Hi, electronics newbie here with a tough one! (Or a dumb one?)

I want and need to build a current limiting circuit, which needs to be relatively small (sub 7x7x7cm) and able to limit high power draw - Up to around 3200W - To stop my batteries (2S Li-Po) from delivering more than 190A at 8.4-7.4V. I would like for them to be able to deliver freely from 0.000---1A-190A, but not higher.

About the application: It's going to be used in an RC boat. I'm running a 31-inch 3.5Kg ( <-- Nevermind the hull aspects) boat with an ESC rated for 80A at 6S (Li-Po serial configuration - 3.7V x 6 theoretical, 4.2V -3.7V x 6 in practice). The problem is that it draws way more Amps than it's rated for... A few months ago I ran it with two 3S Li-Po's, in series, capable of delivering 125A continuous each - And both batteries were subject to Amp draw over 125, so they puffed (were damaged and rendered useless). This is because the boat jumps in and out of the water due to it's weight, scale and speed (50mph+) paired with the size of normal inland waves, which lifts the propeller in and out of the water, massively changing the draw of current in short periods of time.

So I need a current limiting circuit for my batteries, preferably between the batteries and the ESC.

At this point, I need to limit

• Two 2S Li-Po's in series with limit 190A at voltages 16.8-14.8 (Limiter after serial connection)
• Each separate battery - one limiter hooked up to each battery before the serial connection, able to limit to 190A with voltages 8.4-7.4

Addendum for the interested: Boat: ProBoat Impulse 31 V2. Boat Specs Link | Video Of The Boat (NOT my property) | Battery To Limit (Specs)

I've considered fuses... But I can't find any with appropriate specs, nor would they be practical. I've also considered just using a resistor - but I can't find one capable of the potential power dissipation... I've looked at power limiting circuits - but haven't found one able to limit such amperage...

I hope I managed to explain this at least somewhat efficiently... Thanks in advance for any help!

 

Finding a PWM motor controller for that high of an amperage draw will make a package to large for your boat.

I would suggest using a bunch of high power low ohm resistors in parallel and mounted in such a way that the WATER your boat is in will supply the cooling needed. However, doing this will change your drag co-efficient on the hull and slow the boat.

The power dissipation will be your problem and cooling with air will require large surface area, again it will increase drag and slow the boat.

The best suggestion is to use a motor that will not over draw the battery arrangement, or increase battery capacity with more parallel units so that each unit stays within its max rating. Making a winning combination is not easy, physics always gets in the way of dreams somehow.

 

I would suggest disconnecting the batteries when overload occurs. Use a relay for minimum currnt drop.

 

A a 200A or greater MOSFET could likely be used as a switched (PWM) type current limiter circuit. Note, however that a TO-220 case is wire-bond limited to about 120A, even though the transistor chip can be rated for more pulse current, so you need one in a higher current case for a continuous 190A or parallel two or more of them.

You could also use a 200A solid-state relay, but those aren't cheap.

 

A a 200A or greater MOSFET could likely be used as a switched (PWM) type current limiter circuit. Note, however that a TO-220 case is wire-bond limited to about 120A, even though the transistor chip can be rated for more pulse current, so you need one in a higher current case for a continuous 190A or parallel two or more of them.

I'm not sure I understand the MOSFET suggestion correctly - Are you suggesting that I build a circuit which supplies PWM power to the ESC, with some type of control mechanism which monitors and limits Amp draw via the mosfet operation? If so, I have to say I doubt it will work or be safe, because the ESC takes battery power which is continuous (Afaik), and how would the batteries react to supplying PWM power? The ESC isn't plug-and-use (Tech-term?), it has start-up protocols for self check and voltage-in check and stuff... Or did I get this all wrong?

Also, total newbie question: Component power dissipation: If rated at for example 280W (Like this MOSFET), how do I convert it to a known temperature unit? I've searched for this before without finding an answer... Guess I've missed a technical term for it

I would suggest disconnecting the batteries when overload occurs. Use a relay for minimum currnt drop.

You could also use a 200A solid-state relay, but those aren't cheap.

So a SS relay as a PWM generator, right?

 

...
Also, total newbie question: Component power dissipation: If rated at for example 280W (Like this MOSFET), how do I convert it to a known temperature unit? I've searched for this before without finding an answer... Guess I've missed a technical term for it
...

The quantity you are looking for is called "thermal resistance". It has units of degree C per watt. It describes the temperature rise between two surfaces like "junction to case" or "case to ambient" in case of a heat sink. So you take a power dissipation and multiply it by a thermal resistance and you compute the rise in the junction temperature above the ambient for a given power dissipation. BTW using a 200 A part for 190 watts dissipation is cutting it pretty close with no headroom.

 

At your current skill level, building something for this application and being successful is not likely to happen, and if it does it will not be cheap and will not be quick.

you need an overspeed prevention device, if I read your original post correctly. The controller and motor operate correctly EXCEPT when the boat skips or comes out of the water briefly?

The motor then draws much more current through the controller than the batteries are happy delivering?

Is that correct?

 

You talk about the speed control, but not the motor. Do you have a link? I guess I'm a little surprised the ESC doesn't blow before the batteries.

 

chjange to a series wound motor, they decrease current on overspeed.

 

So do brushless. That's what is strange about this.

 

At your current skill level, building something for this application and being successful is not likely to happen, and if it does it will not be cheap and will not be quick.

you need an overspeed prevention device, if I read your original post correctly. The controller and motor operate correctly EXCEPT when the boat skips or comes out of the water briefly?

The motor then draws much more current through the controller than the batteries are happy delivering?

Is that correct?

Not entirely,

The motor and controller always operates correctly but when the boat is rushing through rough water at high speeds it jumps in and out of the water, while it is being pumped with lots of Amps, which creates massive power spikes because when the propeller leaves the water it starts spinning extremely fast due to the significantly decreased rotational resistance when in the air, and when it then hits the water again at extreme RPM rates it is slowed down again due to the resistance of the water to the rotation, which again provides huge spikes in load...

The boat does this 30+ times at stretch-runs lasting (normally) under 10 seconds at speeds above 40mph, jumping to heights above water (completely out of the water) ranging from 3-15cm in lenghts of 20-150cm... Which causes the massive load spikes which causes the overdraw of Amps which my batteries cannot handle.


Listen to the sound of the motor as the boat here (just an example video of a similar boat) jumps in and out of the water. Note: This is a gasoline powered boat, but the same happens here, of course. The extent to which the RPM increases out of water is greater with electric boats, as mine is, since they have less torque but much higher RPM rate potential, making it even worse:

(I recommend starting at 1:50)

You talk about the speed control, but not the motor. Do you have a link? I guess I'm a little surprised the ESC doesn't blow before the batteries.

Yep, it is this one

Not much more info is available, it's just a motor for a hobby grade boat

• 6-pole
• 1500Kv (1500 RPM @ no load per Volt applied)
• 3630 - size&form factor
• Water cooling jacket comes with it
• Takes up to 6s Li-Po (25.2V max)

Here is the ESC if it is of help

• Multi-chemistry battery compatible
• 12AWG input wires
• 80A at 6s (25.2V) throughput capable

chjange to a series wound motor, they decrease current on overspeed.

?

It's a 6-pole motor, how the configuration of use of those poles is, I don't know. There is no information on it available. What is a series wound motor? Are you talking about a 3-pole triangle/star configured motor?

So do brushless. That's what is strange about this.

Are series wound motors and brushless motors different types of motors?

 

refering to a dc motor. an ac motor should decrease current with decrease of load when the prop comes out of the water.

 

Yes, series and brushless are different.
I did a google of puffy lipos and there seems to be several causes.
Overcharge. Storing fully charged. Discharging below 3 volts, to name a few.
I'm not sure that having it come out of the water is a problem as the current is low when there is no load on the motor.
The battery says it can do 380 amps in bursts. that is like starting a car. I would think the wires, motor resistance and the ESC would limit the current below this.
I would look for another cause, but you could add a few inches of wire (say 12 awg) to add a little more resistance to the whole thing.