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I have a device that pulls about 10 amps and is typically powered with a car battery. It runs fine with less amperage but needs 9 to 14 volts. I want to power my device directly from a solar panel but haven't accomplished anything if the solar charger requires a bulky battery.

It works with a DC DC buck converter but inefficiently, pulling ~70 watts from a 120 watt panel. I want something more efficient.

I found a mppt solar charger that powers my device efficiently when connected to a 5 amp-hour battery, but again I don't want the bulky battery.

Can you suggest a way to power a solar charger with a very small battery and not have it discharge when clouds block the sun?
or

Is there an MPPT out there that doesn't require a battery?

edit:

Many of you asked "what is the load?"...... I have four titanium electrodes, spaced about 1/4 inch apart, and place them in salt water, to produce bleach. The bleach is used for sanitation or disinfecting water in developing countries.

If I had 12 volts and 10 amps the batch of bleach would be done in an hour. I don't need to run at night and it doesn't bother it to stop when a cloud covers the sun.

As far as the buck converters.....The ones I purchased don't have any adjustment..... Someone asked for the circuit diagram for the buck converter. I just hooked the input to the solar panel and the output to the bleach making unit.

The unit will operate fine with less than 10 amps, it just takes longer to make a optimum strength of bleach. It likes 12 volts but will work fine at 9 to 14 volts.

I have been told that the problem is similar to powering a water pump directly from a solar panel.

I didn't know there was a MPPT "controller" that would not need a battery. That's why i tried the "solar charger". I'll look for one or maybe you could point me in the right direction.

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    \$\begingroup\$ What @winny said. If your load can handle 9 to 14 volts adjust the buck converter output voltage to maximize power output. You should be able to find the peak power output point for your conditions. (Which might not be as good as true MPPT but may be better than what you originally observed.) Do you have the panel I/V characteristics? \$\endgroup\$ – John D Aug 18 '18 at 20:21
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    \$\begingroup\$ @JohnD What is "true MPPT"? It is my understanding that actual MPPT circuit has nothing to do with the battery. It's only goal is to maximize power output regardless of the load. The specialized MPPTs that also double as chargers take battery charge profile into account to further increase efficiency. Point being - the "true" or "normal" MPPT not designed to charge batteries could be much better fit for OP application. \$\endgroup\$ – Maple Aug 18 '18 at 22:54
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    \$\begingroup\$ @Maple Right, if your load can operate from 9-14V and perhaps isn't constant power (say like a pump), then constantly adjusting the output of the buck powering the load to extract the highest power from the solar panel will result in maximum power output. (True MPPT, no battery need be involved.) What I suggested above was to try to maximize the power from the panel at (say) maximum or average illumination manually. It wouldn't truly track the maximum power point perfectly under varying conditions. \$\endgroup\$ – John D Aug 18 '18 at 23:27
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    \$\begingroup\$ MPPT is something that just tries to get as much power as the panel can give in any condition. So I have some fixed quantity of power. On the other hand the load takes the power it needs. These two facts cannot be on its own, there must be something in between acting as a buffer to match energy fluxes and hold true conservation principle. So a MPPT cannot be connected to an unknown load without some buffer (e.g. one battery) The case of a solar or wind inverter is totally different, they work against the "bottomless" power grid which can in any moment take whatever power the MPPT supplies. \$\endgroup\$ – carloc Aug 19 '18 at 11:45
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    \$\begingroup\$ @carloc "as much power as the panel can give" correct. "load takes the power it needs" wrong. load cannot take more than available. "there must be something in between" yes. It's called DC-DC converter. "MPPT cannot be connected to an unknown load without some buffer" all you need to know about the load is required voltage (variable voltage requirements of battery charger is exactly the reason charging MPPTs exist). The solar panels with built-in MPPT have fixed 5V or 12V output and work just fine without any battery. Of course adding battery to the system increases reliability. \$\endgroup\$ – Maple Aug 19 '18 at 17:01
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I found a mppt solar charger ... but again I don't want the bulky battery.

If you don't want a battery, why did you look for a charger?!

Can you suggest a way to power a solar charger with a very small battery and not have it discharge when clouds block the sun?

First, when a battery powers something it discharges. That's how it works. Second, most likely there is a way to hack charger schematics to keep it "operational", i.e. powered up without the sun. But what would be the point of that if there is no sun to get power from and no battery to charge?

Is there a mppt out there that doesn't require a battery?

I believe they called "Direct-coupling photovoltaic systems". You have to research those yourself. The MPPT controllers (not MPPT chargers!) are commonly used for loads that do not have strict power requirements, for example water pumps, water heaters etc.

However the fact is, most loads cannot operate in the wild output power range of the solar panels. Using them without battery basically negates the efficiency gains of the MPPT, because they will shut down in low light when just a little extra juice from the battery could have kept them working. At the same time in very bright light the extra available power is wasted when it could have been used to top up the battery.

So if your device (you haven't given us enough details) is the kind that can operate intermittently and you want it to draw as much power as available but don't mind shutting down in low light, then MPPT controller is what you need.

If you want your device to have stable operation in variable light conditions then what you really need is a bigger battery and MPPT charger. Then you can limit power available to your device at say, stable 100W 10V and use the remaining 20W (when they available) to charge the battery.

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  • \$\begingroup\$ @old_timer Unfortunately OP did not provide any details on the "device". However from the description the goal is to maximize output power when it is available and no worries about losing power in darkness. Pretty much how solar-powered water pumps work. My answer covered both approaches though. \$\endgroup\$ – Maple Aug 20 '18 at 0:11
  • \$\begingroup\$ But... if it is cloud/night situation and OP does not care if "device" would shut down, what would you need that "storage" for? MPPT shuts down as well and waits for sun to come back. \$\endgroup\$ – Maple Aug 20 '18 at 0:39
  • \$\begingroup\$ Ok I see the interpretation of the question that implies no need for shade/night operation. But you went down that path and your highlighted statement that implies an MPPT requires a battery is incorrect, it requires storage of the spare energy collected and a way to return it for that use case. A battery is likely the most common solution but is not the only solution and most certainly an MPPT without a battery does not automatically negate the gains, neither in the without a storage case as you are getting more out than you would with your load and in the storage other than battery case. \$\endgroup\$ – old_timer Aug 20 '18 at 12:36
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I have a device that pulls about 10 amps and is typically powered with a car battery. It runs fine with less amperage but needs 9 to 14 volts.

So the device presumably pulls 10 amps when at the full 14V and less at lower voltage, but it wouldn't hurt to know what it is, since if it contains a voltage regulator of it's own, it may actually draw more current at a lower input voltage.

I want to power my device directly from a solar panel but haven't accomplished anything if the solar charger requires a bulky battery.

To be clear, is it weight or bulk you're more concerned with? They go a bit hand in hand, but it wouldn't hurt to know. You can store substantial amounts of charge at vastly lower volumes and weights than a car battery for sure, but...

\$Efficiency, Low\,volume\,and\,weight, Low\,complexity\$

Pick any two.

It works with a DC DC buck converter but inefficiently, pulling ~70 watts from a 120 watt panel. I want something more efficient.

First, have you measured the voltage and current going into the converter as well as the voltage and current coming out of it? You cannot assume that a 120W panel is putting out a full 120W. Also does it require adjustment or is it automatic as the name implies? Answering this might help you.

Buck converters are generally better than linear regulators, but it's good to be aware that with most topologies, PWM switching converters only reach maximum efficiency at some specific value, usually close to max load, so you may simply need a smaller switching converter.

I found a mppt solar charger that powers my device efficiently when connected to a 5 amp-hour battery, but again I don't want the bulky battery.

This suggests that the solar panel may simply not be powerful enough. Is it perhaps actually the battery doing most of the work? Whether it is or not, you could switch to a smaller battery bank.

10A at 12V (120W is a pretty substantial size of panel. If you want your load to run all day and night, you'll need quite a bit more than 120W rated worth of panels and a substantial battery bank.

Can you suggest a way to power a solar charger with a very small battery and not have it discharge when clouds block the sun?

to answer your question, normally a small volt drop diode is used on each series set of panels to prevent dark current losses (current flowing back to the panel from the battery when there is no light).

There is at least one company (Powerfilm) that makes panels that have better low light performance. I use mine for camping (shorter daylight due to mountains) and live in Canada (Light is at less of a good angle than at equator so overall brightness at a given level of cloud cover is lower). It's been several years since they started production so they likely have competitors by now.

Be aware that when panels are arranged in series, a shadow falling on part of the array will decrease the performance of the entire array. Arrays that are in parallel with each other will each still work when the others are shaded. Also be aware that to produce their rated wattage, the light the panels are exposed to has to be very bright and the panels have to remain at a specified temperature. Direct sunlight on a low pollution cloudless day with substantial wind if you're not actively cooling the panel.

Is there a mppt out there that doesn't require a battery?

Your MPPT device is a "Maximum Power Point Tracking" Solar charge controller, meaning it charges a battery by optimizing the both the input voltage and current AND the output voltage and current to charge the battery. If you draw power too fast from a solar panel, output voltage drops and output current goes up. This makes the panel much less efficient, so assuming it's fully automated, it limits the current it draws from the panel to keep the voltage up and maintain an ideal VA. At the same time it is converting the pulses it outputs to the correct voltage to charge the battery.

This means it is likely designed for a particular type of battery and you likely need a different one if you want to switch battery types.

Apparently MPPT is simply a property a voltage converter can have, and the output does not necessarily have to go into a battery, but without storage, because the input must vary as conditions vary, the output must either waste power, vary in voltage or be intermittent, so not using a battery will likely only work for you if your device will only be used in consistent bright lighting conditions.

I'd suggest putting your load on a variable voltage regulator and see how low you can power it and still be satisfied with performance. Find a voltage converter that can produce this output at a high efficiency.

Measure the actual input and output wattage to your MPPT controller and confirm its efficiency. Figure out in a worst case scenario for sunlight, how long you need it to be able to run, and make sure your battery bank has that amount of capacity. If you need it to be lighter, you may need to switch to a more complex and expensive battery type.

Figure out roughly how often the worst case lighting scenario is going to occur, and choose panels whose actual output will add up to enough extra wattage to run the device and fully charge the battery bank in that time frame.

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    \$\begingroup\$ An MPPT is a "Maximum Power Point Tracking" Solar charge controller. Nope. That would be "MPPT charge controller". The MPPT by itself is a technique or a device implementing that technique for maximizing power output from solar panel. \$\endgroup\$ – Maple Aug 19 '18 at 17:07
  • \$\begingroup\$ @Maple Hmmm. Missed that. Thanks Maple. I'll make some corrections later. \$\endgroup\$ – K H Aug 19 '18 at 19:44
  • \$\begingroup\$ I'd expect a non-charger MPPT controller to vary the output voltage depending on the available power - and I'd only use it with a load designed for that. \$\endgroup\$ – immibis Aug 20 '18 at 1:03
  • \$\begingroup\$ Good to know. There could be merits to discontinuous output as well, although the first thing that springs to mind is to charge a battery or cap or something. \$\endgroup\$ – K H Aug 20 '18 at 1:09

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