Charging a battery in the bow? Lots of questions. Any thoughts?

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madabouttheboat

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My boat currently has an electric windlass run from the starter battery at the other end of the boat. The wires are currently significantly undersized given the nearly 16mtr return trip. I am thinking of adding a bowthruster, which again will need some hefty cables given the distance.

So, what I was thinking is why not install a battery in the bow to run both these items. The wires can then be much smaller, given the short distance, with smallish wires running to the helm for the control gear.

I am thinking of using something like an Optima AGM battery due to its totally sealed nature and ability to be more resistant to vibration as well as being good at coping with high levels of discharge.

Current battery set up is a single 85ah starting battery and single 120ah domestic both of the wet cell variety and charged through a 60a alternator. I have a battery charger fixed just to the domestic battery for when I am on shorepower.

So my problem is, how do I charge the new battery at the Bow.

I can think of three solutions, but just as many problems.

Option 1. stick it on the same circuit as the other batteries so it gets a charge from the alternator. But does it matter that it would be AGM while the others a lead acid? Is the alternator big enough for three batteries of total 300ah. Also what size wire would I need to run forward and how do I stop large power draws along that wire should the bow battery run low?

Option 2 Have the bow battery as a stand alone unit charged with a solar panel. How big a panel would I need given the high draw of the items using the battery, and is solar charging really worthwhile in the UK

Option 3. Again isolated battery with its own battery charger. With this method the battery will always be charged when I leave the berth, but will it have enough power, in a worst case scenario of say bow thruster off the berth, anchor out, anchor in and bow thruster back on the berth, before it gets recharged.

Are there any other options or any comments on the above methods.

Thanks
 
Option 1 - You've hit the nail on the head! You will still need large cables to charge it, as if it runs low the full current will draw down the cables - may be easier just to run the big cables and run off the engine battery (which may need to be upgraded?)

Option 2 - If you need to reanchor 3 or 4 times, the solar wouldn't put the charge into the battery quickly enough (where as the engine would likely be running so the alternator would keep up with the current draw) - this is assuming you don't have a super anchor which always holds first time (hopefully this doesn't turn into another anchoring thread!)

Option 3 - If you are away from a marina (ie sail in the Med / Caribbean) you will have no way of charging this battery.

I have been advised when installing my windlass (still to be decided which one!) just to run heavy cables from the engine battery.

Hope this helps?

There was a whole thread recently on battery vs cables for a windlass - here - seemed to favour cables.

Jonny
 
Last year on my boat in the Med we just did a season with a windlass battery only charged by solar panel, but I don't think this will be enough in the UK. Fit the windlass batt as far fwd as poss with thick "starter cables" to the windlass. Then connect the windlass batt to the domestic batt via a voltage sensing relay. This will not need such heavy cables, at least no heavier than the ones coming off the alternator in the first place. On our boat this year it supplied more than enough charge, so much so that I was tempted to disconnect it sometimes to ensure the domestic batt kept more!
 
Thanks for the answer. Lots of useful stuff on the other thread although I did get a bit lost when it went all techy.

My thoughts of puting a battery in the bow were this: (if any of this is rubbish, please say as I only think I know the basics of this stuff)

2000w thruster @ 12v = 166A
500w Windlass @ 12v = 42A

I won't need to use both at the same time.

with a 16m round trip, the cable to run the thruster would need to be 180mm2

If I run a charging cable forward, I assume the cable would only need to take the 60amps that the alternator chucked out therefore it could be 64mm2.

I accept if the battery was low, it might try to draw 166A up the charging cable, but is there something I could put in the charging line to protect it, say a 65A breaker.

Or am I talking nonsense?

Thanks Again
 
Surely if you operate the windlass with the engine running the VSR will be engaged and the current could be drawn from the undersized cables? I guess this isn't an issue in this case 'cos the cables have survived so far!

But I was wondering if VSRs are capable of protecting batteries as well as alternators?
 
Option 4: Use a battery to battery charger located near the bow thruster battery at the bow.

Any charge combining, or diode, system will fail over those distances.

Sterling do one, but I have not used it. The key is that a thruster needs huge currents for short times and can then be charged over longer periods. The lowest amp charging version will do probably. The currents for a thruster ranges from 200A to 600A so the output of an normal alternator is not going to help much during the thrusting. They are limited to about 4 minutes of operation so that is 14AHours to 40AHours.


However, the windlass is a different problem. If the battery becomes exhausted then you can not drop the anchor easily. So what to do in that situation is a problem. I would just have a windlass that has a manual release. There are electrical methods which would just require large-ish cables (you seem to already have them) to the bow and a contactor switch.
 
That's a cool bit of kit. Sounds like it will do exactly what I need. Anyone have any thoughts on these. It is being supplied by Merlin, a company I have spoken to at boat shows and seem really knowledgable on the subject. I get the impression they wouldn't put their name to something they didn't believe in.
 
I agree, a DC to DC charger would do the trick...

... rather like the device that keeps a GMDSS VHF dedicated battery fully charged and independant in a properly coded charter boat.

As to windlasses, although it is common (& bad) practice to power the chain out (down) I can't think of one yacht windlass that DOESN'T have a clutch and therefore a "manual release".

Steve Cronin
 
[ QUOTE ]
I accept if the battery was low, it might try to draw 166A up the charging cable, but is there something I could put in the charging line to protect it, say a 65A breaker.

[/ QUOTE ]

Yes, that would protect the cable. The issue with motors is that they take huge currents in starting and stall conditions. Work out the voltage drop around the loop for 65A and that will tell you how low the bow battery would need to be before it tripped. Quick calculation for 25mm2 would be 2.5V so assuming the engine is charging at 14v the bow battery will trip the breaker when it falls to 11.5v under operation. So it might be a continuous problem. Go with Option 4.

The simple rule is EVERY lead connected to the positive of a battery needs a fuse. The only lead that this rule does not seem to apply is the starter motor. But then the solenoid is usually placed near the starter battery so the length of permanently powered unprotected cable is small.
 
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Is your battery to battery charger the same kind of thing Pembrokeshirepromise linked to.

[/ QUOTE ]

No they are different but similar. I would use the Sterling in this application. However, I just read their manual and the voltage sensing is from the bow battery so the unit must be fitted physically at the engine end of the boat. (The one I made sensed alternator charging). The other one has no remote sensing and so can only work for batteries that are close together.
 
[ QUOTE ]
The other one has no remote sensing and so can only work for batteries that are close together.

[/ QUOTE ]

Do you mean the Merlin one, cos it says in their bumf that is is ideal for chagring a bow battery from the domestics placed elsewhere in the boat. It seems to me that when the domestic battery is getting over 13V, the Merlin thingy will start putting power into the bow battery. Am I reading this wrong?

Cheers
 
Yes Merlin do what is called an echo charger which is a battery to battery charger I used a 44AH Redflash for the bow thruster works well on a 40 foot yacht.
 
I just read the echo charge manual:

[ QUOTE ]
Select a location that is within two feet of the house battery and within two feet of the starter battery, for maximum charging current

[/ QUOTE ]
[ QUOTE ]
The longer the wires, the less current will flow and the longer it will take to charge the starter battery.

[/ QUOTE ]

It seems it is a "voltage follower" which means the charging battery will never get charged at a voltage above the input battery. So at the charged state there is no current flowing and so the unit senses the real voltage.

However, you have to get there. In the state when it is charging the the current will cause a voltage drop. So the output voltage will be lower than it should be (assuming the input wires are long). However if the output wire is long the voltage at the unit will be correct but the battery will see a lower voltage.

Why did they not just fit a seperate sense wire and it would have been perfect.
 
Hairbox

Can you elaborate?

Do you use the Echo Charger with your setup?
Is your Red Flash in the Bow?
If you use an Echo Charger, is the other battery also AGM or is it standard wet cell?
What sort of distances are your batteries apart?
What size cable have you used to run from domestic battery to bow battery?

I had a look on the red flash site, I can't see a 44ah. Do you mean the 42ah?
http://www.dmstech.co.uk/marine/pdf/redflashdata.pdf

Thanks
 
Havent read through the whole thread so I apologise if I am duplicating something.

There is another option. That is run smaller wires forward (sounds like the existing ones will be fine, and fit a 12v battery charger - yes there is such a thing, it works similar to the 12v DC coverters for laptops, just draws a lot more power. (and is a lot more expensive!) cant remember who makes them off hand, but you end up with a 3 stage battery charger for the bow battery.

Personally, I ran decent sized cables forward to enable the battery to charge at the same time as the normal ones, but then put an isolating switch into the line (and a big fuse) so that when using the bow thruster I wasnt trying to draw excessive current over this line, but was fine for normal charging - seems to work fine.
 
your cable size of 150mm sq, will surely give a very low V drop of 0.3 V, assuming the 166A and 16m run of cable, yes? Desirable of course, but rather lower than necessary for the thruster to give good results.

Is the thruster really only 2KW ? ( given the batts are a run of 8meters away)


just wondering if all the complexity of a battery & charging gear stacks up well against fitting bigger cable - what problems does the present setup give you?
 
Two thoughts...

First, you don't need 180 sq mm cable for a 2kW bow thruster, 50 sq mm should be fine, or 70 sq mm if you're a belt & braces man.

Second, if you put a dedicated battery in the bow, you need to ensure that you protect the cable in both directions (ie with a breaker at both ends of the positive cable), and as high-amp breakers are expensive this starts to affect the economics. By the time you tot up the cost of all the bits, you may well find (as lots of others have) that it's cheaper just to fit big cables. If there's money left over, increase the size of your domestic battery bank.
 
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