Bowthruster battery charger

[matt1]

My boat is on a mid stream pile mooring, with no shore power. I often use the bowthruster when retuning from a trip. Will the battery remain fully charged whilst the engine is running eg taking its current front he alternator (but via the battery) or should I leave the engine to run on for a length of time after I’ve finished mooring to allow the battery to recover charge?

 

[pvb]

A thruster will typically draw much more current than the alternator can supply, so it will be taking current mainly from the battery. So letting the engine run for a while after mooring will help to top the battery up.

 

[[2574]]

If your bow thruster draws 500A then it pulls about 8.5AH from the battery every minute. If you were to use it for 15 seconds in getting back on your pile mooring you'd consume about 2AH. If you alternator is charging at 10A it'll replenish the consumption in 12minutes. That's all a bit theoretical but sets the scene. The thing is making sure that the alternator is prodcing that kind of charge - it'll depend on how it is sensing the demand from the batteries.

 

[rogerthebodger]

You say you are on a pile mooring, with no shore power so I would consider fitting a solar panel to keep all your batteries topped up while you are away so you boat is ready when you want to go sailing

It would also replace any charge used from your bow thruster battery PDQ whit the need to run you engine any longer.

 

[TernVI]

If your bow thruster draws 500A then it pulls about 8.5AH from the battery every minute. If you were to use it for 15 seconds in getting back on your pile mooring you'd consume about 2AH. If you alternator is charging at 10A it'll replenish the consumption in 12minutes. That's all a bit theoretical but sets the scene. The thing is making sure that the alternator is prodcing that kind of charge - it'll depend on how it is sensing the demand from the batteries.

Alas not.
Drawing current from say a 100Ah at 500A, i.e. a '5C' rate, its capacity is a fraction of the nominal 20hr rate, you'd actually lose a lot more capacity.
Maybe 8Ah worth.
The alternator would then need to take into account that charging is not 100% efficient, you'd likely need to put back over 10Ah.

A solar panel to recharge over the following day or so is a good idea.

Online calculator: Battery discharge time depending upon load

 

[[2574]]

Alas not.
Drawing current from say a 100Ah at 500A, i.e. a '5C' rate, its capacity is a fraction of the nominal 20hr rate, you'd actually lose a lot more capacity.
Maybe 8Ah worth.
The alternator would then need to take into account that charging is not 100% efficient, you'd likely need to put back over 10Ah.

A solar panel to recharge over the following day or so is a good idea.

Online calculator: Battery discharge time depending upon load

Interesting. Surely Peukerts formula is about the capacity of the battery to supply the energy, not about the energy drawn by the load?

 

[matt1]

I do have a 110w solar panel which seems to work well in summer. This is connected to the domestic bank though. In the winter it’s enough to run the heating for a couple of months and has generally recharged the batteries by the time I check on the boat a month later.

 

[rogerthebodger]

I do have a 110w solar panel which seems to work well in summer. This is connected to the domestic bank though. In the winter it’s enough to run the heating for a couple of months and has generally recharged the batteries by the time I check on the boat a month later.

Just get yourself a twin output solar regulator with each output connected to your domestic and the other connected to your bow thruster battery

 

[jwfrary]

Just get yourself a twin output solar regulator with each output connected to your domestic and the other connected to your bow thruster battery

unless your already using a voltage sensitive relay to charge the bow thruster bank, then the solars likely already charging it!

 

[PaulRainbow]

unless your already using a voltage sensitive relay to charge the bow thruster bank, then the solars likely already charging it!

Indeed, would be useful to know how the thruster battery gets its charge from the alternator.

 

[luke collins]

It may be if you have a charging regulator for the bow thruster the solar is not developing enough voltage to excite the regulator to start the bow thruster charging. I found with mine I needed 13.5v from the alternator to get the bow battery to charge and Reg to activate. The mains charger gave 13v (2 outputs) one for port and one for stab) and the bow battery get going down if the boat was not used for a few months.

 

[matt1]

So I have to confess I’m not entirely sure because being a new boat everything is PCB panels and not good old fashioned battery switches / emergency parallels and that kind of things. That’s the next thing to find out but I suspect there is a VSR. Now I wired in the solar panel, via a Victron MPPT to the domestic bank, so I’d imagine the thruster just gets its charge from the alternator and shore power (unless the domestic banks or wired to the thruster?)

 

[Nom de plume]

Slight aside - Is there any general rule of thumb regarding the size of the thruster battery bank versus the power of the thruster (Hp or Kw)?

 

[[2574]]

Slight aside - Is there any general rule of thumb regarding the size of the thruster battery bank versus the power of the thruster (Hp or Kw)?

I’m struggling to provide 1300 CCA for our 8hp thruster whilst keeping the battery size practicable. The attached thruster spec might give indicative guidance.

 

[pvb]

Slight aside - Is there any general rule of thumb regarding the size of the thruster battery bank versus the power of the thruster (Hp or Kw)?

Thruster manufacturers give guidance in the installation manual.

 

[pvb]

I’m struggling to provide 1300 CCA for our 8hp thruster whilst keeping the battery size practicable. The attached thruster spec might give indicative guidance.

Use two batteries wired in parallel to meet the CCA requirement.

 

[luke collins]

AGM batteries are good for CCA.

 

[[2574]]

Use two batteries wired in parallel to meet the CCA requirement.

The issue is weight, easy to parallel batteries but I don’t want 100kgs in the bow.

 

[pvb]

The issue is weight, easy to parallel batteries but I don’t want 100kgs in the bow.

100kg?? Which batteries are you looking at? As an example, a couple of Exide EN750 batteries would give you over 1300CCA and would weigh only 36kg (for both).

EN750 Exide Start Marine Leisure Battery

 

[[2574]]

100kg?? Which batteries are you looking at? As an example, a couple of Exide EN750 batteries would give you over 1300CCA and would weigh only 36kg (for both).

EN750 Exide Start Marine Leisure Battery

Looks like a good option, thanks. That’s CCA EN, a little less than CCA SAE, but close nonetheless. I might need three but still a lot less than 100kg. I’ve AGM elsewhere so would need a clever charging regime to cope with two technologies.