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Daox 08-21-2019 11:20 AM

Adding regenerative braking to the Mirage
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I have finally gotten the alternator delete done on my Mirage. It has been working consistently and perfectly ever since I tweaked a few things. It was a heck of a time getting it to work good, but I am pretty sure I got it all nailed down. Much more info in my thread here:

Help me kill my alternator - it is stubborn and won't die

To get this alternator delete to work, I had to create a circuit to trick the alternator into thinking the battery is fully charged. With the next phase of this mod, I plan to bypass this circuit when braking. This will give me some regenerative braking capability.

Here is the rough and not finalized diagram of the circuit I plan to use to do this. The 7805, capacitors, and dc-dc converter are the circuit that tell the alternator the battery is fully charged. The relay and switch are there to bypass the circuit to get the alternator to charge normally. This allows me to turn the alternator on whenever I want, or just when I am braking.

Finally, I know we've talked about this over the years, but I've never seen someone actually post about doing this. Have any of you guys actually done this? I'd love to hear more about how you did it and the results you had with it! :thumbup:

mpg_numbers_guy 08-21-2019 06:09 PM

I've strongly considered doing something like this on my Insight, as I've seen about a 10%/10mpg improvement with my Meanwell pseudo-alternator turned off. Problem seems to be that the 12V battery can't hold any significant amount of regen, so one would need to drag the brakes for a long time to get ant usable regen - one couldn't just tap the brake once and throw in as many amps as would be necessary to charge the battery. I also rarely brake, so my braking isn't more than a quick tap every now and then anyway.

Supercapacitors sound like a possible solution, but I am not knowledgeable enough on them to know how they would work in a situation like this. If supercapacitors would work, and actual regen could be used to the 12V battery, that in combination with a small solar charging setup might be able to eliminate alternator need except on highway trips. The real goal would be to eliminate having to plug in the 12V battery at night, since saving fuel cost at the expense of additional electrical cost isn't a money savings.

What deep cycle battery did you go with?

Daox 08-21-2019 11:10 PM

Right now I am running a group 24 deep cycle 12V lead acid battery. Its rated for 80ish amp hours.

I agree, it'll be interesting to see how much it'll actually regen. I think I may pick up a 100a ammeter so I can see what is actually going on. I did some rough calculations that showed I could possibly regen roughly 2-3Ah per commute. My commute would use about 8Ah though (again guesses as I don't know my current draw).

The main reason I'm considering it is I have a few fair size hills on my commute. Both have stop signs/lights at the bottom. So, I have several opportunities to regen brake a bit.

I also just want to try it. :D It sounds cool to have a normal car with regenerative braking.

oil pan 4 08-21-2019 11:15 PM

The max you are going to be able to push into a big AGM battery would be maybe 1hp worth of electricity.

Stubby79 08-21-2019 11:58 PM


Originally Posted by oil pan 4 (Post 605071)
The max you are going to be able to push into a big AGM battery would be maybe 1hp worth of electricity.

That's about all the alternator will make anyway.

mpg_numbers_guy 08-22-2019 09:20 AM

IIRC lithium can handle much greater amounts of regen than lead acid. I wonder if a tiny 6ah lithium battery could handle all the regen you could throw at it, and then use a DC-DC converter with a voltage cutoff (to prevent the lithium from discharging too far) to slowly charge the deep cycle lead acid battery from the lithium battery. Then you might be able to regen at a higher amperage - say 300 amps of regen versus just 50 or 100 (which might damage the lead acid battery). One would just need to do something to make it pull that 300 or so amps whenever braking. Obviously like in a hybrid any regen is still an overall efficiency loss compared to EOC, but if you have to brake, why not charge the 12v while you're doing it?

A small, 25w solar panel exposed to 4-5 hours of sunlight a day, running at average efficiency, theoretically should provide 5-6ah of charge per day: 20 watts * 4 hours = 80 watts / 14v = 5.7ah. Even with efficiency losses that's still about half of your electricity use.

Daox 08-22-2019 09:34 AM

The alternator on the Mirage is rated for 85 amps. Will it ever really pump out that kind of amperage with a battery that is anywhere near charged? I have no idea. I will have to get an ammeter setup to test that.

While stumbling through my process of killing the alternator, I did discover a way to get the alternator to pump out a solid 15V. This is higher than it's normal output of 14.4V. A small boost, but it will help pump in more amps. I do plan on testing the higher voltage to see what difference it makes.

Perhaps with a small solar panel and the regen I will not have to plug in. That would be pretty cool. But one step at a time for now.

Thanks for the ideas guys!

oil pan 4 08-22-2019 10:14 AM

Alternator ratings tend to be optimistic.
To handle 85 amps you would need at least 6 gauge wire, to handle it continously it would need to be 4 gauge.
You will need at least 15 volts to over come wire resistance between the alt and batt if you want to run higher amps.

I have found that car alts typically have about 10 amps available for charging a battery at idle speed.
That alternator is only going to be able to make anywhere near 70 amps is spinning it 7,000rpm.
Even my modified DR44G welding alternator only makes about 25 amps at idle speed externally powering the armature with 13v. The DR44G is a pretty good size medium duty commercial truck alt.

Daox 08-22-2019 06:59 PM

Sounds like some testing is in order. I'll get an ammeter and shunt on order.

mpg_numbers_guy 08-22-2019 07:03 PM

Someone I know "overclocked" their alternator with a different pulley size. Might be consideration if you don't plan on using the alternator for normal driving. I surmise there would be a definite amount of additional load from doing that, but you might be able to get a bit more "regen".

Also what about when your battery is full? Any idea on how to stop charge from going to the battery when braking if the battery is full to avoid cooking it?

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