Solar Panel blows globes. Battery doesn't

Do you have a 12v regulator on the solar panel? Unregulated the panel voltage can be about 20v, too high for the bulb?

Volts are pressure, 12v to 20 volts is a lot, nearly double .Try blowing up a balloon to max then doubling the pressure. That's what you just did to the globe . Amps has no relevance here.

Bill D
A globe only accepts the amount of current the globe can use while on 12v, therefore, 55w @12v will be drawing around/close to 4.5 amps depending on how hot the filament is actually running. Because the battery voltage doesn't rise the globe stays alive as designed to do. ie, the globe is a resistance and resists current flow to 4.5amp at a 12v "pressure"
A solar panel in good sun with an 8amp ability at around 18v , the 18v being 50%more voltage than the battery and globe combination means the globe will have around 80w being forced through it and a current of 7 to 8 amps instead of 4.5 amps. That will fry the globe, the same as an alternator will do if the regulator of the vehicle goes way too high in voltage. Someone mentioned a regulator, great idea, BUT, solar regulators are designed to have a battery on the wires and they regulate to that. The globe is then just a draw on the circuit. Just a solar panel to regulator to globe may not end well as the regulator doesn't know what is what at it's output side.

Have a look at OHMS LAW as it is the basis for all electrical flows and wattage in circuits. BASICS- 1v will push 1 amp through 1 Ohm resistance.

The graphic demonstrates the dynamics of a solar cell. The pannels that we commonly use for charging our 12 V batteries have 36 cells, so multiply the voltage on the bottom line by 36 to reflect how a full "12 V panel" works. A vehicle alternator works roughly the same way, people don't realise that as we rarely see an alternator without a working regulator on it. The curve for an alternator is different from the solar cell but the big difference is the capability of the two. An alternator is generally capable of supplying very much more power than we use in any item we power with it. The solar panels we use often are not capable of powering the total load we use of a night time if we switched it on during daylight hours.

Looking at the curve below you will see that when no current is being drawn from the cell its voltage is zero (the green line.) At that time the power is zero (the red line.) When we start to put small loads on the cell you will see the voltage starting to fall as the power starts to rise. As we increase the power we draw from the cell the voltage continues falling as the power increases. This continues until we reach the maximum power point (MPP.) If we continue increasing the load on the cell you will notice that the voltage commences dropping rapidly and the power curve drops away as the panel is not capable of delivering full power.

When we mount panels on our vans we use controllers to ensure those panels do not overcharge the battery (or pop your incandescent globes.) The simple ones just regulate the current so that the system supplies a suitable charging voltage to the battery. In our case, the cell would be operating ar around the 0.4 V point (depending upon the exact absorption we choose to set in the regulator/controller.) You can read the value of current the cell supplies from the curve - in the ideal condition. However we can not achieve this as out panels are not pointing directly at the sun and also the cell performance drops off as they are heated up by the sun. These are called PWM (pulse width modulation) controllers. You will see from the red curve that when using these controllers we cannot achieve the maximum power that the panels manufacturers say their panels are capable of generating.

There are a second type of controller that manipulates the voltage and current relationships so that operate the panel at its MPP. These are called MPP controllers. They work well in our household grid connect systems and the panels can produce their maximum power potential. However, they do not provide much power gain in our caravan systems and in many cases they provide less power than PWM controllers.

Simply too much voltage!

Thanks to all, especially Nomadic Navara and RMD for that detailed discussion. And to Shane and Gary for summarising it all, ie too many volts forcing too much current through the globe. A great example of how forums should work, with the detailed replies giving us all food for thought, and nobody slinging off at the silly old bloke who asked the question in the first place, ie me. Bill.