Bow thruster battery & wiring

[LittleSister]

I want to wire up a new boat thruster. I am contemplating a separate thruster battery located in the bow (a change from my intentions in a more general wiring query a few weeks ago), charged via the existing 10mm2 cables that, er, currently power the windlass.

The system is all 12v, the thruster has a nominal current draw of 315A, but 250A fuse or breaker recommended. The windlass is IIRC 700w, (= c60A).

I know it's generally a mistake to ask more than one question in a post (to leave room for all the answers to question one hasn't asked? ), but -

Q1
How would the charging cable normally be protected from current overload from the main batteries aft when the voltage falls in the thruster battery during operation?

Q2
Would the existing thermal breaker (c60A?) for the windlass wiring be a practical and safe limiter for the charging cable? Would it likely trip every time the thruster was operated?

Q3
Is a high current/low ah AGM battery (e.g. Red Flash) suitable as a thruster battery?

Q4
Do such 'sealed' AGM batteries (which can be used in any orientation) vent hydrogen?
My reason for asking this question is that for space etc. reasons the battery would be mounted in the same locker as the thruster, but I wouldn't want to mix any electric motor and hydrogen. (I think I could potentially create a small, separately vented compartment within the locker with the thruster, but would rather avoid this if possible.)

What does the team think?

 

[PaulRainbow]

Q1 - Fused at both ends, rated for the cable. Breakers are OK too.

Q2 - Should be fine, fit it at the aft end of the cable, but make sure you sit another one, or a fuse at the thruster end too, or a short between that breaker and the thruster battery will be unprotected.

Q3 - Perfectly suited.

Q4 - Won't vent anything if you don't cook it, so no problem.

Don't pay RedFlash prices, plenty of sensibly priced AGM car batteries around now.

 

[Boathook]

For Q1 answer - I would use a battery to battery charger with fuses at each end to protect the cables and charger. Adverc do one (along with other companies). I have one on my boat and it only allows the BT battery to charge once the main ones are. Nice bit of kit imho.

 

[LittleSister]

Thank you.

 

[Ian_Edwards]

When you are designing a system with both batteries banks connected in parallel you need to consider a few things:
Fuse the connecting cable between the batteries at each end to protect the cable, which has already been covered.
The current flowing through the connecting cable will depend on the several factors:

• The state of charge of each battery.
• The internal resistance of the batteries.
• The resistance of the connecting wires.
• The load on each battery.

In normal operation, I'd expect the power taken by the thruster to come mainly from the battery in the bow, because the resistance of the cable will limit the current from th main bank. It's also likely that the AGM in the bow will have a lower internal resistance, than the main bank, especially if the main bank is older and standard wet lead acid.
During charging the thruster battery will charge at a slightly lower voltage than the main bank, because of the resistance in the connecting cable.
The thruster battery is connected in parallel with the main bank, so will discharge at more of less the same rate as the main bank. This may mean that the bow battery bank will be in the same state of discharge as the main bank when you need to use the thruster, this may result in more current in the connecting cable, because the main bank, although at a similar state of charge will still have more remaining capacity, just because it's bigger.
When I considered this, I thought it better to electrically separate the two banks.
There are two obvious ways of doing this, a voltage controlled relay and a battery to battery charger.
The voltage controlled relay will work OK when the main back isn't been charged, it''l be open circuit, but if the engine is running and the relay is closed, it may see a high surge of current as the main battery bank supports the thruster battery. The relay may well dropout as the voltage of the combined banks drops under the high load of the thruster. This give a problem with how to size the current rating of the voltage controlled relay. Simplistically, you'd rate the relay as at a little more than the maximum charging current, but this may lead to early failure if the relay opens during a current surge.
After, thinking about all this (and perhaps overthinking it) I decided to use a battery to battery charger. It's a more expensive option, but probably more reliable, it overcomes the problems I've outlined above.

 

[[176172]]

Q1 - Fused at both ends, rated for the cable. Breakers are OK too.

Q2 - Should be fine, fit it at the aft end of the cable, but make sure you sit another one, or a fuse at the thruster end too, or a short between that breaker and the thruster battery will be unprotected.

Q3 - Perfectly suited.

Q4 - Won't vent anything if you don't cook it, so no problem.

Don't pay RedFlash prices, plenty of sensibly priced AGM car batteries around now.

I am a devotee (along with many liveaboards) of the Trojan 6v batteries as fitted in golf carts. They tolerate if not thrive on both high discharge and charge rates. They are some 240ampH x 6v, (a pair in series 12v x 240ampH)
Their lifespan makes them a firm favourite. with liveaboards. More expensive initially but long term a good buy.

 

[[176172]]

When you are designing a system with both batteries banks connected in parallel you need to consider a few things:
Fuse the connecting cable between the batteries at each end to protect the cable, which has already been covered.
The current flowing through the connecting cable will depend on the several factors:

• The state of charge of each battery.
• The internal resistance of the batteries.
• The resistance of the connecting wires.
• The load on each battery.

In normal operation, I'd expect the power taken by the thruster to come mainly from the battery in the bow, because the resistance of the cable will limit the current from th main bank. It's also likely that the AGM in the bow will have a lower internal resistance, than the main bank, especially if the main bank is older and standard wet lead acid.
During charging the thruster battery will charge at a slightly lower voltage than the main bank, because of the resistance in the connecting cable.
The thruster battery is connected in parallel with the main bank, so will discharge at more of less the same rate as the main bank. This may mean that the bow battery bank will be in the same state of discharge as the main bank when you need to use the thruster, this may result in more current in the connecting cable, because the main bank, although at a similar state of charge will still have more remaining capacity, just because it's bigger.
When I considered this, I thought it better to electrically separate the two banks.
There are two obvious ways of doing this, a voltage controlled relay and a battery to battery charger.
The voltage controlled relay will work OK when the main back isn't been charged, it''l be open circuit, but if the engine is running and the relay is closed, it may see a high surge of current as the main battery bank supports the thruster battery. The relay may well dropout as the voltage of the combined banks drops under the high load of the thruster. This give a problem with how to size the current rating of the voltage controlled relay. Simplistically, you'd rate the relay as at a little more than the maximum charging current, but this may lead to early failure if the relay opens during a current surge.
After, thinking about all this (and perhaps overthinking it) I decided to use a battery to battery charger. It's a more expensive option, but probably more reliable, it overcomes the problems I've outlined above.

+1 I see no good reason to connect the bow battery to your main domestic bank, for me the whole purpose of it is to service the thruster. Keeping it charged is not a big deal as explained. (possibly a small solar dedicated to it) In fact your bow battery could be used for both windlass and thruster as it is hard to imagine them both being used at the same time.

 

[neil1967]

I also have Trojans for my domestic use, but an Odyssey AGM as my engine start battery, which is great - small form factor, sealed, high CCA and the charging regime is so similar to the Trojans, that I have the smart charger (from the alternator) and the mains battery charger set to Lead Acid settings with no problem at all. In the application that the OP is proposing I would have thought that an AGM would be ideal.

 

[LittleSister]

Rather than the cost/complication of a B to B charger, how about a diode (Schottky?) in the charging supply to the battery? This would allow the battery to be charged, and maintain full voltage by not allowing discharging to meet domestic consumption.

On the (very occasional, short burst) use of the thruster, the (suitably specced) diode could handle an up to 60A 'contribution' from the alternator and domestic batteries supplementing the power to the thruster from the bow battery.

In the event that the bow battery voltage fell so low that over 60A was drawn from the domestic batteries and alternator, the breaker would trip to protect the charging cabling and diode.

 

[Paulg25]

+1 I see no good reason to connect the bow battery to your main domestic bank, for me the whole purpose of it is to service the thruster. Keeping it charged is not a big deal as explained. (possibly a small solar dedicated to it) In fact your bow battery could be used for both windlass and thruster as it is hard to imagine them both being used at the same time.

Completely agree with this. No point in over complicating things.

 

[EugeneR]

For Q1 answer - I would use a battery to battery charger with fuses at each end to protect the cables and charger. Adverc do one (along with other companies). I have one on my boat and it only allows the BT battery to charge once the main ones are. Nice bit of kit imho.

+1

Given that you are already adding a battery, using a battery to battery solution avoids most of your problems for little effort. I used a 10A Echo Link charger to do exactly the same. Keep your cables from battery to thruster thick and short, with good crimps.

 

[PaulRainbow]

Rather than the cost/complication of a B to B charger, how about a diode (Schottky?) in the charging supply to the battery? This would allow the battery to be charged, and maintain full voltage by not allowing discharging to meet domestic consumption.

On the (very occasional, short burst) use of the thruster, the (suitably specced) diode could handle an up to 60A 'contribution' from the alternator and domestic batteries supplementing the power to the thruster from the bow battery.

In the event that the bow battery voltage fell so low that over 60A was drawn from the domestic batteries and alternator, the breaker would trip to protect the charging cabling and diode.

Too much over thinking in this thread.

Firstly, the suggestion of using 6v Trojans in incorrect. For a bow thruster you want all the power quickly, Trojans are not best suited to this. You also most likely won't want 56kg of batteries at the bow, or spend £300.

Post #5 raises some valid points. Connecting the thruster battery to the engine battery, not the domestic bank, overcomes them though. You won't be using the thruster without the engine running. When away from the boat, whatever charging system you use for the engine battery will keep the thruster battery charged up. You could add a VSR but i would question its value. If there is no charging regime for the engine battery when the boat is unattended, a small solar panel for charging is not a bad idea, but i'd connect it to the engine battery and fit a VSR.

 

[lw395]

Rather than the cost/complication of a B to B charger, how about a diode (Schottky?) in the charging supply to the battery? This would allow the battery to be charged, and maintain full voltage by not allowing discharging to meet domestic consumption.

On the (very occasional, short burst) use of the thruster, the (suitably specced) diode could handle an up to 60A 'contribution' from the alternator and domestic batteries supplementing the power to the thruster from the bow battery.

In the event that the bow battery voltage fell so low that over 60A was drawn from the domestic batteries and alternator, the breaker would trip to protect the charging cabling and diode.

As they say, 'do the math'.
A bow thruster draws what?
The bow battery will drop to what voltage?
The main battery will give a current of what?
It's actually quite hard to design a system which is robust.
If you consider what happens when your bow battery is getting on a bit for instance.
The worst thing might be for the cables from the main battery to cover up the reduced performance of the bow battery until suddenly a fuse or diode blows and then you crash the boat because the thruster doesn't do its thing.

Personally I wouldn't want to put my name on a system which only worked assuming all the batteries are new and in a good state of charge.

There's also a saying in the world of audio amp design, that the semiconductors will often blow to protect the fuses. The kind of over current which blows a fuse in millseconds, blows a diode in microseconds.

In general, I think the most bullet proof solutions are running the thruster off the house bank or starter battery via serious buss bars, or a bow battery with B2B charger. The latter allows a 24V thruster to be considered in an 12V boat or v/v.
But the maths of sharing current front to back vary a lot. Small changes in cable length or dia can matter.

 

[LittleSister]

Too much over thinking in this thread.

I didn't get where I am today without over-thinking things!

 

[[176172]]

Too much over thinking in this thread.

Firstly, the suggestion of using 6v Trojans in incorrect. For a bow thruster you want all the power quickly, Trojans are not best suited to this. You also most likely won't want 56kg of batteries at the bow, or spend £300.

Post #5 raises some valid points. Connecting the thruster battery to the engine battery, not the domestic bank, overcomes them though. You won't be using the thruster without the engine running. When away from the boat, whatever charging system you use for the engine battery will keep the thruster battery charged up. You could add a VSR but i would question its value. If there is no charging regime for the engine battery when the boat is unattended, a small solar panel for charging is not a bad idea, but i'd connect it to the engine battery and fit a VSR.

Too much over thinking in this thread.
Agree with that!
Having used Trojans for a long time I cannot agree with the contention they are not suited to give power quickly, they most certainly do. Golf carts use similar and they do not take all day between holes...
As to the extra weight, hardly noticeable with the added weight of the thruster and lots of chain in the locker.
The £300.00 is small beer, well worth making sure your thruster investment of over 10 times that or more is properly fed when required.
I have no argument on charging, I am a 1 2 Both man myself.

 

[PaulRainbow]

Too much over thinking in this thread.
Agree with that!
Having used Trojans for a long time I cannot agree with the contention they are not suited to give power quickly, they most certainly do. Golf carts use similar and they do not take all day between holes...
As to the extra weight, hardly noticeable with the added weight of the thruster and lots of chain in the locker.
The £300.00 is small beer, well worth making sure your thruster investment of over 10 times that or more is properly fed when required.
I have no argument on charging, I am a 1 2 Both man myself.

Deep cycle batteries are great for domestic banks, not so good for engine starting, winches or bow thrusters. Standard FLA/SLA batteries do better, AGMs even better still.

The OP has a 28ft boat, adding a bow thruster plus 56kg of batteries right at the sharp end would be a noticeable, i'd say. As would the £300 hit on his wallet, as opposed to the £100 AGM which will do the job just fine.

I have customers with 50 + foot boats, weighing 40 tons or more, those have bow thrusters with two AGMs. and stern thrusters with just one. I reckon if a single AGM can swing the stern of a 56ft, 40 ton motor sailor, it'll move the bow of an LM28

 

[Norman_E]

I don't know what calculations were done, or even if any were done but my house battery bank is connected to the bow thruster and windlass battery with at least 50mm cross section cables. I don'y know the length but I guess about 10 metres each way. I have never measured it but its at least 50mm. Even when the thruster battery was dying nothing failed and nothing got hot. Its a system of great simplicity, and it works.

 

[pvb]

Having used Trojans for a long time I cannot agree with the contention they are not suited to give power quickly, they most certainly do. Golf carts use similar and they do not take all day between holes...

Golf carts only draw about 50-70A from the batteries. Bowthrusters draw, in the OP's case, 315A. His bow thruster's spec probably says the minimum battery requirement is 500CCA (EN). What's the CCA of a Trojan T105? I don't know, because even Trojan don't quote it.

 

[LittleSister]

I don't know what calculations were done, or even if any were done but my house battery bank is connected to the bow thruster and windlass battery with at least 50mm cross section cables. I don'y know the length but I guess about 10 metres each way. I have never measured it but its at least 50mm. Even when the thruster battery was dying nothing failed and nothing got hot. Its a system of great simplicity, and it works.

With sufficiently heavy gauge cables a thruster can be run directly from batteries aft, eliminating the need for a battery at the bow. That was my original intention, and would have required minimum 70mm2, and recommended 95mm2, cables for the c6m each way (c12m total round trip). For various reasons too complicated/boring to explain here I'm now revisiting my original strategy.

 

[lw395]

I don't know what calculations were done, or even if any were done but my house battery bank is connected to the bow thruster and windlass battery with at least 50mm cross section cables. I don'y know the length but I guess about 10 metres each way. I have never measured it but its at least 50mm. Even when the thruster battery was dying nothing failed and nothing got hot. Its a system of great simplicity, and it works.

You can also make a system that works with quite thin cables, the resistance limiting the current to safe values.
But I wouldn't advise doing that without careful calculations.
If you're going to put in 50 sqmm cables, it might be more cost effective to put in really thick cables and save the cost of a battery.
But then you may want to worry what it's going to do to the alternator....

It is quite complicated and the variables from one boat to another could mean that what works well for you might not work for someone else.