Spontaneous Deceleration

If you bought an Optima 12v battery, they have a reputation for re-selling bad batteries, that could create lots of problems... Only time I've experienced what you're describing at freeway speed is when I was working on a hybrid battery pack and the warning lights came on and the problem was severe enough that computer shut off access to electric motor part of the car, which is a deceleration issue, but you'd have error codes and warning lights if that happened. Other situation with loss of power is "traction control" while accelerating from a stop briefly shuts off electric motor to protect drivetrain if wheels start to slip on ice or uneven pavement. But again, the traction light control comes on and that doesn't happen at freeway speed unless you start sliding/hydroplaning. How about acceleration to get up to freeway speed? Any issue with that?

 

My bet would be a glitch in the 12 volt system. As other people look mentioned some people have had bad luck with the Optima batteries. Also could be one of the connections back at the battery is loose.
Or perhaps a malfunctioning wheel speed sensor or yaw sensor. Basically the car thinks it's slipping or spinning. But the traction control light should come on if that is what's happening.

Posted via the PriusChat mobile app.

 

I would recommend a meeting between the car and a code scanner to see if any trouble codes are stored. There are a number of conditions that can trigger the car to enter a limp-home mode (enough power to do some limited driving, but no more). It sounds as if your car entered that mode. Next order of business is finding out why.

-Chap

 

how many miles on her?

any dash warning lites when it happen?

 

My bet is that with a driving style common in older folk used to driving big V8's for decades (like you have) is that you just ran the battery low. The Prius has an extremely low powered engine. It is made driveable because of the electric motors that assist the output using the HV pack. The HV pack in your car is only charged via the engine.

If you are used to pressing the gas pedal with your toe then moments later the thing lurches forwards about to rocket to the moon, your driving style reflects that. I have seen it lots of times when people then drive lower powered cars they basically become binary. I want to go, it is not going fast enough, keep pushing more. Eventually you're pedal to the metal and flooring it then realize it's still going, back off to idle and repeat every time you want input. On a highway with passing or even terrain differences this becomes all the time. Now in a Prius when you do this you use up a bit of the battery. Over and over and over. Eventually it will become depleted ("1 bar") and now you have no electric assist and you're driving around something with 76hp that weighs almost 2 tons. It ain't moving anywhere in a hurry. This then feels like a lack of power. You press on the accelerator the car will lurch for a split second using any reserve of battery it has, it depletes it, the engine roars and you go nowhere extra.

This is common going from super power to prius power because you've most likely never given it any thought. You don't need 500hp to take the kids to school but it sits there and you always have hp available. Now you have something that makes 76hp and up to 110hp when assisted. If you stop modulating the pedal as much and just pick a power level and stick with it, the car will adjust and give you what you want for long periods of time. But constantly searching up and down for a power band means depleting the battery quickly and then taking a loooong time to charge it back up.

Next time this happens, look at the Energy Monitor screen. I bet it will be 1 battery bar and you'll see no arrows coming out (or going into) the battery pack.

Living in the mountains, I see this all the time because going up a 7% slope for 2+ miles at 80mph will 100% deplete any even brand new battery. But you know it is happening so you just apply power and keep on it. If you modulate, you will never gain speed back.

You will read about it all the time in threads from people going up the Rocky Mountains on I-70 that their Prius couldn't maintain over 50mph or 60mph or whatever. The car can do it just fine, but you have to know the machine. I do those same slopes at 80mph and pass lots of other vehicles with standard shifting transmissions that can't find a gear for that grade at that speed. But I know if I let off the accelerator even for a second, I will never get that speed back.

 

Also remember she drove to Michigan on the old 12v battery that went bad just after she arrived. She also mentioned driving at high speeds 75-85. I wonder if the 85 mph speeds up slight inclines could drain the traction battery? I know when I’m climbing hills the battery discharges much slower if I stick to the speed limit. Lower speed, less air resistance, less horsepower needed.

 

Higher speeds exacerbate the problem. Our posted speed limits are 75mph so 80mph traffic flow is the slow lane, and really even that rarely goes that slow. When going those speeds on the highway slight variations drain the battery quickly.

 

When I was driving up the hills to Jackson for thanksgiving there was a gen 3 in front of me and a gen 4 behind me. 3 Prius all pretty much driving the speed limit up the hills.