Trickle Charge idea for Winter.

[MrAl]

Hello there,

I was going to use solar trickle charging too using a solar panel mounted in the window and a direct but fused line to the battery. Unfortunately it would take a pretty big panel. Here's what happened to that idea.

First, probably the min current should be 100ma. That is because the charge efficiency of a lead acid battery isnt very good, maybe around 70 percent with a good charge current, and since charge acceptance goes down with current that means it could be much lower than that. Let's say 25 percent just to have something to work with. That leaves us at around 17.5 percent but lets round way up to 20 percent to make this simpler and we can always adjust later.

Since 100ma at 20 percent means 500ma would be required, we'd need a 500ma panel, which means a 6 watt panel, ideally.

The second problem that comes in is the angle of the sun vs the panel face. Large systems make up for this by rotating the panel both for the angle of the sun and the second angle that changes as the seasons pass. So that's a two angle rotation platform. Assuming we dont want to have to rotate (i dont and that is much more complicated to do) then the current drops by the cosine of the angle:
I2=I1*cos(Th)

where Th is Theta which is the angle, where 0 (zero) degrees is max where the panel is perfectly perpendicular to the suns rays.

So what happens with a fixed panel is we get a variable current output. If we start from 500ma at say 9am then at 12 noon we have a different angle and that means less current. If we assume we have 8 hours of sun each day that points in a direction that is consistent with the panel face, then integrating for the average over -45 to +45 degrees we get about 90 percent of full current on average, which isnt that bad really, so if we use a panel that can put out 556ma in direct sunlight we might see something work good.

But now there is one more problem, and that is the fact that the sun is not out for 24 hours per day it's only out for our assumed 8 hours per day. That means we are only charging for 1/4 of the time. That means we need a panel with 4 times the output current:
556*4= approximately 2.2 amps.

which means we need a panel of about 26 watts.

Now there is a chance that we might get something useful with a lower wattage, but i did not get to try it yet.
I have a 10 watt panel that i did not connect yet. Another problem is that snow coverage must be cleaned off right away in the morning so that i can get the morning sun energy as that's the angle that is good for my location and car orientation.

So in the end a 26 watt panel should get you there, but i dont have that size yet so i didnt do it yet.

[mecheng]

Hello there,

I was going to use solar trickle charging too using a solar panel mounted in the window and a direct but fused line to the battery. Unfortunately it would take a pretty big panel. Here's what happened to that idea.

First, probably the min current should be 100ma. That is because the charge efficiency of a lead acid battery isnt very good, maybe around 70 percent with a good charge current, and since charge acceptance goes down with current that means it could be much lower than that. Let's say 25 percent just to have something to work with. That leaves us at around 17.5 percent but lets round way up to 20 percent to make this simpler and we can always adjust later.

Since 100ma at 20 percent means 500ma would be required, we'd need a 500ma panel, which means a 6 watt panel, ideally.

The second problem that comes in is the angle of the sun vs the panel face. Large systems make up for this by rotating the panel both for the angle of the sun and the second angle that changes as the seasons pass. So that's a two angle rotation platform. Assuming we dont want to have to rotate (i dont and that is much more complicated to do) then the current drops by the cosine of the angle:
I2=I1*cos(Th)

where Th is Theta which is the angle, where 0 (zero) degrees is max where the panel is perfectly perpendicular to the suns rays.

So what happens with a fixed panel is we get a variable current output. If we start from 500ma at say 9am then at 12 noon we have a different angle and that means less current. If we assume we have 8 hours of sun each day that points in a direction that is consistent with the panel face, then integrating for the average over -45 to +45 degrees we get about 90 percent of full current on average, which isnt that bad really, so if we use a panel that can put out 556ma in direct sunlight we might see something work good.

But now there is one more problem, and that is the fact that the sun is not out for 24 hours per day it's only out for our assumed 8 hours per day. That means we are only charging for 1/4 of the time. That means we need a panel with 4 times the output current:
556*4= approximately 2.2 amps.

which means we need a panel of about 26 watts.

Now there is a chance that we might get something useful with a lower wattage, but i did not get to try it yet.
I have a 10 watt panel that i did not connect yet. Another problem is that snow coverage must be cleaned off right away in the morning so that i can get the morning sun energy as that's the angle that is good for my location and car orientation.

So in the end a 26 watt panel should get you there, but i dont have that size yet so i didnt do it yet.

All valid points, however you forget one fact. Any solar trickle charger is better than none at all. I've noticed it helps the car start easier, not quite as good as my plug in charger but I don't have to worry about running wires through the doors, opening the hood, running extensions and theft.

[wizechatmgr]

Don't worry about proper solar system design, there is minimal drain here. The idea is to just attempt to counter the draw and self discharge in between runs.

We're not talking 2-7 KW here on an off-grid system running a full residence

Personally my goal would be ~90% SOC total after 21 days. Enough to get her started up and the alternator can make up any difference.

Anything over ~15W and I'd suggest running a MPPT charge controller. You don't want a couple of great days starting to boil/cook that battery by over-charging.

[MrAl]

Hello there,

I was going to use solar trickle charging too using a solar panel mounted in the window and a direct but fused line to the battery. Unfortunately it would take a pretty big panel. Here's what happened to that idea.

First, probably the min current should be 100ma. That is because the charge efficiency of a lead acid battery isnt very good, maybe around 70 percent with a good charge current, and since charge acceptance goes down with current that means it could be much lower than that. Let's say 25 percent just to have something to work with. That leaves us at around 17.5 percent but lets round way up to 20 percent to make this simpler and we can always adjust later.

Since 100ma at 20 percent means 500ma would be required, we'd need a 500ma panel, which means a 6 watt panel, ideally.

The second problem that comes in is the angle of the sun vs the panel face. Large systems make up for this by rotating the panel both for the angle of the sun and the second angle that changes as the seasons pass. So that's a two angle rotation platform. Assuming we dont want to have to rotate (i dont and that is much more complicated to do) then the current drops by the cosine of the angle:
I2=I1*cos(Th)

where Th is Theta which is the angle, where 0 (zero) degrees is max where the panel is perfectly perpendicular to the suns rays.

So what happens with a fixed panel is we get a variable current output. If we start from 500ma at say 9am then at 12 noon we have a different angle and that means less current. If we assume we have 8 hours of sun each day that points in a direction that is consistent with the panel face, then integrating for the average over -45 to +45 degrees we get about 90 percent of full current on average, which isnt that bad really, so if we use a panel that can put out 556ma in direct sunlight we might see something work good.

But now there is one more problem, and that is the fact that the sun is not out for 24 hours per day it's only out for our assumed 8 hours per day. That means we are only charging for 1/4 of the time. That means we need a panel with 4 times the output current:
556*4= approximately 2.2 amps.

which means we need a panel of about 26 watts.

Now there is a chance that we might get something useful with a lower wattage, but i did not get to try it yet.
I have a 10 watt panel that i did not connect yet. Another problem is that snow coverage must be cleaned off right away in the morning so that i can get the morning sun energy as that's the angle that is good for my location and car orientation.

So in the end a 26 watt panel should get you there, but i dont have that size yet so i didnt do it yet.

All valid points, however you forget one fact. Any solar trickle charger is better than none at all. I've noticed it helps the car start easier, not quite as good as my plug in charger but I don't have to worry about running wires through the doors, opening the hood, running extensions and theft.

Hi,

Did i?

Now there is a chance that we might get something useful with a lower wattage

I guess i forgot to forget that we might get something useful with a lower wattage

[RickHaleParker]

The load, in this case the battery, determines the current not the Voltage source.

I suggest you use a buck-boost converter to buck-boost the voltage of the solar panels to a steady float voltage of 12.8V. This way the voltages will not cook the battery and the battery will draw whatever current it needs up to the maximum power that the solar cells can deliver at that time or the maximum current the buck-boost converter can deliver, whichever comes first .

A buck-boost converter will provide the regulation you need. You can get adjustable buck-boost converters suitable for this application on eBay dirt cheap.

One idea I have is to not use one big panel. Instead install a array of smaller cells on the trim between the dash and windshield. Wire the array to a Buck-boost converter then to the battery. One could possibly do a neat job that looks like it was a factory option.

[wizechatmgr]

The load, in this case the battery, determines the current not the Voltage source.

I suggest you use a buck-boost converter to buck-boost the voltage of the solar panels to a steady float voltage of 12.8V. This way the voltages will not cook the battery and the battery will draw whatever current it needs up to the maximum power that the solar cells can deliver at that time or the maximum current the buck-boost converter can deliver, whichever comes first .

A buck-boost converter will provide the regulation you need. You can get adjustable buck-boost converters suitable for this application on eBay dirt cheap.

One idea I have is to not use one big panel. Instead install a array of smaller cells on the trim between the dash and windshield. Wire the array to a Buck-boost converter then to the battery. One could possibly do a neat job that looks like it was a factory option.

If someone does all that work, they my as well start selling them... There is more than a handful of folks that could likely use them.

[wizechatmgr]

Who will be the first to use one of those solar cell mats you can cut to shape and put it over their dash pad?

Where is a patent attorney when you need one?

[cn90]

- I have used this very same solar charger on my previous 1983 BMW 735i. It worked great b/c I left the solar panel on the dash. During the winter, it still keeps the battery in good shape. Without the solar charger, the battery became dead in about 3-4 weeks. With the solar charger, one month later, the car fired up without any issues.

- The BMW cigarette socket is HOT all the time, so plugging the Solar Charger in is no problem.

- The Volvo cigarette socket is HOT ONLY when key is on. Glad you found the solution.

- As others said, you can buy a cigarette socket separately and run the wires through the firewall ---> battery.

[abscate]

Don't buy cheap electronics on eBay thinking they will regulate and be reliable. Failure rates of this stuff range from 30-50% per year. Keep your solar panels small enough to eliminate overcharge by design, not cheap dreck.

If you want 5 watts of charge, buy 20 watts of panels to deliver

[RickHaleParker]

Don't buy cheap electronics on eBay thinking they will regulate and be reliable. Failure rates of this stuff range from 30-50% per year. Keep your solar panels small enough to eliminate overcharge by design, not cheap dreck.

The Chinese have gotten good at building electronics. A buck-boost is so easy, any electronics company that cannot build a good one would be laughed out of the market. That is why they are so low cost, nobody has an upper hand and anybody can make a good one.