Charging EV portion of the battery going down mountains

A couple of things I have found or at least I think I have found.

I can hear the engine being spun when coasting down hill in B mode. I assume no gas is being used.

I can fill the battery from coasting down hill, I have gotten it to 100%. It seems, though I am not positive, that when filling the battery in EV mode I can use HV to save the miles until I want to use them but when charged in HV mode the miles will be used first when in HV mode before using the ICE.

And a question: I have noticed some advertising using time to charge to 80%. Does the battery charge slower to get from, say 90% to 100% as compared to from 80% to 90%? Is it harder or less efficient to get the last part compared to the first part of the charge?

 

The last 20 minutes or so are slower to charge. But that's only the last few percent.

 

Am I correct to think that is a design feature to cool the battery or something and not because it is harder to charge the last bit into the battery?

 

The car does use interior air to cool the battery, but the reason the last bit is slower is to keep the voltage down.

 

Lithium batteries can have problems when over charged or when fully depleted. Toyota is very conservative about this, because they want your battery to last a very long time. So when the battery reads 100% full, it's really only about 80% full. And when it reads empty, it's really about 20% full.

 

the last 20% would be 20%. yes, the charger slows way down at that point to protect the battery. so when they say 80% of charge in x minutes, they mean 80% of potential miles. the last 20% may take longer than the first 80, depending on the car.
on my pip, i can get 80% (around 12 miles) in 45 minutes, but 90 minutes for 100%.

 

This is more to the point I was wondering about. To help my unsophisticated understanding is charging the battery more like filling it up with a liquid, last bit like the first bit, or adding the last 10psi to a tire compared to the first 10 psi.

 

Nice metaphor!

The latter, although the battery fill-up speed is influenced a lot by the battery temperature as it fills and at 3.3 kW the temperature changes little.

 

i don't know the physics, or chemistry. i would say the opposite of the tire analogy, because it takes a long time for the first 80+% and not time when adjusting a few psi in a full tyre.

 

If tire pressure is analog to battery voltage and amount of air in it to charge, then maybe a better analogy is to inflate a bicycle tire with a hand-pump, as we used to do when we were a lot younger.
At the beginning I could pump air quickly into the tire, towards the end my weak hand worked very slowly in order to produce the necessary pressure to overcome the back pressure in the tire.

 

I am thinking it might be an idea on a long trip to run the EV charge to 50% to make sure to be able to use any long downhill free charging and even better to lower the charge to less than that to make it easier to add charge. But still leave some charge in case it is needed for a long uphill.

I may be getting carried away here though.

 

No, I think this limitation which causes very slow charging of the last few percentage is something deliberately done by Toyota engineers

 

The implementation of "B" mode in the Prius (Plugin or standard) is a perpetual topic of (sometimes intense) discussion. There are several long threads on this in the Gen I Plug-in subforum. In conventional cars, "B" mode is used to activate engine breaking: using compression with little or no fuel to slow down the vehicle without using the brakes. The energy is dissipated heating the air by compressing it. In the first generation Plug-in, "B" mode would sometimes just enable more aggressive regeneration, at least until the battery was full, and depending on HV/EV mode. This is pretty much exactly equivalent to slightly pressing the brake pedal. Other times, it would use the traditional engine compression braking, by turning the engine without fuel. I think that how the Prime does this is yet to be determined.

Some people feel that there are very few situations recommending use of "B" in the Plug-in. Others point out that it may only be there as a regulatory holdover for "automatic transmissions", since with a manual gear box, you could just down-shift to get the engine braking, and one needed a way for vehicles with automatic transmissions to avoid burning out their brakes on long downhills.

 

Actually it is standard practice with a Lithium battery to charge this way. It has to do with resistance to holding additional charge and heat dissipation from my understanding. When a battery pack is near empty, it is easy to shove more power into it, this causes very little heat loss in the system. As a battery gets closer to full finding room for the additional charge is harder, there is more resistance and thus more heat is lost. This heat isn't good for the system and can harm the battery over time. So rapid chargers tend to take a battery quickly up to 80% and then trickle charge the last 20%. You will find this on many cell phones today with rapid chargers. The battery can be safely and efficiently charged up to 80% rapidly but that last 20% has to be more moderately charged to protect the battery from damage. A balloon may be a better analogy but even that is off, when you fill a balloon, the initial portion that you fill you can push the air in fairly rapidly, it can hold that capacity easily, as you get near full you want to back off a bit to give it time to stretch to its new size more moderately, push to hard and it may burst.

 

The main reason is charge voltage. More voltage to pump more current can cause certain chemical problems that aren't related to heat or resistance.