Hi All,

I'm looking for some information around stepping down voltage for large amperage loads.

The load is for a a bow thruster (electric motor) on a boat. The manual says it's 24v and a maximum of 690amps. It's used sporadically and in bursts. Normally these are hooked up to a couple of 12v car/marine batteries. As the boat has a full off-grid 48v lithium battery bank to run the rest of the power requirements, I was wondering if I can leverage this for the 24v motor.

Can I use something to step down the voltage directly from the lithium 48v battery bank to 24v and can you do it to handle the high amperage demand of this motor.

Thanks in advance

 

A buck converter can be used to reduce 48V to 24V. However, this will not be a simple device, as the current is relatively high.

 

Can your battery bank deliver that kind of current (or at least the 350 A that would still be needed if you could step things down to half the voltage with 100% efficiency)?

Remember, we have no idea what "rest of the power requirements" might be on your boat.

 

The battery bank is made up of 12 x PowerPlus Eco4847P batteries. They have a continuous discharge rate of 63A each. I have 12 batteries so will this be enough?

 

Firstly, you have to consider whether it would be cheaper to find a 48V motor, or to add a second one in series than it would be to build a stepdown converter.
As it is a motor, presumably a brushed motor, then a simple PWM circuit would do, because you don't need the inductor which would be a major part of the cost.
When I say "simple", it is the concept that is simple. Switching 630A is never simple.

 

You have 12 piece of 4V Lithium battery. It gives 48V, therefore they are all in series. Their maximum current is 63A, it is not enough to supply the necessary ( 690 / 2 = 345) 345A.
Additionally, I don't know how large Your boat is. I have seen already buck converter with 500A output capability. It was as large as a cabinet in the kitchen. Or, 8-10 microwave oven packed together. It's weight was about 50-60 kg. I've seen its inside, there was not much free space and it was modern enough, so the size is not due to the obsolete construction.
My advice is: forget it.

 

You have 12 piece of 4V Lithium battery. It gives 48V, therefore they are all in series. Their maximum current is 63A, it is not enough to supply the necessary ( 690 / 2 = 345) 345A.
Additionally, I don't know how large Your boat is. I have seen already buck converter with 500A output capability. It was as large as a cabinet in the kitchen. Or, 8-10 microwave oven packed together. It's weight was about 50-60 kg. I've seen its inside, there was not much free space and it was modern enough, so the size is not due to the obsolete construction.
My advice is: forget it.

The manual says it’s 48V

 

The bow thruster 690 amp max draw is over ten times the current rating of your present battery system. But the voltage is only twice as much. So the short answer is that a 100% efficiency voltage conversion it is far short.
BUT there is a simple scheme that is probably cheaper than a different motor, which, if it were the same power, would in theory draw 345 amps at 48 volts.
The alternative will be a separate battery group, probably four 12 volt batteries in series/parallel, installed near the bow thruster system, with short power leads from the very nearby monster power contactor. Then a small charger to keep that pack ready, fed from the supply for the 48 volt battery pack.
This is based on the similarity to the cranking requirements of the diesel engines on larger trucks. The battery pack should not overheat if only used for short bursts. BUT you certainly will need to fuse he circuit.

 

700 A @ 24 V
Wow!
That's 17 kW! Even my house doesn't ever draw that much power.
You need a separate 24 V power source for that.

 

700 A @ 24 V
Wow!
That's 17 kW! Even my house doesn't ever draw that much power.
You need a separate 24 V power source for that.

Certainly Mr.C. is right, thus my suggestion for the four high current batteries.
Then I went back to post #1 and saw:" Normally these are hooked up to a couple of 12v car/marine batteries. "
The reality is that switching that kind of load is a "very big deal." Both the wiring and especially the switching must be capable of surviving some serious arcs.
And as that was stated as the maximum current, the running current might be half of that, maybe 345 amps, only six times the rating of the 48 volt stack.

Now I am wondering if that 690 amps is the "LR" (Locked Rotor) current rating. What sort of boat would have that heavy duty a power supply on board?? ALSO, now I am wondering if the TS is in the process of considering the purchase of such an item, or has already purchased it, or has purchased a craft that has it in place but not installed as for the wiring part. 17KW is about 20 HP, which is quite a big DC motor.

What sort of "boat" is this bow thruster installed on, anyway?? And what sort of control system did it have originally?? OR, is this a part of a retrofit of a much older boat? I was part of one of those projects many years ago. And I have declined invitations to be part of other similar projects since then.

 

Why didn’t I think of using a series resistor? It only needs to dissipate 17kW.

 

Why didn’t I think of using a series resistor? It only needs to dissipate 17kW.

By my math the battery bank can only deliver 2835 watts for a short while. There may be problems with using a resistor.

 

Certainly though, the 48 volts battery stack, or the generator that charges them, could supply a 24 volt charger for that monster 24 volt motor. OR, a cheating trick would be a switch scheme to put the 4 batteries in series and charge the stack directly. Once charged, that 24 volt pack could provide quite a few minutes worth of short bursts.

 

I came across exactly the reversing relay for the job recently. It should be standard repair part for a power driven pallet mover, one of those that works like a fork-lift, but is steered, not driven. It has two coils and four bolt terminals .
It is a Curtis/Albright brand type LC88-4 DC motor control double relay control. It is only about 2 inches by 3 inches by 3 inches high, with the positive terminal and the two motor terminals on top and the negative terminal on one side.

 

As an alternative option, you can make a buck converter with several high current mosfets of the same type in parallel such as irfp4468. Of course with an inductor design that suits your power needs.

 

As an alternative option, you can make a buck converter with several high current mosfets of the same type in parallel such as irfp4468. Of course with an inductor design that suits your power needs.

An inductor is not required (see post #5) because there will be enough inductance in the motor.

 

Think carefully about what it would take to build a switching converter able to deliver 690 amps!! Even if that current is just the starting inrush, the wires alone will be massive!! The other issue, which is much more serious, is that the 48 volt battery system IS NO WAY able to deliver

As an alternative option, you can make a buck converter with several high current mosfets of the same type in parallel such as irfp4468. Of course with an inductor design that suits your power needs.

The only big problem with a switching converter will be the requirement for 850% efficiency!! Even if you locate the design, this website has an issue with systems exceeding 100% efficiency. And certainly 17,000 watts out with 3024 watts (48V X 63A) in would be an engineering marvel.

NOTE TO MODERATORS: I am NOT suggesting that this is possible!!!

 

Think carefully about what it would take to build a switching converter able to deliver 690 amps!! Even if that current is just the starting inrush, the wires alone will be massive!! The other issue, which is much more serious, is that the 48 volt battery system IS NO WAY able to deliver

The only big problem with a switching converter will be the requirement for 850% efficiency!! Even if you locate the design, this website has an issue with systems exceeding 100% efficiency. And certainly 17,000 watts out with 3024 watts (48V X 63A) in would be an engineering marvel.

NOTE TO MODERATORS: I am NOT suggesting that this is possible!!!

Where did you get those figures?
The TS said "The battery bank is made up of 12 x PowerPlus Eco4847P batteries. They have a continuous discharge rate of 63A each."
The PowerPlus Eco4847P is a 48V battery. He has a 48v system, so those batteries are in parallel, and therefore can supply 756A.

 

OK, what I did was I guessed that they were all in series to provide 48 volts. Which is what others have also assumed.


Aside from that, consider that high current wiring is always a big deal! Consider that even at 48 volts, that power would require 345 amps to a 100% efficient step down switcher. THAT is why batteries and a control relay close to the bow thruster motor make sense.