LEDs in parallel .......Cells in parallel

[treez]

You can put cells in parallel (as in a battery of cells) without needing to add circuitry to equalise the cell currents, so why can't you put LEDs in parallel without circuitry to equalise the LED current?

 

[Syncopator]

There are inevitable differences in the individual characteristics of l.e.ds, in particular the forward current vs Vf curves.

They are sensibly similar enough for current limiting resistance values to be calculated, but if l.e.ds are connected in parallel any difference will result in one diode passing more current than the other/s.
This can, and often does, result in the diode's failure. The result of that means the current which was taken by the failed l.e.d. is now flowing through the other ones.

What do you think will happen next? And next, etc?

 

[treez]

thanks, though its the same with cells, if we have two in parallel, then they will tend to wear out at different rates, and when charging them in parallel, one will charge up quicker than the other one, and will be getting overcharged as the charge continues for the one thats still charging......the result will be even more wear on the one thats worn out.

is it true that leds in parallel is then a worse situation than cells in parallel?

 

[Tahmid]

Let's say you have 5 LEDs in parallel. You want a current of 15mA into each LED. 20-25 mA is the max current that you can give.

Let's say you choose a current limiting resistor such that total current = 5*15mA = 75mA

Now, due to different specifications/characteristics of the LEDs, let's say 2 of them burn.

Now you have 3 LEDs and a current of 75mA going. This could cause all 3 to burn out, either at once or one after the other.

When cells are in parallel, differences during charging and discharging cause the life time to decrease. However, with LEDs, you may damage the LEDs immediately.

Just use a current limiting resistor for each LED to limit current to each LED. So, when a LED gets damaged, it doesn't damage the others in a chain.

Hope this helps.
Tahmid.

 

[IanP]

You can put cells in parallel (as in a battery of cells) without needing to add circuitry to equalise the cell currents

Connecting batteries in parallel is also not a good idea, although here and there you can see people doing this ..
In case of batteries "better" cell(s) can be discherged by one faulty cell.
That will "equalise" the resultant voltage but currents will be flowing amongs cells without you even knowing this ........
:wink:
IanP

 

[treez]

Tahmid

When cells are in parallel, differences during charging and discharging cause the life time to decrease. However, with LEDs, you may damage the LEDs immediately.

.....thankyou for this, you have confirmed that LEDs in parallel are worse than cells in parallel......i used to work at a LED company that insisted on putting LEDs in parallel with no current matching whatsoever, ..when i pointed it out, they just said it was ok to put cells in parallel so why not LEDs.


Anyway, cells in parallel is also not good.....but neither is cells in series....since a fully charged internal cell will kepp getting over charged where the cells round it have not charged up yet...........

so woul danyone like to vote on which is worst out of the following two?..........

1....cells in parallel
2....cells in series.

 

[Tahmid]

In case of cells, when multiple cells must be used, it's better to connect them in series and use a DC-DC converter to bring down the voltage to the required level. Keep in mind the loss in the DC-DC converter. Still better than cells in parallel. This is during discharge. While charging, charge each cell separately. If you're using constant voltage charging, then you can connect the cells in parallel.

Even better is to use a single cell with higher current rating. :lol:

 

[treez]

Ok thanks, so cells in parallel is worse.......but its an interesting point, and i suspect that cells in parallel is just as bad......because i think of supercapacitor stack chargers....the supercapacitors are effectively cells in series, and they need voltage equalisation on each capacitor.......so i think this is an indicator that cells in series is pretty bad news (unless each cell has voltage equalisation circuitry?)

 

[Tahmid]

If the cells that are in series start with the same capacity and discharge together, maybe it isn't too bad. After all, all around you you can find circuits all around you powered off multiple (rechargeable and non-rechargeable) AAA or similar cells/batteries in series. I would think that as long as the cells have similar capacity and discharge together, there shouldn't be a problem.

After all, all the high power inverters (usually above 600VA) in Bangladesh and India are powered off 24V batteries which are made by connecting 2 12V batteries in series. They are charged and discharged together. Of course they are lead-acid type batteries that do require maintenance. Inverters running off 48V are made usually by connecting 4 12V batteries in series.

 

[Syncopator]

Of the two connections you ask about in your post #7, connecting cells in parallel is not a good idea at all.

But series connection is done everywhere; in batteries and all manner of electronic and electrical equipment.

 

[treez]

Thanks, series connection is widespread, as you kindly say, though in how many cases do serial cells have voltage equalisation circuitry?

 

[Tahmid]

Thanks, series connection is widespread, as you kindly say, though in how many cases do serial cells have voltage equalisation circuitry?

For lead-acid batteries, equalization is done once every few months. Sometimes even this is omitted.
Check this:
http://www.thesolar.biz/Battery_charging_article.htm

Many of the inverters (I mentioned in my last post) don't have equilization circuitry, but still have batteries running for at least 2.5 to 3 years.

I'm not sure of other battery types though.

 

[treez]

Thanks, i agree with you, but i must admit it surprises me that cells in series dont tend to need voltage equalisation circuitry for when theyre charging, but supercapacitors always do need voltage equalisation circuitry when theyre in series

 

[Syncopator]

... in how many cases do serial cells have voltage equalisation circuitry?

I don't know the answer to that question, but I suspect that it is very rare.

I worked for a while maintaining stations in a microwave system. While the repeaters ran from on-site diesel electric generators, the main sites in towns were normally powered from the national 3-phase system with a standby of lead-acid batteries formed with enormous cells, and large inverters. There was no voltage equalisation in those batteries.

I also worked on the electronics of a research satellite. That battery had no voltage equalisation either. It would get charged from solar panels after launch.
In the lab we gave it a number of charge/discharge cycles. The voltage of each cell was monitored during discharge. When the terminal voltage of a cell had decreased to 1V, a relay would short that cell out, and so on until all cells had a short across them.

During the charge cycle, only the battery voltage was monitored.

 

[Tahmid]

Thanks, i agree with you, but i must admit it surprises me that cells in series dont tend to need voltage equalisation circuitry for when theyre charging, but supercapacitors always do need voltage equalisation circuitry when theyre in series

It could be due to the leakage currents of the cells and mismatch between the cells of the supercapacitors that they need voltage equalisation. Batteries tend to have much lower leakage currents.

 

[Tahmid]

Go to this page: **broken link removed**

Read the sections What is leakage current? and What is cell balancing and do I need it?.