If a battery pack is 12V @ 90Ah and the motor draws 42 amps, how long will it run at full speed

Announcement - Help us fight the BOTS! Please report all spam including stuff in your inbox!

ajw22

Well-known member
Joined
Nov 16, 2018
Messages
635
Somewhere around 2 hours, right
very very roughly, yes. But it depends on a lot of details and factors.
Some of those factors are:

Depends on the manufacturer, but the 90Ah are likely when tested at much slower discharge rates.
If it's a lead acid battery, it's usually measured over 20hours. The usable capacity may drop by 1/2 if discharged at ~10x that rate.
Li-Ion cells are usually measured over ~5 hours. The drop in usable capacity won't be that great at ~2.5x that rate, but still noticeable at perhaps ~10%.

Temperature

Actual voltage of the battery (higher voltage -> motor will run faster)

That's just on the battery side, and the load(motor) side will have a lot of factors, too.
 
Last edited:

B8rez 2g4

Member
Joined
Jun 21, 2021
Messages
32
Battery: 90ah @ 12v = 1080wh Load: 42a * 12v = 504w. If steady 504w, then 504w * 1hour = 504wh.

At 80% DOD - that's .8 * 1080wh = 864wh of useable power in the battery.

864wh/504wh = 1.7hrs estimated run time.

Perfect ! Thanks man

very very roughly, yes. But it depends on a lot of details and factors.
Some of those factors are:

Depends on the manufacturer, but the 90Ah are likely when tested at much slower discharge rates.
If it's a lead acid battery, it's usually measured over 20hours. The usable capacity may drop by 1/2 if discharged at ~10x that rate.
Li-Ion cells are usually measured over ~5 hours. The drop in usable capacity won't be that great at ~2.5x that rate, but still noticeable at perhaps ~10%.

Temperature

Actual voltage of the battery (higher voltage -> motor will run faster)

That's just on the battery side, and the load(motor) side will have a lot of factors, too.

Willing to bet constant draw at max throttle is much less than 42 Amps.

Yeah man ... I'm doing what I can to increase range

The biggest factor was getting a PWM speed controller : difference was night and day on low speeds

Working on adding another battery to get to 120 Ah
 

ajw22

Well-known member
Joined
Nov 16, 2018
Messages
635
Saying exactly what type of battery and what motor you're using and for what, might get you more useful answers.

Yes, adding more batteries to increase capacity will help, more than simple calculation will suggest.
The individual batteries will discharge at slower rates, so will be able to deliver closer to their rated capacity. They'll also heat up less, leading to longer lifespan.
 

B8rez 2g4

Member
Joined
Jun 21, 2021
Messages
32
Oh hey. Yeah they're 18650's . 3 modular batteries in parallel ( 3s14p ) adding a fourth

The motor is a Minnkota Endura Max 45 with pwm
 

ajw22

Well-known member
Joined
Nov 16, 2018
Messages
635
Oh, that one... found the other thread:

Have you solved the issue with the motor cutting out prematurely due to the lower working voltage range of 3S LiIon batteries?
 

Korishan

Moderator
Joined
Jan 7, 2017
Messages
6,509
864wh/504wh = 1.7hrs estimated run time.
This is assuming 100% efficiency, which we know is not possible. Actual run time may be more like 80%, or 1.3-1.4hrs. And even 80% is a bit optimistic, to be honest.
 

B8rez 2g4

Member
Joined
Jun 21, 2021
Messages
32
Have you solved the issue with the motor cutting out prematurely due to the lower working voltage range of 3S LiIon batteries?

Not yet . I haven't drained the new pack low enough yet on the new motor . Hopefully I'll get on the water and test it this weekend.


This is assuming 100% efficiency, which we know is not possible. Actual run time may be more like 80%, or 1.3-1.4hrs. And even 80% is a bit optimistic, to be honest.

I'm building and adding packs as fast as I can test them. Anything else I can do to improve efficiency ? Increase range ?
 

ajw22

Well-known member
Joined
Nov 16, 2018
Messages
635
This may be obvious, but air/water drag quadruples when you double the speed. So very roughly speaking, reducing speed to 1/2 will double range. Might not make much difference below a certain speed, and possibly even counterproductive when going against the wind/flow.

You could wire in a toggle switch to reroute power through a boost converter when the battery voltage gets too low for the controller. That way you have no conversion efficiency loss when the battery is above 30% charge (or whatever), and can still wring out energy out of the battery when below 30%.

Is adding solar panels a possibility? Perhaps one of those lightweight thin flexible plastic ones?
 

Korishan

Moderator
Joined
Jan 7, 2017
Messages
6,509
Anything else I can do to improve efficiency ? Increase range ?
Not really, unless you buy some really expensive equipment.
Instead of using 6 gauge wire, for example, you'd use 2 gauge.
Instead of running at 12V, you'd go with 48V, or 96V (most places going over 100VDC requires special permits or is flat out illegal for DIYers and need EEs to do the work)
Much more efficient buck converter, or inverter, or charger.

Generally speaking, efficiency on power conversion/storage is around 80% on average. You can get a little higher, but it starts to begin to become dimensioning returns.

You could wire in a toggle switch to reroute power through a boost converter when the battery voltage gets too low for the controller. That way you have no conversion efficiency loss when the battery is above 30% charge
It probably would be easier, cheaper, and more efficient to go with higher string count, 4s instead of 3s, and use buck convertor to run everything. Going with 5 or more in series would be even better.

A buck convertor is far more efficient than a boost convertor. A boost convertor has to use magnetic fields to recreate a higher voltage. This is inherently loosing power during the boosting as a good chunk is dispersed as heat generation.
A buck convertor on the other hand just pulses the power flow and a capacitor to smooth out the output. Think of filling a water tank that has a drain hole. To keep the water at a certain level the valve is either on, or off (no flow regulation in this example with partially open/closed). So to keep a certain amount in the tank, you would turn on the valve until the water line reaches a certain level, then turn off and wait until the power drops to a certain level, and repeat. The output drain would never see a difference.

Running a DC motor would be far better to run it on a buck converter, and just have a beefy enough convertor to handle amps required. There are some out there that can do 50A on the output @ 12V
 
Top