I'm planning to run 2 DC motors in parallel for a project.

Each of my motors has rated power of 250 W and a rated current draw of 14 amps at 24V (so 28 amps from 2 motors).

After researching around, it seems that a used car battery is a good solution (versatile and cheap). I would need to use 2 in series. However, are they okay to use for such high continuous currents for long periods of time (50-60 minutes in my case)?

Any help would be appreciated. Thank you.

  • \$\begingroup\$ try a deep cycle marine battery if you can get one. you can find good info on websites dealing with DIY wind and solar power \$\endgroup\$ – jsotola Nov 4 '17 at 4:39
  • \$\begingroup\$ power ratings on motors will not tell you how much energy they will consume, their power cosumption depends on how heavily they are loaded. 28A no problem for a car battery, sustaining it for 50 minutes could be however, but it not yet known that 28A is required. \$\endgroup\$ – Jasen Nov 4 '17 at 5:25

The car batteries are designed to supply 300 amps or so for a few seconds. They would not ordinarily be discharged more than 30% or so very often. They will not last very long if they are discharged nearly completely every time they are used. There are batteries that are very similar to car batteries, but they are designed to be discharged more completely every time they are used. They will not do well at 300 amps, but that is not what you need. The proper battery is called a deep discharge battery or a marine battery.

Marine batteries are not widely available for 24 volts, so you will need two connected in series plus to minus just like flashlight batteries.

You are not going to get electrocuted by 12 or 24 volts, but you could be badly burned or even explode a battery is it gets shorted.

Two fully charged batteries connected in series should provide 25 or 26 volts with no load. Loaded as you propose, the voltage will be closer to 24 volts. The motors will just run a little faster or slower if the voltage is a little above or below 24 volts. As long as the motors don't continuously draw more current than they are rated for, motor power should not be a concern.

  • \$\begingroup\$ I see. The issue is that I need a very high current (~160 A) to provide the stall torque to get the motors going. And the motors will be turned on and off periodically (on for 2, off for 2-3 minutes). I know that car batteries are capable of outputting such currents, but can deep cycle batteries do that too? \$\endgroup\$ – user2999870 Nov 4 '17 at 9:51
  • \$\begingroup\$ You will need to look at some deep cycle specifications. There are dual-purpose marine batteries that might be more suitable than deep cycle batteries. There are also starting duty marine batteries. I saw a comparison on the internet somewhere. I will try to post a link or additional information later. Perhaps more importantly, you should consider using an electronic speed controller (ESC) to control the motor current. Allowing the full stall torque, particularly with that duty cycle will be hard on the motors. \$\endgroup\$ – Charles Cowie Nov 4 '17 at 14:16
  • \$\begingroup\$ In the time when DC motors with electronic speed control were commonly used for industrial equipment, the de-facto standard for controller current-limit adjustment was up to 150% of rated current. That leads me to believe it is not advisable to operate a DC motor above 150% of rated current. If you need more that 150% of rated torque for your application, you need to carefully review the motor's overload ratings. You may need a bank of parallel and series connected batteries or some other battery technology such as lithium batteries. \$\endgroup\$ – Charles Cowie Nov 4 '17 at 16:42
  • \$\begingroup\$ I am using the motors to build a pitching machine. The motors accelerate wheels to their equilibrium (my target is 2750 RPM). The machine will be used to practice for 2 minute sessions, 3 minute rest and repeat. So, the motor will go over the rated torque only during the time when the wheel accelerates (if ideal, it will go from stall torque to rated torque). I really appreciate your insight so far. \$\endgroup\$ – user2999870 Nov 4 '17 at 18:03
  • \$\begingroup\$ If the current is not limited, the motor will indeed go from stall torque to normal running torque every time the motor starts. You need to calculate the time required to accelerate the inertia and convert that to a time vs current duty cycle. The current will be limited by the internal resistance of the battery, plus the resistance of the interconnecting wiring and the motor's armature resistance. You can determine how much energy is dissipated in the battery and motor, but it may be difficult to determine how much the battery and motor can tolerate. \$\endgroup\$ – Charles Cowie Nov 4 '17 at 18:39

Short answer - possibly.

Batteries have a wide range of capacities, typically measured in Amp-Hours (sometimes Watt-Hours) which is more-or-less how many hours the battery can supply 1 amp. The battery in my truck is 70Ah, so it could supply 28 amps for around 2.5 hours. Of course, this is a very rough estimate - the battery manufacturer will specify the allowable discharge rates and times. It's possible that my calculation is nowhere near accurate for that particular battery. You would have to check the rating of your batteries, which should specify the capacity and continuous amperage capability.

Two car batteries can safely be put in series to get a total voltage of 24 if

  1. they have the same ratings
  2. they are both charged

To ensure this, you should use two brand-new, identical batteries in series. Remember, series would be negative of battery 1 connected to positive of battery 2, with the total voltage across positive of battery 1 and negative of battery 2. Better yet, you could buy a single 24 V automotive battery (yes, these exist for special purpose vehicles).

Required PSA

First, and foremost, let me state that automotive batteries are designed with the specific purpose of delivering an instantaneous current of hundreds (sometimes thousands) of amps in order to start a car. That kind of power can kill you.

There might be better options

More commonly, car batteries are rated in CCA (Cold-cranking amps) which just means how many amps then can deliver at a freezing temperature for about 30 seconds without dropping the voltage too much. Car batteries are designed to put out a LOT of current all at once, then recharge while your car is running, not discharge slowly down to nothing then be recharged like other common batteries.

Lead acid batteries are really tough and versatile, and they will work for this purpose if they are rated for the power you need. A better option might be lithium batteries, many of which are designed to deliver a high continuous current load.

You also have to remember that car batteries see a high voltage drop under load. They are typically charged up to around 14V and can drop as low as 7.2 during extreme current usage. Without a voltage vs current chart, you would have to measure the voltage across your batteries during the expected current draw to know what to expect. The voltage will also drop over time as the charge depletes. Ensure your motors can handle this potential range of source voltage.

Double Check your Ratings

Power = Current * Voltage: 14A * 24V = 336 W. You stated their rating at 250W.

The current a motor draws and the voltage across its coils is also dependent upon the motor load. There are many types of DC motors that behave a bit differently. You'll need to provide more information about your motor and the batteries you have in mind to get a more precise answer.

  • \$\begingroup\$ I neglected to mention deep-discharge or marine batteries, but why down votes? Is any information incorrect? (I am known to make mistakes late at night...) \$\endgroup\$ – Kurt E. Clothier Nov 6 '17 at 18:55

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