Anonymous
Anonymous asked in Science & MathematicsPhysics · 2 months ago

# What happens if you use a weak battery for a powerful motor?

To be more specific, I want to throw together a cheap ebike. I want the highest voltage to make the most efficient usage of the amperage of my battery (which will be the most expensive part of the whole thing), yet the wattage rating of the motors vary significantly for the 36v and 48v kits I come across. Since P=IV, makes sense that a higher voltage will draw less amperage to produce the same amount of power. I want to use the cheapest battery I can, and those seem to be rated at 500w max delivery (for whatever that's worth.. they're all chinese unbranded). So my question is, what are the dangers or drawbacks to using a relatively weak battery with a powerful motor? I'm assuming the motor will just draw too much current and wear the battery out quicker. Does anyone know what would happen though?

Update:

The voltage would be the same for the battery and the motor, but the battery amperage would be relatively low, while the motor is rated to pull many amps.

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• cosmo
Lv 7
2 months ago

The battery is a constant voltage source up until its maximum rated discharge current, beyond which the voltage will drop while the high current output likely damages the battery.  So you don't want to exceed the maximum discharge current.  Yes, a higher voltage battery produces more power for a given current, but higher voltage/lower current just means there are more pairs of battery plates, whereas lower voltage/higher current means there are fewer plates but they're bigger.  The total energy storage is pretty much just going to be proportional to the total mass of the battery (and the type of battery it is --- there's a reason why electric cars use expensive, high-tech lithium batteries which have a high energy storage per unit mass).

Now, motors are somewhat funny electrical elements.  The torque at the motor shaft is proportional to the current supplied, regardless of the speed of the motor (it can be stalled out, or turning at high speed).  The power produced by the motor is the motor speed times the torque. The "back EMF" of the motor means that the voltage at the motor is a measure of the speed of the motor shaft. Approximately --- since the power is proportional to IV, and the torque is proportional to I, and the power is also equal to the torque times the speed, the speed is more-or-less proportional to V.   A DC motor will have a rated top speed and a corresponding peak voltage.    In an electric vehicle, what you want is to be able to control the acceleration of the vehicle in order to avoid jerkiness and to have control over the speed of the vehicle so its speed does not run away and ideally gives some breaking force, perhaps even regenerative breaking, on long downhills.  For that, you really need a motor controller, and a well-designed controller will not let you exceed the rated output of your battery.

Absent a motor controller, the vehicle will simply want to accelerate up to a maximum speed that is determined by the voltage of the battery and the windings of the motor, provided that the load on the motor from inclines, wind resistance, etc. is within the rated torque.  The drain on the battery will depend on the torque needed to maintain that speed (if possible) against the forces of resistance.  The motor at stall should not draw more current than the rating of the battery (the rated output power of the battery divided by the nominal battery voltage).

You need to choose the motor and motor controller first, then the battery.

You could implement a stupid "bang-bang" controller, where you the vehicle driver simply connect or disconnect the battery from the motor with a switch, depending on whether you want the motor to provide forward torque or not.  That, at least, would likely prevent you from killing yourself on a run-away vehicle.  You'd want a "dead man switch" that disconnects if you let go of it.  For that, you just have to work out the current corresponding to the stall torque and the battery voltage would correspond to the motor voltage at its nominal top speed.

• D g
Lv 7
2 months ago

Batteries hold so much amperage and give it at a specific voltage

If you had a 40 amp motor and ran it with a battery that only gives 20 amps you would not last long plus the battery would likely be damaged from the severe drain

• Vaman
Lv 7
2 months ago

Calculate the power needed. This you can guess by your (weight+cycle weight)*1/2 v^2* time you want to travel. The energy content is known. You know the voltage of the battery by amp/hr.

• Anonymous
2 months ago

You want to create a great thing, but you only want it to be infinitesimally expensive.  Add to this unlikely scenario, all your misused buzzwords.  What does that leave?  Your fantasy.

If you want a high power, low cost battery (power source) then talk to Elon Musk or Ann Marie Sastry.  They are far past the fantasy stage, but just what do they have?  Anything powerful AND inexpensive?  Not that I know of.

You're not the only person who wants a better mousetrap.

• 2 months ago

If a battery cannot supply sufficient current it is because every battery has internal resistance.  As you increase the current more of the power is lost in the battery and less is given to the motor.  The efficiency falls.  Whereas a low powered motor with a higher current capacity battery is significantly more efficient.  Meaning that for the same storage capacity it will go further and faster than the cheap battery. It also lasts longer too.  Basically undermotoring is a far more satisfactory design than overmotoring.

• Mmm J
Lv 7
2 months ago

If you use a weak battery with a powerful motor, the battery power won't last very long. We don't know how long you expect the battery to provide power to the motor or what the weight of the bike and the rider will be or if you'll be pulling any hills (up).

I've made a couple of solar generators... I started a few years back using one 100 amp-hour lead-acid deep cycle battery. You don't want the battery to get less than about 50% capacity, so you basically have 50 amp-hours to work with. The 100 amp-hour batteries weigh about 65 pounds each.

In the last couple of years, I've moved to using Lithium Iron Phosphate (LiFePO) 100 amp hour batteries. The "smart" ones are about \$900, use up to 90%-95% of their power storage without damaging them and they weigh more like 25 pounds. They last longer per charge and are much more forgiving when recharging or reconditioning. They are about 5x more than the lead acid deep cycle batteries, but I've found they are well worth the investment.

You have a choice: Do it the right way the first time, or expect to do it again (which ends up costing more in the mid-long term).

• Jim
Lv 7
2 months ago

All batteries are "weak" even in Teslas. What you need is a combo of both serial (for voltage), and parallel (for current).

• 2 months ago

It is a bad idea to use a weak battery to drive a strong motor.

Think of the battery as a tank. When the tank runs out it all quits.

The tank has a pump if the pump won't supply fuel fast enough the bike won't move very fast. Use a battery big enough to drive the bike. The motor will spin very fast. You will need to gear it down to a manageable speed.

• 2 months ago

If you overload the battery, the voltage will drop excessively due to its internal resistance and it will get hot.

The battery will degrade very fast. It's false economy compared to using a properly rated battery that should have a good lifespan.

• 2 months ago

The battery can be thought of as a constant voltage source.  It has a finite charge capacity which is why batteries used for motor usually have an Amp-hour rating (current x time  =charge).  If you short the battery, it will pump out as much current as it can and likely ruin the battery in the process.  And of course as you increase the load across the battery it will deliver less current but at the rated voltage, hence the output power goes down.  If you have a motor that has a current rating at some fixed voltage, and you use a higher voltage battery, it is likely you will damage the motor.  If you use a lower voltage battery, you won't pull enough current to run the motor