MacGyver a flat 12v battery from the HV ?

If the 12v battery is dead, how would you boot up the computers for the HV battery to charge the 12v battery?

 

Well, this is chicken-and-egg time. The Prius computers need 12 vdc, and the only way to get that relay in the HV NiMH pack to close is to have the car boot up first.

If the 12 vdc battery is flat, nothing happens. The car is completely dead. You need a boost to get going

 

Is there a way to fool the 12v regulator/converter to give a charge to a flat battery from the HV battery?

There is no simple or standard way. If you made a direct connection between the 12-volt and the traction battery the 12-volt would probably go off like a bomb. Wisely, Toyota has made it hard for you to do this.

It would save a lot of grief if you are stranded far from help with a dead battery.

The 12-volt doesn't do all that much work. I've jumped my car with the battery out of an emergency light. You might look into getting a little 12-volt battery, Ni-Cad maybe and setting it up to trickle charge when the car is on, or have a solar panel in the window to keep it topped off. It would be enough for a self jump or two. I'm not sure if you could use the cigarette lighter plug to connect them.

 

some 'gator clip leads and a really big resistor?
.
_H*

 

LOL... You're talking about a BIG resistor, and you're talking about melting it anyway. You really could design a voltage regulator for it, but then you'd need access to the hybrid battery taps which would PISS OFF the computers when you fired the car up; I'd bet every single alarm you can think of would go off, including some none of us have ever heard of. Moreover, you're safer just getting a lantern battery and clipping it onto the 12V battery in parallel with a couple 10ga wires. And it would cost a tenth of that voltage regulator.

~ dan ~

You could also get one of those quick battery chargers that plug into the wall and include a 50A "quick start" option. Then just jump the car from your wall.

 

OK, here ya go. Open the HV battery box. Find the relay. Install a plastic rod through the case to the relay contact assy. When you press on the rod, it closes the relay, energizing the charger, with will then allow someone else to press "start" to fire up the computers, which will then take over the job of holding the relays closed.

But don't do it. It's very dangerous, could blow some of the electronics, and is just silly. A simple boost will suffice to do the same. The Prius takes a 45 Amp 1/2 second pulse when you press the brake pedal, then another when you press "start". After that the HV system takes over and charges up the 12V battery with 13.8V.

If you want to be "self sufficient" get a portable car booster. They are inexpensive, fairly small, and work well with the Prius. They will also allow you to boost other cars, something you shouldn't do with your Prius.

 

Which is of course why ships never sink or are left dead in the water.

I know what you are saying, but just had to poke a little fun. For any backup system you have to do a cost vs benefit analysis, and also look at the problems that could be introduced by using the backup. With the Prius, the control systems are powered by the 12V bus, so you need a source of 12VDC before you can fire off the inverter. It would be relatively simple to provide a backup 12V charging system, but that adds extra cost and weight, and offers the risk of drawing down the HV battery through misuse.

Tom

 

My backup battery

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

 

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?

 

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!



DON'T TRY THIS

 

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

 

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

 

i think

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?

 

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

 

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

 

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