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Old 12-03-2012, 06:49 AM   #1 (permalink)
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Default Regenerative Braking

Hello all,

I think I know about regenerative braking (thanks to the threads here), but there is still one item which is not clear to me.

You have to press the brake pedal slightly to get the most out of Regen, I got that.

But some stated they would downshift (either 2nd or 3rd gear), now I red the in the Gen1 forum.

So my question is, since I am in the process of getting an Insight (current model), does downshifting do anything for Regen on a Gen2? It would only slow down the car, but no brake would be applied. Or does this mean that, due to a higher rpm, the Regen is maximized?

As many here, I drive mostly city and am 'concerned' about keeping my battery charged. I usually just lift my foot and let the car roll to a stop, with regenerative braking I will have to change something, I know.

I appreciate any input. Thanks all.
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Old 12-03-2012, 10:17 PM   #2 (permalink)
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Regeneration starts immediately when you lift your foot off the accelerator significantly so the pedal is less depressed than was needed to maintain speed. Somewhat like the drag on a manual transmission car. The moment you press the brake, a switch is activated with about a 1/4" pedal depression that increases the level of regeneration, as you press the brake more regeneration is increased as well as the brake pads engaging. As you are coming to a stop when the car reaches 7 mph regeneration stops and the engine disengages and stops, you coast with braking from there on in. All of this is quite smooth, there is a slight bump in speed at 7mph when the engine drops off, similar to pressing the clutch in as you come to a stop in a manual.

Using the paddle shifters to increase the RPM as you are coming to a stop is counter productive. The engine is hard linked to the generator, and is always turning when the generator turns, increasing the RPM dramatically increases the engine drag for not much more charging.

If you currently coast to a stop when approaching the light, this is already the optimum approach, there will mild regeneration as you slow down, the most efficient.

As far as keeping the battery charged, this is completely done for you by the car's computer. If there is not enough braking to keep the battery charged, the computer simply starts charging, causing a slight drag on the engine. If there is a lot of braking so the battery is fully charged it will tend to assist more. The computer strives to keep the battery 50% - 75% charged. Hope this answers your questions.
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Old 12-03-2012, 11:18 PM   #3 (permalink)
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Quote:
Originally Posted by insightfulone View Post
Regeneration starts immediately when you lift your foot off the accelerator significantly so the pedal is less depressed than was needed to maintain speed. Somewhat like the drag on a manual transmission car. The moment you press the brake, a switch is activated with about a 1/4" pedal depression that increases the level of regeneration, as you press the brake more regeneration is increased as well as the brake pads engaging. As you are coming to a stop when the car reaches 7 mph regeneration stops and the engine disengages and stops, you coast with braking from there on in. All of this is quite smooth, there is a slight bump in speed at 7mph when the engine drops off, similar to pressing the clutch in as you come to a stop in a manual.

Using the paddle shifters to increase the RPM as you are coming to a stop is counter productive. The engine is hard linked to the generator, and is always turning when the generator turns, increasing the RPM dramatically increases the engine drag for not much more charging.

If you currently coast to a stop when approaching the light, this is already the optimum approach, there will mild regeneration as you slow down, the most efficient.

As far as keeping the battery charged, this is completely done for you by the car's computer. If there is not enough braking to keep the battery charged, the computer simply starts charging, causing a slight drag on the engine. If there is a lot of braking so the battery is fully charged it will tend to assist more. The computer strives to keep the battery 50% - 75% charged. Hope this answers your questions.
Great explanation! Yes don't down shift your car it's not a truck and you're just going to put more wear on the CVT...
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Old 12-04-2012, 02:03 AM   #4 (permalink)
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I drive mainly in the city as well and I have no problem keeping my battery over 60%. The car will even regenerate on idle and I have loads of red lights on my commute.

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Originally Posted by insightfulone View Post
All of this is quite smooth, there is a slight bump in speed at 7mph when the engine drops off, similar to pressing the clutch in as you come to a stop in a manual.
This got me by surprise the first time, I thought I wasn't adding enough pressure on the brake pedal at first (because of this idea I had about braking as slowly as possible to regenerate the battery).
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Old 12-04-2012, 07:16 AM   #5 (permalink)
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Thanks, that was a very detailed explanation, thank you very much!

This might sound stupid, but what about the battery charge? Wont the battery ever be full? You mentioned '60%' or '75%', is that about the stage where the battery will be at (since it has to assist the petrol engine)?
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Old 12-05-2012, 12:14 AM   #6 (permalink)
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Default Full battery not necessarily a good thing

The battery will occasionally be full. This will typically happen at the bottom of a very long downhill that required braking, and sometimes when the car's computer is running a full charge cycle to detect the correct charge level. Most of the time the charge level will be 40-75%. The reason for this is that NIMH batteries work most efficiently in this range. They will last longer if they do not spend much time fully charged or fully discharged. To accomplish this, the cars computer will do more assisting if the charge gets near full, and have the engine run the generator if the level gets low.

You need to think of the battery in a different way from a plug-in electric vehicle. The battery is only temporary storage for energy derived from braking on downhills or coming to a stop, that can be used to start the car moving again or climb hills. A full battery is not a good thing when coming to a stop or descending a hill, as there is no more room for regenerative braking. Only energy derived from braking is 'free', if there has not been enough braking to charge the NIMH battery after assisting and charging the 12v to run the accessories, then the extra charging done by using the engine takes additional gas.

Using more of the range of the battery would only be helpful if you lived in a mountainous area where the battery frequently becomes full charged on a downhill. It is a rare occurrence here in Mass, but descending some mountain roads in VT I have had it happen.

It seems to take little energy to hold the car in auto stop mode, so I have never seen the car restart from auto stop due to a low charge level, so using a wider range would not help with auto stop either.

You cannot electrically tell the charge level of a battery when it is partially charged. You can only tell it is fully charged by a rise in voltage as it nears its full charge level, and a drop in voltage as it nears depletion. So the cars computer keeps track of the charge level by measuring the current used to assist and current sent back in regenerative braking. Over time small errors in this tracking build up and the voltage levels of the battery make the computer 'discover' it is fully charged or nearly discharged. It will then adjust its idea of actual charge level and the battery meter will jump to full or empty. To avoid the problem of 'discovering' the battery is lower that it thinks, the car seems to occasionally keeps charging with the engine until it detects full. This is one of the occasions you will see a full battery.
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Old 12-05-2012, 09:36 AM   #7 (permalink)
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"Full" is a relative term. 90% is what you mainly see as its constantly being used to power your tradational electrical system since we lack an alternator.

Ive found that by engine braking the battery fills up faster and it gets the engine in the sweet spot if you need to suddenly switch to agressive acceleration.

Plus with my straight pipe muffler I like hearing the engine wind down.
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Old 12-05-2012, 09:43 AM   #8 (permalink)
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Quote:
Originally Posted by Cobb View Post
Ive found that by engine braking the battery fills up faster and it gets the engine in the sweet spot if you need to suddenly switch to agressive acceleration.

Plus with my straight pipe muffler I like hearing the engine wind down.
I guess this is something I have to 'play' with, to see which ones works more efficient. Something new to play with.
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