Powering a 12V 11.5A motor with a 5V battery

Other Hardware Motors, Mechanics, Power and CNC
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I'm making a project that requires me to power two 12V 11.5A(max) brushless motors with a max power of 130watts each. I currently have two 3500mAh 18650 batteries for a total of 7000mAh with a holder for the batteries that can output 5volts at 3Amps max.

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I have a dc to dc boost converter that I can use to increase the voltage to 12v.
Since the motors can draw up to 11.5 amps, will this still run and simply drain the battery quickly/run the motor at slower speeds? or do I need a different setup?

If so, what's the best way to achieve this? Different batteries or a circuit to increase the amps?
I only need the motors to run for 30mins max, probably closer to 15 mins.

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That's 15 watts. You need 115 more watts to power a 130 W motor.

Actually more, to start the motor moving.

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You need a different power source. To power the motors you would need about 30 Amps at 5 volts.

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12V x 11.5A = 138 Watts
5V x 27.6A = 138 Watts

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Is there a way to not use the battery holder, just wire the batteries and force it to output at 12V 11.5A while draining it faster? If so, what would the circuit look like?

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It would consist of a different power source that can supply more than 130 W at 12V.

A motorcycle battery would be one example. What you want to do is not feasible with the parts you have.

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12V * 11.5 A = 138 Watts

So two motors at 130 Watts = 260 W

260 W / 5V = 52 A ! Do you have a rough idea of how thick the cable has to be for 52 A?
Diameter of the copper 10mm !

If course there exist circuits where a voltage can be transformed to a higher value.
Though a DC-DC-Step-Up-converter that is able to deliver 300 Watts will cost 5 times more than a 3S lithium-polymere battery (in short LiPo).

3S delivers 12.6V (if fully charged down to 11.1V ) straight out of the battery-contacts

Your batteries have 3500 mAh. If they are Lithium-polymere the voltage is 3.7V
2*3.5 Ah * 3.7V = 25.9 Wh.
From a lithium-polymere battery you should only discharge 80% = 20 Wh
Your motors draw 260 W
This means with the motors running at full power your batteries will be discharged down to 20% withing

20 Wh / 260 W = 0.08 hours = 4.8 minutes.
They will be fully discharged after 6 minutes.

Is this battery-runtime long enough?

You wrote that you use

For driving brushless motors you use electronic speed controllers in short ESC

These are recommended for 1S, 2S, 3S, 4S etc where 2S, 3S etc means two lithium-polymere cells in series or 3 cells, 4cells etc.

This would be two 3S 15A ESC. Two because each brushless-motor requires its own ESC.

3S means 3 cells in series which is 3*3.7V = 11.1V which will be well enough for your 12V motor.

Though I have doubts that your motor specified as 12V 11.5A is a brushless-motor.

Brushless Motors are never specified with a voltage. They are always specified as
1S, 2S, 3S, 4S...... and a value with unit kV
Example 3S 1400 kV max. 20A

If you want decent help you should post much more information about your project.

This means post a datasheet of the brushless motor or at least the link where you have bought them or at very least a picture and write the dimensions of the motor

Write a pretty detailed description of what the two motors will drive

best regards Stefan

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StefanL38 this is an Excellent explanation!

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Use a 12V 20Ah lead acid battery

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Thanks for taking your time to explain this to me. This is a great explanation!

And maybe, 6mins might be enough for what I’m trying to do; I’m only trying to run a few short tests regarding these thrusters in a swimming pool.
But as you’ve mentioned, I’m assuming the cables would still have to be 10mm thick if I want to use my batteries? I don’t think it would be worth the hassle of sourcing those cables.

As to my project, here is a link to the motors: http://www.fwdenergy.eu/product/t100-marine-thruster/

They’re underwater thrusters for rov use. These particular models are discontinued, so the data sheet link doesn’t work anymore, but I’ve checked with the wayback machine and these draw around 16amps at max thrust.

https://bluerobotics.com/store/thrusters/speed-controllers/besc30-r3/

This is the esc for my thrusters. This one actually might be a newer version thats rated for a higher amperage than the one I have, but they look identical.

I have an arduino uno hooked up to the escs.

The batteries are 18650 3500mAh batteries, but now I understand that these are basically useless for these motors unless I plan to keep them running at ~1amps.

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Are there specific batteries you would recommend for these thrusters?
Do you have any other suggestions for my project?

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The stator of the thruster is sealed. The green covering of the coils.
But the ESC you linked to is not sealed. This means you have to make connections
from the battery and the ESC to the motorcabe inside a housing and then the cable that goes outside to the outside mounted thruster has to be sealed.

If the housing is carefully sealed against water, inside the housing you can use normal components and normal contacts.

So any kind of 3S-Lipo manufactured for the use in RC-cars or RC-planes will do.

Motors for RC-cars and RC-planes easily draw 20, 30, 40 , 50 amperes and need a high amperage battery.

The discharge-current of these Lipos is specified as "X"C where the number for "X" is the factor you multiply the nominal capacity of the battery

Example:

A lipo 3S 2000 mAh rated 20C means:
the maximum discharge-current is
20 x 2 A = 40 A (20C meaning twenty times the capacity)

Almost any RC-car / RC-plane Lipo will have minimum 20C.
Most of them will have 30C or 40C and you can buy Lipos that go up to 70C

3S means 3 3.7V-cells are connected in serial which means 3.7V + 3.7V + 3.7V = 11.1 V as nominmal voltage.

A Fully charged Lipo-cell goes up to 4.2V so 3S = 3 * 4.2V = 12.6V

Attention ! For charging LiPo-batteries you need a special suited LiPo-charger.

If you ever load a LiPo to more than 4.2V per cell the LiPo-battery will catch fire!

The voltage of each cell has to be monitored to not exceed 4.2V. This is the reason why such LiPos have so called balancer-plug. With this balancer-plug the voltages of each cell will be monitored.

If you do pretests of the thrusters with longer wires and the battery above water
a 12V lead-acid-battery that is able to dis-charge with a current of 30A will be sufficient.
You should not reach the maximum dis-charge-current of the battery.
A battery with max dis-charge-current 25A is too close to the limit with 2x 11.5 = 23A

So my recommendation is a 12V lead-acid-battery dis-charge-current minimum 30A
Any even used 12V car-battery with a capacity of 40 Ah will do.
(though these car-batteries need 12 hours to re-charge.

Or a LiPo of minimum 30A / 20C = 1.5Ah = 1500 mA.

If a 20 Wh battery is empty after 5 minutes highly depends on if you were running the motors on full throttle continiously or not.

For your pre-tests another option would be to use a high amperage power-supply 12V 30A

Or using a Lipo 3S 3000 mAh which would have

3 * 3.7V * 3 Ah * 80% = 26.64 Wh

Something like this
https://hobbyking.com/de_de/turnigy-battery-3000mah-3s-20c-lipo-pack-xt-60.html
Such batteries can be charged in one hour

plus a LiPo-charger
https://hobbyking.com/de_de/turnigy-accucell-6-80w-10a-1-6s-balancer-charger-suitable-for-lihv.html

No idea where you are located. If your town has an RC-model shop you could look there for a special offer

best regards Stefan

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