How Long Will This Battery Last...Is My Math Right?

[rs14smith]

Hey everyone,

I purchased two 24VDC electric wheelchair motors a couple of months ago, and each motor uses 10Amps, according to the label on the side of the motor.

I found on eBay (**broken link removed**) a set of two 12v batteries that are made for wheelchairs and are rated to have 35Ah. My main question here is, is my math correct for determining how long the motors should last off the batteries if I hook the batteries up in series to give me 24v total:

Calculations:
Motors
P = V x I
= 24v x 10A =240Watts per motor
Total Watts for Motors
P = 240W x 2 = 480Watts total for both motors

Batteries
P = V x I
= 24v x 35Ah = 840Watt hours

The total time batteries will last:
840 Watt hours / 480 Watts = 1.75

So, the system should run for about 2 hours?

 

[MikeMl]

If you are PWM'ing the motors with any reasonable controllers, then you should get more time because the motors will not draw 10A all the time. Also, with a more sophisticated controller which includes dynamic braking, you should be able to back some of the energy... OTOH, you should not discharge the batteries all the way to zero, if you expect to get reasonable life out of them.

 

[rs14smith]

If you are PWM'ing the motors with any reasonable controllers, then you should get more time because the motors will not draw 10A all the time. Also, with a more sophisticated controller which includes dynamic braking, you should be able to back some of the energy... OTOH, you should not discharge the batteries all the way to zero, if you expect to get reasonable life out of them.

Yeah I'm aware of the effects of fully discharging batteries, my main concern and question here is, are my calculations correct provided above, which I assume they are ? Also, according to the eBay post, I would assume these batteries are full deep cycle batteries but I could not find that stated anywhere on the auction page, but I have contacted the seller.

 

[DemonicSandwich]

Yes, in an ideal world they would be correct. But an ideal world this is not.
SLA and AGM batteries have their capacities rated at a 20 hour discharge rate.
Another words, these 35Ah batteries would supply 35Ah of capacity to a 1.75A load for 20 hours.

But the higher the discharge rate, the less capacity is available.

If these were used to supply a 20A constant load, they would yield only about 21Ah, or about 1 hour of run time before being effectively flat.

Of course you're likely not running the motors wide open the entire time so the average current will be lower, and thus capacity (and run time) will greater than 1 hour.

 

[crutschow]

Your math is right but not your assumptions.

The motors should draw 10A only at maximum load and maximum speed. Is that how you will be using them?

As DS noted, the battery will have significantly less than its rated capacity at a 20A load. I would think that 1 hour maximum operation at that load sounds about right.

 

[rs14smith]

Your math is right but not your assumptions.

The motors should draw 10A only at maximum load and maximum speed. Is that how you will be using them?

As DS noted, the battery will have significantly less than its rated capacity at a 20A load. I would think that 1 hour maximum operation at that load sounds about right.

Right, I know that they will not be using 10A the entire time, but I just wanted to calculate the max and worst case senario time I could use them if they were, and then based on that I could get an idea of how long they would last.

 

[audioguru]

Do fat people drive an overloaded wheelchair at full speed up the side of a mountain??
I wonder what an overloaded Toyota Prius will do driving up the side of a mountain on its battery because it ran out of gasoline.