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BMW regen braking

New BMW x5 only charges battery while braking,coasting. So it uses the vehicles momentum to drive motor which is connected to alt of course to generate elec which is stored in battery. It can gain approx 3% in fuel economy using this process. So does voltage readout fluctuate or read lower while stopped at light? I bet the volt gauge is adjusted by computer so driver does not see up/down swings.

There’s a voltage regular circuit I expect they use. The alternator’s magnetic field (which opposes the belt’s turning force) is adjusted by this circuit so it always tries to put out the same voltage, independent of how fast it is spinning or how much current is being used by the car’s electrical system. If the car was stopped at the light, then if you could somehow measure the force needed to turn the alternator, it would be harder to turn, because it wasn’t receiving any help from the braking/coasting system. The car’s motor would make up the difference, burning a little more gasoline.

Gee, that’s a good one. So the alternator is “shut off” under normal ops, except when braking? If you did that in a normal car, you’d have 12ishV without the alt, and 14+ with it.

Dunno how to get around that…batteries charge by having a “stronger” voltage pushing the wrong way.

(Maybe the charging circuit is isolated from the rest of the car?)

You could get one of those voltmeters that go in the cigarette lighter to monitor volts, I suppose…

The alt is disabled during acceleration so you would expect to see 12v or system voltage during this time. But what if you went up an infinite hill? The car would eventually override the scheme and have to charge.

In order for this scheme to work with only the main battery it would have to be kept less than fully charged.
There has to be a place to quickly put that energy.
This would be bad news for a standard lead-acid starting battery.
So maybe it uses a deep-discharge battery instead of or in addition to the conventional one.
Even better would be a small nickel or lithium battery with high specific power.
I don’t think super capacitors are quite there yet.

Haha, good one. Commenting on how the experts do things at BMW. Maybe we can direct their next design project for improved results?


Stoveguyy…I have never seen an “infinite hill” and I’ve driven everywhere except Hawaii and Vermont. A good battery will keep a vehicle running for hours if you don’t use the headlights. Don’t ask me how I know.

People here have posted that BMW may use multiple batteries in their models that use regen charging. May be true, maynot be true. BMW says battery will not charge during acceleration. That is normal, short interval acceleration. But it says nothing about LONG periods of uninterrupted acceleration. That is a purely hypothetical situation.