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.