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Hello,
   I have a strong geared DC Brushed motor that I want to control on-off, single speed, single direction @ 12VDC.  Normally, I would use a PowerFET to control the on-off action.  However, I'm running into a problem.  I have a 12V NiMH 10AH battery that I'm trying to run this motor on and the motor seems to be stalling out my battery.  With it fully charged, I'll only get about 30 seconds of continuous motion before the battery reads around 1-2V.  After the initial 30 second run, I can only get very short bursts of motion.  Even after this the battery reads >13V with no load.  I have multiple batteries that exhibit the same behavior. The motor in question has startup current at ~20A and normal running current at 3-4A when attached to the gearing.  Without the gearing (ie no load on motor), the battery can run the motor fine and it draws about 13A inrush and 2A continuous.  By the way, this behavior is seen with the battery hooked up through a switch directly to the motor.  No FET or no arduino yet.  Can someone help explain what is going on and how to fix it?  I would think that a 10AH battery would be able to handle a motor like that for about 2 hours (5A/hr).  Maybe I don't understand the properties of the battery enough.

Also should note that I have a 12VDC 30A power supply that drives the motor in it's gearing like a champ.

Any help would be greatly appreciated!
Jeremy
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Batteries operate by chemical reactions.  One property of the chemistry determines how much charge the battery holds leading to your amp-hour spec.   A 2nd property determines how fast the chemicals can usefully react limiting the maximum surge current.  A battery is not a capacitor.  You cannot draw current faster than the chemistry without damaging the battery.

In general people are using hi surge LiPo batteries.  LiPo's sold for RC planes and helicopters often quote the max safe surge current.

If size and weight are unimportant,  you cannot beat the lead/acid battery.

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Thanks so much for the info!  You mentioned that batteries are not capacitors.  That gives me an idea.  Would it be a good idea then to put in a large capacitor like a motor starting cap to take the initial inrush of current?  If so, would this cap go in series or parallel to the battery?  I'm sorry for the somewhat simple question, I have only dealt with small decoupling caps of 10uF or less to filter out noise in the line and maintain voltage.  Using a cap to store energy is a bit foreign to me.
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Thanks so much for the info!  You mentioned that batteries are not capacitors.  That gives me an idea.  Would it be a good idea then to put in a large capacitor like a motor starting cap to take the initial inrush of current?  If so, would this cap go in series or parallel to the battery?  I'm sorry for the somewhat simple question, I have only dealt with small decoupling caps of 10uF or less to filter out noise in the line and maintain voltage.  Using a cap to store energy is a bit foreign to me.

In principle you could use a capacitor in parallel with the battery to provide additional short-term current. In practice, the amount of capacitance you would need (several Farads or maybe even a few tens of Farads), coupled with the voltage (too high for supercapacitors, unless you use a lot of them in series), makes it not practical in this case.

I suggest you use a lead-acid battery if weight is not an issue, or a LiPo battery if it is.
« Last Edit: July 24, 2013, 02:57:58 pm by dc42 » Logged

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Quote
I suggest you use a lead-acid battery if weight is not an issue, or a LiPo battery if it is.

Good advice. 
Lead-acid is safer as long as you don't eat it smiley

If you have not used hi-power LiPO before, be sure to readup on safety precautions. You must prevent excessive discharge with protective circuit, often built-in (but check).  An alternative is to attached a LiPo low voltage buzzer available very cheap on ebay.  Some people use Arduino A/D input to watch the battery.

Joe



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I have some drycell power sports batteries.  Would this work?
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If you mean alkaline cells, then they are less good at providing high currents than similarly-sized NiMH cells.
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Not all NiMH batteries are the same - some are designed for high current and others aren't.
You need to try and find proper datasheets for the batteries you use (not always easy), and
find the recommended max and continuous discharge rates.

You'll also find that the effective capacity will be reduced for higher current use, so you may
have to choose a bigger battery than you think.

People often seem to assume that NiMH are a direct replacement for the old NiCd cells,
they are not, they are usually lower current, somewhat higher voltage, much greater variation
in voltage with SoC, less memory effect.

In general discharging any secondary cell system into the ground severely damages the cells,
reducing capacity dramatically - you shouldn't let this happen - monitoring battery voltage is
vital to long life.
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Most comments here are very good. I have 2 suggestions to add.

Try parallel connecting the batteries and see if it works.
It may be stupid question but are you sure the batteries are fully charged before use? Do you have quality peak charger or cheap 24 hour charger?

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