Power loss & low battery during climb

I'm also VERY interested in hearing what TMC comes up with. I just bought my Prius a week ago and took it on a long trip with mountain passes with 4 people, and your problem description fits what I experienced 4 times during the trip to a tee. Checked the 12V battery per the link, all OK there.

 

My guess is you were on I-70 with a loaded car correct? Seeing as how you only owned it a week, it is quite bold to say you know it is a problem. The OP has been told it is not a problem by numerous people and sources. My guess is Toyota just took the car to shut him up.

When your battery dips to 40%-ish (1 purple bar) you aren't going to have battery power assist and you only have engine power. So you are powering a really heavy fully loaded vehicle up a very steep hill with a lawn mower. What do you think is going to happen?

 

Who is the "OP" and the sources that say it is not a problem, and what credentials do these sources have to assert it is not a problem? You'd have to drive it to understand. The problem is very pronounced and would certainly be reported much more and well documented if it was by design.

I've owned my share of small, underpowered cars, and understand the ICE in the Prius has even less power than those once the battery is too low to assist. What happens is you're cruising along up the hill at perhaps 65 for a few minutes, then the car suddenly slows down like you completely let off on the accelerator, letting you go no more than around 30 with the engine reving much higher than before the problem arose. This happened both on I70 and also I15 in Utah over passes that were not nearly as steep as on I70 but very long.

I found another thread on this forum from 2007 (see below) that looks like the same issue but listed no resolution, just a lot of conjecture. There are plenty of posts from other people that drive their Prius over the same kind of terrain without seeing any problem. It would be nice to be watching the SOC display to see if the issue starts when down to one bar or well after that, but I was not watching for that each time it happened. BTW, I had the car checked out including the health of the battery packs before I bought it with no issues found.

forums/gen-ii-prius-technical-discussion/35709-lost-all-power-going-up-mountain-passes

 

OP being the guy who started the thread you obviously have not fully read.

Very consistent with low battery and or overheated battery. Mountain passes means lots of current out of (up hill) and into (down hill regen) the battery pack over and over. It is not 100% efficient and the waste is heat. This raises the pack temperature. Toyota makes it so that if there is a problem like high temperature or low state of charge, it basically "shuts off" the battery so it isn't used as much or all.

This thursday when I had it parked in 105F heat down in the springs the car was not happy. It would not use the traction battery because it was too hot. Turned on the AC and by the time I was heading back home, all was well.

 

Hearty, my father in law saw the same thing you do in his Civic Hybrid. Basically, near the top of esp. Vail Pass, the traction battery is fully depleted, and the car enters a state where the engine is sending some charge to the battery as well as power to the wheels, and it eventually creeps over the top. His battery finally bit the dust and I fixed it for him over the winter.

I question whoever "checked" the health of your traction battery. Very few people know how to load test the battery. It is not that you have a bad battery, but rather, a weak/aging battery. I have wondered if a weak battery in a Prius triggers some kind of forced charge, which certainly happens with Honda. If you install a ScanGauge, you could monitor the battery current and possibly cell soc delta (maybe a SG user out there can confirm that both can be monitored) to see if the engine is having to force charge the battery when all power is lost.

Try turning off your A/C before you begin ascending a pass. Try to avoid any situation where you are accelerating.

 

Battery health was checked by owner of Boulder Hybrid Conversions where I plan to convert it to a plug in. He had his laptop connected to the maintenance connector using dealer software and we watched the SOC and voltage of all the cells during acceleration, coasting and braking as well as loading it while in reverse with brake on. We mainly looked for single cells differing in voltage from the others. We did not look at current flow, not sure the capability to do that is in the stock car. He did not discuss specific voltage specs at various loads (which would probably require a dyno), so it is possible the battery is tired.

 

Sounds like he knows what he is doing.

You can measure cell voltage but you cannot measure cell current. You can measure overall current though. The cell voltage will sag with cell current so basically just check that they all sag and come back up similarly. That's all that can be done with the stock pack and stock battery ECU.

You doing a Hymotion conversion?

 

If you do PHEV, then these problems will go away.

By the way, the HV battery is a string of 168 cells connected in series, so to first order, all cells see the same current. Now, to second order, for the purpose of maintaining state of charge balance in the string, the battery ECU can send current through the sense wires. This much smaller current is unfortunately is not a direct observable.

 

I'm going with BHC's Plug-in Supply based kit. They don't offer Hymotion. From what I've read the Plug-in Supply kit won't make this problem go away since it is an adjunct battery and does not replace the OEM battery. Once the adjunct battery is depleted that's it. It does not get recharged by the ICE. The adjunct will deplete before I ever hit the big mountain passes, and I'm back to running on just the OEM battery. If Tone88's battery replacement fixes the problem it is likely my OEM battery is weak and causing the same problem.

 

Okay, then your problem will be even worse because of the extra weight, unless you choose to save your PHEV for only the uphill portions of I-70.

 

We are dealing with a fundamental limitation of the car's design. The Prius, being a hybrid has 2 motors, 1 gas and 1 electric of nearly equal power, about 70 HP each. A 70 HP motor cannot keep a 3000 lb car going up a good grade at a constant 70 MPH, it just will not happen. This is basically what happens in the Prius. It starts the hill not knowing high long it is, and uses battery assist. Once the battery is depleted, and you are still going up hill, the battery will not recharge, and you basically just have 70 HP. If you could program the car to know in advance you were doing a steep climb, the power sharing algorithm could be ajusted to maximize the ICE output from the beginning of the climb, which would extend the life of the battery, letting you climb futher at the needed power. As the car ages, the capacity of the battery decreases, but still stays within specification, so you will not climb as far on the same hill as when the car is new. Generally, a battery is considered at end of life when it is at 50% of the new capacity.
By the way, this problem is not nearly as severe in a mild hybrid, such as the Honda, since the electric motor is a smaller amount of the total power available. This is the price we pay for getting the best overall mileage.
Maybe we should strart a write in campaign to get Toyota to add a hill climbing button to modify the power sharing algorithm to optimize it for going up long steep hills.

 

I believe the problem in these specific cases is worse, where the engine is supplying current to the battery. It is possible that there is a mismatch between the computer calculated SOC and the actual capacity of an aging battery, when there is sustained and heavy demand. This may trigger a process similar to the dreaded negative recal in Honda hybrids. That would explain why a particular car that could normally scale e.g. Donner Pass at 70 now crawls over it at 50.

Or it could be something easier, like having the battery fan clogged with lint, fur, etc.

 

Dude, what do you consider a good grade? I have no problem doing 70mph+ up the grapevine or Donner Summit. I have 140,000 miles on the battery.

 

I haven't driven over Donner since the 70s, so I didn't try it in a Prius. My Datsun 510 did slow down somewhat if I remember correctly. Anyway, the point is that at some point the SOC on the battery will be too low, and then you need just the ICE. The ICE will be working harder after a while, and when you just have the ICE, the car just is more sluggish feeling.

 

I don't disagree but I wanted to point out that it doesn't happen all the time. I've only experienced a similar situation once since I've owned the car and that was a winter trip to Reno and I ran the battery down to a purple bar or two and the car was definitely low on power but it was a very steep section of road and it was around 20deg out. So I'm not saying you are wrong or that it doesn't happen but it has been very rare in my experience.

I also traveled up from Bishop CA to the Bristlecone pine forest (just short of Barcroft Station) which goes from 4,147ft. to over 11,000ft. and didn't have a problem nor did my friend in her 2009 Prius.

 

Glad to hear Toyota is doing the right thing!

 

He drove the same Prius on the same route without issue the first few years of ownership. What will be interesting to me is driving the repaired Prius on the same route. While it is good for him that other things beyond the battery are also being replaced, I was disappointed to hear that since it may no longer be clear what fixed it if the issue does indeed go away.