Loading a test meter for testing
 

I understand that the V will be lower under a load and that flight batteries should be tested under load. I purchased a Hobbico multi-test meter with this function. Well I got what I paid for in quality....This 'test under load function' no longer operates, (the leds do not light)

I also have a Fluke testmeter that I would like to set up to test batteries under load but have no idea of just what would be involved. Is it a simple matter to put one or more resistors together and attaching it to one of the test probes to get a loaded reading? If so, what resistance should be used for 4.8 and 6V battery packs?

RE: Loading a test meter for testing
 

I also prefer to use a Fluke meter. The Hobbico meter I have has a 27ohm load.

Yes it is a simple matter of putting a load resistor across the battery. A 20ohm load resistor will load the pack in the range of 240 to 340ma depending upon battery voltage.

V = I * R
4 cell = 220 to 280ma
5 cell = 275 to 350ma

For convenience the resistor was mounted in a film canister with a momentary switch so both no load and load voltage could be observed.

Technically the resistor value might be matched to the current draw of the airplane but that seems like an over kill. In addition 6v packs are generally used in higher load applications.

All this fly/no fly decision based on voltage is sort of an educated guess anyway.

RE: Loading a test meter for testing
 

First, go to Red Scholefield's Battery clinic web site for battery info. He suggest using a #44 lamp which can be gotten from Radio Shack. Also buy a bayonet socket for it. Wire the lamp across the meter leads such that the lamp is in parallel with the meter. The #44 lamp will present approx a 200 ma load to a 4 cell battery, slightly higher for a 5 cell pack. I like the lamp as a load because I can see it is working.

RE: Loading a test meter for testing
 

i prefer a simple circuit made up from a resistor and LM317 wired in constant current mode. will always draw the same current no matter how many cells you're testing or what voltage the pack is at.

i wire it with a pushbutton so i can see what difference the load makes on voltage.

dave

RE: Loading a test meter for testing
 

Well until a few minutes ago LM317 didn’t exist in the vocabulary.

But that is a good piece of information. Are you sizing for a specific current draw? It would be interesting to know the factors used to determine load current used to represent the airplane?

It seems that fly/no fly is sort of a fuzzy logic problem anyway. Apparently it is very fuzzy for the beginner. For the old-timer’s the problem seems to be fuzzy in a very different way.

RE: Loading a test meter for testing
 

BIllS,

The only plane I have that I did any testing just for curosity is an 8 foot Telemaster which uses four Futaba S148 servos. I cycle checked the battery first to determine their capacity and then flew the airplane for about four flights and recorded the time for total flight time. Then back at home I cycled the battery again to see how much was left. The plane used the equivalent of 180 ma per hour. That is it used 180 ma out of the pack. So a 600 mah pack should be good for approx 3 hours. However, two hours would be real safe. Notice, this is practical field type testing and not lab testing.

Now that is a slow TM, your mileage will vary greatly due to the type of plane, the type and kinds of servos and your flying. The best thing to do is to do what I did with your plane.

RE: Loading a test meter for testing
 

I understand and have used your approach for many years but believe there might be a more sophisticated method. Most of us long in the tooth do use the method that you describe but not all.

The methodology described doesn’t fit the beginners very well for a variety of reasons. Beginners generally have small budgets and a limited understanding of batteries and most will not have cycling equipment at the start. After a few crashes some will acquire the necessary equipment for checking the packs. Many remain glossy eyed for years when batteries are mentioned. In addition many acquaintances that are old timers are simply ‘battery clueless’ and in addition turn glossy eyed when batteries become the subject.

RE: Loading a test meter for testing
 

Anybody got a wiring diagram for a resistor and a LM317 circuit?

RE: Loading a test meter for testing
 

I searched Google to find out what LM317 meant and found a wiring diagram but don’t remember where. Sorry.

RE: Loading a test meter for testing
 

Dave:
A schematic of how you put it together would be appreciated. Googled to TI site for them and they show a variety of configurations.

RE: Loading a test meter for testing
 

Check these out:
resistor value calculator
http://www.electronics-lab.com/articles/LM317/

self powered fast battery tester
http://www.electronics-lab.com/proje...012/index.html

This shows 11 RC Model related DIY projects:
http://www.electronics-lab.com/proje...els/index.html

The avenues this hobby opens up!

RE: Loading a test meter for testing
 

Mike, the calculator in your first link is for using the LM317 as a voltage regulator not as a current regulator. figure 10 [link=http://www.st.com/stonline/books/pdf/docs/2154.pdf]here[/link] shows how to wire the IC and how to calculate the resistor value.
i use a 1.2 ohm, 2 watt resistor for 1 amp current or a 2.4 ohm, 1 watt for 0.5 amp....the resistor doesn't need to be so heavy for lower currents. for higher current you can use the LM350 that handles 3 amps. a piece of aluminium as a heat sink is always a good idea.


dave

RE: Loading a test meter for testing
 

All this is really a little excessive, for a loaded meter. The exact value of the load you apply is irrelevent as long you always use the SAME load to compare your batteries with. Testing a battery at 1/10th of it's capacity as the load will give you a quick and dirty charge check. 1/5th of it's capacity for a load will give you a very good idea of the batteries remaining charge. It's rated capacity as load is basically a medium duty stress test. Twice it's capacity as a load is a really good stress test for a packs overall condition (stress tests should be done on packs that are at least 3/4's full)

50ohm 1/2 or 3/4's watt resistors are perfect for the job. (The higher the tollerance the better)

Quick and dirty example. 1 50ohm resistor connected between the test leads of a multi meter when you attach them to a battery will give you approximatly a 100ma load on a 4.6volt pack. For every 50ohm resistor you add between the leads you increase the load approximatly 100ma's So on a 1000mah pack 1 is good for a 'quick check' 5 is good for a good check, and 10 is good for a medium stress test, 20 being good for a full stress test. Double that (2 10 20 40) for a 2000mah pack. This might sound like a lot of resistors, but you can wire the whole thing up on a piece of perfboard in an hour. Testing light medium and heavy loads and writing down the voltage every time you test the pack will warn you when the packs going bad. I'd stress test every 5 charges, use the 1/10 test for a 'is this pack useable' check and a 1/5 test if you think the pack might be lower than the 1/10th test suggests. If you don't know how to interpret the voltage reading learning is easy. Test the pack fresh off the charger for the high reading, and test it right after a run for the low end. After doing that a few times you'll get a feeling for what the voltage should be. If you do this with your batteries on a regular basis you will NEVER have a pack go bad on you before you know it's going to go bad.

RE: Loading a test meter for testing
 

oops, I ment 1/2 not 1/5th in that last post, both times.