Other concerns addressed?

The headlights staying on after the door closing may be a safety issue. If the lights are on for illumination of a tire change or to light up a person waving for help, an auto off may be worst than an eventually dead battery.

 

Any auto-off system I have owned has an override. You twist the delay around to the detent and the lights stay on.

Tom

 

Wrong, you are mixing the ions and the triones. You've just got to work smarter than that.

 

Regarding revers jumping and damage, one important difference between Prius and other cars is that Prius doesn't need high current for jumping. Therefore reverse polarity protection could be installed very inexpensively at manufacture.

But it is not, and so we have these discussions.

Incidentally D cell batteries in series are not sufficient to jump a Prius. Try it and see.

A larger number of cells in serries/parallel would manage no doubt, but it has always seemed simpler to use a lead acid gel cell 'brick' for this. The 7 AH size works, and at least one report that the petite 4 AH does as well. They just have much lower internal resistance than D cells (of any chemistry) and there's the rub.

 

It would depend on the type of D cells. People often think batteries only differ by form factor, but there are a lot of different types. While I haven't bothered to do the math on this one, standard D cells would probably lack the necessary current for booting a Prius. You need a battery that can source a good amount of current for a short time. It's not as bad as running a starter motor, but it's still a lot of current for a small battery. Photoflash D cells might work. Photoflash batteries were designed to deliver a large pulse of power for firing flash bulbs. I don't know if they are still made.

Tom

 

NiMH rechargeables could probably handle the current, though you would need more of them to compensate for the lower voltage.

Alkaline batteries have a very high internal resistance. I don't have any fresh D-Cells on hand to test, but google is giving me internal resistances between .15 and .5 ohms. Call it .22 to make the math easy, though this is likely overly optimistic. With 9 cells, you wouldn't be able to charge at much more than 2 amps; if voltage & resistance didn't sag at all it would take you 5 hours to discharge a meager 7Ah into to the battery. (1/4th of a charge assuming 100% efficiency). In reality the voltage will start to drop immediately after you start discharging. You'd be very lucky to last an hour at that rate. And this is all assuming a compliant Prius that doesn't try to draw more than 2 amps.

NiMH on the other hand have very low internal resistance, so at high power ratings their voltage drops much less under higher currents; not only does this allow you to use more of the capacity but less power is wasted. (Note- NiMH have lower voltages; a 10th cell would likely be needed). At 3 amps, a decent NiMH AA battery will last as long as a good D-Cell alkaline. Take 10 11Ah D-Cell NiMH, and they could handle a much more realistic 10 amps for an hour; that would be enough for 1/3rd of a charge even in the real world. In fact, they'd likely do nearly as well at 20 amps, over half an hour, but it would hurt their lifespan.

 

Yeah, I was assuming 11000mAH cells. 2500mAH "D"s would not do as well.

If you really wanted to do this regularly for some crazy reason, your best choice would really be 14000mAH F cells. Enough power for half a charge, and more capable of handling the high current without hurting lifespan.

 

One question; exactly what current does it take to start the computer? After all, that is what does not start when the 12 volt battery runs down. The main battery is protected from being run down so that is not the problem. Starting the 12 volt computer circuit starts the car.

My thought is; not more than the output of single honest D cell, say 10 in series for sufficient voltage to overcome the drag from the low 12 volt battery. Series multiplies the voltage, current is no higher than the weakest cell in the series.

That stick of D cells would be paralleled to the weak 12 volt battery. Some of the current will try to flow into the 12 volt sink but, if you act fast, you will start the computer and it will use the traction battery through the inverter/converter to remain running; it ain't as dumb as some people think.

 

Peak of 30 amps, apparently.

Prius Startup Current

 

The article includes the estimate:

"This is a tiny amount of energy, so tiny that one could imagine jump-starting a dead Prius with a stack of six or seven AA batteries if one didn't think too hard about the 30-amp current peaks required. (I've seen internal resistances of 0.1 to 0.15 ohms quoted for alkaline batteries, which would limit the peak current to much less than that.)"

A healthy stack of "F" cells should provide all the energy needed for such millisecond perambulations. Of course, the brake pump might be a problem but it has a removable fuse.

Using an O-scope and laptop, reasonable tests might be disconnecting the brake pump fuse, and/or the 12 volt battery, and using a 10-cell stack of "F" and/or "D" cells. Variations of arrangements would be informative. How bizarre.

But it would be worth the effort to test with variations of 12 volt battery state of discharge. I'd do it, but I left my O-scope in Colorado about 20 years ago.

Bringing a well charged booster with the car when you leave your Prius at the airport will immensely ease your mind on that month long vacation.

 

The 30 amps is inrush current when the brake pump starts. Depending on the pressure already in the system, 30 amp peak currents may or may not occur. The current measured is the current out of the 12 volt battery. I verified the above referenced currents by placing a DC current probe around the wire from the battery to the front jumper post and looking at the currents on an oscilloscope. Note that the high current flows for only a few milliseconds which is easily produced by the Prius 12 volt battery.