Featured Toyota’s Redesigned Aqua uses a new Bipolar Nickel-Hydrogen Battery

My guess is the switch to lion in the rav4 hybrid may have something to do with this.
Are 2019 and 2020 Toyota RAV4 Hybrids Suffering Battery Issues?
Lion batteries don't lose charge as fast when parked and can keep the 12V lead acid battery tended better. It also may have been a quality problem in the lead acid or nimh batteries. We don't know.

In the prius the lithium battery only has 60% the capacity as the nimh choice. Of course that will hurt cold weather performance but it comes from choosing a much lower capacity battery. Still that makes it even lighter. In the rav4 the lithium battery is 70% (1.1kwh) the capacity of the nimh(1.6kwh). That seems like a better plan if cold weather performance is important. All of the rav4 hybrids are awd. Usable energy and power are higher in the lithium than the nimh in the rav4 hybrid. When it gets cold these decrease, but that size seems like a good compromise. Resistive heating can be used to quickly warm it up if need be ;-) with power coming from the engine and mg2 on very cold days. That change could be inexpensively implemented on a refreshed design.

 

Whatever the cause for the Li-ion switch last year, the 2021 production models are back to using NiMH. Toyota has designed the packs to be interchangeable, with the better power and energy density of Li-ion being leveraged for weight savings. Which is used for a sales advantage, whether better MPG rating in the Prius Eco and Camry hLE, or to counter weight gain of features in higher trim Prii.

As for the starter battery, I recall reports here about some dying quickly in other hybrids, mainly the Auris. Exact cause was never determined. A bad battery is possible even in a new car, and a minor defect in the electrical system could lead to a higher than average drain. Whatever it was, and smaller than ICE sized battery would exacerbate any issue. While the hybrid system doesn't need the output of a 'full-sized' 12V to start, the accessory load on the battery while the car is off is the same as in an ICE car. A few days parked that isn't a problem for the ICE model becomes one for the hybrid, because the same accessory drain equals a larger depth of discharge on the smaller battery, which leads to shorter life.

 

The 12V battery in my gen III prius died within the first year. Toyota replaced it under waranty. That one died in 18 months. Thought there was something wrong with my car. I think it would have been replaced under waranty again, but I was sick of the problem and bought an after market optima battery. I didn't have 12 V battery issues for the next 7 years. It must have been bad batches of 12 V batteries.

 

Yes your Gen 3 Prius sounds like a bad battery, although it may be that they did a software update to fix a parasitic drain problem at the same time you updated to Optima. In my Gen 2, the original battery lasted 6 years and the replacement, which was from Toyota (Panasonic), has lasted 10 years and still going. I suspect the issue in the RAV4's is some component is not turning off properly and it drains the 12v battery.

 

OK so that is interesting and supports the theory that the decision on which battery to use is about supply and cost. Maybe the Li-Ion that was in the RAV4's is now going into RAV4 Prime's.

 

I've only experienced it in video games, and never once wished I could have that experience in a real car.

Does the presence of the feature mean that this model no longer needs $3k worth of brake blender hardware under the pedal? Can it use the same $65 cast iron master cylinder as an '86 Tercel?

If so, it might not be what I truly desire but I could still respect that as progress.

 

They are different types. Hybrids use battery formulations with high power to give that burst of acceleration when needed. Plug ins have ones for high energy density for increased EV range.

Switching the assembly line between the two might be difficult, but I suspect the switch last year was because of the pandemic causing an interruption to the NiMH supply chain that is now fixed.

 

The problem turned out to be the multimedia software continuing to search for something, bluetooth or wifi, I don't remember which. Toyota issued a TSB that fixes it.

 

On the new Aqua, the single pedal mode (Power+ Mode) is a configurable by the driver, so the 'brake blender' hardware is still required for Normal Mode. Tesla's use single pedal and it's common on most BEV's and so I guess Toyota recognise that people want this feature. Having said that, many drivers coming from conventional ICE cars may be turned off by single pedal braking and so I can't see Toyota getting rid of the 'brake blender'.

 

It isn't real one pedal driving, as the battery pack isn't large enough to allow a braking force large enough to stop the car. It is going to be far less than what even a PHEV could possibly provide.

Accelerator off normally provides enough braking to simulate the drag of an automatic transmission. In Power+ mode, that braking force is doubled. It might seem like driving in a lower gear. The force being doubled was slight to begin with, so some may not notice a difference. Perhaps you can modulate the braking force with feathering the pedal, if the pedal map gets a braking region to make up for the increased torque demand rate for the Power mode.

 

I would be very hesitant to have any car with "real" single-pedal mode enabled until every car in the household worked the same way.

I mean maybe we (speaking for other drivers in the family) might get used to switching back and forth but from here it just looks like a real opportunity for danger.

 

Regenerative braking has always been a core component of Toyota hybrids. Assuming the Aqua weighs 1200kg and is travelling at 70km/h ... it has 0.13 kWh of kinetic energy. Assuming the battery is 0.9k kWh, then regenerative braking to a stop would only charge the battery an additional 14%. So one pedal braking should be fine.

In the cases where the battery did get full from repeated braking and you were in Power+ mode (one pedal), I suspect Toyota would keep the one pedal experience consistent by using the regular brakes in the background, because you don't want to be a in situation where Power+ works differently based on battery state of charge.

I suspect Toyota set the battery size to recover most of the braking energy. i.e 80% or more. Maybe with the bipolar battery they will be able to make it larger and recover a greater amount.

 

It should be obvious that the car is in one pedal mode, as it takes pressing the accelerator further to get the car to go. But that is the last point to realize the EV is in.

Not every BEV has one pedal driving. Of those that do, it is optional for nearly all of them. It has to be actively turned on, with many models doing so through the gear shifter. To use one pedal driving on your BEV, you'd likely have to to put the car into a gear other than D to get it.

One pedal driving is a different way of driving that not everyone will like. The car makers seem to have taken those factors into account with the design. The actual safety issue involved might be getting the brake lights to come on when needed; it was possible to get one model to stop without them coming on.

You aren't considering the maximum safe charge rate for the battery. This is dictated at the cell level, and influenced by temperature and time.

At a certain speed, a car has an X amount of energy. When regen braking, that energy is divided among the battery's cells. How quickly the car decelerates sets the charge rate. Faster stop, faster charge rate, the more heat is generated. Too much heat, and the cell fries. Since the incoming energy is split among the cells, the more cells, the less energy being pushed into an individual cell. Which means lower charge rate and less heat per cell.

A BEV with one pedal driving can stop the car with just regen from 70mph at a medium hard brake rate that will require the brake lights to come on. A Model 3 SR has 2976 cells. A gen2 Prius has 168. It stopping from that speed, at that rate, is too much energy, over too short a time, for the battery cells to safely take in. Decelerate slower, and it might be able to do it, but the the rate would mean coasting, not one pedal driving.

The gen2 had a regen recovery rate of around 30%, IIRC. Later generations improved that, but not substantially. The batteries were all around the same size with just air cooling. The bipolar battery should do better, but it is only enough for Toyota to double the meager braking while coasting off the accelerator.

 

I got about half that, I think ... were you using ½mv² with m in kg and v in m/s to get joules, then converting to kWh?

 

Yes my mistake as I forgot the 1/2. So it's 0.063 kWh or 7%, In normal traffic that energy will be spent accelerating again .. so it can be managed within the hybrid battery fairly well. If there was much lost energy then Toyota would increase the battery size.

 

So do Tesla's only offer one pedal ? ... or can they do regeneration through the brake pedal ?

 

They are the only one with just one pedal driving, but I've heard they have the hardware to do blended braking with the brake pedal.

 

Good point .. yes I did not consider peak power. My calculation was based on battery capacity and not battery power. According to Priups.com, the Gen 2 Prius battery is rated at 27 kWh.

Now moderate braking at say 0.3 g would give about the following regenerative charging;

- 50 mph - Peak Power = 87 kW, Total energy recovered = 46%
- 70 mph - Peak Power = 117 kW, Total energy recovered = 35%

For the bipolar battery, peak power rating is doubled to 54 kWh giving;

- 50 mph - Peak Power = 87 kW, Total energy recovered = 75%
- 70 mph - Peak Power = 117 kW, Total energy recovered = 62%

So now we see the advantage of bipolar NiMH battery in terms of regenerative braking, whether you use the new Power+ (single pedal) mode or not.

 

Wait, what are those 'h's doing in your power units?

Well, the hardware under the pedal is what it takes to do your ABS, brake force distribution, traction and stability control, and do it without relying on engine vacuum for power assist.

The additional 'blender' complexity from regen just means the computer in this module has a way to talk with the power management control ECU and say "hey, how much of this can you do for me?", which maybe added about a nickel to what's already there.

 

my leg gets tired. i'd prefer paddles behind the steering wheel for throttle and brake/regen. my hands have to be on the wheel anyway.