# experimenting charging a car battery with solar



## budgetprepp-n (Apr 7, 2013)

I just got done doing something with my old battery. So I put a load on it and ran it way down (like 11.o volts)

How long should it take to charge it back up till the controller starts flashing to indicate the battery is full?

I have two ways of doing this I can use a 235 watt solar panel or two 235 watt panels. 

Or I could even move everything back down to the other end of the porch and use a 100 watt panel. 

I have heard some people talk about how long it should take but it seams like no one knows for sure. 

I am going to wait until we have a sunny day 

Want to find out? You guys tell me which panels do you want used to see how long it takes?


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## 8301 (Nov 29, 2014)

What is the VOC for the panel? What is the amp hr rating (20 hr discharge rate) for the battery?

Assuming they are 18v Voc with a PMW controller they will put about 13-15 amps into the battery in strong sunlight until the battery is about 85% full. After that the charging rate will gradually reduce as the battery gets fuller. 

Personally I'd like to see the single 235 watt panel buy without knowing the battery size and Voc the test won't be very useful.


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## budgetprepp-n (Apr 7, 2013)

FoolAmI said:


> What is the VOC for the panel? What is the amp hr rating (20 hr discharge rate) for the battery?
> 
> Assuming they are 18v Voc with a PMW controller they will put about 13-15 amps into the battery in strong sunlight until the battery is about 85% full. After that the charging rate will gradually reduce as the battery gets fuller.
> 
> Personally I'd like to see the single 235 watt panel buy without knowing the battery size and Voc the test won't be very useful.


 Well then these test would have been of no use to anyone then. I don't have all the information that you would need to 
find out in general how long it takes to recharge a car battery with solar power. I was just going to do it just to see.
What was I thinking?


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## Smitty901 (Nov 16, 2012)

We use to mount a small solar charger/maintainer on the hood of the M996 it was not but maybe 6x3 inch it seemed to help when parked long term The m996 was a 2 volt.


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## 8301 (Nov 29, 2014)

ok, lets assume a 100 watt panel that is rated at about 18v (Voc). Let's use a 12v 80 amp hr battery that is half discharged (50% DOD) so the battery needs about 40 amps at 14v+ to be recharged. Keep in mind that with a PMW controller excess voltage is not fed to the battery.

100 watts x .8 derate so the panel after flowing through the charge controller is capable of putting out 80 watts of electricity at about 18v but we only can use about 14v of that voltage so... 14 / 18 = about 77% of the electricity will actually be at a voltage the battery can accept if you are using a PMW charge controller. So 80 watts X .77 = 62 watts going into the battery. 
side note: if you were using a MPPT type controller you'd get use of almost all of that excess voltage raising that 77% number closer to 98% for faster battery charging.

62 watts / 12v = 5.2 amps current at 12 to 14v

We needed 40 amps to top off that battery so.... 40 / 5.2 = 7.7 hours of strong sunlight will get the battery close to a full charge. The reason it's only close and not fully charged is because in the last 10% of charging the internal resistance of the battery increases meaning you'll need a bit more electricity to fully top off the battery so... I'd guess about 8 1/2 hours of strong sunshine.

BudgetPrep, I really appreciate some of the experiments you do on this site but a lot of people here have little 13 amp motorcycle batteries and a few may even have forklift batteries. Panel and charge controller efficiencies are also very important as well as having a good idea about exactly how deeply the battery is discharged and it's size if the experiment is to provide useful information.


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## dwight55 (Nov 9, 2012)

Actually there are so many variables, . . . including dirty connection, . . . battery size, . . . battery condition, . . . panel capabilities, . . . 

Best way is plug it in and start up a timer, . . . at the end you will know.

May God bless,
Dwight


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## budgetprepp-n (Apr 7, 2013)

dwight55 said:


> Actually there are so many variables, . . . including dirty connection, . . . battery size, . . . battery condition, . . . panel capabilities, . . .
> 
> Best way is plug it in and start up a timer, . . . at the end you will know.
> 
> ...


Yes just do it and see then you know,, Sometimes what works out on paper doesn't do so good in the real world


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## 8301 (Nov 29, 2014)

budgetprepp-n said:


> Yes just do it and see then you know,, Sometimes what works out on paper doesn't do so good in the real world


true, but I prefer to understand how it works,,, not just if it works or not with undefined perimeters. That way I have realistic expectations and some knowledge about how to modify or repair the system.

Like you I actually tinker and try variables to my solar system but I prefer a slightly more scientific method. Solar parts and batteries are too expensive to just buy and hope they work well together.


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## inceptor (Nov 19, 2012)

To get the info you need about the battery, find the manufactures website and look up your particular model. You should be able to find the spec sheet and that will have the info you require.


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## budgetprepp-n (Apr 7, 2013)

Look ,,I was just thinking as a prepper would post SHTF you got a run of the mill car battery
And a working solar panel -no specs- just junk you can scarf. In general how long to charge
an old battery back up? 

"I prefer to understand how it works,,, not just if it works or not with undefined perimeters."

After it hits the fan if it works will matter understood or not. "undefined perimeters" wont exist. 

Lots of people will get things going without understanding how they work. Seeing what someone else 
did with common stuff may be a big help to someone some day. 

I cook some of my food in a microwave I don't know how it works and I don't care just so long as it works.


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## inceptor (Nov 19, 2012)

budgetprepp-n said:


> Look ,,I was just thinking as a prepper would post SHTF you got a run of the mill car battery
> And a working solar panel -no specs- just junk you can scarf. In general how long to charge
> an old battery back up?
> 
> ...


So run some tests that way and let us know what you find. I have several batteries, a couple of AGM's and a couple of car batteries I use for backup for my ham gear. I don't have a solar set up yet but I am interested in one so I will look forward to what you find out.


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## 8301 (Nov 29, 2014)

budgetprepp-n said:


> I cook some of my food in a microwave I don't know how it works and I don't care just so long as it works.


Microwaves excite the molecules creating heat. Water molecules are especially active in microwaves, that's why super dry foods don't heat as well in a microwave. But microwaves can only penetrate so deep. That's why cooking a big chunk of frozen meat on full power can leave the center still frozen. Instead cooking on a lower power allows time for the heat in the outer inch of the meat to travel in through convection warming the center.

Yes, you can drag out an old car battery that's half drained, hook a 100 watt solar panel to it and in about 8 1/2 hours of strong sunlight the battery will be charged. Give it another 8 hours and the battery will be overcharged and possibly damaged.


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## Mad Trapper (Feb 12, 2014)

If you have a deep cycle 12V and a 40W /3A panel it can be charged without anything else, except a multimeter to test charge level. You can't leave it overnight or let it overcharge.

Done this for 6V DC in series used in game cameras, And starting/deep cycle 12V for cars.


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## dwight55 (Nov 9, 2012)

I'm using a couple of 18 AH batteries for a project for a church.

The battery would not discharge below 11.25 volts, . . . so I had to bring it up from there.

Took about 4 hours in pretty good sunlight with a 12 Volt / 100 watt panel.

I'm thinking I could recharge a regular car battery in probably 6 hours, . . . at least enough for it to try and start the vehicle.

May God bless,
Dwight


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## budgetprepp-n (Apr 7, 2013)

John Galt said:


> Microwaves excite the molecules creating heat. Water molecules are especially active in microwaves, that's why super dry foods don't heat as well in a microwave. But microwaves can only penetrate so deep. That's why cooking a big chunk of frozen meat on full power can leave the center still frozen. Instead cooking on a lower power allows time for the heat in the outer inch of the meat to travel in through convection warming the center.
> 
> Yes, you can drag out an old car battery that's half drained, hook a 100 watt solar panel to it and in about 8 1/2 hours of strong sunlight the battery will be charged. Give it another 8 hours and the battery will be overcharged and possibly damaged.


I'm going to use a charge controller so it can't over charge


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## budgetprepp-n (Apr 7, 2013)

OK I had the battery down to 9.5 volts and I hooked it up to two 245 watt panels to see how long it takes to charge up 
hooked it up at noon


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## dwight55 (Nov 9, 2012)

You DO want to make sure you have a charge controller............

OR, . . . what would you like served with your fried battery???

May God bless,
Dwight


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## 8301 (Nov 29, 2014)

FYI If the battery is down to 9.5v it may take a charge but not hold it. Taking a battery below 10.5 - 11v is extremely hard on the battery and will often make a margional battery fail. 

Let us know the results!


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## Mad Trapper (Feb 12, 2014)

Anybody try just using an old car voltage regulator instead of a charge controller? Hard to find in junkyards these days


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## dwight55 (Nov 9, 2012)

Mad Trapper said:


> Anybody try just using an old car voltage regulator instead of a charge controller? Hard to find in junkyards these days


They're built in to the alternator now, . . . bought one for my tractor, . . . found out I didn't need it, . . . may wind up sticking it on the bottom end of some kind of makeshift windmill to charge 12 volts.

It's called a single wire alternator.

AND, . . . FWIW, . . . used a 100 watt solar panel today, . . . took two 12 volt batteries (18 AH) in parallel, . . . charged them up from 11.5 to 12.75 in just shy of 3 hours.

May God bless,
Dwight


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## inceptor (Nov 19, 2012)

dwight55 said:


> They're built in to the alternator now, . . . bought one for my tractor, . . . found out I didn't need it, . . . may wind up sticking it on the bottom end of some kind of makeshift windmill to charge 12 volts.
> 
> It's called a single wire alternator.
> 
> ...


Good to know....... thanks. :vs_closedeyes:


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## budgetprepp-n (Apr 7, 2013)

budgetprepp-n said:


> OK I had the battery down to 9.5 volts and I hooked it up to two 245 watt panels to see how long it takes to charge up
> hooked it up at noon


Ok it took about 3 hours to charge it back up and it seems fine


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## inceptor (Nov 19, 2012)

budgetprepp-n said:


> Ok it took about 3 hours to charge it back up and it seems fine


That was with a charge contoller?


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## dwight55 (Nov 9, 2012)

Inceptor, . . . I didn't know if your question was to budgetprepp-n or me, . . . but my answer is yes, . . . used a little charge controller.

Tell you the truth, . . . I bought a panel, wires, controller, mounting brackets, . . . one of the "kits", . . . and kinda wondered if that little controller would work. It's really pretty small all things considered, . . . 

But it does the job like a 7th Cavalry trooper, . . . 

Don't remember exactly where I read it, . . . but someone else really wrote hard on NOT charging a battery you wanted to keep, . . . by using a panel without a controller.

I suppose if you monitored the thing every 10 or 15 minutes you could get away with it, . . . but I'd rather not take the chance.

May God bless,
Dwight


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## inceptor (Nov 19, 2012)

dwight55 said:


> Inceptor, . . . I didn't know if your question was to budgetprepp-n or me, . . . but my answer is yes, . . . used a little charge controller.
> 
> Tell you the truth, . . . I bought a panel, wires, controller, mounting brackets, . . . one of the "kits", . . . and kinda wondered if that little controller would work. It's really pretty small all things considered, . . .
> 
> ...


I was asking him if he used one.

Just know that your response also told me want I wanted to know. I am in the middle of setting up my ham station. I tore it down to install new cabinets and counter top out in the man cave (my garage). All is in but the counter top and I wanted to add a solar charger to my back up batteries. I have 2 AGM's and 1-2 car batteries. The AGM's will probably be enough but you can never have enough back up.


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## budgetprepp-n (Apr 7, 2013)

inceptor said:


> That was with a charge contoller?


 Yes I used a charge controller .. I don't see how you could charge a battery without one. I tried that and
it didn't work well at all. I used a volt meter and the panel was trying to charge the battery with way to much
voltage. Like 25 volts. The reading jumps from like 12.2 to 20 volts before it has any time to put a charge of any kind
in the battery. Maybe if you had a 12 volt panel ,,,,maybe

This is the charge controller set up I use to experiment with. I have it mounted to where it's really easy to unplug
one or two of the panels and plug them into the controller and there's a table there for using for like a work bench
if I need it. Not all but most of my "solar testing equipment" is made from repurposed stuff. I might take some pictures
and show you guys what I use.

This is from when I was trying to see if I could build a 12 volt outside security light.
It worked fine and is in use now. 


This is a better shot of the controller. Most of the people on here give these controllers a bad wrap and 
say they are junk. I have had mine for years and it's been droped (many times) rained on, Snowed on,
and had the corner chewed off by my sons dog and it still works just fine. And it has some cool features.
It will turn the lights on when it gets dark and shut them off when the sun comes up.
Or it will turn on the lights at night for 2-to-12 hours and then shut them off. Cost= $14 


If your new to solar don't be afraid to give it a try. It's really simple to hook the panels up to a controller
and battery. You don't need to use the two wires for DC from the controller unless you need to have your
lights automatically come on and go off. For power I just hook up right at the battery. 
And the controller will know if you need 12 or 24 volts.


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## budgetprepp-n (Apr 7, 2013)

inceptor said:


> I was asking him if he used one.
> 
> Just know that your response also told me want I wanted to know. I am in the middle of setting up my ham station. I tore it down to install new cabinets and counter top out in the man cave (my garage). All is in but the counter top and I wanted to add a solar charger to my back up batteries. I have 2 AGM's and 1-2 car batteries. The AGM's will probably be enough but you can never have enough back up.


 For what you are doing I would think a 100 watt panel would be all that you would need.
Low cost and easy to set up


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