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I have the option of replacing my aging and almost dead lead acid batteries with Li-ion to run a 24 volt motor. The data sheet for the Li-ion cells report the cells as having a nominal voltage of 3.8 volts. If I use 6 of these cells in series, the average voltage will be slightly lower than what the lead acid provided, but if I use 7 cells in series, the average voltage will stay above 24 volts, (starting at 29.4 volts).

The lead acid batteries are rated and 18Ah and the Li-ion are rated at 30Ah and tested at a 100 amp discharge rate to provide 28.3 amps, so they can handle a high current!

I am leaning toward the 6 cell battery because when I look at the datasheet for the battery, the discharge curve shows the voltage to stay at about 3.8 volts or higher (at a 20amp discharge) until the last 15 minutes out of a 1.5 hour discharge, and for my purposes, it wouldn't be discharged that low. So this would keep the voltage at around 22 volts.

So is it better to run a DC motor with more volts than rated or less? - if you have no choice?

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    \$\begingroup\$ As an alternative idea, why not use 7 batteries and an efficient buck regulator to deliver precisely 24volts. \$\endgroup\$ – Andy aka Feb 2 '14 at 10:19
  • \$\begingroup\$ I looked on EBay, but couldn't find buck or boost converter capable of handling the current my motor requires ! (1200watts) \$\endgroup\$ – Filek Feb 3 '14 at 4:57
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You don't say what you use the batteries for, and if you are running your motor at full speed all the time. It's often better to have more voltage than you need and then modulate or regulate the current to run over a longer period of time. Then as the battery voltage decays you can go longer without recharging. A d.c. motor will rotate at a speed creating a counter voltage that equals the source voltage. Often motors are under some form of feedback, like a servo amplifier. I have controlled high inductance motors that put power back into the control amplifier. That extra energy needs to be put somewhere when slowing down the motor - it acts like a generator. Extra energy needs to be released as heat, or put back into the powering battery. You don't need any extra voltage as long as you can run at the speed you desire. Don't forget that voltage times current equals force, so can you usefully use extra torque? Do you mind accelerating faster than you have before? The insulation on a d.c. motor is at least twice the rated voltage, usually much more. The constraints have more to do with force, top speed and control. Lighting a lamp with overvoltage will burn it out sooner, but that is not usually the main constraint in motors.

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  • \$\begingroup\$ The li-ion battery will be replacing lead acid batteries in a lawn mower with a 24 volt dc motor. I'm considering the 7 cell for the higher voltage and likely higher speed and thus better grass cutting, but if I burn out the motor my wife will be somewhat unimpressed with my "improvements" to the lawm mower. \$\endgroup\$ – Filek Feb 3 '14 at 5:08
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Motor voltage ratings are designed to allow you to run the motor at its rated voltage continuously, without overheating and burning out the motor. So if you value the life of the motor, you will run it at or below its rated voltage, which in this case means six cells.

Further Reading
https://bricks.stackexchange.com/a/799

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