Voltage when discharging battery with PV going? (1 Viewer)

[e9999]

Hope this is not too EE-nerdish for this august assembly of red-blooded truck techers, but wondering about something and maybe somebody here may have done some measurements or seen it in passing. Or you're a battery and PV whiz. I'm curious what happens if you are set up to charge a battery with a PV system and charge controller but the load exceeds the energy production of the panels. This may be the case for example if your fridge is running when there is some sun but not enough, or perhaps all the time if your panel is too small.
I'm wondering how the charge controllers handle that. I imagine an MPPT one might be smarter about it than PWM, but what do you see either way? I imagine the voltage at the battery -and controller output- will decrease. If so, up to what point? Does the controller kick out at some low level cuz the voltage gets too low for it to run properly or safely? (I'm assuming here also that the controller does not have a low voltage cutoff on the load side to prevent overdischarging.)

 

[Cruiserdrew]

That's not a bad question, but as you might imagine, this is exactly what the charge controllers are set up for.

A PWM controller will simply go to maximum panel output. Because the voltage may drop slightly, the panel voltage may drop slightly lowering it's output marginally. But basically the "Pulse" width goes to infinity and the panel output goes into the battery.

An MPPT controller just goes to the max it is capable of. It will maintain the panel voltage at it's most efficient voltage for power production and the output will go into the battery system.

If you think about it, this happens every night. As the sun goes down, the panel output drops until it drops below the load in the system. The panel still makes the max output it can given the amount of incident light.

None of this is a huge deal. Again Eric, build yourself a system, and most of these questions will have obvious answers. Don't get lost in analysis paralysis.

My big panel is slightly more efficient with MPPT control, but it isn't much. A cheap PWM controller does a decent acceptable job for less $, as long as you are using "12 volt" panels. I have both types of controllers and have messed around enough to know what they do. Either is adequate in a small system designed to supplement battery power and running small loads. Now if you're using 24 volt panels (rare in our set ups with smaller panels), to get rated output in a 12 volt system, you will have to run an MPPT controller.

Here is a better question to ponder, and one I recently answered in Death Valley. What happens if you leave your panel and charge controller connected all the time, even when the truck is running, and the charging voltage is near or above the float voltage of your controller?

 

[e9999]

re: the original question, it seems you're assuming the battery is not depleted much, but what if it is? Say the sky is cloudy for a while and some big load is going (without a low voltage cutoff), the panel and controller are connected, the battery is draining, so the battery voltage will go down and I imagine drag the voltage of the controller output down with it (does it?). What does the controller do? Just tolerates the low voltage -which would not change the panel current much- or disconnects at some point?

 

[george_tlc]

Dear god man, it works, hook it up, move on and let the beer get cold and have a drink.

With PWM the most the charge controller can do is let all the solar panel output go to the battery. So, even with a 140W panel you'll get maybe 10A or so going to the battery (you'll be below the panel's max power point anyway). The PWM controller can't do much more than just go 100% duty cycle (emulating a piece of wire).

With an MPPT the controller will try and keep the panel at its maximum power point and it'll then put whatever current that is (at the output of the DC : DC controller's output) into the battery to try and charge it up. Since there's a DC : DC conversion going on the output current can be higher than the panel output current (the whole point of an MPPT controller). Still limited by the maximum power of the panel (less conversion efficiency losses).

Since you have a small & finite input power source (the solar panel), it's not like either PWM or MPPT controllers have any issue if your battery voltage is low - they can only draw at most what the panel can provide. i.e. if you are worried about a melt down of the nuclear power source (the sun) or the charge controller, you have nothing to worry about...

cheers,
george.

 

[Cruiserdrew]

Dear god man, it works, hook it up, move on and let the beer get cold and have a drink.

cheers,
george.

Lol. It's OK, E9999 is always imagining problems where none exist. Seriously Eric, do the experiment and report back!

But if there were a problem, you could always hook up the panel to the battery directly and let it charge to a voltage where the controller would certainly work. Just keep a close eye on it and pull the plug before it hits 15 volts!

 

[george_tlc]

^ yep, I've been here long enough to be aware of the mental disorder that e9999 has and we are all suffering due to

cheers,
george.

 

[e9999]

Cute, but no worries on this side, just curious about how the controllers handle very low battery voltage. I would imagine there is a protection built-in. Not the same thing, but I do know that my Battery Tender charger will simply not charge at all if the battery voltage is too low. One of my dumber chargers will overheat and trip a thermal breaker and repeat the cycle. (That would be a meltdown of sorts I take it... ) But if a solar charge controller were to stop charging because of low battery voltage it would not start again if the load keeps drawing power, which is not good if you want to get your battery up eventually. They got to be smarter than that.
Will call a manufacturer or do an experiment myself when I get the hardware I guess, if all-knowing MUD doesn't know...

 

[george_tlc]

Please read above again and again until you get it...

Re-iterating (with my EE hat on):

PWM will go 100% duty cycle (they are essentially just an on/off controller). So full solar panel output goes to your battery. If your panel can put at 5A max, then at max 5A will go into your battery. The controller doesn't 'cut off', it doesn't care and won't melt down.

MPPT will try to get maximum power out of the panel and drive the maximum current it can from that power (less dc : dc conversion losses) into your battery. At most it will have a current limit on what the dc : dc converter can supply.

Neither will suffer or die if they are feeding a discharged battery. They are specifically designed to deal with that, they are designed to charge a battery. Consider a 12V battery is pretty well totally discharged not much below 12.0V... Dead would be anything below 11.5V. Any solar charge controller will and must deal with that and suffer no harm. Though why would you ever want to discharge your battery that low I have no idea unless you like having to buy replacement batteries.

Not sure what you are trying to decide or are worried about. Even the 'free' China charge controllers (pwm) that come with many camping/rv solar panels will work just fine in any of the discharged battery scenarios you have brought up.

Again... NOTE: The solar panel has a maximum output, if the battery voltage is low, the panel can at most output that maximum. The panel becomes the limiting action for the charging current and inherently protects the charge controller.

Your mains battery chargers (battery tender etc) have a VERY large input power source (110VAC x 15A or more...) and so they need to limit charge current or disable charging or overheat - this issue does NOT occur with a camping solar panel setup.

Obviously you need to choose your charge controller to be able to handle MORE than you panel's maximum output current - easy spec to understand and easy to choose based on that. Most folk choose a panel in the 80W - 120W for camping/charging with typical camping power requirements - fridge etc. So choose a charge controller in the 10 - 15A capability, they are very common.

So, stop worrying or being concerned about something that is a non-issue and doesn't even exist... Unless you feel this is a new quest for Don Quixote to tackle.

cheers,
george.

 

[e9999]

Again, not worried. Just curious.
I'll certainly take your word for it that the controllers handle the low voltages in practice with no problem.
I'm going to try to find out what kind of algorithm the MPPTs use to handle the battery voltage feedback. That should be interesting (in a EE nerdish kind of way, sadly, but for EEs, they keep our beer cold!).

 

[george_tlc]

Again, not worried. Just curious.
I'll certainly take your word for it that the controllers handle the low voltages in practice with no problem.
I'm going to try to find out what kind of algorithm the MPPTs use to handle the battery voltage feedback. That should be interesting (in a EE nerdish kind of way, sadly, but for EEs).

GOOGLE MPPT - you can learn as much as you want. There's many ways to implement the specifics of MPPT - so not like you'll get ONE answer. Each MPPT algorithm implementation is dependent on who wrote the firmware and what they felt was the better way to do it.

Simplistically the controller measures the input voltage and input current into its input and calculates the product (watts). The controller will then adjust the DC : DC converter output current to maximise the input watts (i.e. it tracks the maximum power point of the solar panel). Of course it is also monitoring the battery (output) voltage to not overcharge the battery. Sampling rate and step size is all algorithm dependent.

I assume you understand that a solar panel (or even just a single solar cell) has a point on its V/A curve where the product of V x A is a maximum (the maximum power point)? If not, google for that info first and read up on it.

After that if the windmill is still looking like a dragon then start your google mppt education.

cheers,
george.

 

[e9999]

The basic principles of MPPTing are straightforward. The implementation not so much, for me at least. You can in principle move on the V/A curve by essentially changing the load seen by the cell, I imagine, but how the controllers do that when they have to juggle at the same time both the energy source with a specific VA curve (essentially a constant current device over most the voltage range, thankfully) and a battery which has voltage and internal resistance constraints of its own thrown in there, possibly at low voltage with high current draw, and do that without losing much energy in the process, or busting a cap, I don't know. And especially since lead acid batteries have complex voltage / SoC curves that are themselves dependent on the rate of charge or discharge.

 

[george_tlc]

It is quite a simple task if you are an electronics engineer. That's why modern MPPT charge controllers have a processor, the algorithm is implemented in software/firmware. Basically the dc : dc converter is running maximum output current when in MPPT mode AND the battery is not charged. So, nothing tricky, it just runs the MPPT part of the algorithm and outputs all it can. Output voltage is dictated by the battery - so the controller doesn't need to control the voltage at all. This for a vehicle battery, a medium panel (in the 100W range +/-) e.g. typical camping setup - the panel can never output more current than the battery can take and all the charge controller needs to do is deal with the final top up and float transition.

Once the battery voltage is high enough that the controller needs to lower output current, then it does that and obviously no longer cares about MPPT since it no longer needs the full output of the panel so who cares if it is at its maximum power point or not (in fact it will invariably not be). As battery voltage increases more as it charges, the output current is reduced even more until the battery is charged and the controller then finally limits output current to maintain the desired float voltage (around 13.8V for a wet cell, temperature dependent). It's no different than any mains operated charger, first run constant current (the most it can output and not exceed the battery maximum charge current or damage itself) and eventually transition to constant voltage to maintain float voltage. With the typical camping load of a fridge and some other electronics, it will be rare that the battery gets charged all the way to the float condition - the fridge will cycle on soon enough anyway.

Go and google some more. There's even DIY MPPT projects along with schematics & source code if you want to understand THAT SPECIFIC implementation. All the information you want is readily available if you have even rudimentary internet search skills...

Oh, and a lead acid batteries actually have a well documented and reasonably simple charging profile. This is quite ancient technology and well understood.

End of lesson

cheers,
george.

 

[e9999]

that certainly makes sense. Nicely written and very clear. Thanks. Appreciate your taking the time.

That is charging, though. My original question was about a discharging battery, however. You said the MPPT controllers are built to handle it. I'll take your word for that. The argument above that the panel can only output so much current is certainly true, but I would think that does not translate to the situation downstream of the controller if the battery voltage gets very low and you are still cranking a finite amount of power out of the panel thanks to the controller keeping the MPP going. The current to the battery or inside the controller could get high I would think. Fine, I understand that is not too likely to happen in practice with well-chosen components and a good battery. I'll just assume that if it comes to that the controller will just blow a fuse or -better- stop working by design somehow. Maybe the "current limit" you alluded to. I'll leave it at that. I'll buy an MPPT controller one of these days and do an experiment with a deep cycle battery. Should be fun.

 

[george_tlc]

The DC : DC controller will have a current limit - it has to limit current under program control anyway to achieve the needed output voltage profile in top up and float. Due to the current regulation it is easy to set the maximum current that can be delivered to be within safe limits of the controller. These are buck dc : dc converters since panel voltage is always higher than battery and if it it isn't charging stops anyway. A buck converter with current limit can drive into a short circuit safely... so a very discharged battery is a non-issue.

Again, these controllers are designed to deal with all battery discharge(d) scenarios since they are designed to charge... Only fuse blowing situation would be a failure/short of wiring/electronics etc and the fuse blows to prevent fire/melted wiring etc.

It is pretty simple to design a constant voltage/constant current capable DC : DC buck controller. Add input voltage/current monitoring (temperature too for correct battery charging compensation) and some firmware and you have an MPPT solar charge controller.

Yes, stop worrying, buy your stuff and get the fridge/beer cold. Sit back, relax and look for another windmill

cheers,
george.

 

[e9999]

Interestingly, the better the beer, the more windmills one feels ready to tackle...

The ironic part about the current limiting scenario you describe is that the controller would have to disengage MPPTing at low battery voltage to limit power generation, which is exactly when you normally would want to have the maximum charging power. And the battery will discharge even faster. Oh well...

 

[george_tlc]

Yes, but MPPT is only to get the most out of the panel *IF* the output can use all that power, if it can't then MPPT no longer is the priority in the firmware. Again, this is only an issue if you were trying to charge a short circuit... Any 'normal' scenario would have the battery voltage well above 11.5V anyway and that would hardly require limiting the output current of the DC DC converter to protect it...

Ok, this dragon/windmill is now officially dead, time for a new one

cheers,
george.

 

[Cruiserdrew]

Like I said eric-if you really thought there might be a problem with the charge controller in a low voltage situation, the panels can charge the battery directly. The problem is, if you forget about it, once fully charged, the voltage will rise rapidly to the VOC of the panel, often in the 20 volt range which will harm your battery and kill it quickly.

You need to be practical about this though. There are virtually no scenarios where you would take a battery below 11 volts, and if you did, you would have bigger concerns than a $40 charge controller. If you're worried take 2 charge controllers.

Please buy yourself a solar set up and go camping for a week. Stop posting on ih8mud about solar power. When you return, you will have learned more than 1000 posts about solar systems could ever teach you, and you will generate an infinity of new questions to post up! Enough to keep you entertained for weeks.

 

[e9999]

No need, I'm going to blow up some controllers! Entertainment for the whole family! Yay!

 

[JCruse]

All I know is that when the 125wpc stereo is playing the battery voltage goes down from 13.2v to about 12.7 in say 3-4 hours depending on volume. That's about my limit for battery battery.... so I shut loads off and let it recover.