MPPT solar controller and LiFePO4 battery for backpacking

[Cabinlakewister]

Interesting topic. My e bike lifepo4 just went 62.6 miles at 5,000k miles 2.5 year old battery. Maybe I shouldn't have just bought a gell cell for my cabin. Anyway I just joined and am trying to post. Where did they hide the post button? How do u post a new topic?

[PNjunction]

Originally, they told me that the battery backs down to a few mV when fully charged, so I assumed that it didn't disconnect in the course of normal operation. But in the course of conversing yesterday by email, it came out that after shifting to CV, when the battery voltage hits 14.6V, the charge current goes to 0 A.

The charge current going to zero amps is due to the *battery* doing the current regulating at CV, not the bioenno! This is what is called the CV "absorb" stage. LFP or lead-acid, the basics are the same. What you got over the phone may have been over-simplified so that the guy can get back to work. Both Genasun and Bienno power could easily go bankrupt unless they hired a full time staff to explain the nitty gritty of it all for all the diy callers.

For example, what you described above is what a Battery-Tender designed for lead acid agm absorb (14.6-14.7v) will do! The point here is that it is NOT the battery-tender doing the current regulating once it reaches the absorb 14.6v stage.

The main selling point to me of Bioenno was that their batteries do internal balancing. To me, this made using LFP in my application possible. I had read that LFP batteries degrade quickly if they're not balanced, and the required input current for the balancing chargers I found was too high in a single-panel solar situation. So internal balancing did it for me.

That's fine. It also has under-voltage discharge and over-current protection. Great. Those are things we do ourselves outside our batteries, with no proprietary circuitry. Your choice.

Be on the lookout however, for cheap bms systems that are essentially baby-sitting poor quality cells to begin with! Instead of using quality cells, crap cells are used and a cheap ass bms is there to make sales of these poor quality cells. I'm not saying Bienno is doing that, but there is a lot of disreputable junk out there, so be careful. A deal too good to be true is the big red flag.

Also understand that there are many configurations of batteries - mine for 12v are merely a 4S configuration - 4 cells in series. Pretty easy to keep balanced once the initial balance is done. If a battery consists of a bunch of 18650 LFP batteries, the more they put in to increase the capacity - things can start to go nutty especially if those cells are not reputable without some safeguarding circuitry.

I wouldn't be inclined to take on a diy LFP project with no solar experience and limited electronics experience.

Ok, it's a start and you know your own limitations. Fantastic!

Except I have one more question: When I asked Kevin what is the max voltage his BMS can handle without frying, he answered 15V. Well, the BMS was hit with much more than that every time I connected it to the GV-5. So, I wonder if the BMS might be damaged. It doesn't seem so. The battery still works fine with the Bioenno controller. I wonder if a steady diet of seeing 18 - 20V will damage it over time.

Ah, once the Genasun is connected to an actual battery, it will properly regulate the voltage with it's own circuitry now that it sees a load. When the Bienno says the battery is full, and disconnects, this is now an abnormal setup for a controller to have no battery attached, and we're back to just seeing the ocv of the panel.

If I think about it, without a battery, (either physically missing, or the Bienno cutting it off) the Genasun may be interpreting the open circuit voltage of the panel as the battery voltage itself - and of course at 18v, IS over voltage from a fictional missing battery standpoint!

I think this is the big trip up in understanding. You are questioning a logic-trap kind of situation, so that is very good actually.

Remember that from this distance, there IS the chance that your Genasun unit, may have been defective and returned to sales. Did you get yours directly from an authorized Genasun dealer, or was it some 3rd party online sales deal?

[karrak]

Guys, any thoughts on my last post?

Sorry, I have have been away.

Here is my take on what is probably happening when you charge your battery using the Genasun taking into account the subsequent commentary.

The Genasun will supply as much power to the battery as the solar panel will supply if the voltage is less than 14.2 volts. When the battery voltage gets to 14.2 volts the Genasun starts to limit the current to keep the voltage at 14.2 volts as it should. Because this is an LFP battery with virtually no leakage the current will drop to virtually zero if the voltage is kept at 14.2 volts. The Genasun is incapable of keeping the voltage regulated at 14.2 volts with no current so the output voltage from the Genasun starts to rise. At some point, probably just above 14.6 volts, we don't know exactly what voltage because of the poor battery documentation the battery BMS disconnects the battery from the battery terminals. The BMS reconnects the battery to the battery terminals when it detects that the input voltage at the terminals has gone down below its safety cutoff voltage.

I am using the word probably because I don't have all the facts needed to make a definite statement.

The documentation that I can find from both Genasun and Bioenno is woefully inadequate.

If the Genasun charger does indeed let the output voltage drift up to the solar panel output voltage if there is no load connected to it there should be a big warning in capital letters in the user manual about if. If i was being polite I would call this a unique design feature. If you tried to connect the Genasun up to an LFP battery without a BMS it could quite easily damage it. This is a good case for buying a battery with a BMS installed.

The Bioenno documentation should tell you what voltage the BMS disconnects the battery from the battery terminals, and should tell you the maximum safe voltage that you can supply to the battery terminals.

While we are on the subject of documentation, Powerfilm should at least tell you what the Vmp (voltage for maximum power) is, and what the power loss temperature coefficient is. Would be nice to have some graphs of power output versus temperature and panel voltage as well.

IMO it is not your fault that has lead to the problems you are experiencing, but problems with the equipment, documentation and customer support. This is familiar story, the manufacturers and others try make it seem like it is your fault when it isn't.

SImon

[Dave C]

Simon, PNjunction, Sunking and others,

Bioenno confirmed that when the battery voltage gets to 14.6V during CV charging, the BMS disconnects the battery. I sure wish they had told me this in September...

[Dave C]

Bioenno Power confirmed that during CV, when the battery's voltage reaches 14.6V, the BMS disconnects the battery. Not sure why it took so long to get this information. Simon, yes, this should all be clearly documented. To be fair, even if everything was documented, the info would probably have gone over my head--I don't think it would have helped me make a pre-sales decision. So, at very least, when I called with problems, both customer service departments should have had the applicable info at their fingertips and shared it with me.

For comparison, can anyone recommend an MPPT controller and self-balancing LFP battery pack that are highly compatible with one another, lightweight, as weather resistant as possible and as hassle free as possible for a non-engineer? Around 5 Ah would be great. The bar is set high in terms of weight and weather resistance--the Bioenno battery weighs about 1 lb., and the GV-5 is marine grade and weighs just 2.8 oz. The system needs to be reliable in a long-term, off-grid application.

[Sunking]

Bioenno Power confirmed that during CV, when the battery's voltage reaches 14.6V, the BMS disconnects the battery.

Hate to tell you this; told you so. That is the only way the Genasun controller can go high voltage. Two ways to work around this, well three ways. My way is get a different battery without HVC and LVD. Your Genasun controller has a built in Load Port with LVD of 5 amps, USE IT as it was intended.

Anyway keeping the battery you already have:

1. Lower the Genasun output voltage from 14.2 down to 13.8 to 14.0 volts. That will prevent the battery going to 100% and activation the HVC circuit.
2. Keep a load device connected to the battery load port.

Also do not be fooled by terms. Even the cheapest of cheap chargers are CC/CV. It is not something the BMS does. There is no special circuitry that switches the charger from Constant Current to Constant Voltage. It is just the physical interaction between the battery open circuit voltage, battery internal resistance, the chargers current limit, and the chargers set point voltage. When the battery charges up, it voltage rises to meet the charger set point. When the battery voltage rises to the charger set point voltage current tapers off and voltage stays constant via Ohm' Law of voltage and resistance. Read this it might help.

[karrak]

Bioenno Power confirmed that during CV, when the battery's voltage reaches 14.6V, the BMS disconnects the battery. Not sure why it took so long to get this information. Simon, yes, this should all be clearly documented. To be fair, even if everything was documented, the info would probably have gone over my head--I don't think it would have helped me make a pre-sales decision. So, at very least, when I called with problems, both customer service departments should have had the applicable info at their fingertips and shared it with me.

I think you made the right decision with choosing the Bioenno battery. From what I can see the Bioenno battery just needs a power supply that will provide 14.6 volts (14.2 volts is adequate).

For comparison, can anyone recommend an MPPT controller and self-balancing LFP battery pack that are highly compatible with one another, lightweight, as weather resistant as possible and as hassle free as possible for a non-engineer? Around 5 Ah would be great. The bar is set high in terms of weight and weather resistance--the Bioenno battery weighs about 1 lb., and the GV-5 is marine grade and weighs just 2.8 oz. The system needs to be reliable in a long-term, off-grid application.

Contrary to what Sunking has been saying you do not need to worry about compatibility between the Bionno battery and an MPPT controller as the BMS can disconnect the battery from the controller if any problem that might damage the battery arises. The compatibility issues only occur if the battery does not have the internal circuitry to disconnect itself from the outside world. In this case the BMS has to be able to communicate with the controller to tell it to stop charging or connect to external circuitry that can disconnect the battery if a problem arises.
If you can't find an MPPT controller to match the GV-5 specs without the annoying problem that the GV-5 has there are ways of getting round the problem.

With your weight requirements, am I right in thinking you are a long distance bushwalker? Have you heard of the Bibbulmun track?

Simon

[Sunking]

Contrary to what Sunking has been saying you do not need to worry about compatibility between the Bionno battery and an MPPT controller as the BMS can disconnect the battery from the controller if any problem that might damage the battery arises.

It just means the battery is incompatible with any Solar Controller. No controllers out there are made to work without a battery connected. Most controllers would be damaged with the battery disconnected with panels connected in sun light. Just a fact and you can squak all you want about it.

Genasun is the only controller made for a Lithium battery and has a LVD built-in, but is not intended to have the battery disconnected, just the load. Will it hurt a Gnasun Controller having the battery disconnected? Answer is NO. But his battery will never reconnect when the controller output goes to 18 volts with an Open Circuit.

[karrak]

I am typing this very slowly Simon so you can understand and follow along. Yes I clicked the wrong converter.

No you didn't just make a simple mistake and click on the wrong converter, you then went on to describe what the circuit does when you turn the FET off. This means that you did not know by looking at that circuit that it was not a Buck Converter. A first year electronics student would know that. You failed the test.

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.

Pulse width modulation goes from 0% to 100%, 0% is turned off, 100% is turned on all the time. A first year electronics student would know that. You failed the test.

[Dave C]

But his battery will never reconnect when the controller output goes to 18 volts with an Open Circuit.

I understand your point. Whether or not the battery will reconnect is unknown. But I don't think the 18V OCV has any causal effect on the disconnect--I think it's purely a result of the disconnect. After the initial disconnect, the battery would reconnect briefly 4 times. Maybe the battery voltage was slipping briefly below 14.6V? Would the BMS be sophisticated enough to test if a load has been connected? Would the presence of a load affect whether or not the battery disconnects?

What kind of load would you recommend connecting to the battery terminals? What kinds of complications would that introduce?

What battery would you recommend?

[karrak]

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.

How the hell do you know this is the case, have you disassembled the Bioenno battery in question or have access to the circuit diagram of the battery?

I doubt very much you will find any relays in the battery. The BMS is more likely two FETs back to back being controlled by a circuit that detects if the cell voltages are too high or low. See this article for more informationhttp://electronics.stackexchange.com...rces-connected

Don't you ever learn from your mistakes, remember this?

Many battery Manufactures make 12 volt LFP batteries as drop in replacements for Pb and most do not have any type of BMS other than a LVD Relay. That includes the almighty A123 LFP 12 Volt Car Battery. Why would a manufacture do that without a BMS? Because it is NOT NEEDED.

and the reply,

WHOA That statement is complete BS....totally untrue. How to you pretend to know what is inside that battery? Tell you what....you tell us what's in there, and then I'll post a picture of one torn down and we can all see how full of it you really are. Your trousers are ablaze.

By the way, wb9k is the head of the warranty labs at A123systems.

[karrak]

I understand your point. Whether or not the battery will reconnect is unknown. But I don't think the 18V OCV has any causal effect on the disconnect--I think it's purely a result of the disconnect. After the initial disconnect, the battery would reconnect briefly 4 times. Maybe the battery voltage was slipping briefly below 14.6V? Would the BMS be sophisticated enough to test if a load has been connected? Would the presence of a load affect whether or not the battery disconnects?

You are correct. I can't say for sure because I don't have enough facts about the battery but I would think that when all the individual cell voltages dropped below 3.65 volts (overall battery voltage 14.6V) the BMS would reconnect the battery to the connector. The Genasun would then charge the battery a little more until the BMS detected an overvoltage and disconnected the battery again, and so on and so on.

By disconnecting the Genasun controller from the battery the voltage at the connector will drop down to below the cutoff voltage and the BMS will reconnect the battery to the connector. Placing a suitable load on the Genasun controller will make sure that it keeps the voltage regulated.

What kind of load would you recommend connecting to the battery terminals? What kinds of complications would that introduce?

I wouldn't connect the load to the battery terminals but to the load terminals on the Genasun controller as it is this that is causing the problems. All it would need would be a small 1/4watt or 1watt resistor. The value of this would be dependent on how much load the Genasun needs to keep its voltage regulated. I would try a 1/4 watt 10kOhm first. If this didn't work try a 1kOhm 1/4 watt and if this doesn't work try a 330Ohm 1 watt resistor.

Simon

[Dave C]

See this article for more information http://electronics.stackexchange.com/questions/79028/understanding-two-mosfet-with-sources-connected"] http://electronics.stackexchange.com/questions/79028/understanding-two-mosfet-with-sources-connected[/URL]

Would this kind of circuit allow a BMS, when wired to a controller, to block incoming charge current while allowing outgoing current to flow to a load? Or, in a case like mine, is it likely that the disconnect at full charge is like an open a switch, in which case this circuit, if employed, controls the direction of current flow only when the switch is closed?

[karrak]

Would this kind of circuit allow a BMS, when wired to a controller, to block incoming charge current while allowing outgoing current to flow to a load? Or, in a case like mine, is it likely that the disconnect at full charge is like an open a switch, in which case this circuit, if employed, controls the direction of current flow only when the switch is closed?

Current can only flow in one direction at a time, it doesn't go into the battery and out of it at the same time. If you have a load connected to the battery at the same time as it is being charged the current will go directly from the controller to the load, bypassing the battery. If the load draws more current that the controller will provide the load will take current from both the controller and the battery.

In theory you could operate the switch circuit so it would only switch current in one direction at a time but due to it not working well like this you probably wouldn't do it. You would either have is switched on to allow current flow in both directions or switched off so no current can pass.

Simon

[karrak]

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.

I beg to differ, maybe I am wrong and the basic operation of commercial MPPT controllers is radically different from what I designed, which I doubt but I would call the main power circuitry of an MPPT controller a Buck Switch Mode Power Supply with the primary feedback control loop being controlled by the output voltage, and a secondary control loop being to maximise the power obtained from the solar panels. They do not necessarily need a load to operate but as jflorey2 has already said the control electronics usually get their power from the load (in this case the battery) with the exception of the Genasun GV-5 which seems to get its power from the solar cell.

But who am I to make such a statement, I am the idiot who doesn't know what i am doing or saying, even though I have designed and built my own MPPT controller over five years ago which is still working just fine.

For those that are interested here is the circuit diagram of the power electronics of my MPPT controller, look familiar?
SolarControllerPower.pdf

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.

There are a few different scenarios when disconnecting the battery from my controller when the output from the solar panels is at its maximum

1. Battery not full and controller providing maximum amps to battery

This is the one that you are getting excited about and would be a challenge to the controller. In my case the voltage control feedback loop would be too slow to stop the output voltage going above its programmed level but my HVD software would detect this and turn off the switching FETs as soon as this condition was detected. This would still be too slow and the output voltage could rise to the Voc of the solar panels, which could be as high as 40 volts. I am fairly sure that all the electronics that would be subjected to the 40 volts is rated at a higher voltage so I can't see any damage happening. The voltage would then drop fairly rapidly due the power consumption of the controller electronics. I am not sure if it would drop to zero or whether the control electronics would start regulating the voltage again.

2. Battery full and controller providing little or no current

It is likely under this scenario that the voltage control feedback loop would be able to keep the output voltage under the HVD trigger and the controller would continue to provide regulated power to the output, although the regulation would suffer.