Loss of power after coming down mountain

Firstly, I don't think there is anything wrong with the car as I have been driving it all week and nothing out of the ordinary occurred since.

I live in the Appalachian mountains and I have observed deep cycling of the battery after coming down a long descent and fully charging the battery. For those who might live in flat areas, when the battery is fully charged from brake regeneration the ICE will begin spinning to act as an engine brake (but low revs unless you have to shift in B). After the descent is over the battery will discharge, usually down to 2-3 bars.

However, after going up to Brasstown Bald to shoot some photography the descent is much quicker, we're probably talking grades between 8-20% and at low speeds, not exceeding 35 mph, some 15 mph corners, etc. So the battery is quickly charged, by my estimation something to the order of 240-300 watt/hrs in just a few minutes. Likewise, the ICE kicks in and in the case of this mountain B must be used. After coming down the mountain I get on a highway driving 55-60 MPH and the battery goes through it's normal deep cycle, discharging the battery. Except this time it went down to one SOC bar and stayed there. I came to a stop sign and went to take off and the car basically didn't move. I had to give it about 75% throttle before the car would move and it felt like the car was out of gear because it jerked a bit. Very strange. My thinking is that the traction battery was almost completely depleted. I stopped for a while and let the ICE idle and before getting home did come back to 2 SOC.

After I got home, I let it sit. Then drove it again and the battery charged back up and since has been driving normal.

My question is, what happened here? Is the traction battery getting too hot to charge or something else going on? I am going with the assumption that something is preventing the battery from charging, either due to temperature or current related. No dash lights or anything came up while driving and the car has about 19k miles on it roughly.

Any ideas? Or anyone with similar experiences? I had a Prius C in the past, gone up the same mountains and never behaved in this manner, the battery would not do a deep cycle in this manner, just would use the battery for 1-2 SOC and go back to normal behavior. Although, the Prius C has a smaller traction battery and therefore probably less heat/current to contend with.

 

That's what it would seem like. It does similar things when I drive from where I am at to Asheville, about 100 miles and have to cross two steep mountains. The battery gets fully charged, or near full charge and ICE spins on the downgrade and then once leveled out the battery deep cycles, usually down to 3 SOC but no brake regeneration for a good 15 minutes. Wondering what is going on. I've stuck my hand down near the vent and the air is no warmer than usual so I am thinking something relating to MG2 or the inverter board getting hot but no warnings no anything otherwise. The jerking taking off from a stop sign was what concerned me the most, I thought the CVT was done. At any rate, I don't think there was a malfunction, but due to the steep descent and so on put it in a state where the battery was depleted more than it should have been.

Well not sure if there was a malfunction here or just something getting hotter than it should preventing the battery from being charged. It did come up one SOC by the time I got home but I wanted to make sure the CVT wasn't trashed either and why I took the car out for another drive and things went back to normal.

What sort of malfunctions were you experiencing? I am leaning towards thinking that powering off the car would have been a bad idea if the traction battery was depleted, might fail to start the ICE on power on...which means you're stranded and will have to have it towed. My thinking would be to continue to let it run in Park and allow 15-20 minutes for it to come back up.

Hoping someone might know something more technical... like should I avoid driving this Prius up there to do any photography, but there's mountains all around and I think that condition is unavoidable, although it seems the less steep and the slower the battery gets its charge, the less of this cycling effect I see, so like bisco says probably something getting hotter and stopping brake regeneration despite the battery being depleted.

 

No doubt it is, any car will wear out climbing mountains. However, it's double duty, sort of, in a descent as it picks up a lot of regeneration downhill, at pretty high current... Actually if the bar graph is anything near accurate, we're talking something in the order of 4.8 kW or 240 W/hr in 3 minutes, something had to have got hot, that's a lot of power in a very short period of time. So what is getting hot enough to half charging the battery, it's surprisingly not blowing hot air, or nothing out of the ordinary. Either MG2 is getting hot, inverter or battery. Something in the car has to be collecting data to know such a condition exists. Not sure how I am to avoid it in this area or how others might, assuming I don't have something going on that might become a serious problem. As for the old Prius C, much smaller car, much smaller battery too, I think (correct me if I am wrong) something like 900 Wh...

 

Good to know. So either I need to find where the air comes out or...take the back seat out and find out how hot the battery is getting under normal conditions and then again coming down the mountain. Since it picks up a lot of charge quickly then discharges, battery is probably suspect #1, the rest of the components, MG2, Inverter board probably have more effective cooling. Just a wild assumption.

 

Do you use either of these products and what would be your preference?

 

Yeah, Park it will charge, never had the car in neutral.

 

That definitely sounds like the problem I have. I mean once you see how the car behaves it's not too much of a problem, but the first time that happened that really worried me that something was wrong. I will however print this off and see if the dealer will flash the ECU with the updated software. Sorry for the late reply.

 

So to follow up on things. I did have the ECU flashed with new software and did fix the problem, the car behaves differently. Since I almost always go down long steep hills the effect was immediate. I did also get a OBD WiFi dongle and OBD fusion for my iPhone/iPad and recorded data both before and after the flash.

First things first, the battery indicator in the Prius lies horribly. Even with a full charge (all 8 SOC bars) the battery is around 75% charged and I have yet to see it go beyond 77%. Most of the time the battery will run around 45-65% charge...which makes sense and is largely why the batteries can last a long time imo, but I'm no engineer. Anyway...

Prior to the update I monitored all cell block voltages, temperatures, inverter coolant, motor/generator temps and so on. I had initially thought it might be something getting to hot, per the data not the case at all. Nothing is getting anywhere near hot enough to cause problems. But what does happen is that after going down a long hill that fully charges the battery, the battery is drained down to 1 bar SOC and that equates to about 25%, which was what was recorded by the OBD dongle. At that point the battery neither is charged or discharged and you're pretty much running only on the ICE and yes you can notice a reduction of power. Not sure if it is/was a software glitch but if you came to a stop, when you would accelerate from a stop the car would basically idle out to the road unless you put the accelerator down to the floor almost and it would then jerk and go. Not until the hybrid system charged the battery would the car start to drive/feel normal again.

Also, important to point out. If you have a 2016 Prius that has a VIN in the range that the TSB corresponds to, despite the information @Elektroingenieur had about the TSB being covered under warranty, Fred Anderson Toyota in Asheville, NC (formerly Jim Barkley Toyota) told me it was not covered under warranty (out of warranty -- I have just over 36,000 miles) but did not charge me for the ECU update. Not sure if that was due to me getting a new set of tires and other maintenance. But would be a good heads up for those who might have the expectation of not being potentially charged by the dealer to fix the problem.

I do feel that it is a safety issue. If you have a car that falls under the TSB and either haven't experienced the problem or if you lend your car to someone. Yeah, I think under the right conditions the loss of power could be dangerous. I would be worried if I let someone unfamiliar drive the car and it happens to them while driving it. That being said, those who live in flat areas, around cities, etc, are less likely to experience it.

That all being said, if anyone else had the ECU flashed due to the TSB I would be curious about your experiences before and after the update.