Electrical Engineering Problem

[nhschneider]

My house battery bank needs replacing and I'm considering moving the entire bank in an effort to put my boat on a better trim. However, to do this will require extending the battery cables about 25 feet. In order to keep the voltage drop to a minimum, I think I'd need bigger than 4/0 cables. Has anyone moved an 880-amp hour battery bank that far? Would I have to recable from the alternators (3 of them), inverter/charger and the solar panels?
Frankly, I wouldn't mind engaging a marine electrical engineer to design the specifications and layout but I am concerned that the entire project could cost thousands.

I'm grateful for any ideas.

[kenbo]

Some thoughts on your questions:

Cable size from battery to distribution point - need to know what your maximum current draw would be to answer the question properly.

Cable size from alternator to battery - no change required if charging voltage is sensed on the battery, yes if not.

Solar panel charge controller to battery - same as alt to battery if charging voltage is sensed at battery.

[Lake-Effect]

Originally Posted by nhschneider

My house battery bank needs replacing and I'm considering moving the entire bank in an effort to put my boat on a better trim. However, to do this will require extending the battery cables about 25 feet. In order to keep the voltage drop to a minimum, I think I'd need bigger than 4/0 cables. Has anyone moved an 880-amp hour battery bank that far? Would I have to recable from the alternators (3 of them), inverter/charger and the solar panels?
Frankly, I wouldn't mind engaging a marine electrical engineer to design the specifications and layout but I am concerned that the entire project could cost thousands.

I'm grateful for any ideas.

A drawing of your boat plan with engines, bank locations and panel location would help a lot. Also a summary of your DC loads, and some idea of your average and peak current draw (excluding the starter).

For many boats, the single biggest draw is the starter, so the closer your batteries are to the starter, the shorter the run of really heavy wire. Or put dedicated starter battery(s) close to the engine(s), and then you don't need quite as heavy wire to the house banks and the panel, and you have more latitude for placing the house banks... but that might reduce the ability of the house bank to act as a starter battery in emergencies.

If your batteries are currently in the best 'electrical' position, you should maybe consider moving other stuff to tweak the boat's trim.

[sailorchic34]

Hum, 4/0 is pretty darn huge. For a typical diesel starter, for a 38foot boat I think 2/0 would be fine for 25 feet to the starter's. You can take more then 3% loss to the high amp load starters as they are very intermittent in use.

You will want the Inverter located close to the battery bank as that is a critical load and long runs are not your friend there So keep the distance to 12-15 feet max and 2/0 is your friend there too.

Of course, you'll have a 4/0 negative from battery minus to the common ground point (normally an engine) with 2/0 from each engine to ground point.

For the typical dc panel with typical loads, fridge, radar, lights etc. #2 + and - wires would be fine for up to a 25 foot distance. Again this does not include inverter. If your inverter is on the dc panel that you're going to want 2/0. Without inverter on DC panel #2 is fine as loads are lower amp wise

For 100 amp alternator output, #2 would be safe to use to 25 feet.

Just my 2 cents and YMMV.

[Jammer]

Originally Posted by nhschneider

My house battery bank needs replacing and I'm considering moving the entire bank in an effort to put my boat on a better trim. However, to do this will require extending the battery cables about 25 feet. In order to keep the voltage drop to a minimum, I think I'd need bigger than 4/0 cables. Has anyone moved an 880-amp hour battery bank that far? Would I have to recable from the alternators (3 of them), inverter/charger and the solar panels?
Frankly, I wouldn't mind engaging a marine electrical engineer to design the specifications and layout but I am concerned that the entire project could cost thousands.

I'm grateful for any ideas.

If your boat is not already 24v, this might be a good time to think about switching it. That would allow you to use much smaller battery cables with the same % voltage drop.

At 12 volts, 500 MCM cable would give you a 1.8% voltage drop from the existing battery location to the new battery location at a design current of 200a which is what your alternator and inverter/charger should be designed for. 3% is the limit for ABYC and you'll already have some voltage drop in the existing wiring. For 500 MCM, distributor pricing in 1000 foot lots is around $17 a foot right now, you would need 50 feet.

If you were to switch to 24 volts, the design current would be lower, and you'd get comparable % drop from 2/0 copper at about a quarter the cost.

With a 25' run, to meet ABYC standards you would need a disconnect means and overcurrent protection close to the battery (I can't remember the distance without looking it up). You could leave your existing cabling in place and connect it to the new 25' run using copper busswork, properly enclosed.

500 MCM cable is larger than most marine suppliers are accustomed to dealing with and is not, as far as I know, available pre-tinned. On the other hand, it is widely used in telecommunications power systems and industrial systems. I personally would feel comfortable using untinned 500 MCM in a marine environment as long as the cables are terminated using a crimp fitting applied with a proper tool and die as that will produce a gas-tight connection that will not corrode. This is large cable and you would want the two-hole termination lugs so that the weight of the wire does not cause it to move on the bus and loosen the bolt. You would want to use pre-tinned copper busbar.

A lot of cost and some ponderously large cable, but it would work well.

If it were my boat my first choice would be to switch the major sources and loads to 24v (inverter/charger, alternators, starters, solar) to avoid using such large cable.

My second choice would be to relocate the inverter/charger to a location close to the new battery bank, and use more modest cable sizes to the alternators and solar charge controller, being sure to run "voltage sense" wires to the battery separate from the current-carrying wires. That way, a larger voltage drop would still provide acceptable performance, at lower cost and without the weight and installation difficulties posed by 500 MCM cable.

[nhschneider]

Thank you very much for all your thoughtful replies. I should have thought to add a drawing and have attempted to attach one here.

Since we use a separate and dedicated starting battery, we shouldn't have too high of a current draw but our charging currents can get pretty high.

Again, I'm grateful for the input.

[Lepke]

I have several inverter 48 volt battery banks. One in the engine room and another 30' aft and another 30' forward. Each is cabled (braided welding cables) for maximum inverter draw. I have had no problems with drawing power or charging, either with an alternator or AC charger.
They're controlled with the usual battery switch. The batteries are all the same, so at anchor, I run as a single bank for a several days between charging. Or when running the mains, charge while supplying the inverter and avoiding running a generator.

[kenbo]

Originally Posted by nhschneider

Thank you very much for all your thoughtful replies. I should have thought to add a drawing and have attempted to attach one here.

Since we use a separate and dedicated starting battery, we shouldn't have too high of a current draw but our charging currents can get pretty high.

Again, I'm grateful for the input.

Don't worry about the charging current, that's not going to change. What may change is the charging voltage at the B+ terminal on the back of the alternator. And that voltage will only change if your voltage regulator is sensing the house battery's voltage. If it's machine sensed then the cable size and voltage drop matter. To what degree depends on the current flow.

[sailorchic34]

Originally Posted by nhschneider

Thank you very much for all your thoughtful replies. I should have thought to add a drawing and have attempted to attach one here.

Since we use a separate and dedicated starting battery, we shouldn't have too high of a current draw but our charging currents can get pretty high.

Hum, I'm not a big fan of putting that much weight up near the pointy end where the added weight will cause the bow to dig in a bit more in waves.

Plus really long runs to inverter/alternators. Though as kenbo said, if your alternator sense line is connected to the batteries (really a nice thing to have/do) then voltage loss in alternator charging wire does not matter.

How out of trim is the boat and is that with or without provisioning weight in galley, full tankage, etc.

[skipmac]

Originally Posted by sailorchic34

Hum, I'm not a big fan of putting that much weight up near the pointy end where the added weight will cause the bow to dig in a bit more in waves.

Absolutely!! Not only will it cause the bow to dig in but in the right wave conditions the boat will tend to hobby horse a lot.

[nhschneider]

I wish I had photos with me that would show how far out of trim my boat sets in the water (the boat's in Virginia and I'm in California). It's been that way ever since I bought the boat but with the boat fully loaded with water, fuel and provisions, the actual water line (where the boat floats) is about 8" below the factory-painted water line. Initially, I thought that the previous owner was just a pack-rat and had raised the painted water line to accommodate all the crap he carried around but after 7 years of full time cruising and collecting quite a bit of junk myself (it's all important!), the bow remains way too high. In addition, we list about 3" to starboard (measured at the water line). To be honest, I don't think moving my 600-pound battery bank is going to make that much difference on the trim - 90 gallons of diesel fuel barely makes a 1/2" difference. I guess it takes quite a bit to leverage against 12,000 pounds of cast iron keel.

[skipmac]

The problem with moving a lot of weight close to the bow isn't the trim IE how level the boat is fore/aft and port/starboard but the moment of inertia increase caused by the weight forward.

If you aren't familiar with moment of intertia, a mass in motion will have inertia. It takes force to make it start moving and force to make it stop moving. On the bow of a boat the force to move up is the lift of a wave, down is gravity. Put a lot more weight in the bow and when a wave comes under the bow the weight resists and the bow can bury into the wave. Once the bow does start moving up it keeps moving due to the inertia, even after the wave passes.

So a lot of weight in the bow will cause the bow to bounce up and down (hobby horse) and bury the bow in waves so take more spray. The farther forward and the heavier the weight, the higher the moment of inertia.

[DeepFrz]

You will want to pay very careful attention to wiring size to your autopilot and your refrigerator/freezer in particular. These items are very voltage sensitive and can draw sig. current.

[Lake-Effect]

Seeing your boat's plan... that location seems to be about optimum for where your batteries should be. I personally would not move them.

For the money you'd expend to make such a change, it would probably be cheaper to take other steps to trim the boat. A wine cellar forward? Concealed gold ingots? 500 lb of kitty-litter?

[cyan]

+1 on the existing battery location. Is there any way to reduce weight in the stern? For instance, are the aft solar panels supported by elaborate stainless steel monkey bars that could be aluminum? I realize such a weight difference might by small overall, but changing the trim while improving the moment of inertia (as mentioned by skipmac) would be the first subject to explore IMO.