MPPT solar controller and LiFePO4 battery for backpacking

[Sunking]

Incorrect. The schematic shows a standard nonisolated buck converter.

Semi correct Jeff. If there is No Load (battery), there is no place for the capacitor to discharge. Read any MPPT controller instructions. Thou shall connect battery first before connecting the panel. Thou shall disconnect panel before connecting battery. There is a damn good reason the instruct you to do that. Solar MPPT controllers are not designed as Switch Mode Power Supplies. They must have a load to operate into. To make them behave like a SMPS would require more circuitry and expense for something that is not required or needed.

Take any MPPT controller you want and try it. Disconnect the battery with bright Sunshine on the panels. One of three things is going to happen:

1. Voltage out will rise to Vin.
2. Let the magic smoke out and never work again.
3. Spills all the ones and zeros out going stupid.

What do you have to loose to prove me wrong? I can tell you without a doubt a Genasun will rise to Vin or about Voc of the panel. It clearly states that in their documentation. Most others will let the magic smoke out, or go stupid. Try it if you got the balls to do it.

[Sunking]

Simon and Sunking,

Could the problem be as simple as the controller supplying charging voltage that's too LOW for the battery? The controller's specs show a CV voltage of 14.2V, and the battery's specs show a charging voltage of 14.6V. Is it likely that the BMS would disconnect the battery because of a 0.4V difference?

No the Genasun is configured to Float a LFP battery just below 100% SOC. 14.4 is the voltage for 100% or 3.6 volts per cell.

Your battery with two ports has a HVC on the input, and LVD on the output. Both input and output are the point electrically across the battery, but use a relay contacts to open/close the two paths. Both the Controller and battery are stand alone units that do not communicate with each other. That is a major problem with Lithium batteries and their BMS. To operate effectively both the charger and BMS must be able to communicate to be effective.

Your battery appears to disconnect the Charge source when the battery is at or near full charge. For a Stand Alone product is OK. Again as I said a long time ago not ideal for Solar Applications.In a Solar Application you want to Float the Battery so when it is fully charged and still usable sun light, you use solar power instead of battery power. If you manage to recharge by say noon, and disconnect the controller, you run off battery until th enext morning when the cycle starts again. Does that work? Yeah it does but a piss poor design concept IMO as you are not utilizing all the power from the panels, and causing excessive wear on your battery that need not be.

You battery judging from the connectors type is intended for Ham Radio operator market to charge it from a bench power supply. When the battery charges up, it disconnects from the charge source. Then use it on a mobile radio. Why do I think that? Because I am KF5LJW. I have built a many DC power supplies in 40 years as a ham operator.

73's

SK

[Dave C]

The problem is NOT that the CC output voltage is too low. All that would do is stop charging the battery when it reached the preset voltage, and the BMS would not mind that at all.

The BMS appears to be designed to disconnect the battery completely when the battery terminal voltage gets too high OR when the battery voltage reaches the safety limit and the BMS cannot shunt enough current to keep charge current from flowing into the battery. It will keep it disconnected as long as there is too high a voltage present on its input terminals. But it should not prevent you from connecting a load to the battery at that point and using it.
Forget any other tests until you have connected a LOAD to the battery and then measured the battery output voltage (with the Genasun disconnected.) At that point, with the LOAD still in place connect the Genasun and see what voltage it produces.

Thank you for your input. I don't currently have a GV-5 on hand to test with. I plan to order one, probably a unit custom programmed to match the battery's charge parameters.

[inetdog]

Guys, any thoughts on my last post?

The problem is NOT that the CC output voltage is too low. All that would do is stop charging the battery when it reached the preset voltage, and the BMS would not mind that at all.

The BMS appears to be designed to disconnect the battery completely when the battery terminal voltage gets too high OR when the battery voltage reaches the safety limit and the BMS cannot shunt enough current to keep charge current from flowing into the battery. It will keep it disconnected as long as there is too high a voltage present on its input terminals. But it should not prevent you from connecting a load to the battery at that point and using it.
Forget any other tests until you have connected a LOAD to the battery and then measured the battery output voltage (with the Genasun disconnected.) At that point, with the LOAD still in place connect the Genasun and see what voltage it produces.

[Dave C]

Guys, any thoughts on my last post?

[jflorey2]

I am typing this very slowly Simon so you can understand and follow along. Yes I clicked the wrong converter. The Transistor in your circuit is constantly switched on/off to some degree. Pulse width is modulated That means there is always voltage on the output. Without a load (battery) the voltage will go to source voltage because there is NOTHING for the Inductor and Capacitor to discharge the energy into.

Incorrect. The schematic shows a standard nonisolated buck converter. (There's also a bridge rectifier on the input to convert AC to DC; it is not relevant to this discussion.) The controller modulates the duty cycle of the switching transisitor to maintain a certain voltage at the output. If there is no load, the controller will go to zero duty cycle, and the output will not go to source voltage. That's why switchmode power supplies can regulate their output voltages from zero to maximum load.

This is an important concept to understand; it is why a well designed MPPT controller will never exceed its set output voltage even if there is no load. That is not true of PWM controllers.

[Dave C]

Simon and Sunking,

Could the problem be as simple as the controller supplying charging voltage that's too LOW for the battery? The controller's specs show a CV voltage of 14.2V, and the battery's specs show a charging voltage of 14.6V. Is it likely that the BMS would disconnect the battery because of a 0.4V difference?

[Sunking]

Wrong yet again, either you are very confused, or you want to confuse everyone else or is it both? The circuit diagram you have shown if for a boost converter that boosts the voltage. Hint, look at the webpage address of the image you used. The buck power supply does the opposite.

I am typing this very slowly Simon so you can understand and follow along. Yes I clicked the wrong converter. The Transistor in your circuit is constantly switched on/off to some degree. Pulse width is modulated That means there is always voltage on the output. Without a load (battery) the voltage will go to source voltage because there is NOTHING for the Inductor and Capacitor to discharge the energy into. The CAPACITOR charges to the Input Supply Voltage and holds it. A first year electronics student would know that. You failed the test.

Like I said disconnect your battery and see what happens. Now get lost. You are only here to start trouble.

[karrak]

If you had any clue how Buck Converters work you would know that. Below is the Buck Converter in its simplest form. If the FET looses it reference signal which is what happens if the battery is disconnected, the FET shuts off and the Input is directly connected to the output through L1 and a forward biased diode. .

Wrong yet again, either you are very confused, or you want to confuse everyone else or is it both? The circuit diagram you have shown if for a boost converter that boosts the voltage. Hint, look at the webpage address of the image you used. The buck power supply does the opposite.

With reference to the circuit diagram below, if we replace the Rectifier & Reservoir Capacitor with a Solar Panel and the Load with a Battery we end up with the basic circuit for an MPPT controller. As you can see the Switching Transistor is between the Solar Panel and the Battery. If the Transistor is switched off the Solar Panel will be disconnected from the battery. A PWM controller is basically the same except that it doesn't have the Flywheel Circuit. For those that want to find out more about buck switch mode power supplies see http://www.learnabout-electronics.org/PSU/psu31.php



Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[Dave C]

You read 18 volts on both ports?

If by "ports" you mean both the barrel connector and the power pole connector that are soldered to the battery, then yes.

No LFP battery would survive that, it would explode into flames before it ever reached 18 volts. Something is not right. Only way that can happen is if the battery disconnected on a Genasun controller.

I agree that the BMS disconnected the battery from the controller first, and this caused the the voltage increase and alarm at the controller. I don't think we've been able to figure out why, though. My wild guess at this point is that the BMS didn't like how the controller handled the fully charged battery. Maybe it was something like the controller taking longer than the BMS expected to back down current as the system transitioned from CC to CV? Is that possible? If not, then maybe something similar? Whatever it was, it seems like the BMS gave the controller a few chances to get it right, because the sequence at the end was actually: 1) LED shows "charged" briefly; 2) LED changes to error, and stays there for about 30 sec.; 3) the LED changes from error to charging; 4) repeat steps 1 - 3 for a total of 4 times; 5) the LED remains permanently in error status. Because the number of repeats equaled the number of cells, maybe it had something to do with the internal balancing circuit? A strike against that hypothesis is that there was no such error cycle when the battery was charged using the wall charger or with the Bioenno PWM controller. Once the battery charged, the LED on the wall charger switched from CC to CV and stayed there, and once the status on the PWM controller showed "charged", it remained there. Again, I'm grasping at straws in unfamiliar territory.

When you connected the panel first, what did you observe? I bet the voltage went high and in alarm? Then when you connected the battery went normal, charged,, then went back into alarm when the battery was charged? Is that what happens?

Yes, that's pretty much what I observed. I've posted this a few times, but in posting again, maybe I'll see something different or word it more accurately. 1) nothing connected to the controller; 2) connect panel to controller --> immediate alarm and high voltage at controller's battery terminals; 3) connect battery to controller --> LED changes immediately from alarm to charging; 4) battery reaches full charge --> first, controller shows charged briefly, then alarm and high battery terminal voltage return. Sure enough, writing out number 4 led to the bulk of my main paragraph above.

FYI, I'm in contact with a Genasun user who always connects the battery first.

I think a good test would be to get a new GV-5 and a new battery and see if the same thing happens.

It would be a lot easier to just stick with the PWM controller that I have...

[Sunking]

This is absolute rubbish.

Try it and see what happens. Be sure there is bright sun out, and disconnect the battery and see what happens. If you had any clue how Buck Converters work you would know that. Below is the Buck Converter in its simplest form. If the FET looses it reference signal which is what happens if the battery is disconnected, the FET shuts off and the Input is directly connected to the output through L1 and a forward biased diode. .

[karrak]

You need to change your way of thinking because it is the nature of the beast. Every MPPT controller I know of does that. This is due to the fact a solar panel is a Current Source and not a Voltage source. With any Current Source when you open the circuit, voltage goes up to supply voltage immediately. That is just the physics and characteristic of a Current Source

This is absolute rubbish. It is the MPPT controllers job to take the output power from the solar panels and convert it to a form that is acceptable and safe for charging a battery. An MPPT controller is basically a buck switched mode power supply and can easily be designed to keep the output voltage below what it is programmed to be. The Genasun is the only MPPT controller that I have heard of that will let its output voltage go above the programmed output voltage. I am sure it is to do with the fact that the Genasun gets it power from the solar panels and not from the battery.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

I don't think the BMS disconnects the battery outright at full charge. It switches from constant current, reducing current to a few mA, and expects constant voltage of 14V (not sure if it's actually 14.2V; I'll find out). In their literature, the default CV voltage of the controller is 14.2V.

Maybe the BMS does briefly disconnect as it switches from CC to CV. I'll see if I can find out.

I would think that the BMS will only disconnect the battery when the voltage on any of the individual cells goes out of its safe operating range, probably something like 2.5 to 3.65 volts. If the battery is balanced and the cells all have the same capacity this will equate to an overall battery voltage of 10 to 14.6 volts.which is what their documentation implies. They specify a charge voltage of 14.6 volts.

When you are charging an LFP battery to start with the battery will take as much current as the charge controller can give it within its specified limits. If you are charging from solar this is as much current as the solar panel can supply and varies depending on the amount of sunlight. With other power supplies this is usually a fixed current and is known as CC. The battery voltage will rise as you charge. When the battery reaches the cutoff voltage the charge controller will start to limit the current going into the battery to keep the battery voltage at the cutoff voltage. This is known as CV charging. With an LFP battery the charge current will taper off to zero.

For lithium batteries the charge controller should never let its output voltage go above the programmed cutoff voltage!

What I think is happening with the Genasun is that it will not let the current drop to zero, because of this the output voltage will continue to rise until the BMS in the battery does its job and detects too high a voltage and disconnects the battery to protect it. It is fortunate the battery had the inbuilt BMS, without it it is very likely that your battery would have been damaged.

You suggested earlier that the open panel voltage might damage the battery's BMS. But if the BMS can handle 20 or more Volts, then there might not be a problem.

Actually, the BMS must be able to handle open panel voltage in the range presented in my scenario, because otherwise, it would fry when you connect it to the controller after having first connected the panel. (Or, maybe a steady diet of this would damage it over time.) Again, this is a question for Bioenno Power.

I also think that the BMS doesn't disconnect in the presence of any voltage within this range. Otherwise, it would remain open and never accept a charge when you connect it to the controller after having first connected the panel in full sunlight, and this was not the case. The startup sequence with a partially discharged battery was always: 1) connect the panel to the controller; 2) the controller errors and measures panel open voltage at the battery terminals; 3) connect the battery; 4) the controller's status LED immediately goes from error to charging. I never connected a load for fear of damaging my equipment. But it would have been interesting to see if the controller would go back to charging as the load drew power from the battery. I suspect it would have.

I agree with you

I don't think either of these could be overcome by setting the controller to a different output voltage as Sunking sugested. That is, unless in my limited understanding I'm missing something...

I don't think you have missed anything. If the voltage at the connector is less than 14.6 volts and greater than 10 volts the BMS will keep the battery connected to the outside world.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[Sunking]

You read 18 volts on both ports?

No LFP battery would survive that, it would explode into flames before it ever reached 18 volts. Something is not right. Only way that can happen is if the battery disconnected on a Genasun controller. .

Something no one has caught yet, but Genasun is the only controller on the narket that requires the panel be connected before the battery. Do that to any other controller and it will let the magic smoke out. When you connected the panel first, what did you observe?

I bet the voltage went high and in alarm? Then when you connected the battery went normal, charged,, then went back into alarm when the battery was charged? Is that what happens?

[Dave C]

Are you aboslutely certain about that? There is no other reason to have two ports.

Yes, that's what the mfr. told me, and I verified.