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Unread 06-02-2013, 16:24
matthewgkerr matthewgkerr is offline
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Battery life

Hi all, I hope everyone is doing well. My team has come to me with what appears to be a fairly serious issue. We have built the Andy Mark drive frame in the long configuration with 6 high grip wheels - all wheel drive. We added everything needed to drive the robot using the large CIM motors and the gear box as per the Andy Mark instructions. On a fully charge motor, if we drive the robot around in circles, the battery drops quickly to a point where the motors can no longer turn the robot. This happens in less than 2 minutes. I believe the robot can still drive straight - it just can't turn.

Last year we built the Andy Mark in the short (square) configuration with 2 wheel drive. In the old configuration we easily got a full game with the shooting components (and other stuff) all working.

I realise we are probably driving on a surface with greater friction, but I wasn't expecting the new configuration to be such a drain on the battery.

So my questions are:
- Have any other teams come across this problem and can they make suggestions?
- Should I put a chamfer on the edge of the wheels?
- Should we lower the middle wheel so the robot rocks on the middle wheel?
- Are the ways we can manipulate the software and the motor controller to optimise battery usage?
- Any general suggestions?

Thanks everyone, and good luck!
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Unread 06-02-2013, 16:47
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Re: Battery life

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Originally Posted by matthewgkerr View Post
Should we lower the middle wheel so the robot rocks on the middle wheel?
Are the center wheels presently lower at all ?

If not, you need to lower them. Bot not necessarily to the point where the robot "rocks" on them.


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Unread 06-02-2013, 16:50
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Re: Battery life

After the two minutes are the motors warm to the touch?

I ask because if you are draining an 18 amp hour battery in two minutes, you're probably averaging a current flow well in excess of 100A. Even if you have four CIMs in your drivetrain, that will definitely warm your motors after two minutes!

To find out more precisely what current you are drawing, try hooking up a voltmeter across your 120A circuit breaker... you'll find in this thread that Bill_B from 2170 has estimated that there is a voltage drop across the main breaker of roughly 1mV per amp.

If the motors are warming up, or you are observing high current flow through the main breaker, then that is because for some reason your drive train is working the motors paricularly hard... perhaps the gear ratio is off, perhaps there is some binding... perhaps it is because if you don't have at least a little bit of rocker (middle wheels dropped a bit) that the sidways forces when you turn are really, really high and require a lot of torque (and thus current) to overcome.

If your motors aren't warming up, then perhaps you have either a battery issue, or a charger issue. Tools such as the "Battery Beak" can help you determine your battery's state of charge and internal resistance quite easily. If you read the related documentation you'll see what tests the beak performs and may be able to replicate some of those tests using materials you have on hand.

In my experience a 150 robot powered by four CIMs should get 8-10 minutes of continuous operation with good batteries before showing a real drop off in voltages. You'll get more time ifyou drive gently, less if you flog it hard. After a second battery, or about 15 minutes of driving you may find the motors getting warm enough that you want to rest them for a bit.

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Unread 06-02-2013, 16:51
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Re: Battery life

What is your total reduction, and what size wheels are you using? If the ratio is low (and the bigger the wheels, the more reduction you need), that means the motors will be running close to stall all the time, which means they draw a high current. The robot will also have low acceleration and low pushing power.

Also, have you checked how much friction is in the system? If you disconnect the wheels (I haven't seen your design, so I don't know if you can do this), they should spin freely. The gearbox should be able to turn without much noise (scraping or binding).

If you post a picture, and details on your gearbox (and pulleys or sprockets, if any), we may be able to help more.

Good luck!
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Unread 06-02-2013, 16:52
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Re: Battery life

If you used the c-channel from the Andymark kit, you'll notice the center axle wheels are not in line with the outer wheels. If you construct the drivetrain with these upside down,the center wheel will be higher thn the ore wheels. In that case you need to flip the long c-channel pieces over so the center wheel is lower than the outer wheels
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Unread 10-02-2013, 21:02
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Re: Battery life

Quote:
Originally Posted by matthewgkerr View Post
So my questions are:
- Have any other teams come across this problem and can they make suggestions? Now you see the difference between the wide and narrow orientation.
- Should I put a chamfer on the edge of the wheels? Two words: Omni wheels
- Should we lower the middle wheel so the robot rocks on the middle wheel? Yes
- Are there ways we can manipulate the software and the motor controller to optimise battery usage? Maybe, but work on the mechanics first.
- Any general suggestions?
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Unread 10-02-2013, 21:13
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Re: Battery life

I would guess the center wheel is raised and not dropped like others have suggested. Out of curiosity what surface are you driving on?
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Unread 10-02-2013, 23:57
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Re: Battery life

What did you do with your batteries after the competition last year? Lead acid batteries like the ones we use must be maintained, not just thrown in a closet. They should be occasionally put on charge and topped up. If a battery was discharged and left to sit, its condition will degrade over time and the capacity will diminish. The addage "Use it or lose it" should be followed when caring for your batteries.

I would bet that your batteries have perished due to poor storage practice or are defective.

Charge your batteries up, then get yourself a battery load tester like this one http://www.princessauto.com/pal/prod...ry-Load-Tester. If the cables and clamps attached to the tester are any smaller than the type of wire we use to connect the battery to the robot (6 gauge) then it's not a proper load tester. Test the batteries under load to see if they are defective. You can also take them (fully charged of course) to an automotive shop and ask them to test them with a similar load tester. Some autoparts stores offer free alternator and battery testing and will also have the equipment to perform a similar load test.

A voltmeter will not tell you if the batteries are good. They must be loaded heavily and the voltage drop measured to determine if they are truly good or defective.
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Last edited by fox46 : 11-02-2013 at 00:19.
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Unread 11-02-2013, 00:15
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Re: Battery life

I searched for "Battery Test Service" in Melbourne Australia and found this - http://batteriesdirect.99on.com/

On their homepage it states
"Free Battery Testing: We offer our customers good old fashioned service by testing your battery FREE of charge."

Eagle Batteries
Paul Street
Croydon VIC 3136, Australia
(03) 9725 9066
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Last edited by fox46 : 11-02-2013 at 00:21.
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Unread 11-02-2013, 00:22
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Re: Battery life

There are a few things you can do to help.
Make sure your center wheels are dropped
Load Test your batteries with a load tester
Check your gear ratios
Lubricate your transmissions (shouldn't make a huge difference,but is necessary if you like gears with the teeth on them)
Test each wheel and shaft and make sure that they spin freely, and that nothing is too tight
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Unread 11-02-2013, 09:18
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Re: Battery life

If your scrub force is too high (this would be the case with the raised center wheel and 6 High Grip wheels), you will be pulling a lot of current during turns; enough, in fact, to trip your auto resetting breakers. Is the robot jerking or can you hear clicking when turning?
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