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

I'm afraid I don't agree. My bow battery is about 7 to 8 metres (of cable - 14 - 16m if you count both directions) from my rear battery and both are fully charged from a Sterling mains battery charger as I write this - voltages are 13.34 on rear battery and 13.31 on bow battery. I can detect NO voltage drop along the power cable - even with my multimeter set to its most sensitive. The inter-battery cables are (I think) 12mm2.

I can see no case for a battery to battery charger.
 
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I can see no case for a battery to battery charger.

[/ QUOTE ]The case is this....your 13.31V will not fully charge your battery. The battery will need to be taken up to 13.8V or 14.5V to fully charge it, and that might not be possible if the main charger has folded back to the float setting before the remote battery has charged.
 
The point is that all the systems suggested will work. The question is IF they work efficiently or are safe.

We have heard that 50mm2 is big enough for 166A over 16m, but that means the thruster will loose 18% of the voltage and probably run slower, hotter, and over its design current. But the big issue is 50mm2 in a conduit is only rated for 170 Amps so exactly what breaker size could be used?

I certainly would not run cables without conduits and breakers through "normal" human accessible yacht interiors.

The breaker must be sized by the next available size bigger than highest usage. Availability is the key. The wire then has to be rated for the continuous current capacity of the breaker. Approx.


Some suggest just charging from the main battery with a VSR using cables no bigger than the Alternator. This assumes that the Main house battery is very small and cannot supply any output. What would happen when the bow battery was depleted? Quick calculations show that the wire could not be protected without nuisance breaking. Any circuit like that should disconnect the charge circuit when engaging the heavy load.


Then there is the size of the bow battery which is not dependent on the AH required but by its internal resistance. You can not just pull 166A out of a battery that is not designed to supply those loads over a period. The battery will get hot. Internal resistance of a 55AH gel is 8mOhms so that is 1.3v lost at the terminals just due to the resitance. Which is 0.25kW heater inside the battery! Also the capacity of a battery discharged at those rates falls dramatically.


OK then consider how fast it will charge if on a splitter or VSR combiner to a bow battery. Initial assumptions is that not much will be removed, but due to the discharge characteristics at high current the actual amount is large. So with the house/starter bank state determining the voltage at the start of the cable run, and the resistance setting the voltage loss along the cable, the initial current is very low. Only when the House battery is fully charged would you get the bow battery to charge effectively. Unless of course you are a motor boat and the engine runs for hours. I normally design systems for 80-90% in the first hour and then complete by the second hour. Otherwise you become a slave to the engine/generator.

However with a battery to battery system the charging is at its highest rate and the bow battery comes up quickly.

As I said all will work, but put to the test cruising a lot will fail. If you are marina hopping then no problem, except the safety.

I will cite one poor new yacht that had all the electrics fitted in turkey. He was heading out of the Med. He only got to Greece before he realized that the generator was running continuously to keep up with demand. The problems was that the equipment was good (Victron) but the installer had not fit any sense wires and the losses were just enough to stretch the charging cycle to infinity due to the consumption of the boat also appearing across the long charge wires.

Yes, they all work but can you live or survive the solutions.
 
The question was about charging the battery in the bow, not feeding the bowthruster or windlass from batteries far away.

If, as has been suggested by several people, he uses this Sterling battery to battery thing which is basically a dc-dc converter with sense lead and in-built charger characteristics, the Sterling will only pass so much current down the cable anyway, i.e. 50A max for the smaller unit which is all he needs.

He doesn't need to worry about all those volts drop calcs, etc., it is dead easy because Mr Sterling (or his little friends in Asia who probably did it for him) gives the suggested wire sizes.

You see, the battery up in the bow, not the main supply supplies the bowthruster and windlass when they are needed, which is only intermittent use anyway.

Expensive solution and £ wise an overkill, but a very neat engineering solution.
 
Voltage drop calculations...

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We have heard that 50mm2 is big enough for 166A over 16m, but that means the thruster will loose 18% of the voltage and probably run slower, hotter, and over its design current. But the big issue is 50mm2 in a conduit is only rated for 170 Amps so exactly what breaker size could be used?

[/ QUOTE ]Oh dear, I guess you're referring to my post, where I said that "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". This is still true.

Many people find voltage drop calculations difficult. At 12V, 166A along a 16m circuit of 50 sq mm cable will only cause a voltage drop of around 1V, ie around an 8.5% drop, not the 18% loss you claim.

Vetus (who know a thing or two about bowthrusters) recommend 70 sq mm cable for a 16m circuit for their 3kW bowthruster, so I'm still happy with 50 sq mm for a 2kW thruster over a 16m circuit.

As for installing windlass/thruster cables in conduits, yes that's a good idea but is rarely done in leisureboats when retro-fitting equipment. The 170A maximum rating you quote is a continuous current rating, and it's important to remember that bowthrusters only have very short intermittent usage.

Installing large cables through an existing boat may not be an especially easy job, but it's a cost-effective and practical solution.
 
I still don't agree. I think you are confusing the effects of current and voltage. As I said in my post my batteries are presently on "float" charge - i.e. fully charged. If the battery charger was putting out a higher voltage I have no doubt it would appear at the distant voltage just as at the local battery. The only circumstances when this would not be true would be if the current in the interconnecting wires exceeded, say, 15amps. According to Nigel Calder's book at 15amps there would be a 3% voltage drop - reducing 13.8v to 13.4v.

HOWEVER - as the battery charges up the current draw will fall (eventually to 0) and so, also will the voltage drop.

Yes it may be true that the distant battery will charge a little more slowly, but I remain unconvinced that the deficiency would justify the cost of a special charger.

In my view it would take some careful economic and technical calculations to make the case. If you decide to do them I'd be very interested to see your results.
 
You're missing the point. To fully charge a battery it needs to be taken to 14.5V or thereabouts for long enough for the current to fall to about 5A or so. If you don't do that then your battery will never be fully charged. You need some means of ensuring that the remote battery is fully charged. It is not good enough to simply parallel it across the main battery because the type of battery is different, and the leads present significant resistance, but MOST IMPORTANTLY the state of discharge bears no relation to the state of discharge of the main battery.

Trust me, I have a degree in electrical and electronic engineering and am not confused by volts and amps /forums/images/graemlins/smile.gif

The cost is a different matter - it is not for me to say whether something is good value though this is not an easy problem to solve, and the battery to battery idea is a neat engineering solution, albeit an expensive one.
 
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The cost is a different matter - it is not for me to say whether something is good value though this is not an easy problem to solve, and the battery to battery idea is a neat engineering solution, albeit an expensive one.

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David,

I'm slowly getting my head around the Balmar solution to charging engine and domestic batteries, but the third battery bank always seems to be a problem, (although Driftgate2000 seem to have something which deals with 3 banks).

You have twice said that Sterlings Battery to Battery idea is neat but expensive, (as well as interfering with radio), so what are the alternatives which are cheaper and dont interfere with radio, (you've probably suggested something already, but it's gone over my head <g>).

Richard
 
You could install a low cost, small, mains marine charger local to the remote battery and drive that from the mains when you have it, which will be quite often when you get your new genny? You could leave the dioded feed in place and this would keep the battery partly charged when the mains is off. You could even have the small charger at the engine bay end of the wires and only charge the remote battery from the mains but you'd ideally need a sense lead, maybe. On our boat that would work fine as we always have mains on for long enough even at anchor.

However, if you are thinking of using Balmar I suggest that you email them and explain your problem - in all probability they have a solution that would fit in with their kit.
 
Apologies for mixing you up with others that don't know amps from amp-hours.

Well, may I ask a question drawing from your expertise (of which I claim none).

If the bow battery is heavily discharged (after running the bow thruster) won't there be a desirable tendency for power to flow towards it, both from the house battery and from the battery charger / alternator?

I assume (perhaps wrongly) that the charger will not be able to distinguish between the two batteries - it will simply see a net resistance into which it can pump electricity. Where the electricity goes depends on the relative resistances of the various circuits. (I am crudely assuming that a charged battery appears to present more resistance than a discharged one - but I think that is a reasonable way to envisage the system).
 
Well, sort of and sort of not. It's often easiest for practical folk with little electrical experience to think of a plumbing analogy.

Think of three barrels of water all connected with a pipe (at the bottom of the tanks). The interconnecting pipes will restrict flow if they are narrow bore but water will only flow from a tank with a high level to one with a lower level. The flow will depend on the bore of the pipe and the height (pressure) difference of the water in the barrels.

Volts is equivalent to height (head) (pressure) of the water. Amps is equivalent to the speed of water flowing through the pipes. Ampere hours (Ah) of the battery is equivalent to the volume of the barrel. Ah remaining in the battery is equivalent to the amount of water left in each barrel. Watts is the equivalent of the volume of water flowing through the pipes. Diodes are non-return valves.

Does it help to think of the batteries using that analogy? It doesn't work for everyone, but it is a great help for many and the analogy can be extended to many other electrical/electronic systems and terms.
 
Is this voltage during charging or after charging has finished, and it is in float mode?

If this is charging mode, then the volts it entirely inadequate to charge the battery. If there is a substantial current flowing, I do not understand how you manage to get such a small voltage drop.

I presume therefore that this is a float voltage - where the current is so small, as not to suffer loss of voltage.
 
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You have twice said that Sterlings Battery to Battery idea is neat but expensive, (as well as interfering with radio), so what are the alternatives which are cheaper and dont interfere with radio, (you've probably suggested something already, but it's gone over my head

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Richard, I have not experienced any radio interference from the Stirling Battery to Battery charger. It is mounted right beside the VHF and radio/cd player.

I am convinced that this route was the easiest to go - if not the cheapest. However from your point of view (and mine) the level of technical compentance was very low! (apart from crimping the connectors on big wires)
 
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Richard, I have not experienced any radio interference from the Stirling Battery to Battery charger. It is mounted right beside the VHF and radio/cd player.

[/ QUOTE ]Unfortunately we have two threads running at the same time discussing this Sterling product! I am not surprised that they have suppressed the device at VHF but a poster told us that he had problems with his SSB, and that Sterling told him that their battery to battery controller is not suitable for use with boats having SSB. Which is understandable as it is more difficult to do. I'd bet that it interferes with Navtex badly, as well, as well as RTTY and other weather receivers. Basically not suitable for use in a sea-going vessel, probably OK for the inland waterways. It isn't acceptable to run noisy equipment on your own vessel if you will be in the vicinity of other vessels....you have a duty and legal requirement to suppress your kit.
 
Testing these things isn't always easy - you need to test it at several different loads, or even right across the range. While the converter is probably fixed frequency, it may have a variable mark-space or pulse-width ratio to control the output and that results in totally different harmonic noise output. To be sure, you'd want to discharge the battery to the lowest level you normally accept then listen carefully to the SSB with plenty of rf gain, while scanning across the whole band to pick out the frequencies of particular concern. It isn't a five minute job.

Remember, Mr Sterling has been quoted in this thread (or the other one) as saying that the battery-to-battery charger is not compatible with SSBs. That would include weatherfax and (prossibly) Navtex.
 
I was only trying to make the point that my two batteries were fully charged even though one is a long distance from the other.

The voltage drop for any given current is easy to determine from tables, or from the specific resistance of copper and the cross-section area and length of the cables.
 
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