Thoughts on 12v backup system.

As a matter of principal, I dislike disturbing the traction battery unless absolutely necessary. In this case, I don't believe it is necessary. It can be inconvenient to have it locked away in its fortress of solitude, but it is reasuringly safe in there.

You could achieve the same result by keeping a small 12 volt battery stored in the trunk, wired so it charged when the car was on, and (normally) did nothing when the car was off. I have started a gen. I with a little handheld 12 volt battery I pulled out of an emergency light.

 

Interesting ideas.

Does anyone know (approximately) the minimum number of amps required to boot the Gen2?

 

This is what you're proposing:

Have a 12 V power supply attached to the traction battery so you can generate 12 volts to start the Prius so that the traction battery can (through the inverter) supply the 12 volts.

All you'd really need is some sort of override that attaches the traction battery to the inverter.

It might be even simpler than you propose. I wonder what could be done with those pneumatic switches they sell for spa motors.

 

So you're saying that a laptop power brick can be used to recover from a dead 12V battery?

I wonder what the best way is to plug into the HV battery?

 

Interesting idea!

I made a (14.4V minus diode-drop, so) 13.8V Lithium battery. Able to supply >1A from disposable and obsolete cells from work. I wanted to parallel 4 or 6 of those packs, but they threw them out when I was with sickness leave so this is all I could make (four 3.6V cells in series)

The wife left the lights on last week. I told her to hook this battery up through a lightersocket, and she said it did make a difference (thus me concluding the lightersocket had been switched ON). But she couldn't start the car.

So my 1A battery was insufficient. But the dome and doorlights alone would probably draw that much already (hence me wanting to quadruple this battery...).

So how much current would I need?

She ended up starting the battery with instructions from me and help from two other carowners, startercables I threw in the car were too big to clamp in the connector-bay under the hood. I have since made a dedicated startercable with two small 25A rated batteryclamps, 5 meters of speakercable (13AWG) and a cig-lighter plug. The previous owner made an always-ON cig-lighter socket behind the left dash-vent. So I could always click that off and plug it in there to make sure it directly hooks up to the battery.

 

The gen II Prius, and probably the gen III as well, needs a 1/2 second 50 Amp pulse to get the system started and into "ready". It may take two of them if the brake pressure is low. Your supply needs to hold about 10V during this pulse. Most small gel cell batteries such as found in small UPS' and emergency lights will do. I just purchased one new, 12V 12 A-Hr, for $20. It's handy to have it around as it's not very heavy and is very portable.

As mentioned above, you -could- run a diode from the accessory line with a series resistor (10 to 20 ohms, 5 Watt) to this type of battery in the car somewhere. Then install a momentary push button switch (a larger one would be best), from the battery positive to the 12V line (boost point or directly to the 12 V battery), to use in case the Prius battery fails.

 

If you connect it to a switched cigarette lighter, it would charge when the car was on, do nothing when the car was off, and would always be ready for you. Just run a line.

I have wondered about putting a big pair of protective diodes on the battery and on the front jump point, but never actually did the research.

Er, what?? Where was that?

 

Rather than focusing on charging the 12V battery in an emergency, the objective is to jump start the car and either leave it running for a long while, drive to a place that has a 12V smart charger, or drive to a place to have a new battery installed.

When the 12V battery is dead, you often have two competing issues: the 12V battery is hungry, and the car is demanding. It can be 50 amps at >10.5V that are required to start it. For example, a portable jump pack may not provide sufficient voltage to boot the car unless the bad 12V battery is disconnected. Nevertheless, a 12V jump pack is extremely handy and should be in everyone's trunk.
You don't need to get one with a compressor, lights, bells, and whistles.

I am a little skeptical that the car would start when jumped from the HV battery without detecting a ground fault. Seems to me like there is a ground loop here that is not being accounted for. In spite of a claim of isolation in the laptop brick, you still have a circuit that connects the HV battery to the 12V battery in two different ways (laptop charger, inverter+dc converter). So you'd have to disconnect the 12V from the car and charge it.

 

Whoa... 1999 PiP :cheer2:

I considered unhooking the 12V battery, but couldn't ask my wife to do that over the phone... And my mini pack wouldn't come close to the needed 50A anyway...

 

/Looks at file, eyes boggle/

Dang, we need to bring that feature back for all these upcoming fourth gen Pri.

 

John et al,

Various concerns including the one mentioned in the above with regard to a hungry 12V adding to the startup demand have caused me to install a low-voltage disconnect switch I wrote about this morning.

I would appreciate any input, particularly, a way to keep the presets (I remember reading somewhere 6V is sufficient to keep them, is this true?) while the 12V remains disconnected.

Thanks.