Poor man's 15 Amp battery charger?

[Skippic]

Forgot to answer the question

No I haven't connected it to 220/240V yet.

[Farfle]

Mine is holding up great. I've been using a 70uF motor run capacitor with 3ohm resistors and something like 3000mF electrolytic caps to stabilize the current. For control I used a voltage divider, an Arduino and the TextStar display. The Arduino measurement is far from accurate, so I've been averaging 100k measurements to display the battery voltage. To turn charging on and off I used an SSR:
http://www.ebay.com/itm/SSR-25A-Solid-S ... 4ab6626024

Now I'm about to try using a 400uF cap as suggested by Farfle (big thanks):
http://www.amazon.com/dp/B005FUXGZG/ref ... _3p_dp_1_B

In the second charger I'm eliminating the resistors (maybe even the stabilizing caps).
Instead of the Arduino I'll use a much simpler and cheaper solution. My idea is to use a SSR, voltage dividers, voltage detectors and a cheap digital display.

Should be lighter, smaller with less connections and components to fail.

I'm attaching my Arduino code (dirty). I tried attaching the file, but ES didn't like it, so I tried renaming it to .txt, but it didn't help.

Don't use that cap! they are falsely advertised as motor run caps, they are only start caps and are not rated for continuous duty. Motor run caps only go up to about 100uf, always have metal cans and are physically larger for the same capacity.

[Skippic]

Don't use that cap!

Do you mean, the 400uF one you suggested?

[Farfle]

yup, the ones i got failed within 15 minutes of use, not an interesting failure, some hot oil leaked out and the current dropped to zero. But they did fail very quickly
*edit*, thought the cap was labeled as a motor run cap, its not falsely advertised, but I did find a similar one that was mis-advertised *edit*

[Skippic]

So then:
http://stores.directhvacsupply.com/-str ... gories.bok

I'm getting some 10 to make 1 portable charger and a fast one for home.

[Skippic]

What do you guys recommend to use as a rectifying bridge? I'm looking for something, that will create as little heat as possible. Say 50A max.

[Skippic]

From my simulations I see a great benefit from having higher mains voltage.

For a 85uF capacitor and 82V battery:
@ 110V 1.2Aaverage 5Amax
@ 220V 3.9Aaverage 9.3Amax
@ 340V 6.9Aaverage 15Amax

So for the same capacity you get much faster charging. You could in theory get caps for smaller voltage, but it looks like 370V and 440V is standard.

Furthermore even if you get more capacity, the spikes at lower voltage are much higher compared to the average current:
@ 110V 700uF 10Aaverage 37.5Amax
@ 220V 215uF 10Aaverage 27Amax
@ 340V 123uF 10Aaverage 21.5Amax

The capacity with these cheap caps is not such a big issue, the max current, if you want a fast charge is.

[Skippic]

I just received the 10 85uF 440V capacitors. While in the US I will connect all 10 to get 12A average with 45A max. My batteries are rated 5C (charging) so with 10Ah up to 50A should be OK.

I will of course put a selection switch for 85uF/850uF (to slow charge when I have time).

My current charger is pretty complex with 70uF cap, SSR, rectifying bridge, electrolytic caps, resistors, Arduino, 5V power supply...

In the next one I'm eliminating the electrolytic caps, resistors, Arduino and the 5V power supply.

The end of charge will be determined by the pack voltage. I'll use a resistive voltage divider to bring the voltage down and compare it to a reference with a LM311. Since the current needed is really small I'll use a resistor and a zener as the power supply. No stabilization as discussed previously - an oscilloscope is on it's way from Hong Kong so I can see what is really happening when one charges Lipo with impulses

[Skippic]

In Europe 5 of those caps should be enough for 19.7A average and 47A max, so I'll be able to make two chargers.

I'll publish the blueprints for the charger when it's ready (still waiting for some components).

[IBScootn]

Thanks for sharing your progress and it will be interesting to see the data from your oscilloscope. My GBS 40H cells have a recommended charge current of up to 32A and a max of <120A. I have no data on how the charging current spikes would affect cycle life; but if I wanted to charge from 240Vac at 30A, interpretating from your data, it looks like my current spikes would be ~64A (yikes). Wish I knew whether that is safe.

[Skippic]

Tested with two different charger setups:
1. 70uF with stabilizing caps and series resistors, that heat up a lot.




2. 425uF without any stabilization.



Please disregard the offset, my setup is far from perfect. In a couple of days I will measure the first charger without stabilization.

For now:
1. 27mV ripple
2. 1.042V ripple - almost 40x more, but I can't hear any noise coming from the battery

What puzzles me is the second spike. I might try putting some electrolytic caps on there, but no resistors - they tend to heat up too much. Can someone with a little more experience tell me what is the minimum capacity needed for those caps?

[bearing]

What puzzles me is the second spike.

Probably caused by some inductance stray. What you see is a small sine wave oscillation on top of the big pulse. You might be able to decrease inductance by running all wires close to each other, or lightly twisted around each other. Doesn't look bad at all though.

[Teh Stork]

I've been thinking a little about the same circuit that's been posted. The mosfet is the only part I'm missing. Here is the schematic:


Just a short explanation: The microprocessor will handle the job with turning on and off the mosfet. The mosfet will need to be able to block rectified mains. During ZVS (and the time mains voltage is less than pack voltage) the battery pack voltage can be accurately measured, or it can be measured all the time when the mosfet is on (but with positive ir voltage swing). 10A over my 10ah pack having 5mohm IR each cell will give a voltage increase of 0,9V. Somehow it will need to measure the pack while not being charged - or it will need to compensate for this. Knowing IR and current could be one way to do it - zvs another. I have some current sensors that could also be easily implemented. That way you could fill the pack with the same ah drained.

Edit: And, another wire coming from the centre of the pack could be used to test to make sure it's in balance.
Edit2: Corrected a flaw. The voltage from mains and battery won't add up - so the mosfet only needs to block mains voltage (370V DC)

[Skippic]

Some progress. I have a better picture of the impulses during charging.

I'm having a problem with hysteresis with the LM311 controlling the SSR. I decided to use a comparator to evaluate when to turn charging off. The problem is that when I connect the SSR to the output, the hysteresis is horribly exaggerated and not suitable for this type of charger. I tried to correct it by connecting a resistor to get negative feedback and reduce the hysteresis, but it now seems unstable and I have worries about it's working under different temperatures.

I'll post my circuit tomorrow.

[fechter]

Look at the schematic I posted on page 3. What I did there was to use a latching circuit so once the voltage limit is reached, the SSR turns off and latches off until manually reset. This avoids hysteresis problems and oscillation.

[Teh Stork]

I've worked some more, with this as a start.

I've decided to make a buck with transformer isolation after I've finished my boost - psu charger. This is the fast scetch I've made on this circuit: (It's missing a lot of resistors, capacitors and ground symbols)


It's duty cycle will look something like this:


Imo this could be a fast and safe charger. The transformer is a 1:1 ratio. It's just there to provide isolation. The switching and deadtime is controlled by a microprocessor. It will use mosfets capable of withstanding 400vdc, the positive is that its load is more or less continous (less EMI) - the negative is increased losses since these mosfets has a bit higher on-resistance. A current sensor is utilized to sense the current - and adjust dutycycle accordingly.

If anyone would like to start, I could help out with the avr software, given that you prototype it with a xplained 1284p AVR (these are cheap and nice to prototype with).

[Teh Stork]

As a "proof of concept" I built this boost converter to charge my battery. It takes everything from 12-20V input - and can charge based on either input voltage sag or current sensor feedback. It monitors output voltage and cuts off once it's reached.

It uses a irfp2907 fet, a small mosfet driver, a homewound toroid (75uH, 6mOhm) and a bridge rectifier being used as a diode. Planning to use my experiences to try to make a buck - ac connected charger.

[bearing]

That's a good idea. I've been toying with the idea of making a multi phase buck directly from mains. If you have more phases than the ratio between battery voltage and peak mains voltage, then the converter will conduct continuously, which means it will be easy to filter the switching noise from the mains, so that it doesn't pollute the grid.

I think this stuff is worth it's own thread, since this thread is about the passive capacitor method.

[IBScootn]

But I wonder if the SCR or motor-run capacitor is even needed. I just found out that the J1772 stations can be instructed to deliver a max constant current. Setting the value of the resistor inline with the proximity switch determines what that current will be:


total resistance PP-PE 1500 Ω 680 Ω 220 Ω 100 Ω
current capacity 13 A 20 A 32 A 63 A
wire cross section 1,5 mm² 2,5 mm² 6 mm² 16 mm²
[edit]

So you can tell it to deliver 13A, 20A, 32A, or 63A based on a low-power resistor. That would seem to make the SRC or AC cap no longer necessary to limit current.

What do you think?