Avoiding 12v battery depletion

I have searched previous threads but this point doesn't seem to have come up.
I can envisage a situation where I may leave the car unused for a number of weeks (certainly in excess of what is recommended without starting the car every 10 days or so) and crucially not where a battery minder can be used (eg no mains).
In this situation is it acceptable to put another 12v lead/acid battery in parallel with the in-car unit thus increasing overall capacity and extending the time the car can be left unattended.

It is acceptable.

An alternate approach for your consideration would be for you to disconnect the negative battery terminal connector while the car is in storage. Or, disconnect the positive cable where it connects to the dedicated jump start terminal in the main relay/fuse box near the inverter.

Sorry to reopen this old thread, but I am interested if anyone has used the trickle charger on a Gen 3 Prius? If so, do you plug it into the V12 outlet and have the solar panel sitting on the dash board? Or directly connected to the battery at the boot? Is it OK to do so?

I am about to go to holiday for at least 2 weeks, and having been in a V12 battery drain experience where I needed to have the Prius reprogrammed for certain features to work again, I certainly would want to avoid this in the future.

I have the solar roof option I wish there was an option/hack to trickle charge my 12v battery.

Aren't our lighters switched on & off by the ignition? If so, then there would be no way for the energy generated from the solar cell to get where it needs to be.

Not necessarily. The headlights still work after shutting off the car and only shut off after you open the door. The question remains whether the V12 socket is disconnected after shutting off the car.

From experience, none of the phone charger works the monent the car was being shut down. Can anyone confirm that?

If cannot recharge through the V12 socket, then there are solar trickle charger that comes with aligator clips, but you will have to open up the boot and expose the battery terminals, connect to the solar charger and stick the panel up against the back window to get some sunlights, still doable but less elegance.

Solar trickle charger seems a good idea provided you leave your car under the sun for the whole time you're gone on vacation. What if I lock up the car in my pitch dark garage? I wonder if buying a trickle charger drawing power from the electricity grid is an alternative solution? I'm sure those battery chargers must have automatic shut-off @ fully charged state.

Yes I think either way you'd have to connect the charger directly to the battery using allegater clips and not through the 12v plugs.

Coming from the motorcycle world where it is very common to store bikes for long durations such as over winter using a trickle charger is the norm. You can find models with long quick connectors leads that bolt to the battery terminals so you put the cover and carpets back on then attach to the charger with ease and no fuss.

You can find quality solar chargers at marine and/or RV stores. You can find really cheap Chinese versions at harbor freight/northern tools stores but as always buyer beware.

So true, that's why I only stick to locally made stuff here and always ask a few questions to ensure they know what they are talking about.

If you are to trickle charge using alligator clips, do you need to disconnect the +ve and -ve terminals from the car first before connecting to the alligator clips? If not, then there might be potential short-circuit happening between the battery and the solar trickle charger?

Thanks a lot for informing us.
I really like to read forums about cars.
This is a great information.
I'll just bookmark this site for more updates.
so keep posting
keep helping me..

There are solar trickle chargers that plug into the OBDII port. Now would that work with a GenIII?

having ran my battery down several times (interior lights ,doors left open etc.)I purchased a small portable battery and can connect to under hood terminal and start car easily. I just leave it in back .Its small and takes very little room.

Agree this is the best way to avoid starting problems due to depleted battery.
I keep in the trunk a 12V battery pack (rechargeable) from an old hand drill, this is all that needed to evoke the system to start the car in the event of depleted 12V battery.
In case of need (happened to me only once) I am connecting the pack to the dedicated terminal in the hood fuse box to start (no need of high current wires).

According to the Optima 12V battery people, the SKS (smart key system) is the thing that drains the battery. They say you want to turn that off (button under the steering wheel) when you go on long trips. This was the last step in their battery installation procedure (just put one in my '06).

This switch doesn't exist on the GenIII; instead, after 5 days, the SKS turns itself off, and after 9 days, only the driver's door will work. It turns itself back on when you actually pull on the door handle.

I found this procedure in the repair manual. Does it sound like it would shut the system off to avoid battery drain, or just keep it from responding to the key?

3. ELECTRICAL KEY CANCEL
The electrical key cancel operation disables the following functions:

- Entry Unlock/Lock
- Entry Hybrid Vehicle Control System Start
- Entry System Back Door Open Function*
*: w/ Entry System Back Door Open Function
- Electrical Key Lock-in Prevention
- Warning
(a) The operation procedure is as follows:
Precondition:
Power switch off, driver door closed and unlocked.
(1) Push the unlock switch of the key once.
(2) Open the driver door within 5 seconds.
(3) Push the unlock switch of the key twice within 5 seconds.
(4) Close and open the driver door twice within 30 seconds. (Driver door: Close → Open → Close → Open)

(5) Push the unlock switch of the key twice within 5 seconds.
(6) Close and open the driver door within 30 seconds. (Driver door: Close → Open)

(7) Close the driver door within 5 seconds.
When electrical key cancel is activated, the wireless door lock buzzer sounds twice. To return to the original condition, perform the procedure again. When the original condition is returned, the wireless door lock buzzer sounds once.

Looking at the wiring diagram, it seems pulling the ECU-B2 fuse would kill power to the door oscillators and control receiver and certification ECU, which seem to be the culprit. Too much chance of buggering the electronics by pulling that fuse?

I've drained the battery after leaving the car at the airport for a couple of weeks, so the built-in anti-"battery depletion" function seems ineffective. (I'd also bought a solar charger, but hadn't realized that the cigarette lighter is disconnected when car is powered off).

So no one knows if running a solar charger through the OBDII port will work? I had a Cadillac that would sit for weeks and the solar charger would keep the battery up so I could start the car no problem.

A charger will not work on the cigarette lighter style outlets in the Gen III Prius. I would like to change that on at least one outlet because my hand-held GPS turns off when the engine is turned off.

If you use a solar charger you need to also use a charge controller. Solar chargers will draw current from the battery in the dark and also the battery could be over charged in high light conditions.


If we leave our Prius for an extended period of time the battery will be disconnected and left on a 120 volt battery maintainer. I've been doing this for many years with my motorcycles.

I think it should. I got a VW OBD2 solar panel for free from a VW dealer. It seemed to work and I could read a voltage above 12 volts off the only 2 pins it had when out in the sun.

The VW solar panel has diode to prevent discharging in the dark. I wouldn't be concerned about overcharging given how low output the panel is.

However, it has a warning (for an unknown reason) "The panel must be unplugged from the OBD2 connector receptacle and removed from any window prior to starting the vehicle." It's unclear if this is for safety reasons so that you don't block part of your view or if the panel can become damaged due to the car being powered on/started. I ended up buying an OBD2 splitter to reduce the # of disconnect/reconnect cycles on the car's OBD2 connector. I'd rather wear out the splitter than wear out the car's connector.

I tried emailing icpsolar.com about why (their sticker is on the panel) and nobody ever replied.