Need help figuring our Charge current for my 12v 12ah ride-on car battery using solar

[jigs]

Hi, I have bought 21 6x6" Kyocera cells for my kids science project. Each cell is rated at approx 4.5w, 0.6v, 7amp. Exact specs are below.

If all the cells were connected in Series, I would end up with an approx ~90w panel, ~10-12v, ~7amp panel. I hope I am correct here.

Now my kids Kid-Trax ride is powered by a 12v 12ah battery which gets charged by a Charging adapter plugged into a 110v outlet. The adapter says 12v and 12w implies it is supplying current at 1ah.

Now, will I fry my battery if I supplied 6-7amp DC current straight to the battery or to a charge controller. The battery does not have any specification on max current !? If true, what would you suggest to step down the current w/o affecting the charging voltage.

BTW, I am already a proud Homeowner of 6kw Suniva/Enphase Solar system. So kids are excited about doing this Solar project. Did not know of any other place to ask this question.

So thanks again for your help.

Cell Specifications:

Efficiency Eff (%) 15.2-15.75
Power Ppm (W) 4.0 - 4.5 watts
Max. Power Current Ipm (A) 7.89 - 8.25
Short Circuit Current Isc (A) 7.73 - 8.85
Max Power Voltage Vpm (V) 0.521 - 0.561
Open Circuit Voltage Voc (V) 0.623 - 0.625

[inetdog]

Hi, I have bought 21 6x6" Kyocera cells for my kids science project. Each cell is rated at approx 4.5w, 0.6v, 7amp. Exact specs are below.

If all the cells were connected in Series, I would end up with an approx ~90w panel, ~10-12v, ~7amp panel. I hope I am correct here.

Now my kids Kid-Trax ride is powered by a 12v 12ah battery which gets charged by a Charging adapter plugged into a 110v outlet. The adapter says 12v and 12w implies it is supplying current at 1ah.

Now, will I fry my battery if I supplied 6-7amp DC current straight to the battery or to a charge controller. The battery does not have any specification on max current !? If true, what would you suggest to step down the current w/o affecting the charging voltage.

BTW, I am already a proud Homeowner of 6kw Suniva/Enphase Solar system. So kids are excited about doing this Solar project. Did not know of any other place to ask this question.

So thanks again for your help.

Cell Specifications:

Efficiency Eff (%) 15.2-15.75
Power Ppm (W) 4.0 - 4.5 watts
Max. Power Current Ipm (A) 7.89 - 8.25
Short Circuit Current Isc (A) 7.73 - 8.85
Max Power Voltage Vpm (V) 0.521 - 0.561
Open Circuit Voltage Voc (V) 0.623 - 0.625

You have a definite problem (Ready, Fire, Aim!)

Your 21 cells in series, if they deliver the rated .6V+, will give you 12.6 volts. That may seem like exactly what you want, but it is not.

1. The .6V figure is the open circuit voltage (V_OC). The maximum power voltage (V_MP) will be as low as .52V . That means that you will not have that 12.6 volts available for battery charging.
2. To properly charge a nominal 12V Lead-Acid battery, you need a charging voltage of more than 14 volts, since the battery voltage rises above the nominal value as it approaches full charge.
3. To get that voltage to the battery reliably, allowing for wire losses and inefficiencies in the charge controller, as well as a decrease in the cell voltage as they warm up in the sun, you need about 18 volts from your panel.
4. That is why a typical panel used to charge 12 volt batteries will be a 36 cell (~18V) combination.

See if you can get about 15 more of those cells (at least 10!) and then come back to us to discuss it further.

You are correct that without some sort of control, the 7A from the panels will fry your battery.

You can get a solar Charge Controller (CC) which will limit the current while the battery is low and reduce the current cloae to zero as the battery becomes fully charged.
For your application a Pulse Width Modulated (PWM) CC will work fine, although it will waste up to one third of the panel power when the battery is at its lowest charge state.

Read the sticky threads and ask some more questions!

By the way, the factory charger is delivering a current of 1A. That also could be written as 1amp-hour per hour. But that would be silly.
It does not deliver 1AH.

Depending on the type of battery (and I assume that it is a sealed cell that does not leak acid when tipped) it could accept a maximum charging current as low as 1A (if GEL type battery), with a reduction as the charging completes or as high as 3A (if AGM type battery), again with the current being reduced as it nears full charge.

[Sunking]

Short story it takes 36 cells to make a 12 volt panel.

Second point is if you had 36 cells would make a 160 watt panel which is normally not a problem unless you have a 12 volt 12 AH battery. Minimum size battery for that size panel is 30 AH if AGM or 60
AH if using flooded. Hitting a 12 AH battery with 7 amps of charge current is asking for big trouble.

[PNjunction]

Ok, the kind of battery you are using is a "ups-style" agm. They are limited to about 0.25C to 0.3C max continuous charge current, so for a 12ah batt, that would be about 3A max charge current. You would have to make your panel a smaller one, only 60 watts ore less to put out about 3.1A or lower. Think Powersonic, UPGI, B&B etc for common manufacturers. This also means that you are looking at a lengthy charge cycle, especially in winter.

BUT, if you finish your panel to provide the required 18 volts, for a best-case scenario of putting out 7A, why not just upgrade the battery to something that can handle it:

Odyssey PC-545 agm ! Oh, it's 12ah capacity all right. But it will handle your 7A panel just fine. To make things even better, that means that for recharging, you are looking at approximately 2 hours or less. That could really be important to keep your kid happy with minimum downtime, and be do-able in the winter if all you have is 2 hours of solar insolation. Note - Odysseys don't like to be tickled. The *minimum* charge current for the 545 is 0.4C, so that means you need to hit it with at least 5 amps current. That means little wall-warts are out and will kill the Odyssey prematurely.

Finish panel - grab a decent quality charge controller that will go up to 14.7v , an Odyssey PC-545 agm battery and hit the road!

And if your kid shows the Odyssey battery mod to those in the know, count on some instant street-cred.

[jigs]

Thank you all for the prompt and very helpful responses. I am def. more learned now.

I wish I could add more cells as I plan to make/add the panel to a custom light weight roof panel attached to the car itself and hence restricted by size. Max it will take is 24 6x6" cells. Newer battery is not an option as this is for a Science fair which normally restricts you to splurge and encourages "make do", also the Car has already cost me over $400 after the last light mod. This project with the CC will set me back another $150-$200 easy.

I had initially looked at 36" 3x6" cells as opposed to 6x6" cells which I bought bcoz of less tabbing. Had I known that 3x6" cells also produced the same .5v, I would have got them instead which leads me to my next question

1. How is that 3"x6" cells and 6"x6" both produced the same .5v (of-course with half the current) ? I looked at many 3"x6" ebay listings and all said .5v. So def. not a typo. This would have been perfect.

Second, I figured from the equation Power (watts) = Volts x I (current), if I reduce (step down) the current some-how, the Voltage should automatically increase.

So why am I wrong in my thinking here ? Sorry, have forgotten all of my electrical science basics learned in Univ.

---------

Also I will read up your responses in more details and do some more research. The above is to just let you know that I have read and appreciate your responses.

Thanks again
-Jigs

[Mike90250]

The way the electronics work, each cell junction, produces .5V Adding more surface area increases the photo harvest, and adds more amps.

[inetdog]

Thank you all for the prompt and very helpful responses. I am def. more learned now.

I wish I could add more cells as I plan to make/add the panel to a custom light weight roof panel attached to the car itself and hence restricted by size. Max it will take is 24 6x6" cells. Newer battery is not an option as this is for a Science fair which normally restricts you to splurge and encourages "make do", also the Car has already cost me over $400 after the last light mod. This project with the CC will set me back another $150-$200 easy.

I had initially looked at 36" 3x6" cells as opposed to 6x6" cells which I bought bcoz of less tabbing. Had I known that 3x6" cells also produced the same .5v, I would have got them instead which leads me to my next question

1. How is that 3"x6" cells and 6"x6" both produced the same .5v (of-course with half the current) ? I looked at many 3"x6" ebay listings and all said .5v. So def. not a typo. This would have been perfect.

Second, I figured from the equation Power (watts) = Volts x I (current), if I reduce (step down) the current some-how, the Voltage should automatically increase.

So why am I wrong in my thinking here ? Sorry, have forgotten all of my electrical science basics learned in Univ.

---------

Also I will read up your responses in more details and do some more research. The above is to just let you know that I have read and appreciate your responses.

Thanks again
-Jigs

In order to charge your 12V battery with your 21 cell panel you will have to buy or build a DC to DC converter to step up the voltage. There is no other alternative for you at this point. Such converters combined with charge controllers are not, AFAIK, available commercially.
Unlike AC, you cannot just plug in a transformer to convert from one DC voltage to another.
Actually, your other option is to build a separate 15 cell panel and find another place on the car to put it. It could even be made with 3x6 cells if you are willing to cut the charging current in half.
You might be able to get away with as few as 10 additional cells.
Otherwise the science project will only demonstrate the hazards of buying and building before understanding. A good lesson, but probably not a winning project.

PS: Imagine a simple 1.5V alkaline battery. Can you get 3 volts from it just by drawing less current?
The PV cell produces a maximum voltage which depends on the material used, at a maximum current which depends on the amount of light hitting the cell.
In a practical situation the current drawn will be increased, lowering the voltage below V_OC until you reach the combination that gives maximum power. (V_MP x I_MP)