Science Buddies: "Ask an Expert"

After finally getting a job i now have the funds to finance a small project i've been wanting to make for a while.
The basic idea of the project is that i have solar panels connected to a charge controller. Then the controller to the battery. Then the battery to a power inverter.
This project so far can be viewed at http://www.instructables.com/id/Solar-P ... 20V-outlet
What i need help with:
Regulating the voltage coming from the solar panels. The voltage is well over 12V but less than 25V usually about 18v to 20v depending on sun conditions.
from what i've read, anything over 14v will damage a 12v battery and possibly lead to an explosion. (i'm using a 12V 10Ah battery in this project) Since i plan on implementing this project into a backpack, explosions just won't do. The charge controller doesn't limits voltage, it limits the amps to 7A and cuts off power after battery is charged. (as well as a couple other minor safety features associated specifically with solar power)
Also, i don't know precisely the level of amps going to the battery. All i know is that it's less than 7A, due to the charge controller.
What level of amps is safe for the type of battery i'm using? Charge time is not important as i plan to "float" charge because fro what i understand, batteries last longer when float or stand by charged as opposed to deep cycle charging.

Other info you should know:
All equipment and materials is brand new never used and still under warranty so no worries there
I own a voltage meter but not an amp meter and would rather not purchase one for a one time use as i have no other use for it. But however if it is crucial to the sucess of the project i could probably find one at school.
Also, the input voltage is consistently above 15v so "voltage drop" isn't a concern

Other info that would be helpful to me:
How many volts should i regulate it to?
What is the precise voltage for a fully charged 12v battery? Like 12.6v or something?
If i regulate voltage to 12v, how would i ever fully charge the battery?


So in short:
How can i regulate voltage?
How many amps is safe to charge the battery?

-Thanks

Hi i.need.help,

There are many questions in your post. I able to address three of them. Hopefully the methodology here and inputs from others on this board will help you with the others.

If you supply "how do you recharge a battery?" to answers.com, you'll get back many links, one of which will most likely answer your basic question.

Re "What is the precise voltage for a fully charged 12v battery? Like 12.6v or something?":
http://wiki.answers.com/Q/How_long_can_ ... _goes_dead
reports that it's 13.8 volts, for a particular kind of battery. Results may be different for other kinds of batteries. (Note, this site was found by supplying "what is the voltage for a fully charged 12 volt battery?" to answers.com.

Re: "All equipment and materials is brand new never used and still under warranty so no worries there" Actually, you may need to worry. Most warranties include a clause about intentional damage.

How many volts should i regulate it to?
What is the precise voltage for a fully charged 12v battery? Like 12.6v or something

You need to get detailed recommendations from the battery manufacturer to answer these questions. Most electrical equipment have data sheets that give the electrical properties. If you can provide the manufacturer and model number, we can attempt to find a data sheet to answer your questions and show you how we came up with the answers.

If i regulate voltage to 12v, how would i ever fully charge the battery?

If the battery is fully charged when it reaches 12v with minimal current draw, they it will be fully charged. If not, then you can't.

So in short:
How can i regulate voltage?
How many amps is safe to charge the battery?

You regulate voltage with a voltage regulator. You probably want to try putting "voltage regulators" into a search engine. Car batteries are nominally 12v batteries and the alternators that charge them typically supply around 18v peak. As to how many amps is it safe to use in a battery charger, you need to go back to the battery specifications. Typical fast charging circuits are designed to supply more current when the battery has less charge and to reduce the charging current the closer the battery comes to being fully charged. The battery data sheet will usually give you the maximum safe charging current and will also recommend a trickle charge value that is safe to supply even when the battery is fully charged.

Thanks guys for the replies. I'm definetly a lot closer to finshing now and understand my topic a lot better. however i can't find any dat sheets for my battery....
this is the battery i'm using
TR10-12 TEMPEST SEALED LEAD-ACID AGM BATTERY
i bought it from this site
http://www.batteryspec.com/cgi-bin/cart ... product=91
no clue where to find data sheets........
thanks for all the help. i see a lot of people on these forums don't really appreciate or thank you guys for all the help you've given everybody. Just lettin you know there's someone out here who still appreciates it.

Hi i.need.help,

This is what you're looking for:

http://www.tempestbatteries.com/html/tr10-12.html

Generally manufacturers are responsible for providing such data sheets, so I googled "tempest batteries" and then poked around on the website for product info. This is also the company to contact if you have a question that we can't answer from looking at the sheet.

When you start looking at the sheet, you'll see that capacity is listed in terms of 'AH'. This is an amount of charge, amperes * hours. Since an amp is charge per unit time, multiplying by time gives you charge. However, you are on the right track with thinking of the voltage that must be supplied to charge the battery: the applied voltage must be higher than the battery's voltage at the current level of charging, or current won't be able to flow in the right direction to charge the battery. It looks like the discharge voltage is never over 13 V, at least at 77 degrees F, so I *think* that 13 V would be enough to charge your battery -- you might need to check with the manufacturer to determine if the safe maximum is really as high as 14 V. The battery is described as '12 V' because over most of its discharge curve, it supplies at least 12 V, so a device requiring 12 V will run well.

I think the sheet has a lot of good info to get you started thinking about your questions -- but do post again if you get stuck.

Amanda

you might need to check with the manufacturer to determine if the safe maximum is really as high as 14 V.

The manufacturer already gave enough information in the data sheet to answer this question. The manufacturer's requirement is that you not exceed 2A of current in the fast charging mode. The range of voltage values given for this fast charge are based on the range of internal battery resistance and internal discharged battery voltage expected. The voltage differential (charger voltage - battery voltage) times the internal battery resistance will give the charge current. So what this information on the data sheet is telling you is that the expected battery voltages and resistances will be such that you will need charger voltages in the range given to obtain a 2 Amp charge current. Your circuit design should be a current limiter circuit so that you never exceed 2 Amps of charging current in the fast charge mode. The manufacturer also gave you a hint at what the "trickle" or "float" charger voltage required is to slowly charge the battery. Please note that if you are using a high powered silicon transistor as the current limiting device, the actual voltage needed will be 0.5 to 0.7 volts higher (based on the Vce voltage drop of the transistor you have).

Since the power source for charging the batteries is solar cells (you need their data sheet as well), I wouldn't count on trickle charging being able to recharge the battery, your choice but a consumer is likely going to want to use the outlet as soon as possible.

Thanks so much for the replies i figure i'm really close to being finished with the project. Only a couple things left:

This is info taken from the data sheets

Charging.................Initial Charging Current 2A (recommended)...................14.5~14.9V/ 77°F (25°C)
(Constant Voltage)
Cycle....................Float ...............................................................13.6~13.8V/77°F (25°C)


So according to this info should i charge the battery with 2A at 14V?

Since the power source for charging the batteries is solar cells (you need their data sheet as well)......

Ummmm..... well.... you see... i kinda salvaged the panels from a couple of solar fence chargers (to keep horses inside the fences) that i know nothing about the brands or model and have no way of figuring that out so any info on them is pretty much outta the question....... sorry. =/
I don't understand why data sheets for the panels are needed.... is there another way to just test them for whatever info is needed? I don't see a need if i were to put a voltage and amp controller on them. I might be missing something though.....

-thanks for the help!

If you don't have a data sheet for the solar panels, then you are going to have to do some testing to see what their behavior is. In bright sun light, you will want to measure their open circuit output voltage. The difference between this voltage and the 13.6 volt float charge condition will let you calculate how much power the current limiter will have to be able to handle. Conversely, if the solar panel voltage output is below 15 volts, then you probably won't have enough voltage difference to be able to make it work well.

Once you know the open circuit output voltage from the solar panels in bright sunlight, you need to add a load resistor and see if the panels can supply 2 amps. A little bit of Ohms law and the corresponding power equation will tell you what resistance you need and how much power it has to be able to dissapate in order to perform the load test.

The information in the solar panel data sheet would have supplied the information from these kinds of tests so you didn't have to do them yourself.

Since the battery voltage during charging and discharging is NOT constant, a simple voltage regulator isn't what you need. Most modern battery chargers measure both the charging current and the charge voltage to be kind to the battery chemistry. With a solar panel whose output voltage is a function of how much sunlight it is in and how much current it is supplying, the need for limiting the current based on battery voltage is even more important.

In other words, you have a variable unregulated power source with a variable output impedance (solar cell array) and you have a variable load (battery) with an variable internal power source so analyzing the circuit at any point in time requires knowing state information related to both the solar cell and the battery. The state of the battery (amount of charge in it) will affect the solar cell and the amount of sunlight on the solar cell will affect the battery charging circuit so figuring out what is happening at any point in time can't be done with a single measurement.

Craig_Bridge wrote:Once you know the open circuit output voltage from the solar panels in bright sunlight, you need to add a load resistor and see if the panels can supply 2 amps. A little bit of Ohms law and the corresponding power equation will tell you what resistance you need and how much power it has to be able to dissapate in order to perform the load test.

I can find out the voltage but what do i need to test amps? Also where can i find a resistor and what kind/size do i need? Also, what is Ohm's law?

Craig_Bridge wrote:Since the battery voltage during charging and discharging is NOT constant, a simple voltage regulator isn't what you need.

I don't understand why not. If i can just limit the voltage from the solar panels to the battery at 14V wouldn't that work? (I have a solar charge controller that cuts off the power from the panels when the battery is fully charged so overcharging isn't a problem)

-thanks

So, the manufacturer says that the current shouldn't exceed 2 A during charging, the implication being that more than 2 A could damage the battery. One way this could happen is from excessive heat: heat production increases as power increases, and power increases with the square of current:

http://en.wikipedia.org/wiki/Power_(phy ... ical_power

This is why Craig_Bridge recommended a current-limiting circuit. But to design that effectively, you need to know how much current the solar panel is capable of putting out. You mentioned that you had a component that would limit current to 7 A -- is that adjustable?

Since you aren't familiar with Ohm's law, I really recommend reading up on it and other basic DC circuit ideas and equations relating voltage, current, power, resistance, etc. It will probably make a lot of this discussion fall into place, and you'll have more fun when you understand why you're doing what you're doing! These look like some good resources:

http://www.allaboutcircuits.com/vol_1/chpt_2/1.html
http://hyperphysics.phy-astr.gsu.edu/Hb ... hmlaw.html
http://en.wikipedia.org/wiki/Ohm%27s_law
http://micro.magnet.fsu.edu/electromag/java/ohmslaw/

As for obtaining a resistor and ammeter: You also mentioned that you might be able to get an ammeter (or multimeter?) from school, and if that came from an electronics lab course, they would also have resistors. It looks like you can also get them for a few dollars at Radio Shack. I'm hesitant to recommend a household item like a lightbulb, even if it has a resistance rating in ohms, because resistance can vary with applied voltage, temperature, etc -- but something sold as a resistor should give you a nice constant value for making measurements. I expect that other experts with more practical experience can give you better advice here.

Amanda

agm wrote:You mentioned that you had a component that would limit current to 7 A -- is that adjustable?

Sadly, no.

Craig_Bridge wrote:Since the battery voltage during charging and discharging is NOT constant, a simple voltage regulator isn't what you need.

I still don't see why not. If the panels put out enough volts couldn't i just limit the voltage to a constant rate of 14V to charge the battery? (The charge controller has a cutoff so when the battery is fully charged it'll stop charging the battery to prevent overcharging.)


So from what i understand, here's my to do list to complete the project:

-Test the Ampere output of the solar panels and find an appropriate resistor

-Test the avg. voltage output of the panels and find and appropriate voltage regulator

that seems like that's about it. anything else?

-hunter

-Test the Ampere output of the solar panels and find an appropriate resistor. -Test the avg. voltage output of the panels

You have things in the wrong order. It is safe to directly measure the output voltage of the solar panels with a volt meter set to its highest DC voltage scale (and then if the voltage is smaller than a lower DC voltage scale, to switch down to a more appropriate scale to get a more accurate voltage reading).

It is NOT safe to attach a resistor across an unknown voltage. Bad things can happen. If the power dissapated, voltage squared divided by the resistance (one of the power equations associated with Ohms law) is significantly higher than what the resistor is rated at, it can explode! Even if it doesn't explode, it can burn your fingers or if it is laying on something that will burn, it can start a fire. Since what you want to determine is if the solar panels will supply 2 Amps at enough voltage to charge your batteries, you need to use a little Ohms law and power law calculations to determine what power resistor you need to make this measurement. To do that, you need to know the open circuit voltage of the solar cells. You get that from the voltage measurement.

For example, if you measured 20 volts open circuit voltage (nothing attached to the solar panels except for the high impedance volt meter) in bright sunlight, then you need a resistance of 20 Volts / 2 Amps = 10 Ohms. The power this resistor will have to dissapate is 2 Amps * 2 Amps * 10 Ohms = 40 Watts. Now that is one big expensive power resistor if you could buy it. If you can buy 2 Watt power resistors, then 2 Watts / 20 Volts = 0.1 Amps and 20 Volts / 0.1 Amps = 200 Ohms. If you hook 20 of them up in parallel with enough space around them so they can self cool, then you have the equivalent of a 10 Ohm 40 Watt resistor. Unfortunately, 200 Ohms isn't a standard 10 % resistance value. 220 Ohms is the larger standardized value, so you will need 22 of them in parallel to get your 10 Ohm resistor and it will be capable of disappating 44 Watts.

You really need to read up on Ohms Law and KirKoff's Law and understand them so that you can understand why it is slightly more complicated that just volts or amps.