Testing Motors, Jags, Batteries

[Tom Line]

Originally Posted by Ether

If you mean 120v household 100 watt bulbs, how many amps do you think a 100 watt bulb would draw if connected to a 12v battery?

Actually, I was thinking about a 12V 100 Watt bulb. P=VI, so it should draw around 8 amps, correct? They are fairly common and inexpensive online. For instance:
http://www.amazon.com/Light-Bulb-12V.../dp/B000KKX44Y

[Ether]

Originally Posted by Tom Line

Actually, I was thinking about a 12V 100 Watt bulb. P=VI, so it should draw around 8 amps, correct?

Yes, thanks for clarifying.

[Mike Copioli]

Originally Posted by Ether

Is there a technical paper somewhere discussing how it computes state of charge and overall health?

Not yet. We are working on a user manual, it should be available shortly. I do not want to get into too much detail but in a nut shell SOC is based on voltage scaled linearly between two points, the data was found empirically and from data sheets on both chemistries and adjusted for NIMH based on number of cells. Health is derived mainly from internal resistance and was also found empirically. Much of the data was obtained from testing the NP18-12B 12V 17.2 Ah batteries used in FRC, the NIMH batteries used in VEX and the 12v NIMH 3000 mAh batteries used in FTC. Other non-FRC/VEX batteries were tested to test for deviations among manufacturers.

[Al Skierkiewicz]

Mike showed me the proto at IRI. If you want a battery tester that will give you a good idea of the relative health of your battery and something that you can stick in your pocket, this is the item for you. We have used something I call the "mini-sun". It is a series of projector lamps that can be used for a battery load. Switch in as many as are needed for the load you want to test with.
For long term, repeatable battery testing with graphing and curve comparisons, you cannot beat the West Mountain CBAIII products. We test all batteries every year using a 7.5 amp discharge. The CBAIII connects to your computer through a USB port, connects a load to the battery, computes amp/hour capacity and displays a continuous discharge curve. This enables a user to see the relative capacity compared to previous years and other batteries while observing if one cell has less capacity than the others. They often will run holiday sales on their products. I have seen the $149 price-tag drop and may include free shipping. If you buy one please tell them you heard about their products from me and you are buying for a FIRST team. I am trying to get them involved with local teams. They are in Waukesha, WI.
http://www.westmountainradio.com/cba.php

[JamesCH95]

Originally Posted by eagle33199

Here is a better test setup that any FRC team can do with items from the KoP:
Hook up a motor to a Jaguar, and into a gear box. On the other side, hook up the output shaft on something that will produce a moderate load on the motor (for example, have it lift a bucket of sand, or a concrete block) and a rotary encoder from the KoP. Control it all through the cRio. You need a moderate load on there to ensure the motor is actually working - without a load, it's a lot easier for the motor to actually do its job, and a lot harder to tell if something isn't right.

Your program should be pretty simple - spin the motor for x seconds, and record the revolutions from the encoder. Repeat a few times so you can have some consistent readings. Swap in a different motor, and do it all over again.

You'll have to record the voltage during the test and recharge the battery often - you'll see as it wears down that the characteristics of your output change, even if the motor/Jaguar stays the same.

If done right, this type of test can help provide your team with valuable insight into how all of these components work, and how they may deteriorate with time.

I like this, though I think it would be a good idea to put the motor under load during the test, otherwise it doesn't really mean much. Ideally there would be a load schedule, where load varies with time. Testing just the free speed of a motor is, IMHO, largely useless because the motor will be generating little power and drawing little current, this won't exercise the system very well.

The load could be a water pump, fan, another CIM motor (used as a generator) connected to an extra 100W lightbulb, a metal disc spinning through a magnetic field (eddy current brake), etc.

[Jon Stratis]

Originally Posted by JamesCH95

I like this, though I think it would be a good idea to put the motor under load during the test, otherwise it doesn't really mean much. Ideally there would be a load schedule, where load varies with time. Testing just the free speed of a motor is, IMHO, largely useless because the motor will be generating little power and drawing little current, this won't exercise the system very well.

The load could be a water pump, fan, another CIM motor (used as a generator) connected to an extra 100W lightbulb, a metal disc spinning through a magnetic field (eddy current brake), etc.

Completely agree... As I said, "On the other side, hook up the output shaft on something that will produce a moderate load on the motor (for example, have it lift a bucket of sand, or a concrete block)". The goal, obviously, was to use items that teams probably have sitting around their build spaces so you don't have to buy anything specifically for this project.

[JamesCH95]

Originally Posted by eagle33199

Completely agree... As I said, "On the other side, hook up the output shaft on something that will produce a moderate load on the motor (for example, have it lift a bucket of sand, or a concrete block)". The goal, obviously, was to use items that teams probably have sitting around their build spaces so you don't have to buy anything specifically for this project.

That's what happens when I jump CD before having coffee. I totally missed that, sorry!