Charging a car battery via a solar panel

lightseeker2009

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Hi everyone.I want to keep a car battery at my camping site. I will use it at most once a week and will tap at most 30 to 40 amps out of it.What is the minimum size solar panel that will put back this amount of power into the car battery over a period of 6 days?What controller will be needed?
 

jasonck08

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Hi everyone.I want to keep a car battery at my camping site. I will use it at most once a week and will tap at most 30 to 40 amps out of it.What is the minimum size solar panel that will put back this amount of power into the car battery over a period of 6 days?What controller will be needed?

First off, car batteries are NOT designed to be deep discharged. You will want a deep cycle marine battery if you want to pull 30-40Ah out of it. Car batteries are designed to be charged almost constantly, and very shallow partial cycles.

By 30-40 amps, I assume you mean amp hours?

If so, 30-40 * 12 = 360-480Whrs.

8hrs of sunlight * 6 days = 48hrs of sunlight

360/48 = 7.5W and 480/48 = 10W.

So assuming 100% Charge efficiency 7.5-10W would be enough. But you should probably go with 15-20W.

Some 12v solar panels can charge lead acid batteries directly, or you could get a charge controller (don't know specific model #'s).
 

jasonck08

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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

Uhhh... Thanks for the condescending response. The information I provided is accurate, yet you suggest disregarding everything. :( I did not say that hours of sunlight = hours of energy. Days during the summer are 14+ hours long in some places... ~8 peak hours is fairly common on a long summer day in some locations... http://www.pveducation.org It obviously depends on the environment. Where I live we get the 2nd most sunlight out of any city in the US.

Also I see you mentioned he is in California, but I don't see any information in his post for that. Where did you get that information from? Or are you confused, and saw my location, instead of his?

I only have a 14watt panel, but the guide says you should use an 7amp battery, so for a 60amp battery, get a 120watt solar panel minimum.

John.

I think that would be probably if you plan on using a lot of the batteries capacity every day. It sounds like the OP just needs a small panel to trickle charge the battery over the course of 6 days.
 
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Knight_Light

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I have used solar extensively over the years. And the one thing that I have learned is that you always get the biggest panel that you can afford and physically accommodate (within reason). Spend money on a good controller.

If you have the budget you can go for foldable panels, they really pack down small. I would recommend a 60 W foldable. Here are the specs for something like that.
•Operating Voltage: 15.4
•Wattage: 60
•Current: 3.6 amps
•Width (mm): 1207 unfolded; 336 folded
•Length (mm): 1308 unfolded; 178 folded
•Width (in): 47.5 unfolded; 13.25 folded
•Length (in): 51.5 unfolded 7 folded
•Weight (kg): 1.45 kg
•Weight (lb): 3.19 lbs
 

AnAppleSnail

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(Folded panel)
•Weight (kg): 1.45 kg
•Weight (lb): 3.19 lbs

Urgh. I was just wondering why there is no folded and unfolded weight. Not enough coffee in the world...

Hours of sun in a campsite are usually much less than on a house rooftop. Most solar guides tend to assume that you are able to place the solar panels away from or above trees. I know that in dappled shade, my solar yard lights have about 1/10th the runtime of the ones in full sun. I do not know how things are in California, but a forested area in the mountains may have 12 (Peaktop) or 6 (Valley bottom) hours of sun in summer. This will be further cut by shade and imperfect panel aiming.

For what it's worth, a bigger solar panel (That you can afford, and that won't be stolen) will almost always make you happier than a smaller one. And if a $200 solar panel will make you 10x happier than a $100, maybe it's worth that jump. However, plan on a coulombic efficiency (Energy in vs. Energy Stored) of about 1.6 on the lead-acid battery. If your charge controller is 85% efficient, then:

"I want (40amp*hours x 12v)=500W*h per week."
"I need to put (500W*h * 1.6) = 800W*h in per week, or 120W*h per day."
"I need to gather (120/0.85) = 150 W*h per day for my charge controller."
"For my region I need to de-rate the solar panel's output by (x)."
"My good, strong sunlight there is (2, 4, 6, 8, 10) hours per day when I'll use this. That gives me the rate the panel should gather energy."
 

ChrisGarrett

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60w of goodness and I'm in Miami, so I get a lot of solar hours down here and I agree that most people assume too many S.H. when they're thinking things through. Unless you want to babysit your rig and I'm in a condo, bigger will be better because it will be faster, IMO. Together, those two 30w with hardware weigh in at ~2kg/11#, but moving up to the 40w panels, things start adding up, when dealing with rigids.

You can find the milspec, foldable 62w PowerFilms, but they're over $800 the last time I checked this past August. Rigids are heavier, but more durable, have longer warranties and those pictured above are mono-crystaline, so a bit more efficient than the polys and CIG/Thin Films portables.

Also, as semiman mentions, you have controller and charger inefficiencies, so you can never assume 100%. I don't think that my Morningstar SS-10L has a 30% loss, but I can see the controller and any of my battery chargers getting close to that number. Figure on 75% useful output when calculating this stuff and you should be close to reality.

Doing a SLA/AGM, you only have to deal with say...half of that number, becuase that Morningstar SS-10L IS a battery charger, but there's still going to be less than the full pie going in.

Also, the smaller controllers have certain limits to how big a battery they can charge and I don't think that they're meant to charge up a totally dead car-type battery, if I remember the manual correctly, so keep that thought in mind when putting something together.

Anyhow, it was a fun project to do and I think something like my rig, or a nice size foldable panel, are necessities where I live and I'll be waiting during this hurricane season, but I'll be ready!

Chris
 

Knight_Light

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60w of goodness and I'm in Miami, so I get a lot of solar hours down here and I agree that most people assume too many S.H. when they're thinking things through. Unless you want to babysit your rig and I'm in a condo, bigger will be better because it will be faster, IMO. Together, those two 30w with hardware weigh in at ~2kg/11#, but moving up to the 40w panels, things start adding up, when dealing with rigids.

Let me start by saying that you have a sweet set up. :thumbsup:

You can find the milspec, foldable 62w PowerFilms, but they're over $800 the last time I checked this past August. Rigids are heavier, but more durable, have longer warranties and those pictured above are mono-crystaline, so a bit more efficient than the polys and CIG/Thin Films portables.

I agree with your more efficient comment but I don't really agree with durability. In my opinion the PowerFilms foldable will be a lot more durable if you are constantly moving your set up around. I have seen rigid panels get damaged in transportation. Now if you go to the PowerFilms rollable then you are totally dealing with an almost indestructible product when comparing to rigid mono-crystaline panels.

Also, the smaller controllers have certain limits to how big a battery they can charge and I don't think that they're meant to charge up a totally dead car-type battery, if I remember the manual correctly, so keep that thought in mind when putting something together.

Smaller charge controllers won't be able to handle large current but as long as you stay within their specifications they will theoretically be able to charge any capacity 12V battery. Plus don't forget that Pb doesn't require a lot of current to charge. Now they may miss termination on a car battery but you can always follow the progress with a meter.
 

ChrisGarrett

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I agree with your more efficient comment but I don't really agree with durability. In my opinion the PowerFilms foldable will be a lot more durable if you are constantly moving your set up around. I have seen rigid panels get damaged in transportation. Now if you go to the PowerFilms rollable then you are totally dealing with an almost indestructible product when comparing to rigid mono-crystaline panels.

I did extensive research on the rollable/foldable solar chargers last August: Brunton, Global Solar Sunique, Goal Zero and Powerfilm. All had either 3 year, or 2 year warranties, in the case of Brunton (IIRC).

The reason for this is that the thin wiring between the cells are not all that durable over the constant folding/rolling cycling that they encounter and they break sooner, rather than later.

Rigid panels have glass and glass breaks, but their warranties are usually 10 yrs at 90% rated output and 25 years at 80%.

Since I only have those two rigid panels, I can't comment on the longevity of the portables, but the warranties aren't all that long, if you look into them and that's done for valid reasons, I"d assume?

Chris
 

IonicBond

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Hi everyone.I want to keep a car battery at my camping site.
Unless you are rich and are willing to trash an SLI (starter, lights, ignition) battery very fast with those deep discharges, then you'll need at least a hybrid dual-purpose marine battery. You don't mention if you are actually intending to start a car with it when you are done. Are you?

I will use it at most once a week and will tap at most 30 to 40 amps out of it.
That means that you'll have to have at least DOUBLE that in overall capacity, (50% SOC after discharge - any further damages the battery, and some gear will not run well under 12v when you have reached that 50% SOC). Get the 20-hour discharge rate. If all you have is the "RC" rating, then multiply that by .6 to get in the ballpark at 80ah or more.

What is the minimum size solar panel that will put back this amount of power into the car battery over a period of 6 days?
Bad idea using maintenance charge levels of current to actually charge, especially longer than a period of 2 days max. All the time that you are below a full charge, the battery will be sulfating almost as much as it is getting recharged. In addition, a deep discharge really means that you need at a minimum C/12 to about C/8 to *really* charge, and not just put a superficial surface charge on the battery with a dinky little panel or maintainer.

As for recharging fast enough to avoid sulfation (the natural process that all LA batteries do when not fully charged), plan on no more than 2 days to get it done from a 50% SOC. Otherwise you'll be "walking down" the capacity faster than you know it by not providing a decent charge level. (stratification, along with sulfation plays a part here also if we're still talking a wet-cell, and not something like a sealed AGM.

How FAST are you trying to pull that 30-40 amps? A minute - an hour, a few days? Also what is actually your load and how are you powering it? Are you running nothing but 12v gear, or are you using inverters (msw or psw?) and are those loads inductive motors that might trip your inverters no matter what?

Realistically, we also don't know your location which will help determine your solar-insolation hours. These are different from just visible daylight hours. A simple example is that solar insolation means that the sun is strong enough to actually do some good and get the rated output from your panels - typically between the hours of 10a to 2pm - but this varies depending on your location.

Search for "solar insolation chart". Some governmental charts may be hard to read, whereas some solar vendors have easier to read charts where you look up your city, and can find the high summer, low winter, and average hours. If you want to give yourself some headroom, use the low winter values.

Let's assume you have 4 hours of solar insolation in the summer. With a 2-day maximum, that means 8 hours available for charging. To replenish 40ah, at first one might think they would need an 80watt 12vdc nominal panel that puts out about 4.5a at best. BUT, chargng is not totally efficient, so lets add 15% to that. Somewhere between a 100 to 120 watt panel with a 10a Morningstar pwm controller on an 80ah approx hybrid marine battery would be my first starting point assuming that much solar insolation is actually available for a 2 day charge.
 
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Knight_Light

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I did extensive research on the rollable/foldable solar chargers last August: Brunton, Global Solar Sunique, Goal Zero and Powerfilm. All had either 3 year, or 2 year warranties, in the case of Brunton (IIRC).

The reason for this is that the thin wiring between the cells are not all that durable over the constant folding/rolling cycling that they encounter and they break sooner, rather than later.

Rigid panels have glass and glass breaks, but their warranties are usually 10 yrs at 90% rated output and 25 years at 80%.

Since I only have those two rigid panels, I can't comment on the longevity of the portables, but the warranties aren't all that long, if you look into them and that's done for valid reasons, I"d assume?

Chris
Chris not only am I familiar with the companies that you listed but I have actually used products from most of them. These foldable panels are not as flimsy as you may think. And with a little bit of tinkering you can make them a lot stronger. Now if you are talking about the rollable PowerFilm those are encased in some sort of material (almost like a laminate/epoxy)and are very rugged. I've seen numerous holes punched right through them and they still work.

And you are comparing apples and oranges. Without a doubt if you have a stationary nonremovable set up, the rigid panels are the way to go. But if you have a mobile solution then they are not as durable as you think. And I guarantee you if you broke the panels by stepping on them dropping them or whatever that 20 year warranty would not cover it.
 

ChrisGarrett

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Chris not only am I familiar with the companies that you listed but I have actually used products from most of them. These foldable panels are not as flimsy as you may think. And with a little bit of tinkering you can make them a lot stronger. Now if you are talking about the rollable PowerFilm those are encased in some sort of material (almost like a laminate/epoxy)and are very rugged. I've seen numerous holes punched right through them and they still work.

And you are comparing apples and oranges. Without a doubt if you have a stationary nonremovable set up, the rigid panels are the way to go. But if you have a mobile solution then they are not as durable as you think. And I guarantee you if you broke the panels by stepping on them dropping them or whatever that 20 year warranty would not cover it.

I specifically mentioned that the warranties on all four brands are three years or less. That tells me something, doesn't it? Sure, people can mod things left and right and improve on them, no doubt, but the foldable panels don't have the warranty that the rigids have, if both are used normally and as intended.

Nobody's expecting me to get warranty service on my rigids if I drop one of them on my way down the stairs.

Those warranties are kind of short and the companies are doing that for a reason, don't you think?

Whether the foldables end up working well past those three (or two) years, I can't say, as I went with the rigid setup, but those warranties were part of the reason I didn't pull the trigger on a PF 30w panel...it was just too short for me at that price.

Now, to be fair to myself, I wouldn't be folding and unfolding it all that much, so those wires wouldn't get abused, as they would if I were out in the field using the panel everyday.

Chris
 

Knight_Light

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I specifically mentioned that the warranties on all four brands are three years or less. That tells me something, doesn't it? Sure, people can mod things left and right and improve on them, no doubt, but the foldable panels don't have the warranty that the rigids have, if both are used normally and as intended.

The key phrase here is "used normally and as intended", but the rigid panels have a different use and intention then the foldable panels. So you are comparing apples and oranges.

Nobody's expecting me to get warranty service on my rigids if I drop one of them on my way down the stairs.
So then you have to ask yourself a question. If you use them under the intended conditions that the foldable and rollup panels would be utilized how long would they last?

Those warranties are kind of short and the companies are doing that for a reason, don't you think?

Because their intended purpose is a harsher environment therefore their lifetime will probably be shorter. Again apples and oranges
 

SemiMan

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60w of goodness and I'm in Miami, so I get a lot of solar hours down here and I agree that most people assume too many S.H. when they're thinking things through. Unless you want to babysit your rig and I'm in a condo, bigger will be better because it will be faster, IMO. Together, those two 30w with hardware weigh in at ~2kg/11#, but moving up to the 40w panels, things start adding up, when dealing with rigids.

You can find the milspec, foldable 62w PowerFilms, but they're over $800 the last time I checked this past August. Rigids are heavier, but more durable, have longer warranties and those pictured above are mono-crystaline, so a bit more efficient than the polys and CIG/Thin Films portables.

Also, as semiman mentions, you have controller and charger inefficiencies, so you can never assume 100%. I don't think that my Morningstar SS-10L has a 30% loss, but I can see the controller and any of my battery chargers getting close to that number. Figure on 75% useful output when calculating this stuff and you should be close to reality.

Doing a SLA/AGM, you only have to deal with say...half of that number, becuase that Morningstar SS-10L IS a battery charger, but there's still going to be less than the full pie going in.

Also, the smaller controllers have certain limits to how big a battery they can charge and I don't think that they're meant to charge up a totally dead car-type battery, if I remember the manual correctly, so keep that thought in mind when putting something together.

Anyhow, it was a fun project to do and I think something like my rig, or a nice size foldable panel, are necessities where I live and I'll be waiting during this hurricane season, but I'll be ready!

Chris


30% loss that I mentioned is due to the fact of voltage mismatch between the solar panel and the battery, a problem with all PWM controllers. Unavoidable without going with a MPPT controller.
 

SemiMan

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Uhhh... Thanks for the condescending response. The information I provided is accurate, yet you suggest disregarding everything. :( I did not say that hours of sunlight = hours of energy. Days during the summer are 14+ hours long in some places... ~8 peak hours is fairly common on a long summer day in some locations... http://www.pveducation.org It obviously depends on the environment. Where I live we get the 2nd most sunlight out of any city in the US.

Also I see you mentioned he is in California, but I don't see any information in his post for that. Where did you get that information from? Or are you confused, and saw my location, instead of his?



I think that would be probably if you plan on using a lot of the batteries capacity every day. It sounds like the OP just needs a small panel to trickle charge the battery over the course of 6 days.



Sorry, but your response was not accurate, even for your location when one consider that not every day or week is perfect.

You can point me to PVEducation.org but I have years of doing this including resource modelling in must work situations not to mention having designed a high performance MPPT controller and putting up my systems in many locations.

If he went with your suggestion, the OP would have ended up with an undercharge and likely damaged battery.
 
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