12v Cable Run (Bow Thruster Battery Charging)

[Biggles]

For my bowthruster I will have dedicated batteries in the front of the boat. I will of course need to charge these and for that I will need some hefty cable to prevent voltage drop.

 

Is there a calculation for this? I will fuse the line at 50amps so this will be the maximum charge the batteries could receive. Would this be enough?

 

Assuming 50amp is sufficient and the run is about 20m (allowing for twists and turns) would 16mm/2 be sufficient?

 

Biggles

[aread2]

For my bowthruster I will have dedicated batteries in the front of the boat. I will of course need to charge these and for that I will need some hefty cable to prevent voltage drop.

 

Is there a calculation for this? I will fuse the line at 50amps so this will be the maximum charge the batteries could receive. Would this be enough?

 

Assuming 50amp is sufficient and the run is about 20m (allowing for twists and turns) would 16mm/2 be sufficient?

 

Biggles

I've not got any figures but they shouldn't be too big if like mine it's wired in parallel with the domestics and if the BT is run for too long the domestics start to supply the BT. Some advocate using a battery to battery charger at the front end to overcome voltage drop on this long cable run.

[WotEver]

Some advocate using a battery to battery charger at the front end to overcome voltage drop on this long cable run.

I think you may be thinking of a mains charger here. A battery to battery charger requires nice big fat supply cables if the batteries being charged want to draw that much. A mains charger however would only require the normal mains wiring, which can be seen as an advantage. Unfortunately this would mean both that the inverter must be switched on, and that you'd then be reliant on the inverter to charge the BT batts,. so perhaps not an ideal solution.

 

16mm² is a bit small, but for an answer to the OP see this recent thread.

 

Tony

 

edited for a million typos

Edited October 4, 2010 by WotEver

[Gibbo]

There's no simple answer to this as it depends upon so many different things so let's start at the beginning....

 

How are you splitting the charge to the BT batteries?

[ChrisPy]

I originally fitted a pair of 60sq.mm. cables with the hope of running my BT off the engine battery (57ft boat).

It wasn't satisfactory so I fitted an additional battery next to the BT and used the cables to have it in parallel with the engine battery.

It works fine, the BT battery is kept charged up OK.

[Gibbo]

I originally fitted a pair of 60sq.mm. cables with the hope of running my BT off the engine battery (57ft boat).

It wasn't satisfactory so I fitted an additional battery next to the BT and used the cables to have it in parallel with the engine battery.

It works fine, the BT battery is kept charged up OK.

 

You do, of course, have fuses at both ends don't you?

[Biggles]

There's no simple answer to this as it depends upon so many different things so let's start at the beginning....

 

How are you splitting the charge to the BT batteries?

 

 

Hi Gibbo

 

I haven't got that far yet. I am having twin altenator so I might charge the BT batteries and the start battery form the small altenator with a split charge relay. But of couse I am open to suggestions. I know you are involved in these systems and need to buy it somewhere. Please PM me with some solutions.

 

Biggles

[MikeV]

You do, of course, have fuses at both ends don't you?

This is one of the reasons why the cable should not be too heavy. Its resistance prevents the charging current being too high so that a modest fuse can be used to protect from a short.

 

I have a 60amp resettable trip at one end (as originally fitted) and added an 80 amp fuse at the other after comments by Gibbo. With 16sqmm cable and a round trip distance of about 35 metres the volt drop required to pull >60 amps will be >2.2V.

 

The BT operation will pull the BT battery voltage down. If we assume the starter battery is maintained at 14.0V the BT battery can be pulled down to 11.8V without the "charge" current exceeding 60 amps. In practice of course the starter battery will also be pulled down to some degree.

 

Once useage of the BT stops the BT battery voltage will quickly rise back to 14.0V as the current will fall off rapidly. Even a 25 second burst of BT (the maximum recommended by my BT manufacturer) is only just over 5.0Ah of battery capacity. The battery (135Ah) is still therefore almost fully charged and will not take much current even from a directly connected battery charger at 14.4V.

 

I don't have any relays (the starter and BT batteries are just wired in paralell) on the basis that the there is no other load on the BT battery. Even if I have used the BT just before switching off the engine the amount of charge to be replaced is small compared to the combined capacity of the BT and starter batteries.

 

You don't of course use the BT without the engine running! Or use it except when really necessary!!

 

I have seen someone wind a boat in marina using the BT alone and when I commented on it he said "it's hydraulic so I can run it as long as I like!"

[Biggles]

Wouldn't the draw on the BT batteries be shared with the start battery if there was no split charge relay. Although thinking about it of course if the engine was running then the rely would become part of the circuit unless a cut off was incorporated while the BT is operated. Thinking on more I suppose if the BT battery is fully charged its not going to try to pull load down form the start battery is I went that route. As you say the draw down on use of the BT will be fairly low and should be replaced fairly quickly.

 

Biggles

[MikeV]

Wouldn't the draw on the BT batteries be shared with the start battery if there was no split charge relay.

And vice-versa! When you start the engine you'll pull a little bit of current from the BT battery. The cable resistance prevents this current being too high.

 

Also worth mentioning your BT battery should be a "starter" type (not leisure) with the focus on its CCA (cold cranking amps) rather than Ah. Lorry or bus starter batteries are readily avaliable with a CCA rating of 800-1000 amps.

[Gibbo]

There are many ways of tackling this problem but you have to look at various different aspects for each particular boat. What works for one boat, won't necessarily be best for another boat. In some cases, what works for one boat, could actually be dangerous in a different boat.

 

Mike's clearly got a grip of the issue on his boat.

 

First thing you have to remember is that the cable has to be fused at both ends because there's a battery at both ends. Those fuses have to be sized to stop the cable overheating at various different levels of overload (that's all fuses are for).

 

So the size of the cable affects the size of the fuses. But if the fuses are too small they will blow when the BT is operated. If they are too big they will not protect the cable.

 

Mike's trick works with certain boats but not others. It won't work with a short boat and it won't work with a very long boat (don't limit your thoughts to narrowboats - there are other types too). It would appear one can always get it to work by using thinner or thicker cables, but this isn't the case.

 

It's a balancing act, and a bit of a possibly dangerous one unless all possibilities are considered.

 

The other options are a split charge diode, but then you have to deal with the voltage drop which usually means an external regulator.

 

The other option is a split charge relay but arranged so that BT operation cuts the split charge to stop the fuses blowing.

 

'tis all good fun this conundrum.

[Biggles]

Why does it have to be fused both ends? Indulge me here. Surely the weak link (fuse) will blow wherever its put?

 

Biggles

[DHutch]

Why does it have to be fused both ends? Indulge me here. Surely the weak link (fuse) will blow wherever its put?

 

Biggles

Because if the leads come of the bow battery and short, you need a fuse near the main battery.

If the leads come off the main battery, you need a fuse at the bow battery to protect them.

And if the leads are shorted (together or to the hull) in the middle you need both of them.

 

 

Daniel

[Gibbo]

First thing you have to remember is that the cable has to be fused at both ends because there's a battery at both ends.

 

Why does it have to be fused both ends?

 

Eh?

 

Edit: Oh I see Dan has answered it.

Edited October 5, 2010 by Gibbo

[ChrisPy]

You do, of course, have fuses at both ends don't you?

??? dunno. I'll check when I'm back in Blighty. thanks

[RichW]

Hello

 

This is one of the jobs I need to look at.

My boat is 58 foot long and has the BT batteries at the bow.

The batteries are charged by the 45 amp engine alternator using 10mm cable. The alternator and engine battery are connected to BT circuit via a relay which is only energised when the engine is running. I am not sure why the relay is there?

I am also not sure where the charging lead should be connected at the BT end of things?

At the moment the BT batteries are connected to the motor via a fuse and then the isolation switch.

The charging lead is connected directly to the BT batteries on the basis that should the BT fuse blow the motor would then not be able to draw current from the engine battery/alternator.

I have no other fuses so I am guessing that I really need another two at either end of the charging cable run?

As it stands the setup works ok, all be it not as safe as it should be.

The problem I can see is getting the fuse ratings correct so that the cable is protected and the fuses not blowing when the BT is operated?

 

Rich

[Gibbo]

The problem I can see is getting the fuse ratings correct so that the cable is protected and the fuses not blowing when the BT is operated?

 

The other option is a split charge relay but arranged so that BT operation cuts the split charge to stop the fuses blowing.

[RichW]

That should not be too hard. The split charge relay is energised by the D+ on the alternator to stop the starter motor drawing current from the BT batteries. So if I open circuit that connection with a small relay energised by the BT control panel it will also stop the BT motor drawing on the engine battery.

 

Thanks

 

Rich