DIY Open Source EV Charger

[bennelson]

Hey Folks!

Since Paul is going on the whole Open Source 144V EV motor controller, we need to come up with an electric car battery charger to match.

One advantage of a higher system voltage on an EV is that it opens up the possibility of using a home-built 120V AC battery charger.

Does anyone here have experience building one of these?

Keep in mind that we want it to:

1) Be inexpensive
2) Safe
3) Actually work
4) Able to be built by a non-rocket scientist

[Intrigued]

Yes, Please! Someone do this! And make kits!!!

(My soldering looks WAY worse than the Forkenswift's welding ever could!!!)

[MPaulHolmes]

For a 120v 6amp charger, the price ought to be lower than 10*20 = $200, as that is the cost of 10 12v 6amp Harbor Freight chargers when they are on sale, or for a 120v 10amp charger, the price ought to be lower than 10*30 = $300 (those go on sale for $30 each sometimes). Well, maybe a higher price is acceptable since the charger would be smaller than 10 12v chargers.

[mcmahon.craig]

There are a few _very_ simple chargers at EV Technical Files.

I would personally only use the "Bonn Charger". To me, bad-boy charging is a rather dodgy affair and I would want to have as many safeties in it as possible.

Also, I think you could change the output voltage by adding a variac or transformer to the input (before the GFCI). Eventually your output voltage will decrease to the point where very little/no current will move between charger and battery pack.

To my mind, these particular circuits are very much sub-optimal for use in actual electric cars, but seem like a legitimate jumping-off point for adding additional controls and stuff to make a pretty acceptable charger.

[MPaulHolmes]

I could make a black box that can read various inputs (signals about state of charge and whatever?), and have a microcontroller make decisions based on that information, putting out any voltage/current that you want, as long as it is less than or equal to the voltage of the input into the black box. Let's say it is a 144v charger. The black box would need to have an input of maybe 165v DC (I'm not sure how high the voltage gets when in fast charge mode. Is it around 13.7v per battery?).

Basically, that's all a motor controller is, is something that reads inputs, and puts out a voltage that is controlled by the user. You can control the voltage output by a micro-controller just as easily (with the human removed. No throttle!). You would only need a single 200v fancy mosfet, because the continuous current output of the charger wouldn't be any higher than 30 amps I would think, and 1 of those mosfets can put out 50 amps continuous.

It would be a PWM based charger. The PWM signal would be smoothed out by an inductor (like a motor) that would be inside of the black box. The inductor would just be a coil of maybe 12 gauge wire.

So, how do you make a steady 165v input source? All the charging algorithm stuff is just gravy with a micro-controller!

[Intrigued]

Originally Posted by MPaulHolmes

I could make a black box that can read various inputs (signals about state of charge and whatever?), and have a microcontroller make decisions based on that information, putting out any voltage/current that you want, as long as it is less than or equal to the voltage of the input into the black box.

I believe that is how the cordless tool guys do their multi-voltage chargers...

[MPaulHolmes]

Really? Hey! You are right about the multiple voltage. If you wanted to charge a lower voltage, you could just choose that setting. For example, if you wanted to charge 72v, the PWM inside the black box would only run at 50%. The average voltage would then be half of whatever the maximum DC that is being inputted to the black box. I think that could work. Thanks, Intrigued!

[mcmahon.craig]

The smart chargers I have used in the past usually cut off at 14.4V on a 12V battery.

[MPaulHolmes]

Originally Posted by mcmahon.craig

The smart chargers I have used in the past usually cut off at 14.4V on a 12V battery.

Dang! A 173v DC input to the black box then? The battery pack could be like the back EMF from the motor. Hmm... The black box could supply very high current to the batteries. You could vary the voltage to control the current going into the batteries (same as on a motor controller, but replace batteries with motor!), so people could choose the speed of charging. Some batteries have very low internal resistance, like my super fancy Sears AGM platinum editions! (Thank you Sabrina!) Those suckers can be charged at 40 amps each! I think current limiting might be a good feature. Imagine charging 144v at 40amps. You might want to check with the neighbors to see if they were going to use the power grid before doing that.

[Intrigued]

Paul, I think you missed your calling.

...or, maybe you just found it!!!