not fully restore a significantly depleted battery.

 

a good jump pack may come in handy.

 

Yeah, especially for airport parking. Another good app: when someone asks you for a jumpstart.

 

bear in mind that running down the 12v one or more times can be the death knell. it is a very small battery that is designed to boot up the systems and close relays, it doesn't suffer deep discharge well.

 

It completely depends on the capacity of the battery and how much accessory stuff you were running.

People have already mentioned the Prius aux battery is small, but there are numbers to these things. The original batteries for Gen 2 (reading from page AP-2 in a 2004 New Car Features Manual) was a 28 amp-hour battery if the car didn't have smart-entry or nav, or a 36 amp-hour if it did have one or both of those.

Just for comparison, batteries in the sizes conventional cars used to lug around a couple decades ago could have amp-hour ratings north of 80.

Toyota does improve the batteries over time. Your battery part number has changed once or twice and the new one probably significantly beats the original. (Toyota's heavier upgrade for Gen 1 was about 45 when originally introduced, and new ones are labelled 51.)

Once you know the amp-hour rating, how long you expect it to last is just a matter of division. If you're running some accessories (including the normal stuff in the car that is on in ACC mode) that total, say, 14 amps, a 28 amp-hour battery in good condition will be fully discharged in 2 hours. (Note: "fully discharged" for a lead-acid battery means about 10.5 volts; taking it any lower than that is getting into abuse.) If it's older with some capacity lost, then less than 2 hours.

Sort of paradoxically, if there is a saving grace here, it's that the Prius battery is not designed to turn a starter motor, it doesn't need the huge CCA ratings of starting batteries for other cars, it is more like a deep cycle battery and actually handles that treatment better. (The way you get huge CCA ratings is to up surface area by making the plates really thin, delicate, and easily eroded. In comparison, a Prius battery's like a tank.)

So it's not really that the Prius battery is especially bad at deep cycling. It's just so small that you already are deep cycling it within an hour or so with modest loads, and that's not what people are used to expecting.

If you do deeply discharge it, one thing you can do for the best chance it comes back ok is to recharge it quite slowly and gently on a smart charger, rather than in the less-controlled setting of driving the car. When I recently drained my new one flat, it just got swapped with the old one, which is now the spare and lives indoors on a maintainer. The drained new one came indoors and went on the smart charger for a nice controlled charge before it went back in the car. It's been fine.

-Chap

 

Joined today, what a steep learning curve required, and a sense of humor to lubricate that climb.
Really the 12 v in a Prius will last all of 15 minutes when fully charged, and kill it once, toast.