Calculating 'regen' with an alternator

[Daox]

Just this past weekend, I finished my alternator delete on my Mitsubishi Mirage. The next phase of the project is to allow the alternator to be re-enabled during braking events to essentially get some free charging.

But, how much power can you possibly get from an alternator during braking only? Lets find out!

The Mirage's alternator is a modest 85A alternator. In a forced mode, I can make it put out 15V. That gives us a theoretical max output of: 85A * 15V = 1275W.

Lets estimate that when you come to a stop, you 'brake' for about 5 seconds. Thus, you get 5 seconds of charging per stop. More math shows us that 1275W * 5 seconds = 8925 watt seconds. Convert to watt hours and you get 2.48WH. Again, do some conversion to bring that back to something we more so understand. Divide that 2.48WH by 12.5V to get .2 amp hours. .2 amp hours of regen per stop with the alternator on this thing absolutely maxed out. Doesn't sound like a ton eh?

But, that is only for one stop. I have a possible 10 stops on my way to work. Lets say I'm having a bad day and have to stop at every single one, and I brake for the same amount of time on each one. Math is pretty easy at this point. I regen 2Ah. Still not much...

I'm about to head home for the day. I'm going to try to keep track of how long I brake per stop and get a better idea of how much I can take advantage of re-enabling the alternator. At this point, it seems very questionable as to weather its worth it,

[Daox]

Alright, recalculation time. I counted about 150 seconds of braking that I did.

150s * 1275W = 191,250WS

191,250 = 53.13Wh

53.13Wh / 12.5V = 4.25Ah

Thats not too shabby.

[oil pan 4]

2 words:
Solar panels.
(And a morningstar mppt charge controller )

[Isaac Zackary]

Remember that the voltage is going to be affected by the state of charge of your battery. Try to feed 85A into a fairly dead battery, you might not get much more than 12V. But try to feed 85A into a nearly full battery, and you'll get way more than 15V (and a few fried electronics too.) There's no way to control both voltage and current at the same time.

But still, 85A at 12V to 13V isn't that far away from 85A at 15V. You do need some sort of voltage protection that can throttle back the current so you don't go over 15V. Also the bigger the battery the better.

[redpoint5]

The alternator energy is only free when you are in DFCO, with the engine being turned by momentum and no fuel being delivered to the engine. The actual brakes have nothing to do with it.

An ideal system would monitor for open loop and then enable the alternator automatically, or any time battery voltage goes too low.

[Isaac Zackary]

Have you checked out the Mazda system? It uses a 48V generator to charge an ultra-capacitor pack which in turn is used to charge the 12V battery. Might be expensive to build though.

[Stubby79]

You could put an amp-hour meter in to monitor exactly what you're getting. They're not expensive. You'll probably be disappointed with the resulting numbers, though. It will also let you know your state of charge, so you know how long you have left, or if you need to override and turn on the alt...

[seifrob]

I see much more potential of this during long downhill DFCO events. I have something like 4-mile downhill ride on my commute and I wish I could use the energy to charge.

[oldtamiyaphile]

I have a smart alternator. I found that as a hypermiler, it was struggling to keep my battery charged - in my city driving there aren't enough revs to really charge up the battery. I have a 1.6l 2.1 ton diesel so my in gear glide distances are much better than a throttled engine in a Mirage.

I added solar panels.

I did the maths on the solar panels 360W saves a theoretical 0.02l of fuel per day - more or less independent of distance driven.

Sometimes it pays to just know when to quit

[oil pan 4]

Saving the battery is more important than saving a little fuel.
Since the cost of a battery is that of at least several fuel fillups.