Bow thruster battery charging

[howardclark]

I have a similar setup but no solar.
the solution which evolved over time was to connect house , starter and bow thruster batteries via a Sterling 3 battery VSR box which connects to all when the engine runs but disconnects when the voltage drops.
its been like that for about 5 yrs now and I’m very happy with it.

 

[Boathook]

Hi there, thanks for your answer.
I get that your system works as you only have a 4ampere B2B on the bow thruster.
That helps with my problem that my 100w solar panels can only provide a max of 8 amperes (100w/12v) so the issue is that if the solar is charging and I use a 10amp or above B2B it would just suck power from the house bank everytime the sun shines.

But is 4 amp enough to charge the Bow thruster? Bear in mind I have a sailboat so only motor for about 15-20mins in and out of the marina. This would only provide about 2-3 amp hours of charge for each use.

Thanks again
Iain

My B2B only allows power to the BT battery once the main bank is charged up, I assume that other makes do the same. As others have said, you only use the BT for seconds at a time and even 30 seconds seems an eternity when in use! I have found it more than adequate and my boat is a sailing one as well.

 

[PaulRainbow]

My B2B only allows power to the BT battery once the main bank is charged up, I assume that other makes do the same. As others have said, you only use the BT for seconds at a time and even 30 seconds seems an eternity when in use! I have found it more than adequate and my boat is a sailing one as well.

You've not met my neighbour then, he uses his bow thruster to steer the boat through the marina !! I don't think he's noticed it's got a steering wheel.

 

[William_H]

For an alternative option assuming BT is only used with engine running and charging. Invest in heavier cables to BT. Then permanently wire the existing BT battery parallel to the engine battery or revert existing BT battery to parallel to house battery and run BT straight off the engine start battery.
Both options will have the alternator supplementing the BT current drain and of course recover the discharge immediately.
You have a VSR to isolate domestic battery but still charge it. i think that is all you need. Of course if you want to be able to run the BT without engine running then yes you need a stout battery just fro the BT. olewill

 

[PaulRainbow]

For an alternative option assuming BT is only used with engine running and charging. Invest in heavier cables to BT. Then permanently wire the existing BT battery parallel to the engine battery or revert existing BT battery to parallel to house battery and run BT straight off the engine start battery.
Both options will have the alternator supplementing the BT current drain and of course recover the discharge immediately.
You have a VSR to isolate domestic battery but still charge it. i think that is all you need. Of course if you want to be able to run the BT without engine running then yes you need a stout battery just fro the BT. olewill

How thick do you think the cables will need to be for a 340a load (not even allowing for inrush) and an 8m run ?

I make it 240mm to get a 4% loss

 

[RupertW]

How thick do you think the cables will need to be for a 340a load (not even allowing for inrush) and an 8m run ?

I make it 240mm to get a 4% loss

If only there was a current limiting VSR, rather than a current based on/off. And I know that should be CSR.

 

[[2574]]

You need to fuse the cable from the VSR to the thruster battery at both ends, to protect the cable and the VSR. To prevent these fuses blowing if the thruster tries to draw excessive current from the charging battery you can fit relays to the VSR negative to disengage it when the thruster is running.

Interesting idea but this would mean that the BT battery bank is never supported by the charging alternator when the BT is energised. But I suppose that’s okay as long as the BT bank is man enough for the job. The engine start battery is not supported by a charging alternator after all - so why should the BT battery be so?

 

[Norman_E]

Surely its just as effective to fit 70mm cables between the house and thruster batteries. At an 8 metre distance (i.e. 16 metre total cable the voltage drop is perfectly acceptable. If the thruster draws 340 amps the battery close to it will supply most of that and draw from the house batteries as its own voltage falls. If it ends up drawing 200 amps from the house batteries the voltage drop is less than 1 volt. That was the system my boat had, with a longer cable run between batteries and it worked perfectly for years. Sometimes simple solutions are the best ones. I suspect that the actual current flow might be within the capacity of 50mm cable, in which case its going to be cheaper than a battery to battery charger.

P.S. I never measured the cables connecting my battery banks, and they may in fact have been 50mm, though I think they were 70mm, but ran nearly 10 metres each way.

 

[TernVI]

Surely its just as effective to fit 70mm cables between the house and thruster batteries. At an 8 metre distance (i.e. 16 metre total cable the voltage drop is perfectly acceptable. If the thruster draws 340 amps the battery close to it will supply most of that and draw from the house batteries as its own voltage falls. If it ends up drawing 200 amps from the house batteries the voltage drop is less than 1 volt. That was the system my boat had, with a longer cable run between batteries and it worked perfectly for years. Sometimes simple solutions are the best ones. I suspect that the actual current flow might be within the capacity of 50mm cable, in which case its going to be cheaper than a battery to battery charger.

P.S. I never measured the cables connecting my battery banks, and they may in fact have been 50mm, though I think they were 70mm, but ran nearly 10 metres each way.

Whether you can get away with that sort of thing depends on what assumptions you are prepared to make about the health and state of charge of your batteries. When the bow battery drops below a limit, the fuse in the long cable goes and your sudden unexpected lack of bow thruster puts you in the side of someone else's Oyster.
Also your bow battery is connected permanently to the house bank, but it's on long wires and possibly at a different temperature.
Battery manufacturers will tell you that's a bad way to wire a bank and may shorten the life of all of the batteries.

In reality a lot of boats get away with it, but does it cost more in battery life in the long run?

 

[Rocksteadee]

How thick do you think the cables will need to be for a 340a load (not even allowing for inrush) and an 8m run ?

I make it 240mm to get a 4% loss

This is the problem I have. Original wiring is 60mm on a 9m run with 4hp and that is why I only get 7 v at thruster making it pretty ineffectual.
So fitting its own battery with a VSR off starter batteries using original cables to charge

 

[TernVI]

This is the problem I have. Original wiring is 60mm on a 9m run with 4hp and that is why I only get 7 v at thruster making it pretty ineffectual.
So fitting its own battery with a VSR off starter batteries using original cables to charge

4HP is 2.9kW which is a lot of amps.
Your bow battery needs to be quite big or it will drop a long way down from 12V.
Thick copper cables are even more expensive than batteries though!

 

[Rocksteadee]

4HP is 2.9kW which is a lot of amps.
Your bow battery needs to be quite big or it will drop a long way down from 12V.
Thick copper cables are even more expensive than batteries though!

Fitting a 850CCA on 1m cables, hoping that should do it
A previous owner fitted a 4 from 3hp as it was ineffectual and made problem worse as now getting 7volt drop

 

[Frk. Jensen]

That's not a VSR, that's a split charge diode.

So, the only time the thruster battery can get any charge current is with the engine running, via the diode, which has around a 0.7v drop, that's not really a good plan Iain.

I'd connect the charging to the domestic bank, then you get a charge to the battery when the Sun is shining. If the thruster battery is charged, it won't continue to draw current. Just because you've connected a xxa B2B charger does not mean that xxa will constantly flow to the thruster battery.

The thruster battery will almost never be very discharged so it's pointless fitting a large current B2B charger, if you fitted a 10a one it would be rare that you'd see 10a going to the thruster battery for anything other than a few minutes, as the battery state of charge increases, its ability to take current decreases.

If you want to fit a B2B fit a 10a one, use 10mm cable and fuse both ends with 15a fuses.

Job done. The thruster battery will now get charged by the alternator, shore power and solar.

I've struggled with my TB now for quite som time.
My motor isn't used enough to kerp TB charged and rarely use shorepower, so have to go back to charge from housebank. Initially TB was directly in parellel with housebank but when changing to Lithium I had to do something else.
Housebank is charged via DC/DC from alternator from shorepower and/or from solar.
Like has been written in this thread the only source with time and energy for charging TB is housebank.

When I first tried this it didn't work. I had a DC/DC between HB and TB, but it was configured to switch on only when voltage came above 14V. But after a summer with lot of load on HB it never came close to 14V even when charging.
My idea now is to put the level as low as 13V which in practice mean it is always on (lithium are still above 13V even when discharged to 50%)
My hope is that TB is soon on toplevel and thus not discharge the HB. A defect TB of course might provide a problem. I probably also during winter with less sun have to disconnect the two batteries.

Anyone having comments on my thoughts?

 

[PaulRainbow]

I've struggled with my TB now for quite som time.
My motor isn't used enough to kerp TB charged and rarely use shorepower, so have to go back to charge from housebank. Initially TB was directly in parellel with housebank but when changing to Lithium I had to do something else.
Housebank is charged via DC/DC from alternator from shorepower and/or from solar.
Like has been written in this thread the only source with time and energy for charging TB is housebank.

When I first tried this it didn't work. I had a DC/DC between HB and TB, but it was configured to switch on only when voltage came above 14V. But after a summer with lot of load on HB it never came close to 14V even when charging.
My idea now is to put the level as low as 13V which in practice mean it is always on (lithium are still above 13V even when discharged to 50%)
My hope is that TB is soon on toplevel and thus not discharge the HB. A defect TB of course might provide a problem. I probably also during winter with less sun have to disconnect the two batteries.

Anyone having comments on my thoughts?

Sounds reasonable, try it and tweak the voltage as needed. I'd also keep the output float voltage low to the thruster battery charges up but when it goes into float it doesn't keep taking power from the lithium bank.