New so need some tutoring
MarkCrCo
Posts: 89
Ok I think there is a simple explanation for this but can't find it on the internet. I have my boe-bot running fine on 4 AA batteries (6 Volt - non-rechargeable). Tried to hook him up to two lithium 3 volt watch batteries (CR2032). He starts up and plays his little initialization tune on the speaker then lurches forward and resets and starts over again. Why is 6 volts from AA batteries not the same as 6 volts from the disk batteries?
Comments
The current available from a couple of coin cells isn't the same as from some AA's.
Just as a battery pack of 6 AA's can deliver a lot more than a 9V "transistor" battery.
PE - If you took 4 3V coin cells, you wouldn't expect that would be enough to start the car with, would you?
Here's a "book" -- http://web.mit.edu/evt/summary_battery_specifications.pdf
I can give you an example. I know others on here can give a better one.
Lets say you have a little kiddy pool( your boe-bot). And you want to fill that pool fast. So you grab the hose, turn on the water, and start filling it. Now you have water coming out of the hose (the battery voltage), but the water is not coming out fast enough. So you open the valve more (add more current) to fill the pool up faster.
Yes, your boe-bot has the voltage (water in the hose), but you don't have enough current (amount of water) to drive the motors.
Hope that helped.
If a CR2032 coin cell has an unloaded voltage of 3 volts and an internal resistance of 20 ohms, and you try and draw 100mA from it, the available voltage (at the coin cell terminals) will drop to something like 1 volt, causing a drop out and reset.
Motors take a heap of current when they "lurch forward"
Here's the datasheet for AA cells.
Here's a corresponding datasheet for a CR2032 coin cell.
Note that a CR2032 is intended to provide maybe 10mA over a period of hours in short pulses, maybe a few seconds at a time. You might look at using two CR123A Lithium cells in a battery holder to get 6V at the sort of current you need. The Parallax penguin-bot used this for power. If you look around, you should be able to find them relatively cheaply.
Here's the datasheet for the CR123A.
You could use one LiPo cell (about 4V) and use a boost circuit to raise the voltage to 5V. Or you could use two (or more) LiPo cells in series and use a buck regulator to drop the voltage to 5V (or 6V depending on your needs/wants).
Those with high serial resistance can not supply current as much/fast as other battery-types can.
If the power requirement are pretty high but very short, a large (low-ESR) capacitor can be your power-buffer. (ESR = Equivalent Series Resistance)
Alkaline coin-cell batteries are like AA batteries, low resistance. But of course due to smaller size have a mAh that is much lower.
(FWIW, "User Name" == too-lazy-to-fill-in-the-box-with-something-clever)
Otherwise you're going to have to take some current measurements. This might be a good idea. Work out how much current is being drawn, and choose a battery that can still supply enough voltage to keep the microcontroller alive under those load conditions.
It is quite simple. In terms of POWER. amps x volts = Watts, which is the measure of power.
Volts alone is not a measure of POWER. Those tiny CrCo cells don't have enought amps in them to deliver more than a tweek to the BOEbot, and then reset.
Right voltage, but size does matter. Bigger cells in general have more POWER. If you want to drive a robot, anything smaller than AAA cells are going to likely not work out.... unless you are very sure of matching the demands for power with the battery size.
When the BOEbot's motors try to roll, the battery just gets too much current drain and the voltage drops below the reset level.
When you increased the Load (the demand on power) beyond what the battery can deliver at a steady rate current, the voltage drops.. somewhat like a short circuit.
I think it is very normal for the average person to thing Voltage is the equivalent of Power, but it is certainly not that. P = V X I
As far as dependendencies, without current you don't have an electrical flow. We see static electric potentials of tens of thousand of volts, but not flow.
And without resistance, the current approaches infinite as the voltage drops toward zero (this is a short circuit scenario)
A POWER supply is a balance between adequate current flow and a high enough voltage to overcome the resisance of the actual load. The higher the load, the less resistance and greater amount of current required to sustain a voltage.
Batteries are limited electrical supplies, not like plugging int a wall socket. The smaller the battery, the less power it is likely to have (though we have been trying to get denser power in batteries by changing chemistry).
Since batteries are limited electrical supplies, big loads with just cause them to drain very quickly. Those coin shaped batteries were never intended to provide a lot of power. They are intended for 'micro-power devices'; such as calculators or clocks or backup power for some forms of volitile memory.
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There is an issue here of misperceptions.
A lot of people think that everything in electronics can get smaller and smaller. But if you want to drive more and more powerful devices (motors, machines, home automation, heating, ventialation), the microcontroller can be quite small. But the interface devices and their power supplies can be quite large. And in some cases, devices need cooling as they can get quite hot.
We are making huge progress with MOSfets to get smaller and cooler running interfaces, but not everything can be tiny.
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I only figued out a little while ago why some bigger power tools in Australia boasted in big writing 2400 WATTS.
our mains voltage is 240 and the current is 10 amps, 10 X 240 = 2400
I always scew my face up when i hear on TV how the police had to use 50,000 volts to stop a suspect,first its unpleasant to hear of someone getting hurt but second, to me the 50,000 Volts is meaningless. i got zapped with a few hundred thousand volts on purpose in science class at school and it didnt hurt me at all!!! almost zero amps. This might be something along the lines loopy was mentioning ....static potential of tens of thousands of volts? I don't remember how the school experiment went, but i *think* it was actually a million volts I was hit with.!
how many amps does a Taser have, and how do they get so many volts out of a small battery ?
basically with an oscillator and voltage multiplication
In other words, while one might think you know everything because you can apply Ohm's Law; one must investigate the amount of power to assure safety and durabilty.
In this case, not understanding Watts resulted in brown out, but the other option is a fireball.
Yes, quite.
Go sleep it off.
Anyhow... Welcome, Mark from Creative Computing!
Since the filter is a flat circle perhaps it would be better to work with that by cutting 2 short (~1-2") sections of pipe with a diameter just a bit smaller than the diameter of the filter and clamp the filter between them. Seal the top and bottom with a flat sheet of plastic and use one section for the inlet, and the other for the exhaust. Makes the robot short for stability and access under furniture, and round for ease of maneuvering.