MacGyver a flat 12v battery from the HV ?

[okiebutnotfrommuskogee]

Here is my answer to the backup battery question. See it at http://www.wbcoxco.com/priusbat.htm

[Danny Hamilton]

Ok, I'm piecing together a variety of things I've read here at PriusChat at various times, and I'm not certain how accurate my memory is, nor how reliable my initial source was. Please don't take anything I say here as being reliable information until someone with a bit more knowledge in the matter backs it up.

That being said:

I believe I've read that the relay is inside the battery case, so accessing the power in the HV batter will mean accessing the HV battery case directly and then opening it up to tap off the leads that are coming from the series of battery cells to the relay. This will probably be VERY inconvenient to do "on the fly" when you suddenly find yourself with a dead 12V. Therefore, you'll need to install these taps ahead of time and find a safe way to access the free end of the taps "on the fly".

Assuming you've done that, I recall that some people have used switching power supplies to switch the HV to smaller DC voltages to power a UPS in place of the battery that is typically mounted inside the UPS. The UPS then generates 120V AC to power household appliances during power outages in their area. They don't want to drain their HV battery, so they connect the switchers outside the case (on the opposite side of the relay from the HV battery), this way they only draw power from the HV when the car is "on" and able to recharge the HV as needed. Since you are planning on only using a small amount of power to boot up the computers, at which point the car will be "on", you won't have the same concerns about draining the HV. Therefore, I would think you could connect your preinstalled taps to switchers that would convert the HV to 12V DC and then connect this 12V DC source to the 12V battery terminals.

Put a switch in there somewhere so that you only draw HV power through this path when you intend to turn the car on with a dead 12V, and I think you've created a way for the car to use the HV to start the car in an emergency. Don't forget to disconnect, or turn off the HV -> 12V switcher once you have the car started, or else later when you turn the car off, you run the risk of draining the HV battery!

Now I'm not advocating that anyone do this, and I'm a bit concerned about issues of safety both for the operator as well as for the various electronic car components. But purely as an answer to the question of "is it possible", I don't see why not.

Notice that this isn't really a "MacGyver" solution in that it all needs to be prepared ahead of time with parts that the average household isn't likely to have just lying around. If you are looking for a true "MacGyver" solution that can be implemented with a few tools and some spare stuff lying around wherever you are stranded, then hobbit's "really big resistor" has a better chance of meeting your needs. Now what everyday material could we use to rig a temporary "really big resistor" that would last the several seconds necessary to get the car started up?

Note: hobbit, my switching power supply idea is sort of a more efficient (and less likely to start a fire) replacement for your really big resistor, no?

[fruzzetti]

A human body would last the required few seconds... I mean, you'd be dead afterwards, but you could probably figure out how much of you you'd have to fry to get the right voltage drop!

:P

DON'T TRY THIS

[satwood]

OK, I can't resist jumping into this discussion. I'm an electrical engineer and as such I can't resist the temptation to keep exploring this concept. Basically there are three battery related conditions when the Prius is "dead". The first is when the +12v battery is dead. In this case, you still have 300 volts with plenty of current available to boot the system and then re-charge the +12v battery. As Adrick71 has explained in detail, this is very feasible if you have the right parts installed. The second condition is when the traction battery is dead (or very low). In this case, you still have the +12v battery to activate the computer. However, I suspect if the traction battery is dead there is no power source to spin MG1 to get the ICE running to then re-charge through MG1 to the traction battery. The third scenario is when both batteries are dead and then you are really screwed! The only obvious scenario I can think of that creates conditions 2 or 3 is where you run out of gas while using the Prius as a power source and the systems stay running until the +12v is gone, at which time the traction battery is probably gone also. I suspect the on-board computer has a fail safe for this.

Of course, if either battery is dead it is worth asking why, but for this discussion lets assume it just happened and is not caused by battery failure or some electronic calamity. Therefore, the universal solution is really the inverse of the www.priusUPS.com project. Simply buy an old UPS system with good batteres. Modify the power supply to produce both +12V regulated, and 250V DC unregulated, and install it in the rear deck of the Prius. Connect the input of the UPS system to the traction battery with diodes, and several large switches, and you have a backup of both systems. You can certainly get the ICE spinning with the power from the UPS and then the ICE will recharge the UPS as soon as the traction voltage gets high enough again.

Although whimsical, if you plan to visit some exotic place like Burning Man or the cold areas of Wyoming and camp in the car, you can certainly create at least the first scenario where you run the +12v battery to zero by accident and then are in dire straits. So having at least a spare 1v battery is not such a bad idea...

Cheers
Steve

[jayman]

For under $70 Cdn, I solved the issue by keeping my Canadian Tire 700 amp car booster box in the Prius at all times. I take it out once a month to recharge it.

Once it gets colder, I get a lot of use out of it by helping out motorists who can't get started at parking lots or - this is fun - at an intersection. I would never take a chance using the Prius itself to boost a dead car, and it really doesn't help the Hybrid cause if somebody asks you for a boost and you tell them you can't. They will then automatically assume all Hybrid drivers are dickheads

When I got my FJ, I picked up another booster box just for it. So far I haven't had to use it, but it makes sense to keep one in a vehicle. Say you venture to a remote area, happen to leave an interior light on, and in a day or two, dead battery. Beats a long walk back to civilization

[statultra]

i think you can get 14 volts out the HV battery by taking it from the opposite sides, i think thats what i measured when i opened it because each module is 7?

[DaveinOlyWA]

yep there is a way... carry a $25 12 volt jump box in your car. weighs 4 lbs, small, takes up little space, only requirement is to top off charge once every 2-3 weeks or so.

*edit*

ahhh Jay... beat me to the punch again!!

[jayman]

Originally Posted by DaveinOlyWA

yep there is a way... carry a $25 12 volt jump box in your car. weighs 4 lbs, small, takes up little space, only requirement is to top off charge once every 2-3 weeks or so.

*edit*

ahhh Jay... beat me to the punch again!!

David

Just keeping you on your toes

I prefer to keep the larger - 700 amp - jump start box around. THis way you can assist motorists who have a dead pickup or SUV, in which case the more amps the better

jay

[DaveinOlyWA]

the one i have is sufficient and other than using it to charge my RC cars, i have jumped several vehicles of all sizes... but i do admit, i do not have to contend with anything much below 0ºC and that is a BIG advantage

[jayman]

David

Lucky bugger. Got down to -24 F last night

When I priced the battery booster boxes at Canadian Tire, there was actually very little difference in price between the baby 300 amp model and the bigger 700 amp model.

jay