Question

NiMH batteries - max safe discharge? Big pts!

Asked by: BlewItUp

I'm working on a robotics project with two 300ma@24VDC gearhead motors. The trouble is, when stalled they rocket up to 2A each!! I plugged in a power supply I had and the short-circuit current was 1.2A (although it was rated 500ma...). However, when I plugged in a motor and stalled it, I read over 2 amps! Holy sh__! (I no longer wonder about the unfortunate results of using that poor 600mA h-bridge) Need to read up on inductive loads!! I plan to use two motors at once, and for a battery I've got 20 1200mAh 1.2V cells, totaling 24 volts. The cell chemistry is Ni-MH and the type is 121AAM (1.2Ah, 3/4-AA size).

My question is:
Is it safe to pull 4 AMPS off this pack?
This robot is for a creativity competition and they're very safety-concerned. If something explodes or cells start venting, the judges might not be too impressed...

I'm adding points...

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Asked On
2003-02-12 at 06:50:46ID20510772
Tags

121aam

Topic

Miscellaneous Hardware

Participating Experts
4
Points
275
Comments
4

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Answers

 

by: SysExpertPosted on 2003-02-12 at 07:16:11ID: 7934140

I would check the battery site, but in general, I would say that it should not be a problem, pulling at 5x the battery rating.
The biggest problem is heat.

Remember that these can be charged at the same rate as discharge .

Check the GE and other battery sites for discharge charts.

I hope this helps !

 

by: jhancePosted on 2003-02-12 at 09:04:31ID: 7935009

NiMH batteries are very much like NiCD cells in this respect.  They can and will supply a very large amount of current.  For this reason it is strongly recommended that there be a FUSE in the circuit with them to prevent a fire in the case of a short circuit.

The MAX discharge of a specific cell varies and you really should study the manufacturer's technical data sheet for the specific cell you are using.  This is the safest bet since different cells are designed for different applications.  Some are designed with short but heavy loads in mind, others for long term low-rate discharge.

I think you'll find that the 121AAM battery is NOT designed for this heavy draw use.  It seems to be an electronic (cordless phone) application where the draw is only 200 mA or so.

I'd again suggest consulting the manufacturer's spec sheet and see what they say is the max safe discharge.

 

by: ridPosted on 2003-02-12 at 12:27:27ID: 7936484

As suggested, a fuse to protect cells from excessive discharge current is a good idea. You could also clamp the stall current with a resistor, limiting the stall current to a "safe" value. You will have to take the reduced stall and start/low-speed torque into consideration in this case.

Regards
/RID

 

by: Krismark01Posted on 2003-02-12 at 22:30:02ID: 7939831

A couple of suggestions. Put a Supercap parallel to your motor terminals. What this does is take the spikes off your battery and prolong the life of your battery. AVX makes supercaps. You want one with Equivalent Series Resitance.

Add a small limiting resistance(3-5 ohms) in series also, to minimize your spike. Now you get a nice RC to slow your spikes.

You also want to add a protection circuit, not a fuse to prevent your 2 amp draw when you're stalling your motor.

Look for an implementation that uses a PTC and and SCR. Make sure it resets itself when you remove the short condition. Having to remove the load to reset will suck during competition.

I would seriously rethink the battery type you are using. I'd advise to go for batteries used for drills. By the way, don't try to charge those 20 NiCads with a 24V charger, you will blow some of them up. If you're not space limited,  Gel tpe lead acids sounds like the type you need for this application. Also, NiCads self discharge and you only have 18 minutes to use this battery at 4 amps.  

20120131-EE-VQP-002

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