Battery Charger help! (sch included)

[Cloud9]

I thought it was oscillations, but I looked at the frequency, and it is 120Hz. I also tried powering it from a 16.5V battery, and the pulsations disappeared, with the current steady at 3.86A. With no pulsation, the dissipation at the start of the charge (battery voltage=12) was about 12W. The pulsation reduces Q4 dissipation to about 2.66W, but, as noted on the plots, average current dropped to about 1.7A.

The 120Hz would be the 60Hz from the wall, doubled by the rectifier, no?

 

[Roff]

The 120Hz would be the 60Hz from the wall, doubled by the rectifier, no?

Yah sure, you betcha!

 

[Cloud9]

The balloons and confetti just dropped out of the ceiling...

I finally got one right. Haha.

Could you tell me some about the sim program you used?

 

[Roff]

It's SwitcherCAD III (AKA LTspice), free, from Linear Technology. It's all I use now that I'm retired. The learning curve is steep, especially if you have no experience with Spice.

 

[RadioRon]

OK, so the sim thinks that the circuit is pulsing at 120 Hz. But it isn't really is it? This is just a sim error of some sort, right? Its hard to accept that this so called negative feedback regulator has gain when it comes to input regulation.

Kudos to Cloud9 for actually researching how to charge these batteries. Nice to see the effort.

As to your question asking if the right way to adjust it is to set the setpoint voltage to 14.5 v or thereabouts with no battery attached, I would say maybe. With your newfound knowledge, I think you will be able to answer this yourself.

 

[Roff]

OK, so the sim thinks that the circuit is pulsing at 120 Hz. But it isn't really is it? This is just a sim error of some sort, right? Its hard to accept that this so called negative feedback regulator has gain when it comes to input regulation.

Kudos to Cloud9 for actually researching how to charge these batteries. Nice to see the effort.

As to your question asking if the right way to adjust it is to set the setpoint voltage to 14.5 v or thereabouts with no battery attached, I would say maybe. With your newfound knowledge, I think you will be able to answer this yourself.

I'm sure it's not some artifact of the sim. If I change the frequency of the power supply ripple, the loop switching frequency follows it. If I change the power supply to DC, the switching disappears. If I increase the AC supply voltage and the size of the filter cap, the switching stops. The basic reason (I wouldn't call it a problem) is the the ripple causes the voltage on the emitter of Q4 to swing so low that it can't sustain the 4 amps to keep the negative feedback loop happy.

 

[Roff]

Cloud9, according to the sim, you can set the final charge voltage by removing the battery and simply measuring from the collector of Q4 to GND. This works because the current through CR5 goes to zero when the battery is charged, and the drop across CR5 is therefore zero.

 

[RadioRon]

Cloud9, according to the sim, you can set the final charge voltage by removing the battery and simply measuring from the collector of Q4 to GND. This works because the current through CR5 goes to zero when the battery is charged, and the drop across CR5 is therefore zero.

On a production line, wouldn't it be easier to measure the output voltage at the connector rather than digging into the board and using a more difficult probe point.? That's what I would do and the result should be the same.

Still not buying that the regulator has so much gain with respect to input ripple, but I will accept that it may be harmless anyway.

Nice work Ron.

 

[Cloud9]

This morning, I set an open circuit to 14V. Hookup my battery and I am charging around 1A. The battery voltage is actually measuring 14.46V now and the current has only slowed to 400ma. Seems like it will be awhile before 56mA is reached to stop charging considering I am already exceeding max charging voltage of my battery (14.1).

Next reading (5 min later).: 357mA 14.56V

Why am I reading a higher voltage on my batt than I set on the open output?

 

[Cloud9]

(3 min later) 340mA 14.6V
(2 min later) 327mA 14.61V
(5 min later) 306mA 14.63V


Seems to be topping off, I still dont like the voltage. Im gonna pull the plug at 14.7V

 

[Roff]

What sort of meter are you using to measure the voltage with?

 

[Cloud9]

I have various meters ranging from a $30 Craftsman DMM, to a 2channel portable fluke scopemeter. I do note abit of variation between the meters but not more than 0.6V or so.

 

[Cloud9]

I note that with 14V at the open output, once a battery stops charging, it tends to want to cut back on (with some small delay). Must have it set too high so perhaps I am trying to adjust the open output to only the final volts req'd on the battery (13.7V or so)