Bow Thruster Voltage Drop

[MarTer]

Hi, I've just had a bow thruster fitted & upgraded the cabling to cope.
I've installed 4 Tudor High Tech 75A/hr 630CCA batteries & upgraded the cabling from the batteries to the battery switch, approx 3 metres, (via a single terminal post) to 70mm, inline with the 70mm cabling to the thruster. The 70mm crimp seemed a bit loose so I crimped the lugs down to 50mm.
When running the thruster I'm getting a voltage drop from 12.8v to 11.75v at the batteries down to 9.96v at the switch. This then drops to 7.8v at the thruster, (approx 14 metre return run).
The battery inter-connections are 35mm on negative & 3 x 35mm & 1 x 25mm on the positive. I was assured this was OK over short, 25cm, cables.
I'm going to double up the 70mm cabling from the batteries to the switch & engine as a last resort but the drop from the batteries to the switch under load seems excessive, 1.8v over 3 metres?
Grateful fo any ideas. TIA

 

[BoyBlue49]

Hi MarTer,

Are you using a solenoid, is the switch upto the current and are you running the engine at 1,000 + revs ?

My set up is with a solenoid and 3x 140 a/h traction batteries and I run the engine on fast tick over.(These double up as the boat service batteries).

They do take a fair whack out of the batteries, but as there has been no need, I have never tested the voltage at the thruster.

 

[VicS]

To really comment on your installation and problems it is necessary to know the current consumption of the bow thruster.
What is it?

To produce a volts drop of 1.8 volts in 3 m of 70mm² cable would require a current of about 2500 amps.

I assume the current is a lot less than this so you should perhaps be looking at the quality of the crimps and other connections.

Your volts drop from 9.96 to 7.8 volts between the switch and the thruster is also too great .

It will be possible to calculate a suitable cable size once the current drawn by the bow thruster is known.

Also confirm that the cable runs quoted are the sum of the positive and negative cable lengths

What is the minimum acceptable voltage at the bowthruster.

 

[MarTer]

The bow thruster is a Snieper SE80/185T with a nominal current draw of 530A.
All crimps & connections are new & have been seen by an electrician.
The 3 metres from battery to switch is a single run, the 14 metres to the thruster is a return run.
The acceptable voltage at the thruster is 8.5v.
Thanks for your time.

 

[VicS]

OKAY
You know the load drops the battery volts to 11.75 volts but you want a minimum of 8.5 at the bowthruster so you can only afford to lose a maximum of 3.25 volts in the wiring.

A drop of 3.25 volts at 530 amps would occur with a resistance of 3.25/530 = 0.0061 ohms.

If I understand you correctly the total length of the cable run, battery positive to switch, switch to bow-thruster and bow-thruster back to battery negative is 17 metres,

This means that the cable must have a maximum resistance of 0.0061 x 1000 / 17 = 0.36 ohms per 1000 metres

On this basis anything heavier than 50mm² or AWG 0 would be satisfactory but only as the minimum.

70mm² gives a small safety margin and should drop the volts only by a total of 2.17 volts.

AWG 3/0 ( approx 85mm²), with a resistance of 0.2 ohms per 1000 metres, should reduce the volts drop to 1.8 volts.

Any volts loss you measure outside these figures will be due to poorly made crimps, bad connections or a high resistance in the switch.

You say "switch" but presumably you are using a suitably rated solenoid controlled remotely?

There is a way in which multiple batteries in a battery bank should be connected to even out the effects of volts drop in the interconnecting cables and ensure that loads, and charging currents, are equally shared between the individual batteries
See http://www.smartgauge.co.uk/batt_con.html

 

[claudio]

According to the technical spec, you are losing more than 20% of the capacity of the bow thruster with the current installation.

Thrust at 10.5V/21V* (kg · lbs) 80 · 176
Thrust at 12V/24V* (kg · lbs) 96 · 212

http://www.side-power.com/public/templates/default.php?page=42

 

[VicS]

According to the technical spec, you are losing more than 20% of the capacity of the bow thruster with the current installation.

Thrust at 10.5V/21V* (kg · lbs) 80 · 176
Thrust at 12V/24V* (kg · lbs) 96 · 212

http://www.side-power.com/public/templates/default.php?page=42

Given that information the cabling ought to be sized to reduce the volts drop even further.

125mm² to reduce it to 1.25 volts

But is this not getting silly. With a bow thruster of this power dontt you need the battery located close to it. Or an engine driven hydraulic system.

 

[William_H]

Measuring volt drop

The trick is to attach the positive terminal of your meter to the battery post or the terminal clamp.
Use the negative volt meter lead on low volts range to measure the actual volt drop. So from batt+ to motor +ve you would seem to get one or 2 volts drop with the thruster running. Now if you go along the +ve line you can check actual voltage of the first cable the switch the second cable and the operating switch or solenoid. Each part will drop a small voltage but there should be no dramatic volt drop measured.
Next check the negative line in the same way.Meter on the battery terminal and check each stage up to the motor terminals. Any fault condition will show up as a big drop over one part. You might want to poke a hole in the insulation to get a sharp probe into the copper wire to check or discount the lug terminations. (seal the hole later)
If you find the majority of the drop is in the wire itself that is your answer. i suspect it more likely to be in switch contacts or terminals. good luck olewill