Battery Indicator Behavior Not Right???

If you are doing a lot of stop and go driving, this can be normal. Remember, every time you slow down or brake it is recharging the battery. When you accelerate, it is using energy from the battery. While it is more efficient at using power than gaining power (otherwise you'd have a perpetual motion machine, and the world would beat a path to your door), if your "going" is about equal to your "slowing" then it will stay in a relatively small area.

I commute at highway speeds over foothills in the Denver metro. My charge indicator ranges from one bar to nearly full (up to one or two from the top) each way of my commute. For my drive this is normal.

Also for your information, what shows on the bars is actually just the "middle" of the battery charge. The computers keep the charge in that range to extend battery life - drain them too deeply and they can be damaged, and overcharging leads to heat and damage. So what you're seeing on the monitor is really only the middle third or so of the entire battery charge range.

The programming in the computers is really very good at protecting the systems from use and abuse first, maximizing fuel mileage second, and maximizing charge third.

 

Whether stop and go driving or not, the battery charge indicator will infrequently drop lower than 2 bars. It gets closer in stop and go conditions, especially with climate control running, but PriusLewis is basically correct.

It seems counterintuitive, but the car tries to avoid using the battery for prolonged propulsion. Excessive charging, discharging, and heat are enemies of battery life, and the car works hard to assure a long happy life for the battery. The best use of the electric motors is for immediate, short-term power (i.e., brisk acceleration) where the ICE (relatively small and very efficient but underpowered alone) would be inadequate.

This will give you a good sense of the relationship between the display and the actual SOC (state of charge):

 

Ann, IMHO, you can assume that as long as the car is performing properly (and you're not getting a check engine light), the battery is doing fine. You can drive a long uphill for peace of mind, but I don't think that's necessary to prove anything. The hill needs to be steep enough that your Energy display shows fairly continuous current flow out of the battery. A mountain road should do it if you have one nearby.

As an option and perhaps easier and less time- and fuel-consuming, you can run it in "EV" mode around a parking lot or a lightly traveled residential area. The car needs to be at full operating temperature first. Feather the pedal just a little until the car starts moving. With full SOC (or more accurately, 80%), you can get some pretty good speed doing that. As SOC drops, the ICE will become more insistent on lighting, but you probably still can get it down to 2 bars.

I suggest NOT regularly using EV for propulsion. That also seems counterintuitive, but I'm sure that you, as an engineer, understand the losses that will occur when kinetic energy is converted to electrical energy, then to stored chemical energy -- and then the reverse when it's actually used to move the car. Generally better is to let ICE power move you as far as possible and avoid deliberately pulling from the battery.

 

Hello Ann: Thanks for this thread, as it reminded me to post a recent behavior that I've seen.

And Jim: thanks for your excellent explanation.

Anyway, yesterday it got above 90 deg here. I had driven the car in the morning when it was cool, parked for a few hours, then drove back. It seemed as though the SOC meter dropped fairly quickly, and was lower coming home than it usually was from that trip (which I've done often). But when I got home, it was back closer to normal (5 to 6 bars, blue).

When I went out today around noon to do an errand, the SOC display actually got down to "lavender" mode very quickly, with only two bars left. I had not sat idling, and had taken off in my usual moderately brisk manner. It was another route that I use frequently, and again, the SOC doesn't usually drop that quickly. Temps were close to 90 again today. But the car had been in the garage overnight, and was not that warm.

Finally, on the way home, the SOC went from 2 lavendar bars back to 6 blue bars very quickly (in a fairly short distance). After this, it seems to be acting "normally" again now.

This is what I think is happening based on info that I've gleaned from other posts: I understand that the SOC display is really just an "estimate" as to the battery charge state. The Prius determines the SOC partly based on the change in temperature of the cells: as the NiMH battery gets over 80% charge, I understand it can warm up quickly. So maybe if a signficant change in the ambient temperature is causing the battery to warm up, the SOC circuiitry needs to recalibrate itself? And after a period of driving, it finishes the recalibration and says basically, "this is my revised estimate of your SOC"?

Please let me know if this sounds correct.

 

My hunch is that your initial brisk acceleration is simply pulling SOC down. One little quirk of the car's warmup sequence is that it uses the battery preferentially for propulsion during the first minute or so, during what we call stage 1. That's why some of us baby the car during the first minute after startup.