DIY Open Source EV Charger

[jyanof]

Originally Posted by MPaulHolmes

I can help with that! There's no way we need a BQ2031! YA! Way to go Joe! I'll study your schematic more later. I have to do stupid school work right now! Makes me mad, way down deep. $140 for a 144v charger would be really really really nice.

Excellent! I taught for two years... I know what you mean.

I can't imagine the control logic is that difficult. Maybe it's tough to have constant current and/or constant voltage control? I don't think the system would be dynamic enough to need fancy PID control... (like the current limiting suggested for your controller) I have a 12V power supply that I can hook up to a 6V battery to charge it and manually adjust to the desired current, so it seems like a computer could do it without missing a beat.

[MazdaMatt]

If there was a current sensor fed back to a cpu, that cpu could extremely easily control the pwm to make a constant current system. I'd use PI control... it isn't a difficult thing to do. Again, i really wish that i was not so busy, i'd jump on firmware development for this project and paul's motor control... all i can do now is yell stuff from the side lines...

[jyanof]

Originally Posted by MazdaMatt

jyanof, can you make a quick description of how the circuit operates? ... (I'm no power electronics engineer!)

Me neither!

But, I think the way you described the circuit matches my understanding of it.

The 47k resistor was left over from a sample circuit in the BQ2031 datasheet. Admittedly, I'm not sure of it's purpose, but I suspect it could help reduce initial voltage differences, or allow the cap to charge slowly when it's plugged in (though that would take forever). I can't see anything wrong with leaving it in, so I haven't taken it out...

But otherwise, the FET closes and the battery is connected to the DC powersource (rectified AC). A rapid rise in current is limited by the inductor. The FET then closes and the inductor releases it's energy through the diode and batteries. I don't know what to expect for voltage across the FET at this point. If my mental exercise is right, the voltage at the bottom of the diode must be greater than the top if current is going to flow through it, so that's something greater than about 180V near the end of charge. I suspect the 500V FET can handle whatever it is, but might be something to keep note of.

[MazdaMatt]

I would not remove that diode. I believe it serves the same purpose as the freewheels on the motor controller. Going back to basics, the voltage across an inductor is relative to di/dt... so when i goes from 10A to 0A in .000001seconds, you get a LOT of voltage. Again, synchronous rectification would be smart here, but i'm sure this is fine.
The resistor might be there to set up the Vds biasing... not sure (mostly slept through mosfet class )

[jyanof]

Originally Posted by MazdaMatt

If there was a current sensor fed back to a cpu,

Currently, that's what the 50mv shunt is for. It'd need amplification though. I suspect a hall effect sensor would work too.

I'm just thinking out loud now...
Perhaps an even better place to monitor current would be on the AC line. I think AC hall effect current sensors are pretty cheap and would allow you to set and max out the power source throughout the charge cycle. Otherwise, if you hold the battery current constant, your power draw will increase a little as the voltage rises, potentially tripping the breaker partway through charging (and likely unknowingly in the middle of the night).

[jyanof]

Originally Posted by MazdaMatt

I would not remove that diode. I believe it serves the same purpose as the freewheels on the motor controller.

Oh, I agree! I was referring to the 47k resistor...

[MazdaMatt]

I would definately keep the current sensing to the battery line. I don't understand what a 50mv shunt is? I have heard of 50mohm shunts in series with the power line and paralleled to op-amps as a current sensor (that's what is on the boards that i'm testing this morning, actually). So as, say, 10A passes by, it generates 0.5v, the op-amp has a gain of ten, so 5v goes to the cpu's ADC... that sort of thing.

Total system design should take into account a maximum power and the input ratings should be specified accordingly.

[jyanof]

Originally Posted by MazdaMatt

I would definately keep the current sensing to the battery line. I don't understand what a 50mv shunt is? ...

Total system design should take into account a maximum power and the input ratings should be specified accordingly.

Oh, maybe better terminology would be a 50V 50mv shunt, which would be a shunt of 0.001 Ohm resistance. (My electrical engineering greenness is showing through)

As for input ratings, I think versatility of a charger is important. My goal would be able to plug in at home on my 240 30A line, then drive to my parent's place and charge off their 120V 20A line, and make a trip by the school and plug in to the 120V 10A outlet. My current problem (pun intended), is that the charger I have now only runs off of a 240V 30A outlet, so I can pretty much only charge at home.

Thus, the need to adjust the input power (current). I guess there's a number of ways to do this. (thinking out loud again...) Probably the most time efficient method I can think of would be:

1. Determine a max battery current based on the charger components. I'd probably conservatively set it at 20-25A for the ones in the schematic.
2. Set the max input current from the power source
3. Monitor both currents and maximize until one or the other reaches its respective limit.

From a 240V 30A source, the battery current would max out first at 20A or whatever, while the source current would max out first from a 120V 10A outlet.

This would of course only be during the bulk charge phase, but would allow for the quickest possible charge.

I guess it isn't necessary. The user could just select the battery amps knowing that the setting would have to be a little below the power source max. But, i think it'd be easy to do, so why not?

[mcmahon.craig]

Originally Posted by jyanof

3. (and this is where the high voltage components come in) Build a 300V (or so) Power Factor Corrected power supply that'll supply 300V (or so) to the charger from an AC input from 100 to 250V. This'll allow the charger to run from any typical voltage source AND maximize the power drawn from the power source

I don't know about the power factor (if you want, it should be able to be corrected by passive components), but this circuit should put out ~300V from a 120Vrms source.

If you want 240Vrms, just supply it to the leads marked "2Vpp" and "(-)".

[bennelson]

I love the idea of a smart charger which would have some adjustments for amps and volts.

One down-side of the gel-cell batts. I am using right now, is that they require being charged at a little bit lower voltage. (higher voltage cooks the gel electrolyte!)

With a smarter open-source charger, the same charger design could be used no matter what style battery - AGM, Gel, Flooded, etc. is used.