Hybrid Battery Slowly Drains While Engine Running

The battery might be too cold, so the car is trying to avoid using much battery power until it warms up. Which, given how long it takes a 1 lb frozen steak to thaw, you might be waiting quite a while for a 120 lb hunk of metal to warm up...

(However, metal is more conductive than meat, and there's no phase transition to go through like the water in the meat will when thawing; so it's possible the battery will actually "thaw" faster than a steak under the same conditions... But I don't have either a frozen steak or a frozen battery, so I won't be testing this any time soon. )

 

Seems the Prius may be trying hard to protect the hybrid battery. The only possible way to warm the hybrid battery is by charging/discharging. If the ICE were to idle & also supply all the power requirements of the vehicle, the hybrid battery sits & does nothing. By slowly cycling the hybrid battery, heat will be generated internal to the traction battery, warming it.

When you have one of these "Abnormal" conditions how discharged is the traction battery allowed to go before it gets recharged? I have found the dash battery gauge quite inaccurate. Instead I use the SoC (State of Charge) pid in Torque, which follows the true SoC quite nicely.

 

I'm a little unclear, between your two scenario's Normal and "Abnormal"...there is situational difference? And that is outside temperature? So your abnormal behavior is only manifesting when it's cold outside?

Also the first marker of what "situation" you happen to be in, seems to be whether when you "park" the ICE either shuts off (normal) or (keeps running) abnormal. Since in both scenario's you say the ICE is at the same temperature when you park.

I guess I have to ask the obvious, and that is how do you define "park". I'm assuming you are in "P"-park...with the parking brake set, but the Prius left ON?

Since you say once you start driving, and come to a stop it seems to "return to normal", that would tell me that it isn't "physically" something wrong with your battery. If it was something wrong with your battery, it would never "return to normal".

I'm sure as you know, the Prius operates in at least two ways, 1. To keep the engine warm, to help maintain clean emissions. 2. To run the engine to maintain hybrid battery charge.

You seem to be noticing a specific scenario where the engine is running but the battery isn't maintaining charge.

So I guess my final set of questions are:

In your "abnormal" scenario, do you ever just let it go? In other words, just let the engine keep running until the battery get's really, really low? In the abnormal scenario you say the engine does NOT turn off? If so does the engine temperature maintain or drop?

You also say when you observe it is behaving "abnormally" you shut down and restart? Have you ever tried NOT doing this? As every time you shut down, your engine is going to cool, and every time you start, the Prius is going to go into "start up" mode.

I guess my advice is next time you are forced to idle, and are observing the "abnormal" behavior, try doing nothing. I don't know how long your idle time is...or how extreme your outside temperatures might be in any given scenario, but the Prius is designed to not let the HV battery get too low, so at some point it should start taking a charge or get so low, that it throws a code.

Also curious...how "cold" is "cold" outside? If we are talking really extreme low outside temperatures you might just be witnessing "normal" Prius behavior in trying to protect itself while parked.

 

check your 12 volt health.

 

Or....this....

 

Well, I'm just really tempted to say I'm not convinced you have a problem at all.

If it's cold outside and you are sitting in park, the vehicle will run the engine to maintain temperature and to charge the hybrid battery. If you are doing this, even if the engine is running but you've never seen the hybrid battery drop below 40%, then I think the system is just doing what it is suppose to do.

Have you ever tried "force charging" the battery in this scenario? Because from what you are describing I really don't think it's a case of the battery not taking a charge at idle.

 

Have you notice a hit in recorded MPG?

 

I could be wrong.
But I think you are paying closer attention to immediate behavior than most people.
If you've never seen the battery drain below 40%, but you think the battery is slowly draining and would never charge? Then I think what you need to do is let it go until it does drop down to where it would obviously need to charge or the system would throw a code. In your case, what I would be curious about would be if I could get the battery so low, 2 bars or less, and then whether it charged itself or not. If you find that there is a point that the battery DOES receive a charge? Then I think the system IS working. Just not exactly the way you think it should. If it "slowly" drains way too low? I would think it eventually would throw a code. Then at least you would have a starting point for evaluation.

As far as to "why" would it be slowly draining. My guess is as long as The Prius is ON, even if parked and with few or no accessories on, there is still probably a minimum drain that happens just in operation.

I'm not an expert on exactly how The Prius software may or may not prioritize how it uses engine generated energy in any specific situation. But I would assume that it runs it to maintain 2 conditions, keeping the engine warm, and keeping the battery charged within a certain range. Seems to me even though you are carefully watching "how" this is happening, the end result is that The Prius is doing both things. Your charge is not dropping below 40% and your engine is being kept warm.

Good Luck.

PS.
In my test scenario, I wouldn't really want to let the battery drop too much below 2 bars. You want to see if it ever responds and starts charging, or throws a code, BUT you don't want to strand yourself or cause any permanent damage. I'd say if it drains down to 2 bars or less and at that point it isn't charging the battery? Then you indeed do have a problem.

 

since you checked the 12 volt health, can you give us the readings? or are you relying on a service monkey's word?

 

i've noticed similar, but the opposite, in hot weather with AC on, sometimes the ICE cycles and sometimes it just runs all the time while sitting ready in park.

 

Stop trying to overthink the cars built-in battery safeguards, and just drive the car.

Same here.