Charging a car battery via a solar panel

[lightseeker2009]

And I just wanted to cut out any and all complexity. Just start her up and plug in what you want. It might even work out cheaper than batteries, solar panels and inverters. Granted, it is not as quiet at an inverter would have been, but it is not loud at all.

 

[SemiMan]

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[lightseeker2009]

100% agreed.
That is why we paid so much for a Honda. The generator will be run inside a shed, reducing a lot of the noise. My father must sleep with one of those machines CPAP, I believe. They draw a lot of power, hence the choice to rather get a generator. When our neighbor is there, I don't sleep there. His generator is the noisiest piece of junk ever built. He refuses to use the gas option for his fridge. So I will not be bothering him, rather the other way around

 

[Norm]

The generator will be run inside a shed

Carbon Monoxide ????

Norm

 

[lightseeker2009]

Nobody will be sleeping in there. That shed is just storage space.

But we found that running the generator outside made the noise even less than running it inside.
My only criticism with this particular generator is the small fuel tank. They should make it large enough to last at least 6 to 7 hours, ie a full night. It is not good enough for what we need it.

 

[engineeringnerd]

This isn't true anymore. New solar panels are very efficient. I have a small 1.5 watt solar panel that I bought from harbor freight that puts out around 17 volts even with heavy overcast. I added a toggle switch to cut it off at night to prevent reverse drain. You could use a diode to prevent reverse drain at night but it's gonna cost you about 3/10ths of a volt. I have a BMW X5 with tons of electronics - Nav, 360 degree radar, front and rear cameras each with dedicated screens, GPS tracker and other goodies. I'm in school most of the day so it gets a good 6 hours or so to charge which is plenty. The alternator picks up where the solar panel leaves off.

No offense to jasonck08, but with the exception of car batteries not liking to be discharge very much, which is true, most of the rest of the post is not accurate.

It is a common misnomer that hours of sunlight = hours of solar energy. That would only be true under perfect conditions and with perfect tracking of the solar panel with the sun. Realistically you will not have perfectly clear skies and you are likely going to have a stationary panel. Hopefully you will not have any shade. If you do, that changes everything!

40 amp hours on a 12V battery with some charging losses (higher losses at the top of the capacity and need to overcharge a bit), so count on needing about 530 - 550 watt hours or at least 45 amp hours.

You are in California so I assume camping close. You are inland, so fairly clear skies, but lets be cautious and say 4 peak sun hours/day. You may have a great week, clear skies, and no shade and get 8/day, but then again you may not so be cautious.

You are likely going to have a PWM controller so at best extracting about 70% of what will come out of the panel.

I would say at least a 30 watt panel. If you can get bigger, you are more likely to have your battery near peak capacity which lead acid batteries like.

In terms of a controller, get a Morningstar Sunsaver10 or other high quality unit. There are dirt cheap ones out there, but do you really want to use a $20 controller with a $100-$200 battery?


Semiman

 

[SemiMan]

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[Keisari]

This isn't true anymore. New solar panels are very efficient. I have a small 1.5 watt solar panel that I bought from harbor freight that puts out around 17 volts even with heavy overcast. I added a toggle switch to cut it off at night to prevent reverse drain. You could use a diode to prevent reverse drain at night but it's gonna cost you about 3/10ths of a volt. I have a BMW X5 with tons of electronics - Nav, 360 degree radar, front and rear cameras each with dedicated screens, GPS tracker and other goodies. I'm in school most of the day so it gets a good 6 hours or so to charge which is plenty. The alternator picks up where the solar panel leaves off.

Good for you, but your 1.5 watts nominal solar panel is probably not doing any good. Those actually produce way less in real life contitions. A 9 watt-hour charge is pretty much equal to zero in practical terms for a car battery when it has just been topped up by alternator power before and will be after the 6 hours. And again, there is no 9 watt-hour charge but much less in reality.

GPS, radars and other stuff that is used while driving does not drain the battery and does not affect the need for standby charging at all. Even the high current draw while driving is mostly not due to fancy electronics.

Additional top-up charging might sometimes be necessary(or at least useful) because of A) self-discharge and B) standby current caused by standby circuits including but not limited to car alarms, remote control etc. Even then it's only useful when the vehicle sits for weeks. 5-10 watts nominal solar panel would be the smallest size where I would start. If that sound like a lot, nothing is needed. Most problems in normal use will be due to faulty battery and or battery contacts, and will not be remedied through gimmicks such as the smallest available trickle charge solar panel. Some people might even install the panel in a way that it only operates while ignition is on(and does nothing as a result). Quoting 17 volt figures actually makes me wonder if this is exactly what you did.

I'm assuming the panel unit is something like this. http://www.biltema.se/sv/Bil---MC/Bil-tillbehor/Bil-el/12-Volt/Solcellspanel-31630/?artId=31630

Pre-heating by auxiliary heater and driving short distances in very cold weather increases the need for trickle or standby charging. At that point solar panels become useless and regularly plugging in will sometimes be necessary.

As SemiMan said, 17 volts is meaningless. You might want to measure the current and see there is virtually zero current even in sunlight.

If the vehicle was something more oldskool and simple than a new(ish) luxury BMW, a master power switch would be much more useful measure for preserving battery during long term parking than a tiny solar panel.