LED Strip Current Question

[nicksydney]

Hi,

I have bought WS2812 LED strips and using Arduino Mini to program it and work successfully but I would like to power it using battery. There are 2 things that I need help with

1) I'm using multimeter to measure current consumed by the LED strips and googling around found on how to use the multimeter to measure the current and noticed the highest current recorded by the multimeter was 185 which I think it means 185mA (am I reading this correctly) as how the knob on the multimeter is dial to the 200m location ?

2) I tried using 4 x 1.5V AA Alkaline battery and the whole thing powered up nicely but when I tried LiPo battery (it says on the battery 3.7V 800mAH) it power up but the LED is not lit up correctly and it's not playing the different colour that the code was supposed to be doing. My question is how come the LiPo that is rated at 800mAH cannot play the LED strips properly ?

Thanks for the help

[alsetalokin4017]

1. Your multimeter appears to be showing 92.3 milliAmps (mA).

2. The WS2812 strips need a regulated supply of 5 to 7 volts to operate. Your battery pack of 4 AA cells provides a little over 6 volts when fresh. Your single cell LiPo provides only about 4.2 volts when freshly charged and goes down to a nominal 3.7 volts while running a load. This is not enough to run the Mini or the LED strip properly.  The "800 mA-H" marking does not indicate "how much current" the battery can provide; rather, it is a measure of the battery's usable energy capacity, not the same thing at all.

[nicksydney]

1. Your multimeter appears to be showing 92.3 milliAmps (mA).

Yes, that's what I can capture using the camera, but the highest it went to was 180+mA. Just want to check whether the way I'm measuring it using the multimeter is correct as pretty new doing this.

2. The WS2812 strips need a regulated supply of 5 to 7 volts to operate. Your battery pack of 4 AA cells provides a little over 6 volts when fresh. Your single cell LiPo provides only about 4.2 volts when freshly charged and goes down to a nominal 3.7 volts while running a load. This is not enough to run the Mini or the LED strip properly.  The "800 mA-H" marking does not indicate "how much current" the battery can provide; rather, it is a measure of the battery's usable energy capacity, not the same thing at all.

I tried using 3 x AA battery and it was not able to show the LED correctly as some code are also not running, does this mean that 4 x AA battery provide more than 200mA ?. I can't find any info about this particular battery so not sure how to know the current capacity that the battery can deliver.

[alsetalokin4017]

There are volts, and there are amps. As a first approximation, you can think of volts as "pressure" and amps as "flow".  The components represent the "plumbing" for the electricity. For your LED strips you need at least 5 and no more than 7 volts of "pressure" to make the required current flow through your system of electronic plumbing. 3 AA batteries provide only a bit over 4.5 volts when fresh, declining from there with use. So that is simply not enough pressure to make your plumbing work properly.
You certainly can get 200 mA, and a lot more, from 3 AA batteries, depending on the resistance of your "plumbing". In your case the resistance is high enough that 4.5 volts of "pressure" won't allow 200 mA of current to flow. But if you were simply to connect the two ends of the battery pack together with a wire (very low resistance) the battery's "pressure" would push several amps of current through that wire.

This is called the "hydraulic analogy" where the flow of electricity is compared to the flow of water in pipes. It is very basic and doesn't describe accurately all the things you need to know about volts, amps, watts, ohms, but it may make a useful picture in your mind so that you can understand what's happening in your system.

A battery rated at 3.7 volt, 800 mA-H, like your LiPo, is supposed to be able to provide 3.7 volts and 800 mA of current for one hour, or 3.7 volts and 1.6 A current for half an hour.... or 3.2 A current for fifteen minutes.... etc. The current depends on the voltage and the resistance of the load: this is Ohm's Law V=IR.  And the length of time the battery can produce the current depends on the energy capacity of the battery and the magnitude of the current: 3.7V and 800 mA-H.

[nicksydney]

But if you were simply to connect the two ends of the battery pack together with a wire (very low resistance) the battery's "pressure" would push several amps of current through that wire.

So from your explanation if I connect this kind of battery http://www.ebay.com.au/itm/1X-3-7V-1200mAh-14500-AA-Akku-Wiederaufladbare-Batterie-Lonen-Li-ion-Battery-KK-/261868861650 in series I will get 6.4v enough to pump for the LED strips ?

[Zero999]

According to the data sheet for the LED strips. The supply voltage can be between 6V and 7V. When those lithium batteries are fully charged, the voltage will be a maximum of 4.2V per cell, giving a total of 8.2V. In order or the LED strips to work reliably, you need a voltage regulator.

https://cdn-shop.adafruit.com/datasheets/WS2812.pdf

[rstofer]

If you check the battery datasheet, you may find that the 2800 mAh is taken over much more than 1 hour.  Maybe 10 hours or so.  If that is the case, you should only load the battery to 280 mA.

Part of the reason is internal heating.  If you try to use the entire 2800 mA the battery overheats.

The datasheet seems to imply that the input voltage needs to be very close to 7 volts and as suggested above, you may need a voltage regulator.  But it's kind of odd that they spec Vdd and Vcc as 6.0-7.0V and then turn around and characterize the device with Vdd at 4.5-5.5.

[Zero999]

If you check the battery datasheet, you may find that the 2800 mAh is taken over much more than 1 hour.  Maybe 10 hours or so.  If that is the case, you should only load the battery to 280 mA.

Part of the reason is internal heating.  If you try to use the entire 2800 mA the battery overheats.

The datasheet seems to imply that the input voltage needs to be very close to 7 volts and as suggested above, you may need a voltage regulator.  But it's kind of odd that they spec Vdd and Vcc as 6.0-7.0V and then turn around and characterize the device with Vdd at 4.5-5.5.

I think the examples given to describe the concept of mAH were simplifications. You're right, battery capacity tends to be lower at higher current draws and there's always a maximum discharge rate, which shouldn't be exceeded, otherwise the battery will overheat.

I agree, it does deem odd that it says Vcc should be between 6 & 7V, then it goes on to specify the characteristics at 4.5 to 5.5V. It's probably a mistake. The absolute maximum voltage rating is probably 7V with 5V being the optimum voltage. It will probably work between 4V & 6V so you could run if off four NiMH cells.

[mariush]

One more thing that may not be obvious to beginners.

The further you go on a strip of leds, the more voltage drop will be on the wires of the strip, because they're so thin. So for example, if you have a 20 led strip and feed 5v on one end, by the time the voltage arrives at the last led through the thin copper traces on the PCB strip, you may only have 4.7v or something like that.

This makes it even more difficult to power such led strips from just 3 AA batteries - non-rechargeable alkaline AA batteries can be up to 1.6v when full and maybe 1.4v when they're discharged enough to consider them dead, so 3 x AA batteries would have a voltage range between 4.2v and 4.8v .. so your strip could work just fine with 4.8v but if there's a lot of leds in the strip, by the time power reaches the last led, it may see less than 4.5v and those chipsmay behave erratically.

The datasheet for each led in the strip is this : https://cdn-shop.adafruit.com/datasheets/WS2812.pdf

As you can see the chip is specified with a maximum operating voltage of 6v to 7v which means you should never give the strip more than 7v. Ideally though, you should keep it around 5v to 5.5v
From the same datasheet, on page 3, you can learn that when the operating voltage is between 4.5v and 5.5v, the typical current consumption with all those 3 leds on is around 18.5mA - so you can multiply this by the number of elements in the strip to figure out about how much current your strip will draw from the power supply.

[nicksydney]

Hi,

Thanks everyone replied to my question, appreciate the help.

I've done a simple experiment powering the whole setup with a mobile power bank as shown in the attached picture. It says that it is 5V (not sure what is the current capacity). I was able to run i from 1.20pm to approximately 10/11pm so it lasted approximately 8-9hours.

Correct me if I'm wrong in understanding the whole thing:

1) Since the power bank was able to power the whole thing nicely 5V is sufficient
2) Since It lasted quite a long time for my setup this tells me that if I can combined battery to give me 5-6V but a higher current it can last longer ?. From reading http://www.zbattery.com/Connecting-Batteries-in-Series-or-Parallel if I can create a series of 2 x 3.7v (18650 battery) to give me 6.4v and then I create a parallel of 4 of them it will give me a higher current that will last the whole setup more longer ?

Thanks for the help

[Brumby]

... if I can create a series of 2 x 3.7v (18650 battery) to give me 6.4v

Check your arithmetic.


But, yes, by putting more units in parallel, the run time can be extended, proportionally.

[alsetalokin4017]

... But not because the battery stack can supply "a higher current".... rather, it will supply the _same current_, which is determined by the load, for a longer time.

Also, the nominal 3.7 V LiPo cell in the battery pack, when fully charged, will have a voltage of about 4.2 volts per cell, so the two in series will reach 8.4 volts which may be too much for the LED strip. This is why a voltage regulator circuit should be used, so that the pack's output is a nice regulated 5 VDC and remains constant while the battery discharges over time to around 3.0 volts per cell (3.2  V to be safe), at which time it should be recharged.

From his posts, I think nicksydney is still unsure about the relationship between voltage, current, power, and energy in the system of battery+load.

[nicksydney]

Check your arithmetic.

Yes, sorry it supposed to be 7.4v

[nicksydney]

From his posts, I think nicksydney is still unsure about the relationship between voltage, current, power, and energy in the system of battery+load.

True, I'm beginner and trying to learn the theory by practising it as bit hard to digest the theory compared to doing the practical.

Thanks for the help

[alsetalokin4017]

You're quite welcome. Please ask about anything that isn't clear to you, you'll get lots of help on this forum. More than you need sometimes!   
 
The problem with the practical approach as opposed to the strictly theoretical is that learning often comes at the cost of such things as blown components. I'd hate to see you blow out your LED strip or Mini due to some misconception about their power requirements. So a good mix of theory and practice is the way to go, in my opinion.

I've done several projects with the addressable RGB LEDs controlled by Arduino and they are really cool. And bright!  They like a good regulated power supply and proper capacitors, as recommended in the data sheets and tutorials.   Many great effects are easy to produce with some simple coding. I especially like the rings.

[Audioguru]

A Lithium battery cell "averages" 3.7V during a discharge. It is 4.2V when fully charged so two in series will blow up the circuit that has a maximum of 7V.
ebay sells many fake no-name-brand Lithium battery cells. Buy REAL name-brand ones from a local distributor.
   

[nicksydney]

A Lithium battery cell "averages" 3.7V during a discharge. It is 4.2V when fully charged so two in series will blow up the circuit that has a maximum of 7V.
ebay sells many fake no-name-brand Lithium battery cells. Buy REAL name-brand ones from a local distributor.

Thanks for posting the video. Very useful for beginner like myself

[nicksydney]

You're quite welcome. Please ask about anything that isn't clear to you, you'll get lots of help on this forum. More than you need sometimes!   

Excellent. Beginner like myself will have to learn from most of the people in this forum. Thanks