Fun with dead 12V batteries and jumper cables

[Nords]

Today I learned an interesting trivia fact about cheap automotive jumper cables.

It turns out that the inexpensive version only feeds power down one half of the clamp (the half of the clamp handle that has the wire going into it). You would think that the metal clamp halves would make electrical contact with each other through their hinge to have power on both sets of teeth, but you would be wrong.

Our Prius has a honkin' big main battery, of course, but it's also started by a 12V auxiliary battery. It's a real PITA to replace so last year I put in a $165 Optima that should outlast the car. The 12V battery doesn't have to do more than power up a dozen computers when you push the dashboard button. It never cranks an engine so if the 12V battery is dying then you won't know until it's too late.

We don't drive much in ER, and our short trips either didn't recharge the 12V battery or we left something turned on. (We still haven't figured it out. We might have an unsolved problem.) The car had been sitting in the garage for over three days after a series of short trips for the week before that. Today I tried to start it and it was totally dead-- just a small orange light on the "Power" button that quickly dimmed out, and a green light on the "Park" button that also faded. Interior lights were dim/dead.

I hauled out my trusty (cheap) jumper cables from our second car. The Prius 12V battery is actually in the right rear fender well but the car has a jumper terminal under the hood. The terminal supports a vertical metal tab to which you clamp on the jumper cable.

I was unaware that only the front of this tab is metal. The back is (non-conductive) plastic, which must've saved Toyota a fraction of a cent. (It took me a flashlight and an inspection mirror to verify this little detail.) I applied the jumper-cable clamp to the metal tab, not realizing that the teeth from the powered half of the clamp were actually only digging into the plastic back and not the metal front. After running the other car engine for a few minutes I tried to start the Prius again-- still deader'n a doornail. At this point the manual suggests calling the service center for a tow.

(Another fun Prius fact: If the car is pulled into the garage and the transmission is in Park, the front wheels are mechanically locked. This means if the car is dead then it has to either be jacked up & dollied out of the garage for the tow truck or pulled across a slick surface. Maybe from now on I'll back the damn car into the garage.)

By this point I was in a fairly grumpy mood when I called our local Toyota [-]monopoly[/-] franchises. One of them is booked until next Wednesday and the other hasn't even bothered to return my call. Since it looked like I'd be waiting for a while, I started searching the PriusChat.com discussion board for threads on dead 12V batteries.

Ten minutes later I found a thread about the jumper terminal's metal/plastic design and cheap jumper cables. Sonofabitch. I reconnected the jumper cables with the live teeth on the metal tab, and the Prius immediately powered up. It took a few minutes for all the computers to restore their settings, but we're fine so far. (Didn't even lose the radio settings.) I'll have to try to power it up again tomorrow morning just to make sure. Or else I'll jumper it again and then go get a new battery (and a refund on the Optima). Maybe I should check the little FM transmitter dongle that broadcasts the iPod output to the car radio.

Yet another fun fact about a Prius with a dead 12V battery-- when the car's out of electricity, it won't unlatch the (electrically-interlocked) back hatch to enable access to the tool area in the spare-tire well. Guess where most drivers keep the jumper cables.

 

[nwsteve]

Such are the "rewards" for being "Green"
Nwsteve

 

[veremchuka]

(Another fun Prius fact: If the car is pulled into the garage and the transmission is in Park, the front wheels are mechanically locked. This means if the car is dead then it has to either be jacked up & dollied out of the garage for the tow truck or pulled across a slick surface. Maybe from now on I'll back the damn car into the garage.)

Maybe from now on you should buy a "regular" car?

Interesting story. I too would have thought juice would cross into the other clamp. And as far as the plastic coating, well maybe from now on you should buy a regular car!

Talk to the local fire department and ask them how much time you lose when they arrive and see your car is an electrocution hazard to them.

 

[Telly]

I would guess that the Prius, like the Ford Escape Hybrid, charges the auxiliary battery via a DC to DC converter that is powered off of the HV traction battery. So unless there is an unusually heavy accessory or parasitic load, I would think the aux battery would have no problem staying charged, even with short trips.
I would check the charging voltage, but you may need to get the system into the proper mode to get the DC to DC converter running. A Prius forum could help there as to what that is. And what the charging voltage should be under those conditions really needs to come from the shop manual. Hopefully, someone on a Prius forum actually has one and knows how to use it.

Concerning the Park lock, and no power to unlock it... If it were just a matter of needing to get it out of the garage, putting a floor jack under a front control arm (if there is a good place) to lift one front wheel off of the floor will do it. Every auto trans parking pawl I have seen works on the output of the transmission, before the differential. Including FWD transaxles. So lifting up one wheel to break contact with the ground allows the car to be pushed on the floor jack. The wheel still on the ground will turn with the direction of travel, the wheel off the ground will turn in the opposite direction. Simply just the differential action when the input is held still.

Back in the days of the Model T, the brake was a transmission brake. If either rear wheel unloaded enough, there went the brakes! Same thing with most lawn tractors... a quick stop on wet grass gives a slide with the lightest rear wheel spinning backwards.

Another idea on unlocking the drive would be to see if there is a Brake Shift Interlock defeat procedure for the Prius, so you could put it into Neutral. The Aisin CVT used in the Prius is also used in the Escape Hybrid, and over there the transaxle is cable controlled for the P-R-N-D-L range selection. So the interlock probably prevents shift cable movement. If there is a defeat procedure, then the cable can be moved again and you could get to Neutral.

 

[Telly]

...........Talk to the local fire department and ask them how much time you lose when they arrive and see your car is an electrocution hazard to them.

I asked this question of our local FD, which I am well aquainted with. I knew that there had been classes put together to give hands-on training for first responders dealing with HV hybrids. They said no problem at all, the mfgs. have done a good job on it, and they are all trained.
But what really scares them is the proliferation of airbags. Like a side airbag suddenly going off while doing an extraction. There are some youtube fire videos of airbags suddenly going off and injuring people AFTER an accident.

 

[kyounge1956]

I killed the battery in my Prius by failing to shut the hatchback completely, and didn't realize it until a few days later when I went to start the car and zilch. Mine was in the garage of my townhouse, nose first, and for the life of me I couldn't figure how a tow truck would be able to get it out if there was anything really bad the matter with it. (Maybe from now on I should buy a "regular" house!) Fortunately the tow truck driver had one of those batteries-on-a-dolly and some jumper cables, and that started it right up.

 

[Nords]

Thanks, Telly, I agree about the battery charging and hope I just left something turned on without realizing it. Otherwise there are a lot of battery-charging experts on that forum, and we'll have to get into the details.

I'm going to have to play around with the floor jack!

I killed the battery in my Prius by failing to shut the hatchback completely, and didn't realize it until a few days later when I went to start the car and zilch.

We had our hatchback open for a few hours between trips, but I don't have any lights turned on. What was using power when your hatchback was open?

 

[kyounge1956]

Thanks, Telly, I agree about the battery charging and hope I just left something turned on without realizing it. Otherwise there are a lot of battery-charging experts on that forum, and we'll have to get into the details.

I'm going to have to play around with the floor jack!


We had our hatchback open for a few hours between trips, but I don't have any lights turned on. What was using power when your hatchback was open?

I'm not sure, maybe the dome light? I just know that's the only time the car hasn't started when I stuck the key in, and that the hatchback was in fact, not closed all the way.

 

[donheff]

Darn, I always assumed these things could tap the big battery for auxiliary power. From the sound of this thread, the 12V is wussy so anything left on overnight is more likely to drain the battery than in a regular car. So can you connect a trickle charger to the charging posts and recharge the 12V battery?

 

[bruce1]

Today I learned an interesting trivia fact about cheap automotive jumper cables.


Yet another fun fact about a Prius with a dead 12V battery-- when the car's out of electricity, it won't unlatch the (electrically-interlocked) back hatch to enable access to the tool area in the spare-tire well. Guess where most drivers keep the jumper cables.

Prius is not alone in this. My Buick will not unlatch the trunk with a dead battery either and it was December in Ontario Canada. Battery charger and a 100 foot extension cord combined with a generous helping of colourful language fixed that though.

 

[clifp]

Prius is not alone in this. My Buick will not unlatch the trunk with a dead battery either and it was December in Ontario Canada. Battery charger and a 100 foot extension cord combined with a generous helping of colourful language fixed that though.

My that is some brilliant engineering on both companies part. What the hell were they thinking?

On a couple of occasions I've got my manual Accura started with a dead battery without even using a jumper cables. It did involves a lot of pushing but once I got it orient downhill on my steep driveway, it worked great.

 

[Brett_Cameron]

We have a 2007 Toyota Highlander hybrid that DW drives.We have a continual problem draining the 12 volt battery because she and the DS-14 like to look at themselves in the lighted mirrors on the tops of the sunshades. These lighted mirrors do not power down when the vehicle is off and they leave the shades down and the mirror lights on when they exit the vehicle. I have replaced the 12 volt battery numerous times.

I finally bought a Black & Decker battery pack that we keep charged and near the vehicle. It has short cables with clamps and are made for jump-starting a car. Usually the car's 12 volt battery voltage is low enough to keep the computers from coming on but has enough power to crank the engine.

I used to have a 1997 Nissan Maxima. It had a battery saver feature that disconnected power to all of the lights after a set time frame. It was a wonderful feature that I have not seen on any other car model since.

I have also seen devices that open circuit the battery when the voltage drops to a set amount that is higher than what is needed to crank the car.

 

[Nords]

I'm not sure, maybe the dome light? I just know that's the only time the car hasn't started when I stuck the key in, and that the hatchback was in fact, not closed all the way.

The car started fine today, and you can't even tell that we had a problem yesterday.

I opened the hatch and looked around, but we didn't leave any lights on. When the hatch is open, the only difference is the dashboard "door ajar" light.

The 12V battery looks fine-- no corrosion or loose terminals. Not even dusty or dirty.

Darn, I always assumed these things could tap the big battery for auxiliary power. From the sound of this thread, the 12V is wussy so anything left on overnight is more likely to drain the battery than in a regular car. So can you connect a trickle charger to the charging posts and recharge the 12V battery?

I'm still digging around on the PriusChat board to figure that out. The main battery was awfully low at startup, too (as low as the charge controller lets it get) so maybe it was trying to revive the 12V battery. But I thought there shouldn't be any power to the charge controller when the car's turned off, although some other aux systems use battery power when the car's shut off. (For example, one computer tests the braking systems once or twice long after the car is shut off.) Yet even for three days in the garage, that doesn't make much sense.

Apparently yesterday's 15-minute three-mile loop was enough to recharge the battery. But all the 12V battery does is boot up a dozen CPUs, so it could still be barely charged. "Cold cranking amps" is irrelevant to this car.

My that is some brilliant engineering on both companies part. What the hell were they thinking?

Apparently they're expecting drivers to store their jumper cables under the passenger seat!

In defense of Prius, this is the first serious problem we've had in over three years of ownership. Heck, this car is so low-maintenance that I barely remember to check the gas gauge, let alone the tire pressure. "It just works". (Hmm... I wonder if that phrase is trademarked.)

 

[Nodak]

If the vehicle is used infrequently you could try a smart trickle charger (Battery Minder is one brand), that will keep the 12V battery ready to go and will not overcharge it.

 

[Telly]

............Apparently yesterday's 15-minute three-mile loop was enough to recharge the battery. But all the 12V battery does is boot up a dozen CPUs, so it could still be barely charged. "Cold cranking amps" is irrelevant to this car.

Since I don't have a Prius, I don't really know... but wouldn't there be a lot more intermittent and constant loads on the 12V aux battery than you mentioned?

Like lights, both inside and outside including brake lights, and power windows, cabin HVAC fan, door locks (like a hatch release ), ABS pump, electric fuel pump (usually a big draw) for the fuel injection, etc?

For battery ratings, I would think Reserve Power would be the most important for the application. Get out the Marine batteries! Prius goes Nautical! That would seem fitting for your previous occupation

 

[kcowan]

Apparently they're expecting drivers to store their jumper cables under the passenger seat!

Our 2005 Ford Escape has no way of opening the rear hatch when the battery is dead, so storage of the jumper cables is an issue with us as well. The only door that opens with the key is the driver's door. When we fly on vacation or snowbirding, I leave the jumpers on the floor for ready access.

 

[DoraM]

If you could find a large enough IGBT you could use a small 8 bit microcontroller to monitor the terminal voltage on the 12V batttery and disconnect it from the car circuit if it drops below a preset voltage. This would be harder with a normal car which has to supply 600 amps or so to a starter but sounds like the Prius only uses maybe a few tens of amps? Of course modern IBGT or even plain mosfets can handle many hundreds of amps now with no problem. Simply put a override button such that you can start the car and recharge the depleted battery or drive to the repair shop to figure out what is draining it.

Would be a pretty cool product if sold embedded in a short cable such that any joe could install it with no electrical knowledge.

 

[packrat44]

If a person suspected they had a current draw with their car shut down they could disconnect the negative battery cable from their battery and bridge the open circuit with their multimeter.

 

[kumquat]

Virtually every car on the road today will draw a few Milli-amps when turned off.

 

[DoraM]

If a person suspected they had a current draw with their car shut down they could disconnect the negative battery cable from their battery and bridge the open circuit with their multimeter.

Be very carefull if you do this! Most multimeters have a 400mA fused current measuring setting and a 10 amp unfused setting. Not so bad if your fuse blows (400mA is not a lot) but could get exciting if you attempt to supply 600 amps to the starter through the typical 22 gauge multimeter leads.

 

[packrat44]

In street rods it is common for the last 50 years to have electric solenoid switches for the trunk release and it is also common to have the battery in the trunk. It is also common to have a cable (emergency) release for cases of dead batteries. (Hobby cars can sit idle for a long time resulting in discharged battery.) However, with a factory made car that extra cost of a redundant release means profit that does not go back to the company.

 

[packrat44]

Be very carefull if you do this! Most multimeters have a 400mA fused current measuring setting and a 10 amp unfused setting. Not so bad if your fuse blows (400mA is not a lot) but could get exciting if you attempt to supply 600 amps to the starter through the typical 22 gauge multimeter leads.

If you are pulling 600 amps with everything shut off you have a SERIOUS problem and you do not need a multimeter to find it. Best call the local fire department and then your insurance company.

Virtually all vehicles pull some load when stored. Most items that can cause a battery to drain down over the course of a day or two are on a fused circuit. If you remove one fuse/disconnect one cicuit at a time and you no longer draw current, there is something down stream drawing current. At that time you can determine if this a normal draw or if something unintended is drawing current.

The multimeter can also be used to measure voltage drop besides measuring current draw. Look for a short or something inadvertently left on if you have a voltage drop greater than 0.2 volts.

 

[DoraM]

If you are pulling 600 amps with everything shut off you have a SERIOUS problem and you do not need a multimeter to find it. Best call the local fire department and then your insurance company.

Virtually all vehicles pull some load when stored. Most items that can cause a battery to drain down over the course of a day or two are on a fused circuit. If you remove one fuse/disconnect one cicuit at a time and you no longer draw current, there is something down stream drawing current. At that time you can determine if this a normal draw or if something unintended is drawing current.

The multimeter can also be used to measure voltage drop besides measuring current draw. Look for a short or something inadvertently left on if you have a voltage drop greater than 0.2 volts.

True about the 600 amps but I was more thinking of someone on here hooking up their DVM in series with the battery lead while their spouse is in the car playing with stuff and they happen to crank it.

It would be much safer to measure the voltage drop across a fixed length of known gauge copper wire. Then you can calculate what the resistance of this wire should be from tables on the web (will be close anyway) and you can compute the current draw that way. IE, 0.1V drop across a 0.05 ohm section of cable means you are pulling 2 amps (I=V/R).

 

[packrat44]

True about the 600 amps but I was more thinking of someone on here hooking up their DVM in series with the battery lead while their spouse is in the car playing with stuff and they happen to crank it.

).

Society can not keep stupid people from during stupid things. That is why we have so much legal jargon in print with every product we buy.

 

[JakeBrake]

If a person suspected they had a current draw with their car shut down they could disconnect the negative battery cable from their battery and bridge the open circuit with their multimeter.

Or you could use a clamp-on ammeter. When I was working, I used a top-of-the-line clamp-on. When I retired, The meter retired with me.

During my career I worked with large industrial batteries. From my experience, I will not be driving a hybrid or electric car anytime soon.

Nothing can spoil your day like finding several gallons of battery acid on the floor leaked from lead-acid batteries and the sealed batteries have been known to go into thermal runaway, which is a slow-motion explosion.