Car going to be dormant for 52 days

I'm not a pro by any means but I think you can disconnect the small 12 V battery in the back of the car while you're gone.

 

You might have to trickle charge or jump start the 12 v battery. Since you own a Prius, you should know how to jump start it or at least know the guy doing it is doing it right. So check the owners manual. A trickle charger can hook up at the same places under the hood. Or get a portable jump start battery instead and have it the car just in case.

Portable jump start battery

 

If you're capable of jumping or disconnecting a battery, you're capable of connecting a charger. Get a decent "battery tender" and follow the instructions.

It's pretty bad for the battery to let it go dead, so this is worth the effort.

 

1. Purchase a battery maintainer. Not a trickle charger, not a charger. Maintainers sense the voltage in the battery and don't overcharge. Trickle chargers just keep on charging. Maintainers are not for quick charging of charging a very run down battery, they are for long term connection. (Some expensive units are all in one but when buying, look for that maintainer function in the advertising.)
2. Connect the maintainer to the "starter" battery terminals in the right rear of the trunk under the mat. Red to plus, black to minus. Route the cable out the back trunk lid closing the lid.
3. Plug the maintainer in.

I used to leave cars plugged in like this for the winter at the end of a 100 foot extension cord and, on disconnect of the maintainer in the spring, they started right up and no settings were lost.

 

there are threads with recommendations and links to amazon and etc.

 

I don't know if this helps or not, but I've observed something that you may want to be aware of. In conventional cars, the voltage regulator is set to 14.7V, give or take, which is fine for a typical SLA battery. However, that's not where the prius DC/DC converter that maintains the charge on the "12V" battery in the rear of the car is set. Morever, where it is set is not constant. I was quite surprised when I noticed that last part.

I use EngineLink and one of the parameters I observe is the Aux Bty reading. If the loads are low, as in headlights off, interior fan off, a/c off, it reads about 13.4V. However, if the loads are higher, I can do this by just turning on the headlights, if goes up to 14.4V. My own conclusion is that Toyota is balancing the desire to avoid overcharging their battery - it really is just a energy storage device for actuating relays and the vehicle computer during the turn on sequence rather than a power delivery device (turning over the engine), so there is no need to shove everything it can into the battery - with the need to operate the headlights to the industry standard illumination level. I think 14.4V does that, but for the rest of the time they just float the battery.

Does this matter to you? I don't know. However, if you use a battery maintainer, you should find out what voltage it is maintaining. What I'd probably do is use a cheap timer and a cheap, low current charger, and set it up to operate for an hour a day. On the other hand, if you used the maintainer it probably is just fine. I just figured this was a good outlet for this info that I had learned and found curious.

 

I think some of us are over thinking this thing. The 12v battery itself is an ordinary AGM sealed battery that any staged battery charger/maintainer etc. will work. It's true that 30 years ago a cheap trickle charger was nothing but a transformer and a couple of diodes. Not these days. Even this $23 walmart unit (below) will work. Personally, I take a portable jump start battery with me when I leave a car for extended periods but have never needed to use it.

Battery Tender Junior Charger - Walmart.com

P.S. You can not use a setup that charges through the cigarette lighter outlets, those 12v receptacles are disconnected when the car is off.

 

We use a CTEK MULTI US 7002 battery manager (p/n 56-353) and have CTEK battery condition LED indicator pigtails (p/n 56-382) permanently connected to the AGM battery terminals on the cars and inverter genset, making access to the visual condition indicator easy, and charging (if required, on condition) as simple as "plug and play".

CTEK Power Multi US 7002 Battery Chargers 56-353 - Free Shipping on Orders Over $99 at Summit Racing

CTEK Power Comfort Connect Indicators 56-382 - Free Shipping on Orders Over $99 at Summit Racing

 

I don't think you will have an issue with such a new hybrid battery, but for folks who have older hybrids, I sell maintenance battery chargers for the hybrid battery. (200+ volts as opposed to only 12 volts)

When sitting unused for long periods, an old hybrid battery can self discharge so low that the car will not recognize it and then the car cannot be started or driven.

With a new hybrid battery, while it still isn't good for it to sit so long, the hybrid battery should not self discharge too much.

If there were any way you could get someone to drive it, even for short periods of time, I would really consider doing that.

When you get back, if you get dash messages/lights that say you need to replace the high voltage battery, ignore it and call me! It will not need to be replaced. It simply needs a good charge and balance from a high voltage maintenance charger.

Good luck on your trip.

 

No it isn't "set" to 14.7.
And no it doesn't really matter to the average owner.
And yes, some here are overthinking the problem.

The variation that you saw is NORMAL for modern, solid state voltage regulators on all kinds of vehicles. The "nominal" float voltage of an AGM battery is 12.8 volts. To keep it from DIScharging once it is charged up, all you need is an extra .4-.6 volts so 13.4 is plenty. And a good voltage regulator is able to tell when and how much charge is needed MUCH better than the old cheap ones.

Some systems won't let the voltage go below 14.2 or so and some will.
Neither one is necessarily "better" than the other because both keep the battery near to a full charge without "cooking" it.

One of the really confusing parts about modern regulators is that the highest voltage you see might be at idle and as you speed up the engine the regulator cuts it back if not needed (the battery is full and doesn't' need any more.)

 

No the 75% probably does not indicate a problem......UNLESS you come back and it still says 75%.
A battery maintainer really isn't much of a "charger" and it may take several days to restore a full charge before it switches to maintenance mode.

I'm a little curious as to why the 75% to start with though........unless it wasn't used for several days before the maintainer was connected.

And how did we get from 12 V batteries to summer tires ??

 

I will have a dormancy report shortly. It will be interesting to see if the Pri fires up (2 weeks dormant) after a drained battery issue - dome light left on!!!

When the Toyota Care guy came to jump start my battery they used a large jump pack and the Prius started up fine. I was thinking that having something like this would be kinda helpful? It's powered by a SLA battery which can probably be easily swapped out, plus they seem easy to tote around: Jump Starters - 900 Peak Amp 12 Volt JNC300XL | Call us at 1-800-328-2921 |Clore Automotive

 

A jump pack sort of misses the point. These 12v batteries do not take kindly to being run dead, so the real solution is to avoid doing that. If you need a jump pack, you've already damaged the 12v battery. When they're new, its not a big deal but a battery more than 2 years old could be in trouble. A decent battery tender is not expensive or difficult to use.

 

Dormancy, or bringing a drained battery back to life is not an ideal. However, given the context where someone cannot tend to their vehicle for stretches of time probably necessitates removal of the 12 volt. The jump start tool simply duplicates what Toyota Care had when they came to start up the vehicle the last time around. To me, given the dinky size of the Prius battery - a little guy ( CLORE starter) like this might come handy in a car's pro-forma tool kit.

The results from my personal dormancy experiment will come shortly.

 

Would a battery really go dead in just 7 weeks? My boat's AGM battery was good in the spring after sitting for 6-7 months out in the cold.

I hadn't thought of the hybrid batteries, how long does it take if unused for them to have an issue?

 

These cars have several systems that are always on. Its a slight drain, but its also a tiny battery. The constant drain also has a negative physical effect in the battery, making it age much faster. They need frequent charge/drain cycles to stay healthy, and a full charge at that. Batteries that never get fully charged are also at risk.
The traction battery will also slowly discharge, like any Nickel based battery will. But I'd expect that to take months, not weeks.