My vehicle's cigarette lighter outlet is limited to 10 amps output. If charging were limited to less than 10 amps, would solar panel -> charge controller -> cigarette lighter outlet work for charging a car battery. Additionally, would running loads (Ex: laptop or 12v fan) from another cigarette outlet create problems with the power going into the battery or potentially cause problems with the vehicle's electronics? Finally, would running loads only when the sun is out be hard on a car battery in the long term?
Why don't you connect it to the battery directly?
Do you know, that mechanical contact with metal springs inside lighter plugs is not very reliable? It has relatively big resistance and will be heating during flow of such high current for a relatively long time. What can potentially result in melting surrounded plastic panels and even cause fire if you leave it unattended.
On my car, the lighter (power) outlet is switched with the ignition, so it is not directly connected to the battery, so you would not be able to charge the battery through that connector if the ignition is off (and perhaps not if the ignition is on.)
Perhaps, on very old cars, the lighter outlet was powered at all times, so could possibly be used to charge the battery.
If the outlet is directly connected to the battery then it should be possible. Make sure to connect the solar panel through a charge controller configured for the chemistry of you car battery, so the charging will stop as soon as the battery is full. This protects the battery and the cars electronics.
Adding another load should be no problem, there is just less current charging to battery. E.g. you're charge controller delivers 5A and you add a fan pulling 2A, the charging current into the battery is only 3A. If the charge controller provides a switched output with battery under-voltage protection, it may be worth to connect the additional loads on there.
- Limit the charge current to the fused current of your outlet. (often 10 or 15 A)
- Use a good quality cigarette connector rated for the current. Also an additional either in the connector or in the line to the connector is a good idea.
- The best solution is still to connect the charge controller directly to the battery, because the voltage drop on the long cables to the battery can cause the controller to stop charging early. You can also connect a (fused!) pair of wires from the battery to a place where it is convenient to connect the charge controller with a high current connector, e.g. Anderson.
I run such a setup myself in my micro campervan with a 120W panel and a MPTT charge controlelr to keep food and drinks cool.
There's a lot in your question, so let's look at it in parts:
My vehicle's cigarette lighter outlet is limited to 10 amps output. If charging were limited to less than 10 amps, would solar panel -> charge controller -> cigarette lighter outlet work for charging a car battery.
Technically, yes. Realistically: not a great idea.
Generally speaking, cigarette lighter ports are bi-directional, that's how memory keepers work. When a shop needs to change your battery, they plug one of these into a cigarette lighter port, then take the battery out. This avoids the clocks resetting, etc. Yes, some are tied to ignition, some are not, the shop will use a permanent-power port for this, so they avoid that problem altogether.
However, 10A at 12V is a very small amount of energy (120W). Car batteries are rated in CCA / Cold Cranking Amps, which is the amperage it can deliver at 0°F for 30 seconds, while maintaining a voltage of at least 7.2V. Generally, a car battery has 48-or-so amp-hours (AH). That means that even at max capacity, your solar panel would take 4.8 hours to charge the car battery, assuming nothing failed before then.
On top of these issues, there is a lot of inherent resistance in a cigarette lighter port, the cabling, and the connectors. These are spring connectors with low connection quality, sometimes the inside of the port will even be corroded which would mean you have to twist-and-mess-with the connector. (This is often the case with old vehicles.)
But, would it work? Most likely.
Additionally, would running loads (Ex: laptop or 12v fan) from another cigarette outlet create problems with the power going into the battery or potentially cause problems with the vehicle's electronics?
So here's where your scenario starts to fall apart, in my opinion: why are you trying to charge through one port, and draw through another? Why not just use a solar panel and converter that delivery 120V for your laptop directly? Are you trying to use the laptop when there is no sun?
But, your question, would it cause problems with the vehicles electronics? Possibly.
What is effectively happening, is you won't actually draw load off of the battery most-likely, because the cigarette lighters are usually part of the same circuit. So, your solar panel will push power into the circuit, and you will pull power off that circuit for the laptop, then the remainder will deliver to the battery.
Assume your laptop has a 120W power-supply, this means at standard U.S. 120V, it draws 1A. That means at a car rate of 12V, it needs 10A. You need the entire output of one cigarette lighter. (And, mind you, if you have a laptop like mine, it needs 150W, so 1.25A at 120V, or 12.5A at 12V.) This will mean that your cigarette lighter circuit will be at capacity. This can cause more damage to the port (because of the same reasons for the above explanation), and can also cause brown-outs in your vehicle system if your battery is dead, and will also not charge the battery. On top of that, we haven't even talked about the inefficiencies of converting the 12V system to 120V for your laptop, which will cause a (small) loss of power output, so you probably get closer to a max of 0.95A with a high-efficiency system. Additionally, I only did math based on the rated draw if the laptop: I didn't calculate additional draw due to the inefficiencies of going 120V -> laptop voltage. (Do you really think your laptop takes 120V input? No, it's probably the standard 19.5V like my Dell.)
Car batteries are built to put out a lot of power for a very short time. They're built to start your car. They're not built for long-draw. Once the car is started, your battery stops working, and the alternator in your car starts recharging the battery. (This is why, when your alternator is bad, your car starts, and stops running within a half-hour: the battery isn't built to run the car, it's built to start the car.) All your current electrical loads run off of the alternator, not off the battery. Don't believe me? Leave your lights on overnight, and you'll see that it won't start. Your low-beam headlights usually use roughly 5A of power in a 12V circuit (generally, 60W or so), so if my math is right, your car battery won't last for more than 10 hours of that. Additionally, after 4 or so you probably lose enough power that the vehicle won't start.
So, don't do that. Don't do this, we'll talk about an alternative.
Finally, would running loads only when the sun is out be hard on a car battery in the long term?
This is irrelevant, we talked about it above.
So, what should you do?
If your goal is to avoid draining the car-battery to the point that it's dead, just get a dedicated portable solar-backup system built for 120V outputs. Efficiency will be higher, and you won't have to worry about the logistics of converting and charging and all that jazz (nor the math involved with calculating how long you have until your car battery dies). They'll have dedicated batteries, so that you can't drain the car battery by using your laptop. (Remember: 48AH, so with your laptop you have 4.8AH at max draw in your scenario before it's completely unusable.)
This should be fine assuming that you never exceed the wiring to the battery. This can be done by checking the fuse to the cigarette lighter which is usually 10 or 20A (vehicle dependent so check).
I own a jump starter that starts vehicles by going from cigarette lighter to cigarette lighter, so I know it's fine to charge or start cars this way.