Is there a kit that would auto start the car if the 12 volt battery gets below a certain level?

If there were a “start to protect” function, the end result would be a duff HV, a duff 12V aux, and an empty fuel tank.

 

The easiest thing to do to protect your 12v battery from camping loads is to install a second larger 12v battery along with a rv style battery isolator. Then run the loads off the second battery. However this is useless if you want to keep food refrigerated or continuously run some other significant load. The second battery, when fully charged, is unlikely to stay charged more than a couple of hours under a refrigerator type load. After an initial 12v battery discharge, the car needs to run for many hours in Ready to recharge it. So the only reasonable answer is to leave the car in Ready.

Autostart based on 12v status is not easier than the remote start. Basically it would be a function added to a remote start. Probably harder and riskier since now you have a weaker 12v battery. Even if a custom autostart was installed, the car would be running several times an hour since the hv battery in Ready is required to charge the 12v battery. Once the hv battery discharges, the engine starts but only partially recharges the hv battery. Since the hv battery is only partially charged, it discharges very quickly as it remains in Ready with your camping loads. Which is the reason RVs have gas generators to provide electricity for continuous loads without shore power.

A Prius can be a gas driven electric generator on its own. If in Ready. So the only reasonable answer is to leave the car in Ready.

Lights go off automatically except for the hatch light. Take it out or leave it switched off. The map lights will stay on if you manually turn them on so don't do that. The headlights will stay on if you manually turn them on with the vehicle off. This concern goes away if the car is in Ready.

 

Timed or measured starts are possible, but trickle chargers are dramatically cheaper and don't have any unexpected carbon monoxide liability.

You can also get cut-off switches to isolate the battery and at least certain models of the Prius have a very easy disconnect built into the native battery terminal.

Ordinary cars are built to be operated a few times per week because that's what most people want out of them, and they're generally fine with that pattern of charge and discharge. If you expect something different, adaptation is needed.

 

no

 

@chuckers what’s your parking situation; could you manage a charger?

 

Small refrigerant-cycle fridges made for cars and RVs can get their consumption down around a couple amps or less (average of the on/off cycling, steady state) once they have cooled down and are holding temperature. (Their literature sometimes calls this figure "amp hours per hour", which of course is just amps after you cancel the hour/hour.)

The car's OE battery is only around 45 amp hours, but if you're talking about some larger deep-cycle RV/marine second battery in the 100 amp hour ballpark (and say the fridge is your only load on it when you're away), you could hope for close to four days of that if you absolutely had to. Naturally, this is where you'd normally apply a generous safety factor and try never to be away for more than a day or two.

 

He did say he was going to live out of the car, so my points are primarily focused on what happens when the 12v battery is discharged.

 

Automatic systems can fail too and leave you stranded without notice.
My take on 12 volt battery health is to use a multimeter and measure the battery's voltage one a week. after learning what kinds of load it's being exposed to on a daily basis and what voltage range the battery typically experiences.
After you lean the typical voltage range, the monitoring is not to inconvenient.
I've never heard of a 12 volt kit to automatically monitor and than also make sure the 12 volt battery doesn't get too low a voltage reading.

 

While not a full solution, you can greatly improve this situation by replacing the original incandescent lamps with low current LEDs. When carefully selected for low current in light-efficient configurations (i.e. unidirectional emission appropriate for the particular fixture, not stadium brightness omnidirectional lamps that waste most of their light inside the fixture), you can slash the current draw by a factor of 10. This greatly extends the time available to discover an accidentally left-on light before it causes a drained battery. With these LEDs, a light left on overnight or all weekend won't prevent the car from starting.

 

LED light conversion does help a lot- we used to have problems with my wife leaving a door open on the old car, and the battery would die before she got back to it. LEDs fixed that.

You can also do appliance-side fixes for the fridge. I've got a portable refrigerator that can be set to stop working when input voltage drops below a user-preset threshold. It will also stop at a lower, non-adjustable point to prevent damage related to overcurrent operation.

 

Lots of cars have things like that, but there isn't anything universal. They all came up with different ways to keep the battery from running down. I don't know of any that will actively recharge, but there are several that open a relay and cut power after a few minutes.

Every Prius came with an owners manual that says (among other things) "disconnect the 12v battery if you are doing service that requires leaving the doors open for a long time"

...guess what still happens at detail and tint shops every single day?

There's not much wisdom to hand down on these because the stock turns over so fast. What was good last week is bad this week, and unavailable next week. So unfortunately everyone needs to try a few for themselves. I found some that work in our car but you can't get them anymore, so it's pointless to offer a recommendation. Good luck!

 

IMHO; Lithium Jump pack; plugged into your cig. lighter - under your front seat. It's available whenever you need it, you avoid all the tinkering, which would add additional draw to your battery and will cost less (time & money) than your experimental system. You'll be able to charge your cell phone and other small accessories.

 

There are monitors that report your 12v status back to a cell phone. They require wifi to work. You can also install a battery isolator that will disconnect a second battery from the primary 12v battery. This is a low cost rv solution normally used with second batteries to supply camping loads.

It would be possible to have a low battery alarm module installed for a local alert but a battery isolator (known in other applications as a low voltage disconnect) is better for unattended use. The rv versions disconnect after the charging voltage lowers because the vehicle is shutdown.

A lithium jump pack is a good solution under $100. You inadvertently discharge your 12v and it allows the car to start. It does not help your continuous refrigeration problem.

An expensive lithium 12v battery can have a second failsafe battery built in that can be used to start the car after discharging the primary section. The Antigravity brand is around $820 with wireless jumpstart. They don't sell one with the Prius battery form factor but you could get a Corolla version and adapt the terminal connectors. I might consider this option for a second 12v battery with a separate battery isolator. You might get several days of refrigeration runtime from the Antigravity battery alone. Recharging it is another story without shore power.

The Prius programming is setup to avoid a high voltage battery discharge. Which would happen if the vehicle kept the 12v battery charged when the rest of the car was off. The high voltage battery can not be jumped or even easily accessed. If discharged the car must be towed to a qualified dealer for a several hundred dollar high voltage battery charge. Hopefully it is still good as a new hv battery is several thousand dollars. The Prius hv battery is actually rather small compared to a plug in version or any ev.

Remember your problem is how to charge the 12v battery before you can not start the car. With no shore power, you have to use a generator of some sort. The Prius has one designed to charge the 12v and is able to run most 12v loads. It can also cool and heat the car but it has to burn gasoline several times an hour to do so. That means leaving the car on. Manually turning it on and off based on some alert will not work well. Once the 12v is discharged it can take hours to recharge it.

 

These cars are designed for safety, to prevent mechanics and emergency first responders from getting electrocuted, without having to learn a whole new set of electrical safety protocols. They already know how to disable the 12V systems for their own safety. These high voltage systems are designed so that taking down the 12V, also takes down and isolates the HV within the battery case.