Traction Battery Discharge in HEV Mode

The traction/hybrid battery powers the electric motors. This is normal. With regular driving, once the hybrid battery runs low on charge, the ICE will kick on to charge it back up in conjunction with regenerative braking.

 

It seems the HV mode in the Prius Prime isn't like a Hold mode in other PHEVs.

Hybrid operation requires taking energy out of the battery and putting it back in. A NoPlug just keeps the state of charge within a range that is good for a healthy battery. A PHEV with no more grid charge works the same way. When there is still grid charge left, a Hold mode will try to keep the battery charge at the level when the driver switch modes.

Toyota's HV mode with grid charge left will make more use of the charge left in the battery instead of keeping it at the level at switch over. It means less use of gasoline, but it will eventually drain the battery to the point you can't switch back to EV mode. If you want to run the engine for whatever reason, the battery needs to be drained to the point that EV mode isn't possible.

 

I recently posted a video in the review thread where a guy was racing one round with the Prius Prime on the Nürburgring racing track. Despite not being in pure EV mode (who would race like that) the Battery depleted from 80% to nearly 30%.

There is now way it can recharge it when you use all the power for driving aggressively.

EDIT: That means too an "empty" Prime or normal HEV cannot utilize its max power for too long - unlike a Prime with charge.

 

Top EV speed was around 80mph in the gen4, and I don't recall news about it increasing in the gen5.

The hybrid has a lower net power rating to go with the smaller battery.

 

I found when i drive in HV mode, it barely touches the battery at all outside of two scenarios

1) heavy traffic - where the car slips into EV mode more often and stays there for longer
2) driving more spiritedly, as the car uses the electric motor to deliver more power - which makes sense given the higher combined HP they advertise for the prime over the non-prime model as the bigger electric motor will pull more power from the battery to operate.

When the battery is down to the 0 EV range "blue" mode, I found that the battery meter barely moves while driving. And similarly, if I'm coasting and driving conservatively with no traffic the EV range goes down maybe 1 or 2% when it has a lot of charge leftover. And this is on the same route as example 1 above, since some mornings I get a clear drive to work but not always (and never clear in the afternoon home).

So while it doesn't have a hold mode, i think if you're just driving the speed limit, and driving in a normal sustainable manner, you likely won't ever have the battery deplete very far at all when driving in HV mode. Maybe if you're pushing the car a lot more and driving at a much higher speed the car will pull more power from both powertrains in its combined operation mode more aggressively though.

 

I also suspect HV mode doesn't try to hold charge when full. It probably will let charge drop to 80% for the same reason Charge mode doesn't go above it. You might end up with an HV usage 'band' just below the 80% mark, which would explain 76%.

I've never seen HV mode letting charge slide, but I've not used it much, and never when full.

 

very possible, I don't really use HV mode from the jump on my drive, I only use it with a high charge on the battery when I am going at freeway speeds to preserve the battery for when I'm off the highway and for when I am driving home in heavier traffic. It also means I don't need to full charge some nights, and am able to ensure the engine gets used and the gas won't go stale. Plus, the full power of the combined output is very good for passing folks and getting up to speed in the on ramps. Every now and then it's fun to zoom zoom a little bit with the accelerator

 

HV mode using EV battery charge was first reported here by someone trying to get a baseline on HV fuel economy. Specifics of the behavior weren't puzzled out. Draining a full battery down some before keeping the charge level could be done for battery longevity. Toyota hasn't shared that info, and no one has done testing.

 

The vehicle could have been in HV Auto. I don't believe I've seen a situation where driving in HV mode (non-Auto mode) lets the battery go down very much if at all.

I've set the vehicle to HV mode when the battery charge is below 20% and above 80% and have never seen a variance more than a percent or two from the initial battery level. If you turn on HV mode while the vehicle is cold, the battery will go down a few percent while the engine is warming up, but the Prime "remembers" your previous battery level and charges it back up to cover the charge used during warmup.

 

In HV mode, it will still draw from the traction battery to assist the ICE when necessary. It's to maximize efficiency. Don't know if the traction battery would be deleted on a long road under normal driving conditions. I will find out soon enough as we're doing a 4,000 mile road trip in two weeks.

 

I dont think top speed is an issue for the battery - once youre there it can probably hold it with the combustion engine quite fine. What deplets the battery in HV seems to be very sprited driving like hard braking and accelerating all the time as in the race track example. Not normal driving conditions and nothing most people here would do but it shows its possible.

 

Yes, it's the same in Gen 4, but what you observed seems to be on the extreme side.

It will sometimes recharge back and sometimes even to higher than what you started at, but you usually lose SOC in the HEV mode. I typically lose a few percent of SOC in long drives. Tonight, I lost 5% on a 17-mile drive, which was on the high side. You are less likely to get back what you lost if you are driving gently.

Yes, the SOC could eventually go down to 0% while driving in the HEV mode even if you started at a decent SOC, bu that's unlikely.

Bue there is nothing wrong with any of this. Using the traction battery is good for the environment.

This is what Toyota says about it for Gen 4 Prius Prime.

 

Could this just be dynamic management?

Consider: You 'always' want your hybrid battery to be around half charged for maximum service lifetime. Given that most cars spend more than 90% of their existence parked, this counts for a lot.

Except, you generally want a full battery in a Prius at departure time for any given moment.

But when you're in motion, at highway speeds, for an extended period? Why not let the battery discharge a little, to get back to the original priority of holding a partial charge for max life?

After, all, hitting the brakes will regenerate the car's kinetic energy back into the battery for that next departure.

 

The top speed is more about the limits of the EV side alone, and not quickly draining the battery.

Could be, but Hold mode came about because people wanted or needed to save EV range for later on in the trip.

 

I've seen no evidence that it fails to hold charge generally. The two cases it seems to let it slide more than a couple of percent are:

1) Repeated trips which means the "band" keeps getting reset on each start - over time, that is going to gradually drift, because you're not going to be turning on with exactly the same SoC each time, and there's no long-term memory - the band is reset to current SoC on every entry to HV mode.

2) Not holding charge when the battery is full. That makes perfect sense - the charging rate is limited when the battery is full, so the hybrid system is not going to be able to operate as well with an HV band right at the top - it will discharge more easily than charge. Maybe if it was prepared to really not use the battery, but it doesn't want to do that - it wants to actually perform HV operation, with normal Prius efficiency.

 

You should not even store the vehicle with a full battery, let alone drive it in the HEV mode with a full battery. This will simply shorten the cycle life of the battery. Only fully charge the battery immediately before BEV driving. The battery should be at no more than 70% SOC for vehicle storage and/or HEV driving. 25% SOC is ideal for long-term vehicle storage.

 

lots of people. only the ones with ocd worry about those things. toyota says not to leave the battery full for long term storage, but they don't define it.
they also say not to leave it plugged in for more than 3 days, or it will put a higher drain on the 12 volt.