Calculating 'regen' with an alternator
 

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... :snail:

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,

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.

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

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.

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.

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.

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...

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.

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 ;)

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.

Yet surely there are times when you can't coast to a complete stop, even for someone heavily using DFCO, due to traffic reasons. My car will coast for a very long time before fully stopping (hell even small gusts of wind start it rolling on level ground) but the last minute or two of coasting before a complete stop I'd be going quite slow. That's not realistic unless you're the only person on the road. Plus, I'd have to time my DFCO very well to never use the brakes.

Regen like this makes needing to use the brakes more forgiving.

@Daox, a fun calculation to do might be to convert those amp hours into gallons of gasoline, and compare it a few ways to idle fuel consumption, or how much speed you can get back from your car using that same amount of energy.


Unfortunately, that's the conclusion many over on Insight Central came to about hybrid batteries. The battery makes the car a lot more drivable, and leaning on the $2,000 battery with a ~10 year lifespan very heavily to save a bit of gas and prolong the life of a $500 engine which easily lasts 300,000 miles doesn't actually make much economic sense. Better to use it as a driving aid than a fuel economy one, if the latter subtracts from its effective life. $2,000 buys about 50,000 miles worth of gas.

Someone on MirageForum asked a great question and I thought I'd share it here.

That is a great question! Not to mention another great way to look at this mod. We will have to run the numbers to check it out for sure.

Math time! Again we are going to estimate the car uses an average of 20 amps while in use since I haven't tested that quite yet. My commute is about 25 minutes long.

20 amps * 25 minutes = 500 amp minutes

500 amp minutes / 60 = 8.3 amp hours

The car uses 8.3 amp hours, but we get 2.8 amp hours regenerated. This washes out to (8.3 - 2.8 = ) 5.5 amp hours used. The trip home would also use about 5.5 amp hours for a daily total of 11 amp hours.

Now, the battery holds about 80 amp hours, but we never want to use more than half of that. That gives us 40 amp hours to use. 40 / 11 = 3.6 days worth of commuting.

So, I can theoretically go 3 days before having to charge. Not too shabby. However, if I use more power than normal, that cuts it down. Eg. using headlights vs just running lights, wipers, more HVAC power, rear defroster, or the PTC heater would probably also suck a lot of power as well. Those all reduce that range.

In the end, I find it much easier to plug in every night. That gives me a full charge so I don't even have to worry about it at all unless I go on a long trip. If that happens, I just turn the alternator back on and get a little worse fuel economy. I also have a block heater that I run in the mornings, especially in winter. This necessitates plugging in every night, so I kind of do this already. Might as well top off the battery too.

Hi, have you ever think to make an alternator swap with one of the continental, mild hybrid belt drived?
I find this looking for else
https://www.continental-automotive.c...rter-generator

Looks good. Do you have any pricing info.

Not really, i can't find any new. Used it's very variable, from 400 to 800 euro.
this it's the part number 4N0903028D.
mounted on A8 and Q7 from 2017, 3000 tdi hybrid.
I find a pictures with the label, will put in the post.
I find some other element made from Valeo, fitted on PSA group ( Peugeot Citroen and DS) and volkswagen group, its not 48v capable but it's used as starter motor and output it's 210 amp.