12 V battery, gone for 2 months

From what I understand, two months is beyond the capabilities of the small 12v battery. The maximum you can leave it with a new, fully charged battery is 3 to 4 weeks.

A summary of the Options available:

1. get someone to run your Prius once a week for half an hour (car in Ready sat in Park).
2. Use a battery minder.
3. disconnect 12v battery (various settings such as radio, mpg will be lost, and driver's auto window will need init).
4. Let battery die, get jumpstart on return then buy new 12v battery.

There's a lot of experience here on PC to get more detail and the pros and cons.

 

What he wrote. If you use option 1 make certain that the someone knows exactly what they're doing, either by virtue of owning a Prius themselves or through several practice sessions with you.

 

The easiest is to just disconnect the 12V -ve cable from the car body (one bolt). It's not dangerous and it's easy.

That being said, I'd just leave it. I suspect it wouldn't drain fully for two months. Unless the 12V battery has been stressed. Switch off the smart key system (button under steering wheel). Make sure all interior lights are out.

Have fun on vacation.

 

If you intend to routinely be away, invest in a "smart" battery tender that will keep a float charge on the 12 vdc battery. I use a VDC Battery Minder.

BatteryMINDer® Plus 12 Volt 1.33 Amp Charger-Maintainer-Conditioner (Desulfator) :: All :: Battery Chargers by BatteryMINDers.com

Assuming you park in a garage, wire up the Battery Minder directly to the battery like I did



And run the supplied harness out the rear hatch like this



No need to disconnect anything or lose presets. I've been away up to 6 weeks, upon return disconnect the unit, step on brake pedal, press Power, and it boots up no problems whatsoever

If you can't or won't use some sort of battery tender - do NOT use a "trickle" charger as that will destroy the battery! - I would NOT depend on somebody to occasionally start the Prius. If they get the start sequence wrong, they can not only bugger up the 12 vdc battery, but also the big NiMH battery pack

In that case, as David suggested, disconnect the negative cable at the unitbody



Note that the hatch release is electric only. Once you disconnect the battery, the only way to release the hatch is to crawl over the rear seats, into the hatch area, pry off the trim cover, and manually move the actuator. That procedure is documented in your manual

The electric release isn't unique to the Prius, however. The rear door lock and glass release on my FJ is also electric only. The actual handle still works mechanically, but if the FJ is locked, then the battery goes flat, the key can NOT unlock the rear door!

 

60 days x 24 hours/day x 0.03A quiescent current draw = 43 AH.

Your 12V battery is rated at 36 AH capacity when new and should not be discharged more than 50%. I would say there's close to 100% probability that the battery will be discharged and damaged to the point that it is not rechargeable when you return, unless you either disconnect it or hook up a tender.

 

Is it possible to charge the 12 Volt battery through the accessory jack? I do this on my old GMC Van to keep the battery up during winter months it sits for months unused.

 

No, it is not. The power ports are disconnected when the car is powered off. You can do a "hack" to keep them live all the time, but I just hooked the harness directly to the battery

The VDC Battery Minder has reverse polarity protection, automatic float charge control, and desulfates the battery

 

I suspect aux battery was mis-installed and isn't being charged while car is being driven. (I'm an EE.) How can I check this out? Would battery voltage monitoring work? (12.75 being charged - considerably less if not?) Could a fuze be missing between generator and battery ?

 

You could measure voltage across the 12V battery terminals when the Prius is READY. It should be ~13.8V, and if so then the battery is being charged, and there's no problem with the DC to DC converter or the fuses that are inline.

Further, if the 120A MAIN fuse had popped, there's no way that the car could move from IG-OFF to any other operating mode. If the 100A DC/DC fuse had popped, then the car's instrument panel should look like a Christmas tree. Those are the only two fuses that are between the 12V battery and the DC/DC converter.

What causes you to believe that the 12V battery is not being charged?

 

I started car OK, drove 5 miles to Dr office, couldn't lower driver's side window after several tries, a sign aux batty voltage is low. Car was parked for 3 hours, then couldn't start, AAA came and charged aux battery at under hood terminal, car started OK, I drove home ok. Next day after using fob lock button to lock all doors, after a few seconds, doors unlocked "by themselves", a sign aux battery voltage is low.

 

Russ,
As an EE, this should be easy. Pat has been giving good advice. You can also check out your battery status/charging system using the MFD in maintenance mode.

1. Get your Prius into "Accessory Mode" - Press the power button without touching the brake pedal.
2. Put the MFD into "Maintenance Mode" - Press and hold the "Info" button on the MFD while turning the headlights on and off 4 times.
3. Press the "Menu" field on the screen.
4. Press the "Signal check" field.
5. The 12v battery voltage will be one of the items listed, it should be above 12 volts.
6. Put a small load on the battery by pressing the power button again without touching the brake pedal. The voltage should stay above 12v.
7. Now get your Prius into "Ready Mode" - step on the brake and press the power button. The battery voltage should immediately rise to 13.8 volts (Your Prius uses constant voltage charging). If it is lower, the battery will never charge; if it is much above 14 volts, your 12v battery will get cooked.
8. Shutting the car down will reset the system back to normal operation.

JeffD

 

Also keep in mind direct physical access to the 12V battery for charging or just powering up the car if it is dead (to gain access to the hatch) can be done from the "boost terminal" in the under-hood fuse box. It has a red plastic flip-away cover. Use the bolt on the firewall above the fuse box for the -ve. You don't need a very big battery to power up the Prius. Even a 12V motorcycle battery will do it.

You can also connect a battery tender there.

Do be aware you -SHOULD NOT- boost another car from there or indeed ever, even directly from the Prius 12V battery. It's not a "starter battery", that is, it's not designed, chemistry wise, to supply starting currents. The Prius only draws about 50A for about 1/2 sec. when you press the brake pedal, and again when you press start. "Normal" cars starter motors draw a minimum of 100 ish Amps, and usually a lot more in colder temps. This will blow the fuse that is between the boost terminal and the battery in the Prius.

 

Hi Russ,

Especially if your 12V battery is original equipment, I'd suggest that you replace it now, after you verify that the voltage when READY is ~13.8V.

 

Just did this, though for my 2004 there is a step between #3 & #4 that you don't list.

Anyway the results for my 4.5+ year old battery were: 12.2V/11.9V/14.2V

 

derh2o,

Are you referring to the DOME fuse?

 

I have been gone for 5 months with the battery tender on attached at the positive in the engine and to a ground point. Can i expect any problems when i try to start the car?

 

I would have preferred the battery tender have been attached directly to the battery. It can be difficult to get a good connection at the jump point in the power distribution box under the hood.

But assuming the battery tender worked properly, the car should boot right up. The NiMH battery may have lost a bit of charge, but usually is good for 6 months. When the gas engine starts, it may or may not run roughly for a bit

 

You can measure 12V bus voltage under the hood, using the jump start terminal in Engine Room Junction Box 1 (the large fuse and relay box by the drivers' side headlight) and any good ground. Check voltage with vehicle OFF (no READY), then ready the vehicle up and look for a bump in voltage as the DC-DC converter comes on line. No need to root around in the back for that measurement.