Motor vs Battery ???

Naphtali Moore

Ok, first Im building a robot and using 2 - 12v 15amp motors to drive it. Heres the problem. I want to be able to run both the motors at full power for at least one hour. (I figure that will give me several hours of real use) The batteries I have are 12v 1.5ah lead acid. (got 20 of them with which to play)

Is it right to think that using one of these batteries for each motor would give me 6 minutes of run time? (1.5 * 60) = 90am / 15a = 6m ·I know IRL its lot more tricky to calculate but in the simplest terms would this be true?

If I use 2 batteries, for one motor, run in series; would that give me 24v for 1.5ah? If I then used a DC-DC converter to go from 24v - 12v would that then give me 3ah? What type of converter should I use?

Would it be possible to just run multiple batteries in parallel to get 1.5ah * number of batteries at 12v?

Thanks for any help you can give me!
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T Chap

You could add batteries to get more time, but you can get 17 aH batteries like below and carry a lot less weight around. More weight = less time. You don't *2 by down converting a 24v to 12, there is heat involved in going down. 2 17aH batteries shoud get both motors running pretty solid for one hour.


www.allelectronics.com/cgi-bin/item/GC-217/160/12V_17AH_SEALED_LEAD_ACID_BATTERY_.html

Post Edited (originator) : 3/10/2007 9:33:45 AM GMT

Bruce Bates

Naphtali -

You have the basic idea right, but forget any converters of any type, as they're quite inefficient. Here is all you need to remember regarding the connection of batteries:

Series circuits increase the VOLTAGE and it's ADDITIVE

Parallel circuits increase the AMPERAGE and it is ADDITIVE

Batteries can be put in series AND parallel at the same time (not the very same batteries however). In other words, some of your batteries can be wired in series to increase the VOLTAGE. Wiring other batteries in PARALLEL with those SERIES wired batteries will then increase the AMPERAGE.

Regards,

Bruce Bates

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Gadgetman

The best you could hope for in converting from 24V to 12V is 85% efficiency(I believe that some Switch-mode control chips are advertised with 90% efficiency, but to get those last 5% is costly... ), and if running at 15A...

you'd need to draw almost 9A from a 24V setup to produce a 15A current at 12V, not the 'ideal' 7.5A. That means 1.5A x 24V = 36W energy lost as heat, for one motor. and if both motors are going at full speed, up to 72W are being converted into heat. That's about what a P4 dumps.

And that assumes you get the DC-DC convertor to work properly.
(Switch-mode converters should be renamed fickle-mode... )
More realistic results are in the range of 75 - 80%, which means it turns the thing into a space-heater...

The battery that originator linked to looks like they could do the trick, but...
I reccommend that you study the datasheets carefully.
Not all batteries can deliver the Amperage you want(or have a reduced discharge time). and some can 'lag' when the draw changes suddenly.
you may have to connect up several in parallell to get the required power.

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slamer

Good old SLA batteries. I'll admit it, I'm a lead slinger. I've used SLA batteries in several mobile platforms.

SLA batteries are very good at dumping a lot of current for a short period of time. One of their limitations is as soon as you put a heavy load on them their voltages starts to sag in a lovely rainbow arc of death. Usually you get a inital voltage drop of a volt or so under moderate loads. SLA batteries also only produce about 1/2 of their Amp hour rating under·heavy loads. So a 12 Ah rating will actually only provide about 6 Ah of actual performance. So you should usually figure you get about 50% performance out of SLA's

As mentioned above putting any batteries in Parallel increeses the battery·Size or useful amp-hours available and the voltage remains the same.

Putting batteries in Series increeses the voltage available but the battery Size or useful amp-hours remains the same.

The performance of your batteries can aslo vary greatly by the load applied. There is a handy online calculator that you might find useful for testing battery performance under heavy·loads. You can find it here.

·http://architeuthis-dux.org/torquecalc.asp

·You can plug in your own values and see what happens.

I use a little different math but my numbers come out about the same.

http://www.teamkiss.com/newkiss1220.html




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Steven Kirk Nelson (slamer)

Team K.I.S.S
Build Safe, Build Mean, Build Strong!

Post Edited (slamer) : 3/10/2007 3:25:06 PM GMT