Power Button Not Working after Dead Battery Reverse Polarity

You might be able to make "dedicated" jump start / charge terminals that were separated from the car by some diodes to provide reverse current protection. But those diodes would have to hold up to 50+ amps during a "normal" jump start procedure.

Posted via the PriusChat mobile app.

 

These were my favorite jumper cables back in the day.



A plug in the middle. LEDs on each side. They light up when each side is attached to its battery the right way. Then you plug it together.

You also didn't have to fuss with finding a non-battery-terminal grounding location. Just put both clamps on the battery posts, because the spark doesn't happen there, it's only made later at the plug when you plug them together.

Modern jump packs seem to pretty commonly have a similar feature. In fact, it's often more than just lights you have to look at, they won't close the connection if the polarity is wrong.

 

Its not the expense, there is one heavy duty fuse at the battery, fuses act faster than breakers, but there are various scenarios involved. While not applicable here, the typical high current battery to battery reversed polarity scenario takes out smaller fuses first. In part this means the 125a buried fuse goes before the 12v battery's 140a fuse.

In reality its an engineering masterwork when replacing fuses usually gets the system running again. Unfortunately Toyota shares very little of the real circuitry, just the modules and wiring hookups.

It appears various modules have reverse polarity protection built-in to shunt the current and blow fuses. That protection could be as simple as high speed diodes on their incoming 12v biased to conduct (essentially short) on reversed polarity. Protective semiconductors in the modules would have to be strong enough not to blow themselves at currents greater than 125a.

It would seem Toyota could do a little better such as making the 125a more accessible and perhaps by sizing things such that the battery fusible link was smaller than the fuse box fusible link. But that goes against normal automotive engineering principles where the battery fusible link is the biggest. If people carried lithium jump packs with initial reverse polarity protection interlocks these things would not happen.

In this thread's case a different scenario may have played out. A low power charger was strong enough to ruin the 12v battery taking it out of the circuit but not strong enough to blow the 125a fuse box link. At that point some module may have fried when its protection overheated or the Op has not found the smaller fuse or connection that gave up.

 

As a reminder, his battery voltage is only 11.9V, which is very low for this car. And we all know the electronics act up badly if the battery is under voltage. He needs to correct that first. And, maybe that’s all he needs to correct.

 

It is, at the same time, well above 11 volts, which is the already-conservative figure given in the Gen 3 Repair Manual to check for before going ahead with troubleshooting. Nothing wrong with charging the battery, but calling 11.9 "very low" seems a stretch to me.

For a good portion of my 2020 COVID driving cutbacks, before I added the solar charging mod, 11.9 was more like the high-water mark for my battery. Surely wasn't great for the battery, and at voltages well below that there were some winter no-starts where I needed the jump pack, but never caused any electronics to "act up". Some ECUs will log codes that say "voltage went below 9.5", if that happens. That's just them doing their job.

 

I disagree with you. Voltage on mine is usually around 13.6V or more so the car could definitely be acting up with that battery. This whole thing started with his battery needing a jump. I first told him disconnect and reconnect the 12V. Lo and behold, it worked, at least partially.

 

That’s maybe on the cars display, in ready mode??

 

I don't disagree that your usual voltage is higher than 11.9. Higher than 11.9 is a fine place to be.

But you seem to be reasoning as follows: "My voltage is usually x amount higher, therefore 11.9 could definitely cause acting up."

Just doesn't follow. There are a bunch of ECUs in the car, and they are built with on-board active regulators precisely so they can have clean 5 volts locally despite the whole typical automotive range of < 11 to nearly 15 and noisy coming in. The regulators do need a couple volts headroom to do their thing, which aligns well with my consistent experience that a no-start is likely if the starting voltage dips below around 7. As usual for such things, the power-ok signal from a regulator is deasserted then, and the ECU stays off, and restarts cleanly when power recovers. Things definitely function just fine down to the levels where the ECUs log correct diagnostic codes for sub-9.5 voltages. Of course various electromechanical parts like the brake pump will sound like dying windup toys at that voltage, but that's a far cry from the "explain any undiagnosed Prius issue" wild-card that some people want to make out of even decimal-place voltage differences.