battery never fully charged - 8 bars

This is my second post to the forum about 2010 3rd gen Prius.

The car has driven about 500 miles but I have never seen the battery fully charged, I mean 8 bars . If you read the toyota book carefully, they do talk about 8 bars meaning fully charged battery.

My assumption was if the car is fully running on ICE (mostly freeway and 70-75 mph), the battery should just get charged completely and immediately. Also, going downhill should kick off the regenerative breaks system and charge the battery fully.

I have seen it charged to 7 bars but never 8 bars. was wondering if any one else has seen the same problem.

The battery is not normally fully charged for a good reason. The hybrid battery serves two purposes: 1) Provide energy for the electric motors, and 2) Store energy recovered during braking. If the battery is fully charged there is no room for more energy, so regenerative braking becomes impossible. The control system tries to strike a balance between having energy for the motors while maintaining space for braking.

Tom

The car is also managing the battery level to get maximum life out of it. Apparently, common public assumptions about batteries are not correct, and keeping the battery perpetually full is not good for its life expectancy. Likewise, very frequent full charge-discharge cycles are not good. So let the battery controller do its job, and don't worry about it.

I have filled up the battery multiple times, but only on lengthy mountain descents. Once the road flattens out, the car likes to quickly burn off the top one or two bars.

I haven't seen 8 bars either. Someone posted about it before and the response was that it is pretty rare even on previous models.

It makes sense to me. With most rechargeable batteries, the charger takes the battery up to about 85% and then trickle charges the remaining 15% to make 100%. The reason for this is that overcharging a battery causes damage to the battery that severely lowers the overall lifetime of the battery. The battery controller attempts to track the current charge level accurately, but over time there will be uncertainty due to age and other factors that prevent the controller from knowing where the 100% mark is and how far the battery is currently charged. Trickle charging (charging the battery very slowly), is a safe way to charge a battery near full without damage.

I imagine that this is exactly what is going on with the Prius as well.

I believe I read somewhere else here that the battery won't go to 8 or even 7 bars until the battery is conditioned, or broken in. I am at 2500 or so miles and have never seen 8 bars and only see 7 occasionally. When I do see 7 it's after long gentle braking slowing from freeway speed or a long downhill coast.

Go downhill long enough and it'll reach 8 bars.

This depends on the battery, but most are completely happy staying fully charged. Deep cycling batteries is what kills them quickly, as does over charging, which is different than keeping a battery fully charged.

Tom

Thank You guys for your valuable response.

I just did not want to go to the dealer with this question until I hear from you great senior members.

Thanks again.

By this logic, the battery is in infinite dump of excess electricity. This isn't correct. If you kept dumping power into the battery, it would overheat and explode (not like a bomb most likely, but its seals would break and begin leaking and likely start on fire). The battery controller may have some other place to dump the excess electricity such as some large resistor, but I assure you that it isn't the battery. I would guess that the car likely invokes the transmission to turn the ICE without fuel going through it in a situation where there is excess power.

Yes.. I went downhill saturday evening (about a 4000 foot drop over 30 miles) and the top bar was illuminated.

Also.. Fyi.. the top bar was a salmon color -- not green like the other bars. I suspect this is a "fyi warning" that the battery is full.

Also.. the "B mode" on the shifter did not seem to do enough braking.. Initially I could see that it caused more "charge" on monitor, but now I'm suspecting that once the battery is charged there's not much load and hence not much regen braking in B mode.

(From using a hand crank generator in the past I do know that a generator requires more energy when there is a load than when there is "no" load. I suspect the same is true for the prius electric generator... but I'm just speculating/guessing. I dont have a prius manual to consult.)

One concern I have is the brakes... if I cant use the regenerative braking, then what about "engine" braking.. does it work?
Imo, the "B mode" did not really do its job on the downhills.
E.g. the Matrix did a better job engine braking at "transmission setting 2" than the prius B mode.

Just seemed like I had to use the brakes much more than I would have liked to.

jeff

Agreed.

Lithium Ion batteries don't like to be stored for long periods of time at full charge. Their life will be severely shortened. On the other hand, NiMH batteries like the ones in the Prius, I believe, don't have this issue. In fact, NiMH batteries naturally bleed power if stored. So, if you put a charged NiMH battery on the shelf and checked its levels in a month, it would not be unsurprising to find that its charge level had degraded. It should still be able to charged back up to full charge just fine, however.

Tom is right about deep cycling. In general, most types of rechargeable batteries would like to just stay at 50-60% charge all the time.

I went on a long and mountainous roadtrip and finally got to see the battery symbol filled up to the top. Actually, the battery was full most of the time which seemed to render B mode pretty much useless.
And no, I didn't take this picture while driving...

That is odd, because engine compression worked well for me.

Last week I descended 6 mountain grades, some in cruise control and some in B mode, where regeneration completely filled the battery. Each time, when the battery filled, the engine automatically spun up in compression braking mode and took over the braking function. It actually started winding up before the battery was full, but its charge acceptance rate was declining and could no longer provide as much brake power as I needed. As the descent continued, the regen rate progressively fell to zero, and the engine topped out at 4700 RPM, with a very strong manifold vacuum and lots of noise.

Compression may not work as well as on many other cars because the Prius engine is smaller, therefore has less braking capacity. I suspect that its Atkinson cycle also gives it less intake capacity than an Otto cycle engine of the same displacement.

Good post! Somewhere I read that the computer maintains the traction battery between 20% and 80% .... as I understand it, this prolongs battery life. I too have seen my battery indicator "fully charged" and like the above post, it was on a long downhill. Actually, I was a bit concerned .... I would rather not see it completely charged. I have no idea what the computer does with a fully charged battery, and the wheels are continuing to generate electricity. On a railroad locomotive with dynamic brakes, excess energy is released in the form of heat from a grid atop the locomotive. (a simplified explanation). It would be interesting to know how the Prius deals with excess electricity. I do not suppose the generator can be "turned-off?"

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See my post just ahead of yours. The 'excess electricity' is used to spin the gas engine in compression braking mode.

Regenerative braking, which I was discussing, by definition stores energy in the battery. When the battery SOC becomes too high, the Prius burns off excess energy by using MG1 to spin the ICE to pump air. This form of braking is called dynamic braking, and has been used for years on diesel-electric locomotives. With the Prius, Toyota refers to this type of braking as simply "engine braking" as the engine is used as a load. With diesel-electric locomotives a large resistor is used as a heater.

Even without engine braking, the battery would never overheat and explode. When the battery SOC becomes too high, the controller simply stops using regenerative braking. Engine braking and friction braking take over.

Wherever did you get the silly notion that the battery would be an "infinite dump of excess electricity"?

Tom