Bowthruster battery also engine starter battery?

[Kinsale373]

If the boat is a 46 footer you probaly have a 4 to 5 Kw Thruster. W+VA so you Current draw for a 4kW is 4000W/12V =333A
The only sensible way to supply this is from a Large Ah battery positioned as close to the Thruster to keep the run of very thick cables from the battery to the thruster to a min length. At over 300 Amps the heat loss and voltage drop due to resistance in thinner cables would impact on the thruster performance.
Don't dream of using the existing Starter battery. This may be just a Diesel Starting battery and not a deep cycle battery. With the current requirement of the bowthruser you always run a risk of flattening even a suitable rated deep cycle battery if you get into a tough spot and need alot of thrusting so it makes sense not to use the starter battery.
With 300A + been drawn from the battery constanty when thrusting your Alternator will need time to replace the energy drawn.
Generally the House Batterys will be toward the rear of the boat near the engines, If you want to run from these you will need to run that very heavy cable the length of the boat , it becomes very costly and increases weight.
The ususal set up is to have a large designated (Deep Cycle) Battery as close a possible to the thruster and supply this from the alternator with appropriate cables. I have a 120Ah battery running a 3 kW thruster and I can get 20 to 30 of approx 10 Sec thrusts without noticing power drop if I need it . The Manual for the thruster should give advise on the installation and how to size cables.
I have the boat 2 seasons and initially could get only 5 to 6 of 5 sec thrusts and then it began to fade. I blamed the battery and put in a new 120 Ah leasure battery. The real problem turned out to be pitting on the contacts of the reverse/ Forward relay. These take a real beating at 250to 300A. Because of the high currents all components are worked hard in these systems. Next time I'd fit proper Deep Cycle battery.
Kinsale 373

 

[pvb]

The ususal set up is to have a large designated (Deep Cycle) Battery as close a possible to the thruster and supply this from the alternator with appropriate cables.

Why a Deep Cycle battery? These usually have a more limited ability to supply large currents. An AGM starting battery would be a better choice. Also, these "appropriate cables" still need to be quite hefty, and as well as the battery there's a need for additional fusing. In many cases, the cost of putting in bigger cables and running it from the existing batteries is much the same.

 

[lw395]

If the boat is a 46 footer you probaly have a 4 to 5 Kw Thruster. W+VA so you Current draw for a 4kW is 4000W/12V =333A
The only sensible way to supply this is from a Large Ah battery positioned as close to the Thruster to keep the run of very thick cables from the battery to the thruster to a min length. At over 300 Amps the heat loss and voltage drop due to resistance in thinner cables would impact on the thruster performance.
Don't dream of using the existing Starter battery. This may be just a Diesel Starting battery and not a deep cycle battery. With the current requirement of the bowthruser you always run a risk of flattening even a suitable rated deep cycle battery if you get into a tough spot and need alot of thrusting so it makes sense not to use the starter battery.
With 300A + been drawn from the battery constanty when thrusting your Alternator will need time to replace the energy drawn.
Generally the House Batterys will be toward the rear of the boat near the engines, If you want to run from these you will need to run that very heavy cable the length of the boat , it becomes very costly and increases weight.
The ususal set up is to have a large designated (Deep Cycle) Battery as close a possible to the thruster and supply this from the alternator with appropriate cables. I have a 120Ah battery running a 3 kW thruster and I can get 20 to 30 of approx 10 Sec thrusts without noticing power drop if I need it . The Manual for the thruster should give advise on the installation and how to size cables.
I have the boat 2 seasons and initially could get only 5 to 6 of 5 sec thrusts and then it began to fade. I blamed the battery and put in a new 120 Ah leasure battery. The real problem turned out to be pitting on the contacts of the reverse/ Forward relay. These take a real beating at 250to 300A. Because of the high currents all components are worked hard in these systems. Next time I'd fit proper Deep Cycle battery.
Kinsale 373

You have to be very careful with this approach, because as the bow battery 'fades', the alternator and house bank will try to power the thruster through inadequate cables. Unless you have a system which disconnects the bow battery from the rest of the system when the thruster is running.
Counter-intuitively, thinner cables can actually be less likely to catch fire in this scenario, however there will be less 'thrusting' available.
There is no susbsitute for doing the maths for the actual installation, but in general, big cables or bus bars to the house battery seems to be the most robust solution.

 

[PaulRainbow]

Deep cycle batteries are absolutely not the correct batteries for bow thrusters. You want lots of amps in a hurry, that means plenty of plate surface. Best batteries for the job are spiral would AGM, but they aren't cheap. Ultima Blue Top batteries work very well.

Working out if it's cheaper to fit batteries at the bow or to run hefty cables requires some calculations and we don't have enough information to do them.

Good point regarding the current draw through the charging cables. It's not just how it will affect the cables, it's how it will effect the domestic equipment too. I recently investigated some problems on a 34ft yacht, including why the nav gear all reset after a few bursts of bow thruster. The "professional" that installed it had fitted a battery and isolator at the bow, but had connected a 10mm cable between the bow and domestic batteries (unfused !). If the thruster was used a few times it tried to draw power from the domestic batteries, pulling the voltage down enough to reset the chart plotter etc. Easily cured by connecting the charging cable to the alternator via a Victron Argofet and to the shore power by using the unused (!) 3rd output on the charger. Suitable fuses added to the cable, of course.

 

[halcyon]

I have the boat 2 seasons and initially could get only 5 to 6 of 5 sec thrusts and then it began to fade. I blamed the battery and put in a new 120 Ah leasure battery. The real problem turned out to be pitting on the contacts of the reverse/ Forward relay. These take a real beating at 250to 300A. Because of the high currents all components are worked hard in these systems. Next time I'd fit proper Deep Cycle battery.
Kinsale 373

Bowthruster's normally have a max run time per hour, greatly reducing battery drain.

Brian

 

[William_H]

The amps and the problems involved in trying to make a 12v system do so much work is why it is far better to go to a 24v system using a large current 24v alternator. ol'will

 

[halcyon]

The amps and the problems involved in trying to make a 12v system do so much work is why it is far better to go to a 24v system using a large current 24v alternator. ol'will

As bowthruster's are intermittent by nature, a battery to manufacture's spec by thuster, with a dedicated solar panel. A VSR on engine battery would give a back-up charge option, but would need interface to drop out when thruster was in use, to keep cable size down.

Brian

 

[PaulRainbow]

As i have a few minutes to spare while i'm stopped for a cuppa........

It is all about your battery capacity - (Amph)

It has absolutely nothing whatsoever to do with battery capacity (ah).

and how much voltage drop you'll get at the terminals near the thruster. You must check your bow thruster specification and see what the rated current is - then apply at least 15% more (that's when protection fuse will kick in and stop the over current).

You do need to check the current draw of the bow thruster and size the cable accordingly, to reduce voltage drop to an acceptable level, at this point fuses/breakers are irrelevant.

A typical 12V marine battery may be like 100Ah - that's when all the cells function so you assume at least 90% of that - i.e. 90Ah at nominal voltage (12V) which all seems to be fine. BUT If your voltage drops then more current is being sucked out of the battery.

Once again, It has absolutely nothing whatsoever to do with battery capacity (ah).

under load condition if you run say 10m 12V cable (low loss) the best you'll get is just above 11V near the terminals in the bow. so that's when the current will increase and not just that by increasing the current the cable will get warmer and more voltage drop will occur to the point that the Thruster's motor won't get the torque and potentially the fuse will pop out...

This is nonsense.

What's "low loss 12v cable" ?

If the correct cable is used voltage drop will be acceptable.

In most installations you'll see a small motorbike battery near the Thruster to avoid the voltage drop which is also installed parallel to your other 12V battery systems.

This is way past nonsense, it's ludicrous. I'd love to hear your reasoning behind this ridiculous statement.

Why do i keep saying ah is irrelevant ?

To save me re-inventing the wheel, here's a random website explaining what amp hours means.

You can now see why it's irrelevant. What you need for engine starting, windlasses and bow thrusters is lots of amps, fast. You don't need a battery with a big ah rating to get lots of amps. For instance, an Optima Blue Top can be 50ah, but over 800 amps.

 

[geem]

We have a 24v boat. Cable sizes are halved as you would expect. We don't have a bow thruster but our 2kw windlass is wired from the house bank (500 amp hr @24v), the windlass load on the house batteries is not significant. I never like the idea of a heavy battery bouncing around in the bow, just where you don't need the weight. It is bad enough having chain and anchor up there as well as the nultidue of other kit in the bow locker. Regardless of weight and size of cables, I would be tempted to wire from the house battery since it should be of considerable size on a Contest.

 

[Mistroma]

If the boat is a 46 footer you probaly have a 4 to 5 Kw Thruster. W+VA so you Current draw for a 4kW is 4000W/12V =333A
The only sensible way to supply this is from a Large Ah battery positioned as close to the Thruster to keep the run of very thick cables from the battery to the thruster to a min length. At over 300 Amps the heat loss and voltage drop due to resistance in thinner cables would impact on the thruster performance.
Don't dream of using the existing Starter battery. This may be just a Diesel Starting battery and not a deep cycle battery. With the current requirement of the bowthruser you always run a risk of flattening even a suitable rated deep cycle battery if you get into a tough spot and need alot of thrusting so it makes sense not to use the starter battery.
With 300A + been drawn from the battery constanty when thrusting your Alternator will need time to replace the energy drawn.
Generally the House Batterys will be toward the rear of the boat near the engines, If you want to run from these you will need to run that very heavy cable the length of the boat , it becomes very costly and increases weight.
The ususal set up is to have a large designated (Deep Cycle) Battery as close a possible to the thruster and supply this from the alternator with appropriate cables. I have a 120Ah battery running a 3 kW thruster and I can get 20 to 30 of approx 10 Sec thrusts without noticing power drop if I need it . The Manual for the thruster should give advise on the installation and how to size cables.
I have the boat 2 seasons and initially could get only 5 to 6 of 5 sec thrusts and then it began to fade. I blamed the battery and put in a new 120 Ah leasure battery. The real problem turned out to be pitting on the contacts of the reverse/ Forward relay. These take a real beating at 250to 300A. Because of the high currents all components are worked hard in these systems. Next time I'd fit proper Deep Cycle battery.
Kinsale 373

I'd have to agree with others that deep cycle batteries would not be my first choice. My bowthruster is rated for max. continuous use of approx. 2-3minutes. A fairly long cooling off time is needed between very long runs. Runs of 5-10 seconds always seem as if the motor is running forever. To be honest, I rarely use the bowthruster and don't rely on running it for extended periods every time I tie up.

3 mins. at 200A is approx. 10Ah and even 300A only brings it to 15Ah. Obviously such a high current draw will effectively deplete a battery quickly as effective capacity will be much lower than any quoted C20 Ah. Even so, normal bowthruster use with engine running won't normally draw much of a batteries capacity.

I used a spiral wound battery because it can satisfy a sustained high current demand better than a flat plate design. Deep cycle batteries are not renowned for their ability to operate in this manner.

 

[alan]

All_at_sea,
I am currently installing the electrics for a bow thruster on my boat and will give you my thoughts (for what they are worth) below:
1. My anchor windlass has 35mm cable and the thruster manufacturer's recommendation is for 70mm (for about 16m total length, positive + negative)
2. I personally do not like to have anything connected to the start battery - except for the engine starter motor!
3. I was thinking of using the service batteries but a friend (who does electrical/electronic work on boats for a living) told me that when using the thruster you risk shutting down (or even damaging) the boat's instruments.
4. In the end I decided to fit a third battery near the thruster.
5. I ran 16mm cable for third battery charging from the battery splitter (had to replace my two output FET splitter with a three output model!!). Do not under estimate the difficulty of running "heavy" cable from the batteries to the thruster - even with 16mm it was hard work in some places and I was pleased I had not gone down the 70mm cable route!! I bought 16mm super flexible welding cable.
6. You need an isolation switch and a fuse (150 amps or whatever is recommended) between the battery and the thruster; or an MCB (thermal trip and switch combined);as close to the battery as possible.
7. In my case I fitted a 100 amp fuse at the battery end of the 16mm charging cable near the third battery to protect the 16mm cable in case of a short between the battery splitter and the third battery. The 16mm cable is rated at 120 amps and my alternator is 70 amps; I would not expect to have more than 20 amps on this cable for charging but went with 100 amps fuse as I am trying to protect against a short on the battery not to protect the charging current.

So, my recommendation is to think carefully before deciding on whether to use the starter or service battery for the thruster or not; and really make sure that you have space to run two 70mm cables or even on a short run 50mm cables the 'length' of the boat.

Good luck,
Alan.

 

[PaulRainbow]

3. I was thinking of using the service batteries but a friend (who does electrical/electronic work on boats for a living) told me that when using the thruster you risk shutting down (or even damaging) the boat's instruments.

It is most certainly a risk that you shut down the electronics, I recently had to deal with a boat that was doing just that.

7. In my case I fitted a 100 amp fuse at the battery end of the 16mm charging cable near the third battery to protect the 16mm cable in case of a short between the battery splitter and the third battery. The 16mm cable is rated at 120 amps and my alternator is 70 amps; I would not expect to have more than 20 amps on this cable for charging but went with 100 amps fuse as I am trying to protect against a short on the battery not to protect the charging current.

If there is a short along the charging cable the 100 amp fuse will protect the cable from the thruster battery, but it will have no protection from the charging end. The cable would likely survive, but the splitter and alternator have no protection. It would be normal to fuse both ends of this cable Alan.

 

[alan]

It is most certainly a risk that you shut down the electronics, I recently had to deal with a boat that was doing just that.



If there is a short along the charging cable the 100 amp fuse will protect the cable from the thruster battery, but it will have no protection from the charging end. The cable would likely survive, but the splitter and alternator have no protection. It would be normal to fuse both ends of this cable Alan.

Hi Paul,
Thanks for your input. I did think of that but thought it was overkill!! But as I have a spare 100A Blue Sea Fuse holder and fuse I will put it at the output of my batt splitter to batt#3 before the long 16mm cable. As I only received my Victron Argofet Batt Splitter (by courier) a few minutes ago, I still have to configure this end of the installation and will follow your sensible advice and protect both ends of the long cable.

Thanks,
Alan.

 

[lw395]

Years ago, I had a boat where the GPS tended to drop out when the engine was started. I.e., just when you need it at the end of a passage.
I got around that by feeding the GPS via some diodes, from both the main batteries and a small alarm battery, which was also the VHF emergency battery. The instruments in those days seemed immune to supply glitches, recovering instantly. These days, you can probably expect the GPS receiver itself to recover in seconds.
Anyway, point is, it can be simple and cheap to address any dropout problems by adding things to the low power circuit which has the dropout problem, rather than compromisng/complicating the high current circuits.

But it is an issue that should be considered. Some modern nav gear doesn't like brownouts. A bit of caution may be worthwhile with phones, TVs, PCs etc, i.e. anything that's full of software.

 

[pvb]

3. I was thinking of using the service batteries but a friend (who does electrical/electronic work on boats for a living) told me that when using the thruster you risk shutting down (or even damaging) the boat's instruments.

Thrusters are usually operated whilst the engine is running, so there should be plenty of voltage. Most modern boat electronics will work down to around 10v, and if the voltage is being pulled down that low the thruster installation hasn't been thought about carefully enough.

 

[Momac]

From my limited experience a bow thruster battery is ordinarily located near the bow to save having heavy duty cables running the length of the boat.
The same for the anchor winch. One battery may serve the winch and the thruster.
I agree with the thought that the battery should be suitable as an engine starter battery. Many batteries these days are dual purpose.

 

[lw395]

Thrusters are usually operated whilst the engine is running, so there should be plenty of voltage. Most modern boat electronics will work down to around 10v, and if the voltage is being pulled down that low the thruster installation hasn't been thought about carefully enough.

It's quite easy to get below 10V for a 'short while' due to the inrush of a ?kW motor.
But then I'm in the business of measuring such 'short whiles' down to microseconds. And beyond.
A 100A alternator won't prevent that.
The inrush of a big motor can be a lot of current with timescales in the seconds arena.

Plus, a rigorous design of this sort of installation out to cover the 'corners' e.g. what happens if your house bank has been run low, or when one of your batteries is getting to 'end of life'.

 

[All_at_Sea]

Thanks all - all very enlightening. I'm down to the boat this week and will have a good look through the system and decide what to do.