MPPT solar controller and LiFePO4 battery for backpacking

[Dave C]

Since my last post, I learned that the battery has a low current threshold of 90mA. Based on that, my understanding was that at its CV voltage, the controller was reducing current past that threshold, causing the BMS to disconnect. Then the overvoltage occurred, and the BMS wouldn't close until the panel was removed. Both manufacturers seemed to think that this would explain what happened. But your point about timing makes that conclusion questionable, and your point about balancing raise a new question: At what point does the battery's internal balancing occur? I know that for at least some batteries, balancing occurs at full charge. If that's the case with my battery, then I wonder if balancing is happening at all when the controller ends charging at 14.2V. Another question for Kevin at Bioenno...

I'm beginning to think that it doesn't make sense to try to mix and match components from different manufacturers that have not been tested and approved for use with one another. If this is correct then I have two problems: Genasun doesn't market any small batteries, and Bioenno's MPPT controller is too big and heavy.

Actually, at one point, Genasun recommended a K2 battery, but not the one that's nearer my weight requirement (although that one adds a full pound to my pack). I'll reach out to Mark at K2 with some additional quesitons.

From a weight perspective, it would be a toss-up between the Bioenno MPPT controller and the K2 battery. The K2's dimensions are a better fit. The saga continues. This is certainly not territory for a sane non-engineer.

Edit: regarding timing of the current drop off: would the fact that the battery is not yet at CV cause the controller to have to "work harder" or faster to get the current down?

[karrak]

Since my last post, I learned that the battery has a low current threshold of 90mA. Based on that, my understanding was that at its CV voltage, the controller was reducing current past that threshold, causing the BMS to disconnect. Then the overvoltage occurred, and the BMS wouldn't close until the panel was removed. Both manufacturers seemed to think that this would explain what happened. But your point about timing makes that conclusion questionable, and your point about balancing raise a new question: At what point does the battery's internal balancing occur? I know that for at least some batteries, balancing occurs at full charge. If that's the case with my battery, then I wonder if balancing is happening at all when the controller ends charging at 14.2V. Another question for Kevin at Bioenno...

It is all becoming clearer now, so I was right thinking that the Bioenno disconnects the battery when it thinks the battery is full. The only way to really check if it is the 90mA threshold or the battery balance would be to log the battery current as it is charging and have a look at the plot. From your point of view it doesn't matter which it is. The issue is what the MPPT controller does when the BMS disconnects the battery from the outside world.

You are right that the point at which the internal balancing occurs is of interest. The balancing occurs when aany cell reaches a preset voltage. If the cell voltage does not reach this voltage no balancing will occur. If your overall charge voltage does not cause any of the cells to go above the balancing voltage no balancing will occur. There is nothing wrong with this as long as all the cells are fully charged at the charging voltage you are using.

Ideally the MPPT controller should just keep the voltage at 14.2V and that should be the end of it. If the MPPT controller does let the voltage rise to the solar panel voltage it also doesn't matter if the BMS can handle the higher voltage. That is why I wanted you to find out the maximum voltage that the Bioenno battery could handle. If it can't handle the voltage it is very easy to keep the output from the GV5 at 14.2 volts by hooking up a small load to the output of the GV5. Putting a load onto the GV5 to solve the problem of the voltage going too high is about the only thing that Sunking and I agree on.

I'm beginning to think that it doesn't make sense to try to mix and match components from different manufacturers that have not been tested and approved for use with one another. If this is correct then I have two problems: Genasun doesn't market any small batteries, and Bioenno's MPPT controller is too big and heavy.

It would be a good start if the manufacturers gave you a list of equipment that their equipment should work with. Also more technical information like Absolute maximum voltages and minimum load currents and other specification that an engineer could look at and work out whether or not the different equipment would work together would also be nice.

This is certainly not territory for a sane non-engineer.

I hope that this has not been too truamatic for you

Edit: regarding timing of the current drop off: would the fact that the battery is not yet at CV cause the controller to have to "work harder" or faster to get the current down?

All not being at CV means is that the controller is supplying as much current to the battery as the solar panel will supply. When at CV means that the controller is limiting the amount of current to the battery to keep the battery voltage constant at say 14.2V. The MPPT controller is in CV mode when it is cutting the current down.

Simon

[Dave C]

All not being at CV means is that the controller is supplying as much current to the battery as the solar panel will supply. When at CV means that the controller is limiting the amount of current to the battery to keep the battery voltage constant at say 14.2V. The MPPT controller is in CV mode when it is cutting the current down.

I meant to say, "the battery is not yet at it's specified CV voltage of 14.6V." In other words, since the battery is not yet fully charged when all is at 14.2V, does the controller have to work harder or faster to bring the current down due to the battery still looking for full charging current? Maybe my question is meaningless. I'm just grasping for a reason why the controller would back the current down from max current to < 90mA so quickly.

[Mike90250]

I meant to say, "the battery is not yet at it's specified CV voltage of 14.6V." In other words, since the battery is not yet fully charged when all is at 14.2V, does the controller have to work harder or faster to bring the current down due to the battery still looking for full charging current? Maybe my question is meaningless. I'm just grasping for a reason why the controller would back the current down from max current to < 90mA so quickly.

The battery has high internal resistance

[karrak]

I meant to say, "the battery is not yet at it's specified CV voltage of 14.6V." In other words, since the battery is not yet fully charged when all is at 14.2V, does the controller have to work harder or faster to bring the current down due to the battery still looking for full charging current? Maybe my question is meaningless. I'm just grasping for a reason why the controller would back the current down from max current to < 90mA so quickly.

The CV voltage is set by the solar controller not the battery. The GV5 sets the CV voltage at 14.2 volts. The lower the CV, the slower that the current will taper off. Within reason, it doesn't matter how fast the solar controller has to reduce the charge current. It is hard for the solar controller to go from supplying maximum current to zero current instantaneously without the voltage momentarily going higher than is should. With this fairly basic setup and with the low power involved I don't think it would be too difficult to design the charge controller to make sure that the voltage didn't go too high.

Can you give more details about the sequence of events, what was the max current, was it sunny all the time that the battery was being charged, what was the battery voltage when you started charging the battery, how long did it take before the voltage climbed to 14.2 volts and exactly how long did it stay at 14.2 volts and exactly how fast did the current taper off when the voltage got to 14.2 volts.

You as a user don't need to know this information unless you are interested in how the charging process works. To try and work out what happened I need this information.

Simon

[Dave C]

The CV voltage is set by the solar controller not the battery. The GV5 sets the CV voltage at 14.2 volts. The lower the CV, the slower that the current will taper off. Within reason, it doesn't matter how fast the solar controller has to reduce the charge current. It is hard for the solar controller to go from supplying maximum current to zero current instantaneously without the voltage momentarily going higher than is should. With this fairly basic setup and with the low power involved I don't think it would be too difficult to design the charge controller to make sure that the voltage didn't go too high.

Can you give more details about the sequence of events, what was the max current, was it sunny all the time that the battery was being charged, what was the battery voltage when you started charging the battery, how long did it take before the voltage climbed to 14.2 volts and exactly how long did it stay at 14.2 volts and exactly how fast did the current taper off when the voltage got to 14.2 volts.

You as a user don't need to know this information unless you are interested in how the charging process works. To try and work out what happened I need this information.
Simon

I am very interested to know how the charging process works. Unfortunately, back when I had a GV-5 I didn't record any of the information you requested, and I won't purchase another until I'm reasonably comfortable with the answers to my pre-sale questions.

It turns out that it's not a 14.6V HVD at which the BMS ends charging; rather, it disconnects at 0.02C. Again, not sure why it took so long to arrive at this information.

I asked Bioenno and K2 about the voltage at which balancing occurs, but have received no answers yet.

If no cell reaches the balancing voltage at 14.2V, in either of those batteries, then the GV-5 has been eliminated. No, wait, it just occurs to me that a GV-5 custom programmed at 14.6V would probably work. That's Bioenno's recommended charging voltage, which I think means that balancing should occur.

I think the GV-5's OCV "feature" is a major drawback. However, it seems to me that it might work seamlessly (i.e., never go to OCV) with a battery that has no low charging threshold, and that has a HVD that protects against only catastrophic over voltage. Not sure how I would find one, given that specs at this level of detail seem to be available only through persistent digging. I suppose I could email a number of manufacturers/distributors with my requirements and ask them if they have a battery that matches.

[karrak]

If no cell reaches the balancing voltage at 14.2V, in either of those batteries, then the GV-5 has been eliminated. No, wait, it just occurs to me that a GV-5 custom programmed at 14.6V would probably work. That's Bioenno's recommended charging voltage, which I think means that balancing should occur.

You are right that if you can get a GV-5 programmed for 14.6 volts that all the cells should be balanced on each charge if the balance voltage is 3.65V. As 3.65 volts is the maximum voltage that you should charge a LFP cell to I would think this would be the cell balance voltage.

I think the GV-5's OCV "feature" is a major drawback. However, it seems to me that it might work seamlessly (i.e., never go to OCV) with a battery that has no low charging threshold, and that has a HVD that protects against only catastrophic over voltage.

The OCV "feature" is a nuisance but is not a show stopper and can easily be circumvented. A battery without a low charging threshold will still have the same problem because the charge current will taper off to virtually zero as an LFP battery has virtually no "leakage" current. To the GV5 a disconnected battery looks exactly the same as a battery that will not accept any more current.

Simon

[Dave C]

You are right that if you can get a GV-5 programmed for 14.6 volts that all the cells should be balanced on each charge if the balance voltage is 3.65V. As 3.65 volts is the maximum voltage that you should charge a LFP cell to I would think this would be the cell balance voltage.


The OCV "feature" is a nuisance but is not a show stopper and can easily be circumvented. A battery without a low charging threshold will still have the same problem because the charge current will taper off to virtually zero as an LFP battery has virtually no "leakage" current. To the GV5 a disconnected battery looks exactly the same as a battery that will not accept any more current.

Simon

Okay, I think I finally understand at least a sketchy, high level picture of how LFP charging works, and how the GV-5 interacts with LFP batteries.

How can the GV-5's OCV problem be easily circumvented? Boz at Genasun suggested that I look into a TVS, but I'm not dealing with transient voltage spikes. Is there some type of fast acting, weather-proof-able circuit that can keep the max load terminal voltage at or below 15V without fail, without adding significant size or weight and without drawing significant current from the battery?

Edit: Would something like this work? If, as described, it adds noise, and a capacitor wouldn't completely eliminate the noise, this might not be good for a ham radio application. On the other hand, the benefit of preventing damaging voltage would outweigh the inconvenience of noise as long as the noise only happens when the circuit is actively limiting high voltage.

Edit 2: According to Bioenno, the battery balances at any voltage as it's being charged or discharged. So I should be able to save money and go with a default 14.2V GV-5.

[karrak]

How can the GV-5's OCV problem be easily circumvented? Boz at Genasun suggested that I look into a TVS, but I'm not dealing with transient voltage spikes. Is there some type of fast acting, weather-proof-able circuit that can keep the max load terminal voltage at or below 15V without fail, without adding significant size or weight and without drawing significant current from the battery?

Edit: Would something like this work? If, as described, it adds noise, and a capacitor wouldn't completely eliminate the noise, this might not be good for a ham radio application. On the other hand, the benefit of preventing damaging voltage would outweigh the inconvenience of noise as long as the noise only happens when the circuit is actively limiting high voltage.

You are doing very well at learning about all this, before you know it you will be an electronic engineer.

Did Boz confirm the GV-5 OCV problem? His comment regarding TVS makes me think that the overvoltage will only be a transitory event.

A better device than a zener is a Transient voltage suppression diode.

Edit 2: According to Bioenno, the battery balances at any voltage as it's being charged or discharged. So I should be able to save money and go with a default 14.2V GV-5.

When you say default you do mean the 14.2 volt Li version not the Pb version?

Simon

[PNjunction]

Oh man. Let's take a break and look at the application for a second from the very first post:

4.5ah LFP battery. (four and a half amp hour)
28 watt panel.

WORST case, it will take about 2.5 hours to fully charge under best conditions with your current gear. That's do-able unless you are using the system in Norway in the winter.

I can understand wanting to shave weight to the bare minimum, but really - how much trouble is it to carry your existing stuff? Would it all be much easier / cheaper to just replace the R28 Powerfilm with the next size up if you wanted more efficiency / faster than 2.5 hours for nearly a FULL discharge?

No offense, but my paranoid red-flags are going off - I see buzzwords being thrown out again which tend to excite the engineering crowd to tear each other apart - mostly from speculating on gear we don't own, and interpreting it all from the distance of a forum.

While I'm not making any accusations, I do see the buzzwords and indications bordering on FUD about Genasun. The questions are interesting, but seem obsessive.

Just calling it like I see (read) it.

[karrak]

No offense, but my paranoid red-flags are going off - I see buzzwords being thrown out again which tend to excite the engineering crowd to tear each other apart - mostly from speculating on gear we don't own, and interpreting it all from the distance of a forum.

While I'm not making any accusations, I do see the buzzwords and indications bordering on FUD about Genasun. The questions are interesting, but seem obsessive.

Just calling it like I see (read) it.

The original post detailed the problem that Dave C was having with the GV5 when used with his Bioenno battery. It is a serious problem if the GV5 or for that matter any MPPT controller looses regulation and lets its output rise to the solar panel voltage. This could easily damage any LFP battery connected to it.

As with most engineering projects "The devil is in the detail". If the detail in this case involves what you call buzzwords and FUD, so be it. These details need to be sifted through to try and work out what exactly the problem is and if it can be solved. Unfortunately this can be a messy process on forums like this but in this case is I hope leading to a simple solution that means that Dave C should be able to use the Genasun GV5 with the Bioenno battery. It has also highlighted some issues to be aware of when charging LFP batteries from solar panels.

Simon

[Dave C]

You are doing very well at learning about all this, before you know it you will be an electronic engineer.

Did Boz confirm the GV-5 OCV problem? His comment regarding TVS makes me think that the overvoltage will only be a transitory event.

Boz mentioned that when a BMS has a low current threshold, it's typical to see the GV-5 go to panel voltage for "a few seconds before settling back down to the CV voltage". (I guess this must be with a battery that has no HVD.) Would a few seconds be considered transient?

A better device than a zener is a https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode&quot;]Transient voltage suppression diode[/URL].

I looked for one, but wasn't sure which specs to look at. I assumed clamping voltage would be the one of interest. I couldn't find one that looked like it would limit the voltage to 15V.

When you say default you do mean the 14.2 volt Li version not the Pb version?

Simon

I meant the Li version and "default" as opposed to custom programmed.

[Sunking]

Simon you have really proven beyond a shadow of a doubt you really do not know much about controllers or batteries. You really need to quit embarassing yourself. An y damn fool can figure out Solar Controllers have no reason to go to ZERO volts, and charger current never goes to ZERO amps. It cost manufactures real money to make a Buck/Boost, and PWM voltage regulators variable all the way down to ZERO volts. In fact you would have an extremely hard time to even find a high quality Bench Power Supply capable of doing that if not impossible to find one. WHY??? There is no reason too you fool. Who the Hell needs a ZERO VOLT POWER SUPPLY except you? If I need one to go to ZERO VOLTS, I would just turn it off. Built to many of them from the age of 12 years old when I started ham radio. I have forgotten more than you will ever know. Never built one that goes to Zero. Nor does anyone else.

[Dave C]

Simon you have really proven beyond a shadow of a doubt you really do not know much about controllers or batteries. You really need to quit embarassing yourself.

So far almost everything he has said has lined up exactly with what I've learned from the manufacturers and what I've observed, and he has adjusted his approach as necessary as new information has come out. You have also added some valuable information. The only embarrassing content in this thread has been the pejoratives.

[Sunking]

I meant to say, "the battery is not yet at it's specified CV voltage of 14.6V." In other words, since the battery is not yet fully charged when all is at 14.2V, does the controller have to work harder or faster to bring the current down due to the battery still looking for full charging current?

No it doe snot work like that. You can set the Controller to 13 volts. CV is just what it sounds like. Once the Charge Current Tapers to les than Current Limit of the Controller, the Controller holds 13 volts or whatever you set it to. As the battery charges up, it voltage rises to what the supply voltage is. When those two voltages are equal, CURRENT STOPS FLOWING. The cells are SATURATED. Does snot matter if the voltage is 13, 14, or no higher than 14.4. Your battery BMS has nothing to do with the charge current.

Your Battery has what is known as Balance Boards, High Voltage Cut-Off, and Low Voltage Disconnect. The Balance Boards aka Vampire Boards. Just like the name implies Vampire Boards Bleed your batteries. When a cell reaches 3.65 volts the Vampire Board Turns ON. When it turns ON puts a small resistance across the cell to BLEED CURRENT. That clamps or tries to Clamp the VOLTAGE on the cell to 3.65 volts. In other words it bypasses CURRENT around the fully charged up cell to pass it around a lower voltage cell.

Here is the deal. At 14.2 volts from the GV5, the BMS never turns on. It takes 4 c ells x 3.65 volts = 14.6 VOLTS for the BMS to turn ON. at 14.2 volts your cells Saturate, and current stops. Again go read this because you do not understand how batteries charge. Simon if confusing you with fiction.