Non Frc: Electrical question series and parallel

[Tri_Lam]

To whom it may concern:
My name is Jeffrey Villar. My fellow class mates and I are doing a project on Solar Cars. Few questions have arrisen to our project.

First is When you wire cells into two diffrent series which endup producing two different voltage out puts, then connect those two series into a parallel connection. what happens to the voltage?

ex
arrange 7 cells w/ .5 voltage with 6 amps into two series one with 4 cells the other with 3 you get 4 cells withg a voltage of 2 volts and 6 amps while the other is 3 cells producing 1.5 volts with 6 amps. then putting both series into a parallel connection in theory gives us 12 amps but what is the voltage output? is it average?

also if you split a cell w/ .5 voltage with 6 amps what happens to the amps and volts????????????

Jeff Villar

[MrForbes]

It's generally not a good idea to put a different number of voltage producing cells in parallel. Exactly what you'll get out of it, depends on some interesting things involving internal resistance, etc.

If you "split" a cell, it will produce the same voltage, with half the current of the original cell available from each new cell (assuming you split it in half)

Each cell will always provide the same nominal voltage, and the current is relative to the size of the cell.

[Al Skierkiewicz]

Originally Posted by Tri_Lam

To whom it may concern:
My name is Jeffrey Villar. My fellow class mates and I are doing a project on Solar Cars. Few questions have arrisen to our project.

First is When you wire cells into two diffrent series which endup producing two different voltage out puts, then connect those two series into a parallel connection. what happens to the voltage?

ex
arrange 7 cells w/ .5 voltage with 6 amps into two series one with 4 cells the other with 3 you get 4 cells withg a voltage of 2 volts and 6 amps while the other is 3 cells producing 1.5 volts with 6 amps. then putting both series into a parallel connection in theory gives us 12 amps but what is the voltage output? is it average?

also if you split a cell w/ .5 voltage with 6 amps what happens to the amps and volts????????????

Jeff Villar

Jeff,
Cells in series add in voltage but keep the same current rating of the least current capacity cell. So yes, 4 0.5 volt cells@6 amps add to 2 volts@6 amps. It is not a good idea to put cells in parallel without some other devices to prevent the parallel cells from self discharging. (the cell with the higheer voltage will pass current to the cell with the lower voltage even if the cells are only a few millivolts different they will pass current) You cannot put two different cell voltages in parallel, the higher voltage will feed current into the lower voltage until all cells are at the same voltage.

[Tri_Lam]

Originally Posted by squirrel

It's generally not a good idea to put a different number of voltage producing cells in parallel. Exactly what you'll get out of it, depends on some interesting things involving internal resistance, etc.

If you "split" a cell, it will produce the same voltage, with half the current of the original cell available from each new cell (assuming you split it in half)

Each cell will always provide the same nominal voltage, and the current is relative to the size of the cell.

if it does retain the voltage if we put the cells into a series does the voltage add together?
(from a cell with .5 v and 6 amps split into two .5 v and 3amps, then put them together does it become 1 volt with 3 amps?????? is that what your saying????

Originally Posted by Al Skierkiewicz

Jeff,
Cells in series add in voltage but keep the same current rating of the least current capacity cell. So yes, 4 0.5 volt cells@6 amps add to 2 volts@6 amps. It is not a good idea to put cells in parallel without some other devices to prevent the parallel cells from self discharging. (the cell with the higheer voltage will pass current to the cell with the lower voltage even if the cells are only a few millivolts different they will pass current) You cannot put two different cell voltages in parallel, the higher voltage will feed current into the lower voltage until all cells are at the same voltage.

yeah were going to have like a power tracker or something to help us not discharge so if we do what happens to the voltage??

[Al Skierkiewicz]

When you put batteries in parallel they will self discharge to each other even if nothing else is connected. You can use diodes of the appropriate current rating but each diode has a voltage drop. Schottky diodes have 0.2-0.3 volts and silicon has 0.6 volt. So we are talking real world, generally batteries have more than 0.5 volt cell voltage. Flashlight cells and other "dry" cells have 1.5 volts per cell. A 9 volt radio battery has 6 1.5 volt cells inside in seeries. In our robot battery there is 6 cells in series of about 2 volts per cell for a an overall rating of 12 volts. Each cell can produce 18 amp hr at specified discharge currents.
When you put batteries in parallel one of them will always have a slightly higher voltage. Even if that voltage is only 0.002 volts higher than the other cell, current will flow into the lower battery. As they approach the same voltage the lower cell will now have moved slightly higher and it will start to feed current to the other battery. This see saw effect will continue until both batteries are depleted (zero volts)

[RyanN]

The voltage stays the same, but the amperage increases by 100% for every extra power source that you use as long as they are all the same. I would use some diodes to keep every battery separate. In this case, the voltage will be the (voltage - (I think) .4V) of the most charged battery, but the amperage will fluctuate as you use the batteries. For example, if you are using a 9V battery, 4AA batteries, and 2D batteries, the amperage at 9V will be much lower than at 6V, and then the amperage at 6V will be much lower than at 3V. Hopefully this makes sense.

Without some sort of power separator device, your voltage should be the average of all the power sources, and the amperage should be steady.

[Al Skierkiewicz]

Originally Posted by RyanN

Without some sort of power separator device, your voltage should be the average of all the power sources, and the amperage should be steady.

Sorry Ryan,
In your example the 9 volt battery will supply all the current and will be the voltage source. Each of the other battery packs will have their diodes biased to an off condition.

[Tri_Lam]

thanks but im not sure if i was clear enough though. we're arranging solar cells not battery cells to help power a solar car. ill reword my earlier question

arrange 7 solar cells w/ .5 voltage with 6 amp hrs into two series one with 4 solar cells the other with 3 you get 4 solar cells withg a voltage of 2 volts and 6 amps while the other is 3 solar cells producing 1.5 volts with 6 amps. then putting both series into a parallel connection in theory gives us 12 amps but what is the voltage output? is it average? does the electricty runover to the other cells?

if my question is confulsing tell me n ill try my bset to simplify it

[MrForbes]

Originally Posted by Tri_Lam

if it does retain the voltage if we put the cells into a series does the voltage add together?
(from a cell with .5 v and 6 amps split into two .5 v and 3amps, then put them together does it become 1 volt with 3 amps?????? is that what your saying????

If you put cells in series, the voltage adds up.

solar cells do behave differently than battery cells....I think Al didn't catch the fact that you're dealing with solar cells. I dont' know what the voltage would be with 4 cells in series, connected in parallel with another 3 cells in series, my guess is it would be somewhere between 1.5 and 2 volts, more or less, depending on the load (the actual voltage you get out does in fact depend on the load applied in any case).

Perhaps you could do some experimenting? be careful...see what happens.

[RyanN]

Originally Posted by Al Skierkiewicz

Sorry Ryan,
In your example the 9 volt battery will supply all the current and will be the voltage source. Each of the other battery packs will have their diodes biased to an off condition.

Sorry, maybe I wasn't clear enough... When I said "Without some sort of power separator device," I meant that without some diodes or something, then your voltage should be the average of all the power sources, and the amperage should be steady.

[Dave K.]

Originally Posted by Tri_Lam

thanks but im not sure if i was clear enough though. we're arranging solar cells not battery cells to help power a solar car. ill reword my earlier question

arrange 7 solar cells w/ .5 voltage with 6 amp hrs into two series one with 4 solar cells the other with 3 you get 4 solar cells withg a voltage of 2 volts and 6 amps while the other is 3 solar cells producing 1.5 volts with 6 amps. then putting both series into a parallel connection in theory gives us 12 amps but what is the voltage output? is it average? does the electricty runover to the other cells?

if my question is confulsing tell me n ill try my bset to simplify it

As others have said, the cells have an internal resistance, and this will become the limiting factor.



The difference in voltage between the two photo-voltaic arrays will result in current flow limited by the internal resistance of the PV cells and the interconnecting wiring.

Given that the voltage output from the PV cells changes based on the amount of energy they receive, even wiring identical PV cells in parallel isn't a good idea for that reason alone.

With the low voltages you are proposing, even a Schottkey diode (which has a lower forward drop than a traditional silicon diode) represents a large voltage drop relative to your output voltage.

It would be better to connect your PV cells in series, and then using a high efficiency regulator, such as a switching supply, to drop the resulting output down to whatever voltage the load would need.

[DonRotolo]

Jeff, you also asked "what happens to the voltage", in the cases described it gets lost, as heat.

For the solar cells, with 7 cells you would get just a hair above 1.5 volts, because the 4th cell would be reverse-biased and essentially a short circuit.

Note that most real cell arrays (not raw individual cells) have blocking diodes to prevent that.

I suggest that you try your experiment just like a physics lab: form a hypothesis (guess what will happen), do the experiment, and try to explain what you observed. Do note that open circuit voltage may be 0.5 volts, short-circuit current may be 6 Amps, but in a real circuit neither will be that.

Oh, and a solar cell outputs AMPS, not Amp-Hours. Batteries are rated in Amp-Hours. The correct units are important.

Don

[Jeff K.]

Also, one thing that I think that was overlooked is the motor. What type of motor are you using?
From my experience with solar cars (1st place win in a Shell Solar Car Race), the amps do not matter so much with small solar cars. Putting the cells in series(higher voltage) is the best way to get the fastest car if you keep it as light as possible and reduce the friction in the drive as much as possible. If there is friction, then you may need to increase the current by having the cells in parallel.

The best way to do it would probably just be to make a prototype vehicle, and then test it to see what works best. It also depends on your area, how sunny it is because that affects how much sunlight the panels will absorb. We both live in SoCal...so that isn't really all that big of a deal, but we were limited to using the motor they provided us with as well as one of the solar panels they provided us with.

To your question though, having cells of different total volts in parallel isn't a good idea. It'll go slower, but have more current. Is the race uphill? or just a drag race?

Good luck with your car.

[Al Skierkiewicz]

Sorry,
I did miss that you were dealing with solar cells. As Dave and Dan have pointed out, these cells are a little different than batteries but not much. You can run in parallel without as much fuss as a regular battery. Unfortunately, the mismatch between 2 volts and 1.5 volts will still cause some losses and these losses could be catastrophic to the cells. Since you are dealing with currents in the 6 amp range, I think we can assume this is a fairly large array. I am also guessing that you are trying to get to a 12 amp supply for some design point. Have you considered a voltage multiplier running at a lower voltage? i.e. put all 7 cells in parallel for 42 amps and feed a 0.5 volt to 10 volt convertor. Even with the convertor losses, you still will be able to get a decent load current. However, adding an eighth cell would do the trick giving you 2 volts at 12 amps or 4 volts at 6 amps in series.
I would be remiss here if I didn't suggest you read some books on the care and feeding of solar cells. When you are dealing with currents this high, heat can be a serious issue and localized heat may cause cracking of the cell. When cracks occur, current production will drop. There is a really good Ham radio reference on solar cell use for powering remote transmitter sites. Let me ask around and see if I can turn up the title for you. It might also help if you let us in on some of your design criteria. What is the actual load?

[Dave K.]

Something else I overlooked was that (most) PV elements are P-N junctions so they behave like diodes. If an individual cell were shaded, the darkened cell would become reverse biased and pass the array's current through it, which may have long term degradation effects on the cell. Therefore, some array manufactuer's include reverse protection diodes across each cell in an array which purposefully carries the bypass current.

So what we collectively have said at this point remains valid, this is just something else to consider when combining individual PV elements as opposed to working with a larger panel.