MPPT solar controller and LiFePO4 battery for backpacking

[Dave C]

For my backpacking solar setup, I use a PowerFilm R28 rollable 28W panel and a Bioenno Power 4.5 Ah LiFePO4 battery with internal balancing and BMS. It's a great battery--it's small, it gives me enough power for my needs, and its internal balancing circuit eliminates the need for a high-power balancing charger, thus making long-term, off-grid use possible. The panel is great, too--it packs down small, and it's weatherproof.

Here's my problem: I currently have a compatible controller from Bioenno Power. However, it's PWM, not MPPT, so it doesn't maximize solar panel performance, plus it requires a button press to display status. Bioenno Power recently added an MPPT controller with LED status indicators to their catalog, but it exceeds my size and weight limits for backpacking. I tried the Genasun GV5, which was small, light, and had great features, but with one fatal drawback: it passed panel open circuit voltage (about 18V if I recall correctly) to the battery terminals when the battery was fully charged. I think this was because the GV5's BMS conflicted with the battery's BMS.

Can someone recommend either: 1) a small, 4-cell LiFePO4 battery that has internal balancing and no BMS; or 2) a small, light MPPT controller with good LED status lights and no BMS?

======================

Edit: from this discussion, it has become clear to me that I misunderstood some of the terms I was using (e.g., BMS), and as a result, I had drawn incorrect conclusions in my last sentence regarding what I need. Having gained some understanding here, I don't think a different battery would be the solution I'm looking for. My Bioenno Power battery is a very good one, and their level of ongoing, personal service is extraordinary.

======================

Edit 2019-03-20: I wanted to bring this thread up to date. My old posts are probably full of errors due to lack of more thorough understanding, but I won't try to correct all of them. The most important thing I want to share is that Bioenno Power batteries and Genasun solar charge controllers are now fully compatible. At some point since 2015, when I bought my first battery, Bioenno upgraded their BMS so that their batteries no longer disconnect near end of charge when charged with other manufacturers' controllers. I think Bioenno batteries and Genasun solar charge controllers are the best products available for portable use in terms of function, value, size and weight. I'm very happy that they now work together seamlessly.

[jflorey2]

Can someone recommend either: 1) a small, 4-cell LiFePO4 battery that has internal balancing and no BMS

BMS functions include things like overcharge protection, overdischarge protection and battery balancing. So that's something of a contradiction. It is, however, easy to build a small LiFePO4 battery with no BMS at all.

a small, light MPPT controller with good LED status lights and no BMS?

Charge controllers don't have BMSes. The GV-5 and GV-10 from Genasun are nice little MPPT controllers; I've used them. I haven't had the problem you describe.

Note that at the voltages you are considering, MPPT isn't critical.

[Sunking]

I tried the Genasun GV5, which was small, light, and had great features, but with one fatal drawback: it passed panel open circuit voltage (about 18V if I recall correctly) to the battery terminals when the battery was fully charged.

That is not possible, no controller PWM or MPPT will do that. As stated small 4S and 8S LFP systems have no need for a BMS.

I too have used Genasun MPPT controllers. There is only one unique thing about them, they only have 1-Stage charging using a Constant Voltage voltage algorithm aka Float Charging. Controllers for Lead Acid use a much more complicated 3-stage algorythim of Bulk, absorb, and finally Float.

The Float Voltage on a Genasun Lithium controller can be set to any voltage you want. So for example if you use LiFeP04 4S or 12 volt, you set the voltage for 3.45 volts per cell which is 4 x 3.45 volts = 13.8 volts. That means the output is always 13.8 volts when the battery is fully charged. It never shuts off until the sun goes down. When you buy a Genasun MPPT controller you tell them what voltage you want. Requires no BMS.

If you use a BMS, say one built into the battery pack like you have, you tell Genasun what voltage the BMS uses. But for such a small battery a BMS is a waste of money and time. Use a Genasun GV-5, connect the battery without BMS to the battery port, and the load to the Load Port. That will protect your battery form over discharge. Set to 13.8 volts eliminates any over charge.

[Dave C]

BMS functions include things like overcharge protection, overdischarge protection and battery balancing. So that's something of a contradiction. It is, however, easy to build a small LiFePO4 battery with no BMS at all.

Thanks for your reply. While researching solutions, I found one manufacturer who differentiates between balancing and BMS functions in their literature. https://www.victronenergy.com/upload...tteries-EN.pdf. Based on that, it was apparent to me that it would be possible to have internal balancing without BMS. It's certainly possible that I've misunderstood something.

Charge controllers don't have BMSes. The GV-5 and GV-10 from Genasun are nice little MPPT controllers; I've used them. I haven't had the problem you describe.

Okay, I'm kind of grasping at straws to understand what happened with my GV-5. I agree, overall, it was a great controller. But again, it did that very strange thing when my battery reached full charge. Maybe it was a defective unit.

Note that at the voltages you are considering, MPPT isn't critical.

Okay, good to know. Although not critical, will I get appreciably more out of my panel with MPPT?

[Dave C]

Thanks a lot for your reply. I appreciate your input.

That is not possible, no controller PWM or MPPT will do that. As stated small 4S and 8S LFP systems have no need for a BMS.

If I still had the controller, I'd post a video. But I don't, so you'll have to take my word for it. It happened. Again, it could have been a defective unit. Actually, jflorey2 said that MPPT isn't critical, not BMS. Based on all that I've read, I wouldn't want to run an LFP battery with no protection. But again, this may be a matter of a newbie (me) misunderstanding terms...

I too have used Genasun MPPT controllers. There is only one unique thing about them, they only have 1-Stage charging using a Constant Voltage voltage algorithm aka Float Charging. Controllers for Lead Acid use a much more complicated 3-stage algorythim of Bulk, absorb, and finally Float.

The Float Voltage on a Genasun Lithium controller can be set to any voltage you want. So for example if you use LiFeP04 4S or 12 volt, you set the voltage for 3.45 volts per cell which is 4 x 3.45 volts = 13.8 volts. That means the output is always 13.8 volts when the battery is fully charged. It never shuts off until the sun goes down. When you buy a Genasun MPPT controller you tell them what voltage you want. Requires no BMS.

If you use a BMS, say one built into the battery pack like you have, you tell Genasun what voltage the BMS uses. But for such a small battery a BMS is a waste of money and time. Use a Genasun GV-5, connect the battery without BMS to the battery port, and the load to the Load Port. That will protect your battery form over discharge. Set to 13.8 volts eliminates any over charge.

Hmmm.... I found no options either online or in the multiple times I worked with Genasun over the phone to set a voltage in the GV-5. I do agree that a battery without BMS would probably have worked better with the GV-5. But the Bioenno Power battery was the only one that I found that had internal balancing, which to me made long-term off-grid use possible. Everything that I read prior to buying a battery said that LFP batteries degrade prematurely if not balanced.

[jflorey2]

Thanks for your reply. While researching solutions, I found one manufacturer who differentiates between balancing and BMS functions in their literature. https://www.victronenergy.com/upload...tteries-EN.pdf. Based on that, it was apparent to me that it would be possible to have internal balancing without BMS.

You can certainly balance batteries without a BMS. There are several manual ways to do it, and simple circuits (like a voltage divider+op amp on each junction, or a flying cap circuit, or simple zeners) can perform the same function. However you need to know what you're doing to get them to work well; done incorrectly, they can easily do more harm than good.

Okay, I'm kind of grasping at straws to understand what happened with my GV-5. I agree, overall, it was a great controller. But again, it did that very strange thing when my battery reached full charge. Maybe it was a defective unit.

If you were seeing 18 volts open circuit it was almost certainly defective.

Although not critical, will I get appreciably more out of my panel with MPPT?

Depends on the difference between Vmp of your panel and max charge voltage of your battery. If you are using a lithium ion that needs 16.8 volts and your Vmp is 18 volts you are going to see almost no benefit. In general the power increase will be proportional to Vmp/Vbat.

I found no options either online or in the multiple times I worked with Genasun over the phone to set a voltage in the GV-5.

They do it for you. You set the voltage when you order it; you can't change it yourself. You use the "custom voltage" option when you order.

[Sunking]

If I still had the controller, I'd post a video. But I don't, so you'll have to take my word for it. It happened. Again, it could have been a defective unit. Actually, jflorey2 said that MPPT isn't critical, not BMS. Based on all that I've read, I wouldn't want to run an LFP battery with no protection. But again, this may be a matter of a newbie (me) misunderstanding terms...

It had to be either defective, or the battery was disconnected by the BMS. As for MPPT being critical, you misunderstand. PWM and MPPT are both accurate in terms of voltage regulation. You could use either. All MPPT buys you is efficiency. PWM at best is 66% efficient, and MPPT is better than 95%. A PWM controller turns your 10 watt panel into a 6 watt panel, and MPPT into 9.5 watts or better.

Hmmm.... I found no options either online or in the multiple times I worked with Genasun over the phone to set a voltage in the GV-5. I do agree that a battery without BMS would probably have worked better with the GV-5. But the Bioenno Power battery was the only one that I found that had internal balancing, which to me made long-term off-grid use possible. Everything that I read prior to buying a battery said that LFP batteries degrade prematurely if not balanced.

Incorrect. You can specify the voltage when you buy them or change it yourself. It is clearly stated on the web site.

I do disagree with Jeff that it takes a ciruit to balance Lithium batteiries. Non are needed. All you have to do is when you recive them is connect them all in parallel and walk away for a day. Done. Would I do that? Yes and No. Every manufacture tells you to connect them in parallel and let them set till they EQ. Bu tthat is mid balanced and only works if the cells are matched like EV manufactures use. You need to choose between Top or Bottom Balance. Regardless of which method requires you to leave them in parallel for the initial aka Bulk Balance. You either charge them up to 3.6 volts until current stops. Or you discharge them to 2.5 volts. If you Top Balance requires cell level monitoring of each cell which means a BMS of some sort. If you bottom balance requires nothing except pack voltage monitoring. You disconnect when you hit 12 volt (3.0 vpc) and limit charge to 3.45 vpc using a Float Charger which is exactly what a Geneasun is.

A BMS cannot balance a battery. They can only maintain balance after the cells have already been initially balanced. There are not enough hours in a year for a BMS to balance new or out of balanced cells. FWIW cells only go out of balance if you use a BMS.

[Dave C]

First, I have to say that I really appreciate the answers you both have provided.

I forgot to mention something about the GV-5. When I received it, I followed the instructions to connect the panel first. I did so in full sunlight. Immediately, the LED flashed red in groupings of 5, indicating that battery voltage was too high. (Looking back, this should have immediately indicated to me that the controller was defective.) When I connected the battery, the LED changed to a rapidly blinking green, which indicated charging as expected. When the battery reached full charge, the LED turned solid green as expected, but within a few seconds began to flash red in groups of 5 again. It then alternated a few times between charging and error and finally remained at error. Whenever the controller was flashing red, the battery terminals were showing open panel voltage. This was the case both when the battery was disconnected and when it was connected fully charged.

The battery reduces the current that it draws to a few mA when it's fully charged. At that point, it expects around 14V, not 18 or 20V.

On the phone, Genasun tech support insisted repeatedly that the GV-5 was behaving as expected, and yet by email, they said that out of the thousands of GV-5s sold, they had never heard of a problem like this. It seemed bizarre to me that any controller would allow harmful voltage at the battery. Now you both confirm that no non-defective controller would do this.

The issue of setting a voltage on the controller probably never came up because the panel voltage and current fell well within the specs of the controller. The specs show that the controller can handle a max panel voltage of 27V and a max input current of 9A. Neither ever got that high.

From the GV-5 manual:

Genasun recommends using a lithium battery with a Battery Management System capable of disconnecting the solar charge controller in the event that any cell in the pack is outside of its rated temperature, current or voltage range. Failure to do so may result in property damage, injury or death. Genasun highly recommends the use of a BMS with cell balancing. Cell balancing is mandatory for lithium-iron phosphate systems

The Bioenno Power battery should have been ideally suited for use with the GV-5, because it has built in PCM/BMS that does balancing, protection against overcurrent, overdischarge, overvoltage, and undervoltage.

I would like to give the GV-5 another shot. It was great in several ways--small, light, marine-grade, etc. But I'm hesitant, because I don't have a lot of confidence in their customer service.

[karrak]

Here's my problem: I currently have a compatible controller from Bioenno Power. However, it's PWM, not MPPT, so it doesn't maximize solar panel performance

Looking at the specs for the panel it states an "Operating Voltage" of 15.4V which I assume is V_mp (voltage to get maximum output from the panel). If the average charge voltage of you battery is 13.2V (3.3V/cell), the efficiency of a PWM charger will be ~86% (13.2/15.4) whereas a MPPT controller will be around 95% or maybe less due to the small voltage difference in the voltages.

I tried the Genasun GV5, which was small, light, and had great features, but with one fatal drawback: it passed panel open circuit voltage (about 18V if I recall correctly) to the battery terminals when the battery was fully charged. I think this was because the GV5's BMS conflicted with the battery's BMS.

I agree that the Genasun should not be putting out 18 volts, I would think this is either due to a fault or because of poor design in that the Genasun is always expecting some sort of load on it.

The battery documentation says that the inbuilt "PCM(protection circuit module) will disconnect the battery if the input voltage gets to high, so although you were seeing 18 volts at the battery terminals the PCM should have disconnected the battery terminals from the battery. I think it would be worth contacting th battery manufacturer to check what voltage the PCM disconnects the battery from the terminals and what is the maximum voltage you can apply to the battery terminals before you damage the PCM.

Can someone recommend either: 1) a small, 4-cell LiFePO4 battery that has internal balancing and no BMS; or 2) a small, light MPPT controller with good LED status lights and no BMS?

If I am correct the current setup you have should not need to be replaced, although I would check to see if the Genasun is faulty. I agree that the battery sounds like a really nice unit and if the protection works as it should is pretty bullet proof.

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 forgot to mention something about the GV-5. When I received it, I followed the instructions to connect the panel first. I did so in full sunlight. Immediately, the LED flashed red in groupings of 5, indicating that battery voltage was too high. (Looking back, this should have immediately indicated to me that the controller was defective.) When I connected the battery, the LED changed to a rapidly blinking green, which indicated charging as expected. When the battery reached full charge, the LED turned solid green as expected, but within a few seconds began to flash red in groups of 5 again. It then alternated a few times between charging and error and finally remained at error. Whenever the controller was flashing red, the battery terminals were showing open panel voltage. This was the case both when the battery was disconnected and when it was connected fully charged.

This makes me think that the Genersun is expecting some sort of load. What charge voltage have you got the Genersun set to?

Simon

[Dave C]

This makes me think that the Genersun is expecting some sort of load. What charge voltage have you got the Genersun set to?

Hi Simon, thanks a lot for your input. I asked Genasun if the GV-5 was expecting a load at the battery terminals, and they never indicated that this was the case. I never set a charge voltage, and I never requested that the manufacturer set it. I didn't know to do that, and support never mentioned this in any of my discussions with them. There were no controls on the unit for me to set it. I returned it for a refund back in October. It seems strange to me that the error LED indicated battery voltage too high when no battery was connected. If this is a design flaw, it seems like a pretty basic, major one that I would expect to be reported often. Again, Genasun said they never heard of this problem, and folks here report they never experienced this.

[Sunking]

I forgot to mention something about the GV-5. When I received it, I followed the instructions to connect the panel first. I did so in full sunlight. Immediately, the LED flashed red in groupings of 5, indicating that battery voltage was too high. (Looking back, this should have immediately indicated to me that the controller was defective.)

No that meant you had no battery connected and is the crux of your problem. Keep reading.

When I connected the battery, the LED changed to a rapidly blinking green, which indicated charging as expected. When the battery reached full charge, the LED turned solid green as expected, but within a few seconds began to flash red in groups of 5 again. It then alternated a few times between charging and error and finally remained at error. Whenever the controller was flashing red, the battery terminals were showing open panel voltage. This was the case both when the battery was disconnected and when it was connected fully charged.

OK now I know what is going on because that is impossible, even if defective. It means your battery internal BMS is incompatible with the Genasun controller. Only possible way for you to see panel Voc voltage on a battery is if the battery went open circuit which is exactly what happened. Your battery internal BMS disconnected the battery from the charger. So I take back what I said, the controller is fine, your BMS is the problem

It is impossible for a battery to go from say 14 volts at full charge, and go to 18 volts in a short period of time. For that to happen would require a charger capable of producing extremely high amounts of current which your panel is not capable of doing. It would take several hundred amps to do that, and it is not possible for that to happen on solar.

The issue of setting a voltage on the controller probably never came up because the panel voltage and current fell well within the specs of the controller. The specs show that the controller can handle a max panel voltage of 27V and a max input current of 9A. Neither ever got that high.

That has nothing to do with your issue. You are talking maximum Input limits. The problem is the output, not input side. So quit chasing that ghost.
From the GV-5 manual:

Genasun recommends using a lithium battery with a Battery Management System capable of disconnecting the solar charge controller in the event that any cell in the pack is outside of its rated temperature, current or voltage range. Failure to do so may result in property damage, injury or death. Genasun highly recommends the use of a BMS with cell balancing. Cell balancing is mandatory for lithium-iron phosphate systems

There is the crux of your problem. The BMS is incompatible with the controller. It is also some nonsense. There are two unique characteristics of the Genasun Lithium controllers.

1. They only have One-Stage charging known as Float. It is a Constant Voltage algorithm. That is really a good thing as it is a lot less complicated that say a 3-Stage charger for Pb batteries.

2. They are intended for you to use the Controller Load Terminal to protect the battery. The load terminal has a built in Low Voltage Disconnect so if the battery voltage gets too low, it disconnects the battery from the load.

I would like to give the GV-5 another shot. It was great in several ways--small, light, marine-grade, etc. But I'm hesitant, because I don't have a lot of confidence in their customer service

Well you just might be able to. You have to resolve the incompatibility issue. Since you did not specify what voltage you wanted for the Genasun, you got a default value that is most likely too high for the battery BMS. What you are learning is most Lithium battery problems arise from the BMS. What I think is going on is the Genasun Controller output voltage is too high for the BMS. When the battery reaches full charge, the BMS cannot shunt enough current to pull the Controller voltage down. So your battery Internal BMS sees that as an over voltage, and disconnects from the charge source. When that happens, your controller is seeing an Open Circuit and the voltage shoots up. So now go back to the beginning when you said:

I forgot to mention something about the GV-5. When I received it, I followed the instructions to connect the panel first. I did so in full sunlight. Immediately, the LED flashed red in groupings of 5, indicating that battery voltage was too high.

That is exactly what is happening, your battery gets disconnected by the BMS when it thinks the input voltage is too high and it is not capable of shunting the current, so it DISCONECTS. Solution, lower the controller voltage.

I bet you a dollar your Controller voltage is either default 14.2 or 16.7 volts. Determine the maximum voltage your battery BMS can tolerate without disconnecting. Then set your controller to something less like 1 volt less.

What was the Genasun Part Number?

Hint: Either GV5-Li-14.2 or GV5-Li-16.7

[karrak]

Well you just might be able to. You have to resolve the incompatibility issue. Since you did not specify what voltage you wanted for the Genasun, you got a default value that is most likely too high for the battery BMS. What you are learning is most Lithium battery problems arise from the BMS. What I think is going on is the Genasun Controller output voltage is too high for the BMS. When the battery reaches full charge, the BMS cannot shunt enough current to pull the Controller voltage down. So your battery Internal BMS sees that as an over voltage, and disconnects from the charge source. When that happens, your controller is seeing an Open Circuit and the voltage shoots up. So now go back to the beginning when you said


That is exactly what is happening, your battery gets disconnected by the BMS when it thinks the input voltage is too high and it is not capable of shunting the current, so it DISCONECTS. Solution, lower the controller voltage.

I don't know how you can make these statements without knowing the operating parameters of the battery BMS.

If Dave C didn't specify which Genesun model I would assume he got the Pb unit which will charge to 14.2 volts then float at 13.8 volts which is just fine for the battery. Have you considered that the battery BMS might disconnect the battery from the charge controller when it determines that the battery is full,and not leave the battery on float charge?

I doubt very much that the BMS would try to drag down the input voltage by shunting current. It will just disconnect the battery terminals from the battery.

None of this matters anyway. So what if the BMS disconnects the battery from the charge controller either when the voltage gets too high or if it decides that the charge cycle has finished. Only problem you get is the Genesun giving you a "Battery Overvoltage Fault" and a full battery.

Simon

[Dave C]

Sunking,

Your last post makes sense. I think you might have closed the gaps in my understanding.

But the weak link was not necessarily the BMS--based on what you're saying, it was primarily my ignorance of the fact that the controller's output can and must be tuned to the BMS. Too bad Genasun's tech support didn't catch this. If they had, it might have saved us both the hassle of doing a return.

I still think a controller shouldn't allow open panel voltage to pass to the battery terminals under any circumstance--it should handle a no battery condition more gracefully.

My part number was GV5-Li-14.2V.

--Dave

[Sunking]

I don't know how you can make these statements without knowing the operating parameters of the battery BMS.

That is because I am a lot smarter than you and have forgotten more than you will ever know about batteries. Been doing it professionally for 35 years.