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Understanding battery capacity: Ah is not A
- 22 April 2014R. Keisler:
I don't know enough about your system to know for sure. There's got to be some limit on your alternator and whatever battery charging circuit is in your car, and while my impression is that lead acid batteries are among the more forgiving regarding charging, you still might limit their life or otherwise damage them if you just do what you are proposing.
- Jan
Understanding battery capacity: Ah is not A
- 22 April 2014Tchopko,
I am concerned that with the level of electronics understanding you have presented, you might be risking damaging or destroying some fairly expensive equipment. It seems like you might be better off looking for some pre-made solution, like a general-purpose external/backup battery for phones or other electronics.
On to your specific case. First off, you should realize that with four batteries instead of the built-in two, you will get at most 2x the battery life. You should not put your batteries in parallel, and if you just put all your batteries in parallel, you would only have the 1.2V individual battery voltage anyway. You should also go through a regulator, too, to make sure you actually give your device the 5V it is expecting since the fully charged batteries might be 5.5V or more and damage the device, and they might get too low as they discharge. Something like this step-up/step-down regulator could work:
http://www.pololu.com/product/2123
With that regulator, it would be better to go with 6 AA batteries if you have the room for it. But, if you go down this path, make sure you get a better understanding of what you are doing and that you are ready for the consequences if you make a mistake.
- Jan
Understanding battery capacity: Ah is not A
- 14 April 2014Dan,
I don't know what that 12A/5h notation indicates. My first guess is that it's a typo or other mistake; my second one is that it's 12Ah at a five hour discharge rate. If the batteries weigh about the same and have similar dimensions, their capacities will be similar.
- Jan
Understanding battery capacity: Ah is not A
- 12 March 2014"How many mAh a battery will last" is not a good question formulation because it sounds like you want to know how long a battery will last just based on its capacity. It's like asking how many days until you need to refuel your car that has a 12-gallon gas tank without saying anything about how much you drive per day.
As I said in the 9V alkaline battery example, you're basically going to get about the same amount of energy per weight for any given battery chemistry, so since a 9V weighs about the same as a AA, it's going to have about the same amount of energy, and since energy is measured in Wh, which is Ah times V, a 9V battery will have about six times lower mAh rating than a 1.5V battery of the same weight. If you happen to know your application for the 9V draws the same current as a specific application for AA or AAA, then yeah, it will not last as long, but that kind of situation is rare since there aren't many products that can run from either 9V or 1.5V.
- Jan
Introduction to Jan's blog
- 7 February 2014Hi, Raymond.
I am happy to hear you have had good experiences with us. Lynxmotion has sentimental value for me since I played with some of their earliest arms and hexapods almost 20 years ago, but I have not really kept up with their more recent offerings. Is there something you see RobotShop actively doing to hurt Lynxmotion? My impression was that Lynxmotion was mostly the effort of its founder and that he retired; if that's the case, RobotShop would be saving it from disappearing completely rather than causing a problem. But I have no inside information and have not even paid attention to public information about it, so for all I know, all the original Lynxmotion folks could still be active in the company and butting heads with their new parent company.
I would like to offer more of the servo-based parts and kits you are asking about, but there is nothing specific in the works, so it's unlikely that we would have anything new for at least the next few months. We might start with something basic like small grippers; for bigger, more complicated systems, it's not clear to me that general-purpose RC servos are the way to go. Something like the Robotis/Bioloid products, with specialized (and proprietary) servos offer a lot more for robotics. How do you think these newer systems compare to the Lynxmotion type of products?
- Jan
Introduction to servos
- 7 February 2014Adam,
Your question has nothing to do with servos, but briefly: cheap RC toys often have separate frequencies, like 27MHz and 49MHz, to avoid interference. More sophisticated RC systems use several dozen channels in the 72MHz and 75MHz frequencies, and you have to use different channels if you want to prevent interference between multiple systems operating at the same time and place. There are also rules about which frequencies can be used for surface models and which can be used for aircraft; these things can all vary by country. Newer 2.4GHz spread spectrum systems automatically jump around several channels, so multiple systems are not supposed to interfere with each other (you will have to do some kind of "binding" process so that a particular receiver knows which particular transmitter to listen to).
- Jan
Force and torque
- 6 February 2014Hi, Inaam.
The 9.81 in your calculations indicates you are getting thrown off by thinking about the force being in Newtons, but I think you are better off thinking of your 2kg as a force since stepper motor torques are commonly specified in kg-cm, anyway. So, if you have a 2.5cm radius pulley wheel directly on your motor and hang 2kg on it, you would need 5 kg-cm to hold it. Keep in mind that the holding torque that is usually specified is a stepper motor's strongest torque, so you will need a motor with a much higher holding torque if you want to be able to lift the 2kg instead of just hold it in place.
The 0.5 factor you have in one of your alternatives makes me nervous that you are talking about some complicated system of pulleys instead of just the single wheel on the motor shaft (what I called a spool in the article). If that is your situation, you would have to say what that pulley arrangement is for anyone to be able to calculate the forces and torques involved.
- Jan
Ten years in Las Vegas
- 31 January 2014John,
You should come to the LVBots meeting next Thursday at 6:00. More info is at:
http://www.meetup.com/LVBots/
If that works for you and you can come early, you should contact us in advance (call or email the general contact address) and someone can probably show you around.
- Jan
Servo control interface in detail
- 31 January 2014Christiano,
I am happy my post was helpful to you. Unfortunately, I'm not sure where you're getting that conclusion, and most of what you are saying does not make sense. So, here are a bunch of quick facts that I hope help clear things up:
1. Current coming from a supply should only depend on what the load (servo in this case) draws. If the power supply cannot deliver the current needed, get a better supply. Therefore we should only talk about what a servo draws, not what a supply sources.
2. Servos do not draw constant current. You should see that from the screenshots in the post.
3. The peak current is a function of the motor speed and the voltage applied.
4. The average current depends on the mechanical load on the servo.
5. There is no fundamental difference between analog and digital servos regarding the above points.
6. The current depends on the the servo and load. For the specific cases in the article above, I say what the currents are and you can see them in the screenshots. In general, it will range from maybe 0.5 A for a small or weak servo to several amps for a high performance servo to 10 A for a really high performance servo.
- Jan
Understanding battery capacity: Ah is not A
- 10 January 2014Hi, Gerard.
First, a little nitpick: a battery that can do 1A discharge for 50 hours does not mean it can get charged in 50 hours at 1A. But still, if you're getting a full charge at 1A in 10 hours and not doing anything special voltage-wise, you can be sure the capacity is nowhere near 50 Ah.
Your supplier seems to acknowledge that the actual capacity is 12 Ah and gives you the basic math that a battery with 50 Ah would weigh over 10 pounds. I don't know what your actual thing weighs, but if it's a couple of pounds, then that would be another sanity check that your battery has nowhere near 50 Ah.
The last part does start sounding like BS. Maybe he's just trying to say that the electronics is good enough to support a 50 Ah battery if you had one there. Perhaps this is some modular product where the battery part can be upgraded or retrofitted.
- Jan