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I've been following this tutorial on Arduinos, for the ones that are interested it's this one.

Yesterday I was struggling to get a servo moving. Sometimes it made small twitchy kind of movements, other times it wouldn't move at all. So I did some research on what the reason for this could have been.

The most common answer was: Don't use a 9V battery for a motor.

I ended up buying 6xAA batteries, which do very well in moving the servo.

But in the tutorial he IS using a 9V battery, so what are the differences that matter here? (Quality, more mAh, alkaline vs non-alkaline, ...?)

My knowledge regarding batteries is still very poor, so I've put three images at the bottom that I would like to compare if possible.

  1. What's the difference with my 9V battery and his 9V battery?
  2. Why are 6xAA batteries (1.5V) recommended over a 9V battery?

Images:

  1. My 9V Battery
  2. His 9V Battery (I believe it says 'Radioshack Enercell 9v Alkaline Battery')
  3. My 1.5V batteries

My 9V Battery His 9V Battery 1.5V Battery

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    \$\begingroup\$ Simple, higher current output from 6xAA batteries. \$\endgroup\$ – Shibalicious Aug 11 '18 at 13:26
  • \$\begingroup\$ @Hypomania I see, thank you. But what are the differences between the two types of 9v batteries I'm showing then? \$\endgroup\$ – Tobias Heuts Aug 11 '18 at 13:28
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    \$\begingroup\$ The difference is probably not the comparison of your 9V battery and the one you saw in the video. Instead it is far more likely a difference of the amount of load being applied to the battery. The servo in the video may very well be a lower current demand servo. \$\endgroup\$ – Michael Karas Aug 11 '18 at 13:38
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    \$\begingroup\$ The Panasonic NEO that you're using is a Zinc-Carbon battery. The Enercell is an Alkaline battery. And THAT's the difference. 'Alkaline delivers more energy at higher load currents than zinc-carbon' \$\endgroup\$ – MartinF Aug 11 '18 at 13:41
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    \$\begingroup\$ If you monitor the voltage, this will tell you if the ratio of load resistance to total resistance including battery . Servo Motor’s have very low DCR when the position error is high dropping the input voltage such that it fails to switch. FETs conduct when Vgs is 2-3x times the threshold Vt . If Vt is 2-4V (std) it may not conduct at 8V or less. Thus a twitch response as battery drops then recovers slowly. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 11 '18 at 19:03
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What's the difference with my 9V battery and his 9V battery?

The difference is the chemistry of your "Extra Long Life" (Zinc-Carbon or more likely Zinc Chloride) 9V battery vs. video battery.

Not sure what is inside your 9V battery. What is known is your "Extra Long Life" has less energy capacity than alkaline. You would need to look at an zinc-carbon or zinc chloride datasheet to see the discharge characteristics.

This is only a 50 mA discharge curve for 9V batteries.

enter image description here

Why are 6xAA batteries (1.5V) recommended over a 9V battery?

This is not totally correct. One might say use AA alkaline rather than AAA or AAAA alkaline. Or use alkaline rather than zinc carbon. I suspect that 6 AA may still be better choice than 6 AAAA or 9v alkaline.

There are 6 AAAA alkaline batteries inside a 9V alkaline battery.

This is a 9V Energizer alkaline battery.

enter image description here
Source: 9 Volt Battery Hack!



From Wikipedia

The zinc chloride cell is an improvement on the original zinc–carbon cell, using purer chemicals and giving a longer service life and steadier voltage output as it is used. These cells are usually marketed as heavy-duty, extra-heavy-duty, or even super-heavy-duty batteries, and offer about twice the service life of general purpose zinc–carbon cells, or up to four times in continuous-use or high-drain applications.

Alkaline batteries offer up to eight times the battery life of zinc chloride batteries, especially in continuous-use or high-drain applications.


From Battery University.

Zinc-carbon, also known as carbon-zinc or the Leclanché battery, is one of the earliest and least expensive primary batteries. It delivers 1.5V and often come with consumer devices. The first zinc-carbon invented by Georges Leclanché in 1859 was wet.

Alkaline-manganese, also known as alkaline, is an improved version of the zinc-carbon battery and delivers 1.5V. Lewis Urry (1927–2004) invented alkaline in 1949 while working with the Eveready Battery Company laboratory in, Ohio, USA.

Alkaline delivers more energy at higher load currents than zinc-carbon. Furthermore, a regular household alkaline provides about 40 percent more energy than the average Li-ion but alkaline is not as strong as Li-ion on loading. Alkaline has very low self-discharge and does not leak electrolyte when depleted as the old zinc-carbon does, but it is not totally leak-proof.

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Let's compare using the figures from this website. I've made actual measurements in the past, and their numbers look plausible enough.

enter image description here

6AA batteries in series will, according to the numbers above, have an internal resistance of 0.9 ohm.

The servo shown at the beginning of your video, the SG-5010, has a current draw of 550mA according to web sources (probably stall- which is start-up current is higher by perhaps 50% or more). So let's say 1A to be safe.

So with a 9V carbon-zinc battery, the battery simply cannot supply 1A. The short-circuit current is less than 260mA. So clearly it will just effectively short the supply and fail to move.

The 9V alkaline cell with 1.5 ohm internal resistance can supply 2A with a 3V drop (to 6V) so it will almost surely work when fresh.

The 6x AA batteries with 0.9 ohm internal resistance can supply 3.3A with a 3V drop so it also will work.

Note that the way batteries work is that the internal resistance increases as the battery is depleted, while the open-circuit voltage stays fairly constant until the battery is about completely dead.


The above is the analysis that corresponds to your question, however the other question is regarding the energy storage and at what point the battery will be depleted to the point where it will no longer work.

We can ignore the carbon-zinc 9V since it won't work at all. Here is a good quality 9V alkaline cell, and looking at the curves (and extrapolating) maybe 1 hour at 300mA is possible.

Using this datasheet for a good quality alkaline AA cell, it looks more like 5 hours operation at a constant 300mA. In practice the AA cells will also be able to start the motor further into their discharge as well, because of the lower initial internal resistance.

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What I found on the web is that the 9V batteries have about 500 mAh, While AA batteries vary from 1.8 - 3 Ah. So in short it's because of the higher current.

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    \$\begingroup\$ The available energy of a battery is measured in Ah (current * time) but that does not neccesarily have anything to do with the current which it can supply. Perhaps you could find out something about the internal resistance of the cells in question and revise your answer. \$\endgroup\$ – Andrew Morton Aug 11 '18 at 13:41
  • \$\begingroup\$ Incorrect assumption. \$\endgroup\$ – winny Aug 11 '18 at 13:43
  • \$\begingroup\$ @AndrewMorton Yeah I see, I messed up things together. \$\endgroup\$ – Mohamed Abduljawad Aug 11 '18 at 13:44
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    \$\begingroup\$ While this isn't directly the cause it is highly correlated. Generally speaking, a higher capacity battery will be able to delivery higher current than a lower capacity battery of the same type. Alkaline 9V batteries are generally made of 6 cells just a bit smaller than AAA's. Carbon Zinc ones are often in a different physical configuration, but still multiple small cells. \$\endgroup\$ – Chris Stratton Aug 11 '18 at 16:22
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I can't find any proper data on your battery. Some google searches did turn up a part number ( 6F22NT ), which in turn turned up a page on Panasonics hong kong site https://www.panasonic.hk/english/products/living/batterychargertorch-light/manganese-battery/6f22nt.aspx but no link to proper specifications. Various sellers claim it's "zinc carbon", on the other hand the panasonic hong kong site lists it under "manganese batteries".

Sadly even major brands are not immune to slapping bullshit like "extra long life" or "heavy duty" on shitty zinc-carbon or zinc-chloride batteries.

As I understand it Zinc chloride and Zinc carbon batteries have a significantly higher internal resistance than Alkaline batteries of the same size. So if your battery is indeed a zinc chloride/carbon based battery that would explain the difference in results.

A 6xAA pack has much physically bigger cells than a "9V battery" (technically known as a PP3). So for a given chemistry and state of discharge it will have a much higher capacity and lower internal resistance.

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I think that you should first figure out the current requirements of your device and then choose the batteries selectively.

To do this you have to get the Operating current and voltage data of the element you want to drive from the datasheet(Here Servo Model-SG-5010).

I found some data here: protosupplies.com/product/servo-motor-sg-5010

From this, We can say current required to start your servo motor should be around:

600mA @ 5V-DC

Now you can easily calculate Max. power required to drive your servo by using the formula:

$$P_{req} = V * I$$

Which calculates to:

$$P_{req} = 5 * 0.6 = 3\ Watts$$

This is the estimated required power and does not include any efficiency losses.

Well, In most cases you are not directly powering your circuit through the battery, you generally use some voltage regulator in between to scale the voltage down to a nominal level, these circuit elements consume some power to operate and hence there is some decrease in efficiency, so the required power is generally greater than the estimated power and can be calculated as:

$$P_{final} = \frac{P_{req}}{\eta}$$

Where eta is the conversion efficiency of the voltage regulator you are using. Let's say it is 80% efficient, so the value of eta would be:

$$\eta = 0.8$$

Now, calculating the final required power.

$$P_{final} = \frac{3}{0.8} = 3.75\ Watts$$

This is the actual required power that your 9V battery should be able to deliver to drive your servo.

Next, calculating the required current @ 9V-DC & 5V-DC (Critical Battery Voltage below which operation will halt)

$$P_{batt} = V_{batt} * I_{batt}$$

$$I_{batt} =\frac{3.75\ Watts}{9\ V}$$

$$I_{batt} \approx 416mA\ @\ 9V$$

So, your battery must be able to deliver at least 416mA @ 9V-DC or 750mA @ 5V-DC to drive your servo properly.

Now there are two questions:

  1. Whether your 9V battery can supply that kind of current or not. If it can then you are pretty much assured that your motor would run smoothly, otherwise, there is a chance of failure.
  2. If the above condition is met, how long will it run? (I will be skipping this part, It depends on the battery capacity and other factors.)

To answer Question 1, My approach is to calculate the maximum allowable internal resistance of the battery, given your requirements. As it will give you a more clear idea which battery to choose.

To calculate max allowable internal resistance of the 9v battery for your application, first, find the critical battery voltage at which the normal operation will halt (5V in our case) and the current o/p required in that state to continue normal operation(750mA in our case).

Next, we need to calculate the max allowable voltage drop across the internal resistor:

$$V_{Drop} = Nominal\ battery\ voltage - Critical\ voltage$$

$$R_{internal} \leqslant \frac{V_{Drop}}{I_{Critical}}$$

$$R_{internal} \leqslant \frac{4\ Volts}{0.75 Amps} = 5.34\ \Omega$$

From the above result, we can say that any 9V battery(or combination of cells) with internal resistance less than 5.34 Ohms will work for your application (The Lower The Better).

Possible solutions are rounded off in the below table, those which are out of the circle cannot be used until or unless they are paralleled in sufficient quantities to lower their resistance.

InternalResistanceTable

Source: learningaboutelectronics.com

(Needless to say, this is the very basic answer to this question, in reality, there are many more parameters you have to look at.)

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