I added a small solar panel to my 12v 7ah battery. Under direct sunlight, my battery voltage reads 15v and steadily drops to 13.2-14v in the shade.

My question, is it safe to use the radio under these voltages without a certain mod, ie, bigger heatsink, or am I good to go?

Anything much over 12v is unsafe. ?See post ??/g/BITX20/message/26492

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A typical equalization charge for a lead acid battery is slightly over 15v. ?A few hours a month at 15v is not a bad thing, though if it's got inspection caps then the water levels should be inspected occasionally. ?He says it's a small solar panel, so likely just a few hundred ma. ?If worried, I'd add an LM2940T-12 between the panel and the battery, with a resistive divider rigged for a 14V ouput voltage as per the LM7805 datasheet. ?(The LM2940 has a lower dropout voltage than the LM78xx series parts.)

This is kind of what I have. The manufacturer does not add a charge controller. But, then again I will take that 12v is probably the best voltage for the BITX40

That looks like a car battery tender, around 5 Watts into a Vmp of 17 volts or so in full sunlight. ? So a max charge current of 5W/17V = 400ma. ? You could verify that by just putting an Amp meter across those alligator clips, the current from the panel is the same whether it's going into a 12v battery or a dead short. ?There's only so many electrons freed up by incoming photons. ?That 400ma should be way more than the battery's self discharge rate, but way less than an equilization charge current. ?For occasional field use, I wouldn't feel a need for a charge controller, just clamp the panel to the battery. ?But ideally, store the battery with a properly designed mains powered lead acid battery charger/maintainer attached. ?Do not just leave it sit when discharged.

Most lead acid batteries are not designed for lots of deep discharge cycles. ?If you use it enough that this battery eventually fails, you might look into investing in a good Lithium Ion battery.

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The 22v you see is probably the open circuit voltage, with no load on the panels. ? If we put a variable resistor across the panel, at infinite resistance we will see 22v across the resistor but no current, and thus the resistor is not dissipating any power. ?At zero resistance, we see several hundred milliamps but no voltage, so still the resistor is not dissipating any power. ?Somewhere in the middle, typically where the panel voltage has enough of a load to bring it down to 17 volts when in full sunlight, the resistor will be dissipating maximum power, where power in watts is volts * amps. ? That's called the Vmp, or Voltage at maximum power for that particular panel. ?Panels meant for 12v batteries usually have a Vmp of around 17 volts, since he 12v battery needs around 14v to be fully charged, and there are voltage drops through wires and charge controllers and such, and you seldom have full sunlight. ?At least here in Oregon.