On a seven mile descent I drive routinely, my former Prius C operated in a very logical sequence. With cruise control set to a speed (60-70mph) , the system would use battery charging to control the speed. When the battery icon indicated a full battery, then the system used higher motor RPMs (essentially gearing down) to maintain the set speed. My new Two Eco is entirely different. Even with a middle to low battery indication, it revs up the motor right away. That would seem to be denying the battery the best chance at a full charge when any given descent might be less than the seven mile one I am talking about. I assume the engineers know what they are up to, but I can;t make sense of it.
Heat is the enemy of traction batteries. NiMH is especially sensitive because charging generates heat so it makes sense to let the engine-brake minimize wear and tear on the traction battery. One of the hardest things to wrap one's head around is the Prius fuel efficiency does NOT come from the traction battery directly. The Prius trick is to operate the engine at 10 kW or greater in the peak efficiency range. If this exceeds the power needed, the Prius 'banks' the power in the traction battery. Later, the Prius draws this power with the engine off. So don't worry about down hill charging. The Prius control laws that give outstanding MPG are at work. Bob Wilson
Doesn't that go against what tzx4 said? The Prius c with the NiMH battery charges but the Two Eco with the Li-Ion battery doesn't.
I don't know enough about the LiON battery chemistry other than over charging leads to thermal run-away. For example, the laptops that burst into flames in the past. So it makes sense for the Gen-4 Prius to be especially careful about overcharging. Fortunately, protecting the LiON is also consistent with the primary control law, operate the engine in peak efficiency mode, always. As for our two, NiMH Prius, I use "B" when descending tall hills because of the battery heating. I also moderate my hill climb speed to avoid drawing down the NiMH traction battery charge. I've advocated this for some time. But if it were a LiON battery with their welll documented, overheating on over charging, I would be 'setting my hair on fire.' The EPA data shows the Gen-4 has substantially improved control laws. Even with the 5% built-in speed/distance error, the Gen-4 has remarkable performance over the earlier Prius. In the past, I had to exploit experimental results to to get outstanding MPG but now, it would take hard work to get crappy MPG. In particular, cold-start has been significantly improved. Bob Wilson
Toyota has said that NiMH is better for regen brake and very cold weather. Perhaps, we are seeing merit to those.
One late thought, I'd mentioned the laptops burning up. There is something more recent, Google "hoverboard fire images": Now this is NOT a Toyota product but it highlights how important battery management is for LiON batteries. Done badly, they can be a problem. Bob Wilson
It may have to do with the LiON battery being such a low capacity battery. The system is trying to maintain -some- room for later regen. by being very conservative at charging it. Without battery temperature data, however, we can't say exactly why (and even with that data we probably couldn't).
The raw capacity of Li-ion battery is smaller but usable capacity should be the same. With my experience PiP with much bigger Li-ion pack, it doesn't like to regen when it is near full especially when the temp is cold.
FYI: Maximizing Automotive Battery Power | Power content from Electronic Design Actually this issue has articles about the hoverboard batteries and EV charging stations. Well actually, they are all interesting (. . . must put down magazine and get back to work . . .) Bob Wilson
What are the number of cells between the two batteries? That also has a bearing on charge rates in addition to chemistry and total capacity.
While an individual Li-ion cell can generally accept more charge than an NiMH one, having triple the cells will be an advantage for the NiMH pack. The electricity coming in can 'spread out' over more cells to be captured.
Having read their material and 'bought the book', I recommend: Basic to Advanced Battery Information from Battery University Every now and then you run into someone who 'loves their job' and that is the case with Battery University. Bob Wilson
I've noticed that the battery meter in the Energy Monitor in the Gen 4 varies more often than in the Gen 3. Of course part of it could be because of the higher engine off speed so it's relying more on the battery for propulsion. But apart from that, it's also more easily topped up. I want to say that the regen is noticeably more powerful in slowing down the car (moreso than the Gen 2 to Gen 3 change) but I don't have enough driving time to confirm. It seems to slow the car down more rapidly and also the increased efficiency allows greater recharging of the battery (li-ion version) Can any Gen 4 (li-ion battery) owner who had a Prius before confirm this?
In my gen 3, there are road areas that I travel where I wind up completely depleting the battery to 0 bars (40%) and then resigning my self to ICE. In my gen 4, it still happens, but much further down the road. But in this car, moments later, a few bars reappear. In some road areas, the battery recovers up to half, where my previous car would still be in ICE. I think that the gen 4 is really really good at recovering energy, much better that the gen 3.
I think toyota sized the lion pack for power not energy, all we can do is guess what is going on here. Nimh 1.3 kwh x 40% SOC = 0.52 usable capacity 40%-80% lion 0.75 kwh x 70% SOC = 0.52 usable capacity 20%-90% only guestimates, please correct. If they both swap to engine braking at 70% clarge the indicator in the nimh would look much higher than the lion. I'm sure when more get their hands on the beats we will know more. I doubt it is explosion protection, more likely longevity in manging the larger range soc. I odn't think we really know toyota's battery management of the lithium. If the lithium was also 1.3 kwh, then it would be able to take a much bigger charge and much more powerful regen braking. Remember it is a small battery and there are probably safety margins. If the nimh was more efficient they would have used it in the ECO. Before the car was released I would have guess a 800 wh or 1 kwh lion, but toyota chose a smaller battery that they thought was good enough. How is mpg stop and go in the gen IV versus the prius c? I'm sure highway cruise is much better in the gen IV.
