Let's Make Robots!

Common Electronics Knowledge Questions

Hi LMR. I have already made a robot, as you may know, but all I did (on the physical/electrical side of things) was plug a few cables into the pre-made servo ports on the Board of Education, as my copy of Electronics for Dummies told me to do. I read the entire thing, but they explained a few things in a way I didn't understand. Like the names for stuff, and what each means. Until now, I thought voltage was a measure of battery capacity. I FAIL, I know. I've sorted it out, so now I know about amps and volts, but I have a couple technical questions regarding electricity. I'm just trying to learn the ropes.

Is Amperage reusable? Say I have a battery putting out 2000 MaH. If I run a single servo drawing 1000 MaH off of it, do I only have 1000 MaH left to work with, or can I use up to the full 2000 MaH?

Can I go over the MaH specified? Say I want to run 4 servos, each with a 1000 MaH draw, off of a 2000 MaH battery. Would I still get 30 minutes of runtime, although I am drawing double the amperage per hour then it can supply? How much more can I draw? Could I do 32,000 MaH on a 2000 MaH battery?

If I link batteries in series, it increases the voltage (i.e. the electricity's speed), but does it also increase the Amperage per Hour? Say if I linked two 2000 MaH, 4.5 V batteries in series. I would get 9V but would I also get 4000 MaH?

When a servo has 3 pins (+, -, Signal) some people call - the ground. Does it go to the positive end of the battery, or an actual ground? There is a pin on my BS2 that's called ground, does that work for this application?

Thanks for your time and patience. It helps a lot (more then electronics for dummies).

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You might be better off looking at racing packs instead of that sparkfun one because of its low output. One of these gives a continuous maximum discharge current of 100A and is already a 2 cell pack.

Edit: forgot the battery link.

I thought the others looked better because size and weight were a concern (not to mention free day) but that one looks better. Thanks for your input.

The reason I asked about the battery in paralell/series is that I would prefer to run 6 V across maybe 32 servos (for torque/speed reasons). I am asking about capacity and mAh because I am trying to learn how I could compute the runtime from X number of batteries, to find the right number to order.

I desire an approximate runtime of 15 minutes (or as long as possible), and find that maybe six or seven of these will get me my magical 32,000 mAh for an hour's runtime. But the catch is that they only can shunt 3.7 volts through the circuit, while I prefer 6. Although a series linkup can provide the 6 V I want, it will break the batteries sooner. The servos will be under quite a bit of load, and therefore, I desire as much power as I can safely crank out. So, as rik says, maybe I put two in series and the rest in paralell, for the maximum life at the 6 V. Just thinking out loud here...

Maybe I use a few lines of power. As rik also stated, my circuit may not like the 6v I pump through it, so maybe I have a 5 V line using a V reg. and a 6 V unfiltered line.

Just so I'm clear on this, I could theoretically attach the ground wire of a servo to the negative of a battery?

By the way, this may be my biggest FAIL yet, but the current coming out of the battery's + end is actually negative current, right?

Be vary careful working with these, as they do easily catch fire with mishandling.

Using 2 of these batteries in series would be 7.4 volts, which servos can possibly handle, but is out of spec for most. Current use will be proportionally increased with voltage. However it appears that max current use by the servos is always expected, something that is rarely the case. Even under moderate loads, current will be somewhat less than the max rated. 

One statement : "maybe I put two in series and the rest in paralell, for the maximum life at the 6 V "  : seems to indicate a bit if series-parallel confusion. If you series 2 cells for a higher voltage, any added batteries placed in parallel must also add up to that series voltage so that the first pack of 2 does not discharge into the added batteries. So 2 batteries in series paralleled with another 2 in series would work to provide both double the current and double the voltage of just one pack. Trying to add a single battery in parallel with a series of 2 would discharge the pair until they reached the 3.7 volts of the single. (Or more likely until a fire started).

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You can't put stuff backwards through an LED, which prevents backwards discharge. You're saying optimum configuration is 2 and 2?

Yes you can "theoretically" attach the ground to negative. They are being used for the same thing in this case. What are you asking with "current coming out of the battery's + end is actually negative current"

I suspect what Benbo is talking about is the fact that what we conventionally call 'current' is actually the opposite of 'electron flow' (i.e. true electric flow), which is the flow of electrons from negative to positive.
Current flows out of a battery's positive terminal and into the negative terminal. Electrons flow out of the negative terminal and into the positive terminal.

Fortunately the convention of using current is so old that you'll almost never need to think about what the electrons are doing.

There are 3 important rules that will help you understand any electrical circuit:
Ohm's Law     V = I×R     Relates the voltage 'consumed' by a component to the current flowing through it and the component's resistance (only works for regular 'ohmic' resistance components, mostly used for standard resistor calculations)
Kirchoff's Voltage Law     Basically states that in any circuit 'loop', all the voltage added by batteries, etc must be completely consumed by the other components in the circuit.
Kirchoff's Current Law     At any junction in a circuit, the current flowing into that point must be equal to the current flowing out of that point. Just be careful using this near batteries/inductors/capacitors/etc, as these components can store or release current.

If you know how to use the 3 laws above, you can work out the currents and voltages throughout practically any circuit. Just remember that voltage is produced or consumed (increased or decreased) across a component, measured from the input to the output terminals. Current on the other hand flows through a component, and the current along any single circuit loop will always be the same.

When you add multiple cells/batteries in series, you add the voltages together to get the output voltage. Assuming you're adding several of the same type of battery together (which you really should, mixing batteries is a bad idea) then the maximum current (in A or mA) and the capacity (in Ah or mAh) will be the same. Kirchoff's Current Law tells us that the same current must flow through the chain of batteries.
If you add multiple cells/batteries in parallel the opposite happens, you add the maximum currents and capacities together, but the output voltage will be the same as a single battery. When all the cells/batteries are identical, the same current flows through each cell and all the output currents are then brought together to form the output current.
If you need more voltage and more current/capacity you can add cells/batteries in parallel to get the current/capacity you need, and then add multiple blocks of parallel cells/batteries in series to boost the output voltage.

Some manufacturers will give you specifications (particularly for rechargeable batteries) on what the nominal capacity of the battery is, and sometimes they'll also give you a maximum current too. As rik mentioned you'll get one hour from a 2000mAh battery if you draw 2000mA from it, or 2 hours if you draw 1000mAh from it. Most rechargeables can supply at least the '1-hour' current (also known as 1C or just C) without straining too hard, but the more current you pull from the cells the less run time you'll get compared to your predicted value.

As far as the terms negative and ground go, they're normally somewhat interchangeable. A fresh AA cell produces ~1.5V between the positive and negative terminals, so if you connect the negative terminal to ground (which is just a reference 'zero' volt point), then you'll get +1.5V from the positive terminal. If you connect the positive terminal to ground instead, you'll get -1.5V from the negative terminal. Ground is just considered the 'resting' voltage of the circuit, it allows us to have a convenient reference point that we can define all the other circuit voltages in relation to.
Think of it this way: you have two fresh AA cells, measured to have 1.5V across the terminals. You know that means that between the positive and negative of cell 1 you have 1.5V, and you also have the same condition for cell 2. Now, what is the voltage on the positive terminal of cell 1? What about cell 2? Are they the same?
The problem is you really can't say; the only thing we know is what the positive terminal voltage is in relation to the negative terminal of the same cell. We don't know anything about the 2 seperate cells.
If we connect the negative terminals together however, they must now be sitting at the same voltage which we'll call ground (zero volts) for convenience. Because we know that the positive terminal of cell 1 is 1.5V above ground, and the positive terminal of cell 2 is also 1.5V above ground, we can now say with certainty that both positive terminals are the same =D
It may seem like nothing has changed just by connecting the negative terminals, but what if cell 2 was carrying a static charge from sitting in your pocket? That would cause a difference in the 'absolute' voltages of cell 1 and cell 2, but we'd never know about it. 'Grounding' the two cells allows us to compare them under the same conditions.

1) Upper/Lower case counts. A lot! Your battery have a "Capacity" expressed in mAh. That is: milli (lower m) Ampere (upper A) hour (lower h, sometimes upper, use you own preference).

An upper M means "Mega" and is 1_000_000_000 time more than a milli. Don't blame yourself for this kind of error. I see manufacturers get it wrong all the time. They are the ones to feel ashamed.

2) The word "Amperage" is an ugly word. Also, you're using it wrong. You are talking about the "Capacity" of a battery here. Measured in Ah or in mAh. A capacity of 2000 mAh (or 2 Ah) indicates that your bot can draw 1000 mA of "Current" (or amperage if you must) for the duration of 2 hours. Multiply current with time for capacity. Or maybe draw 20 mA for 100 hours. Same product: mA * h = mAh.

Second: a bit of useless theory.

Now in theory you could draw as much as 200_000 mA (two hundred thousand mA) or 200 A for the duration of 10 mh (ten milli hour or 36 seconds). 

Third: practise ain't perfect.

We both understand that a tiny AA battery simply cannot deliver 200 A. Not even when you shortened the terminals with a copper wire as thick as your, uhm, wrist. The battery would heat up and maybe even smoke before your 36 seconds were up. So in practise there is some realistic limit on the maximum current any battery can safely give. Yes batteries have datasheets too.

Also in practise: turns out that capacity suffers more as the battery is asked to give more current. No matter how brand new and freshly charged your battery is. When you overwork your battery, it will not give up all its juice. And it also won't live as long.

Fourth: solving problems makes you an engineer.

So spread the load over several batteries. They will live longer and serve you better. Putting them in series will increase the Voltage (rather: electrical tension), but your circuit may not like that. Putting them in parallel will keep the tension the same, but your batteries may not all work as hard. The first may be depleted before the second even starts to feel there is work to be done. This is the field for practical engineering. Consider your pros and cons. Will you use a voltage regulator? Is it an efficient one? Will you step up or down the voltage? How many Volts are rrequired by your circuit anyways? Sounds like you need to put four AA in series anyway, in order to make a 5 V servo happy.

Yes, four batteries combined have four times more Capacity than just one. But is your circuit able to get it all out? I'll let the others answer that one.

You sure about that AAA battery? I have pretty thin wrists...