How to charge a battery bank a long way from charging sources

[Warpspeed]

But how else can one charge a large battery bank with DC charge sources when the battery is a 30 mtr total cable run from the charge source? Sure massively thick cables are one way, but it is that practical ?

Massively thick dc battery cable may not be necessary.

The issue of course is voltage drop.
One solution to that particular problem is to use a voltage regulator at the engine end that uses remote voltage sensing.
The voltage regulator then acts according to the voltage measured directly at the battery, not the voltage at the generator/alternator end.

This requires a three wire (or sometimes four wire) connection between the battery and the voltage regulator. Two thick wires large enough to carry the main positive and negative charging current, and a third thin voltage sensing wire connected to battery positive.

The voltage regulator will then source an excess of voltage at the source that exactly compensates for voltage drop along both the positive and negative battery supply cables. This can be done with only three wires, because the regulator assumes equal voltage drops along both positive and negative, which will usually be the case. Four wire remote sensing is also possible if the regulator is designed to work that way.

The battery charging cables can then be made thinner and at lower cost. The resulting voltage drop will produce a power loss, but that may still be less than the combined loss of dc to ac conversion, and ac to dc conversion. You will need to work out some actual figures for all that.

You could still run a second inverter at the engine end, but you would need to ensure the dc voltage there will never exceed the maximum voltage input rating for the inverter. That is very likely not to be a problem, but must be checked.
This is the general idea:
https://www.delcoremy.com/alternators/alternator-features/remote-sense

 

[Warpspeed]

No reply ?

 

[Timmytimmy]

I am trying to figure out how to charge a battery bank in the bow of a sail boat. The charge sources are at the stern of the boat (as I guess most charging sources would be in a sail boat).

The battery bank up front would be around 30-40kWh at 48vnom. The DC cabling should support up to 200 amps of current. To run a DC cable from the stern charge sources to the bow battery bank would be about 15mtrs of cable each way, so a 30 mtr run of DC cabling. Not really very practical.

It seems that an AC source to charge the bow battery bank would be best. I am thinking that perhaps a Victron AC charger up front close to the battery bank would work. So that means I would need to run an AC cable set from the stern to the bow. Is there any reason I couldn't power the AC charger up front from a Victron inverters output in the stern?

I plan on putting an Inverter up front fed from the batteries up front as I want to have 2 inverters onboard for redundancy.

Early days on this thought, so before I go to far wondering if powering an AC charger up front from the output of an inverter in the stern makes sense I would like to hear your thoughts on the pitfalls of this type of setup. I am sure there are issues here I am not thinking about.

I would think its very inefficient for one, what with the losses involved in inverting from battery in the stern to send AC to a charger up front to turn the AC back to DC to charge the front battery bank. Then the battery bank up front powers the Inverter to turn it back to AC. Got to be horrible losses involved in all that.

But how else can one charge a large battery bank with DC charge sources when the battery is a 30 mtr total cable run from the charge source? Sure massively thick cables are one way, but it is that practical ?

Some thoughts:

• 40kWh is ~30 12.8Vx100Ah at 30+lbs each which will be around 1000lbs in the least buoyant part of the boat. That won't be much of a sailboat anymore.
• Why 200A charging? The only practical source of that much power is from a dock AC connection; just add an extra AC inlet right where you need it and run a heavy AC charging cord.
• Even at that, most marine connections are 30Ax120V or a bit more than 3kW. A few are 50Ax240V. But you are likely charging overnight so why not charge at the rate that takes 10 hours to go 20%-80% and use a smaller system including smaller wires, etc.
• Unless its 240V, running AC won't save much in wire cost; 120V will only current the current by about 1/2 vs 48V DC. There is also the safety issue.
• Will you seriously spend 10s of $1ks so you can have an electric stove and rid yourself of a $50 propane tank?

I have built a 10KWh bank to drive a pair Epropulsion 6kW motors. The fridge and the stove are staying propane and I have not yet made up my mind about hot water.

Good luck!

 

[0xkruzr]

Why all electric? Diesel or propane are extremely energy dense. Could be an issue for extremely long voyages and running out of cooking fuel but cooking with electricity takes a LOT of it. Maybe cook with fossil fuels but have a small electric cook setup should you run out of fuels. If you nix the all-electric galley, you might find you don't need as large a battery bank or maybe don't need as big of currents to run from the stern to the bow.

I see many people convert their RV's to be 100% electric. Electric hot water, cook top, oven, microwave, etc. I just don't get it. Maybe they have a desire to be completely free of fossil fuels, but currently (ha!) it doesn't make sense to me on a practical level. We have quite a small solar system in our camp trailer, which mostly just powers our fridge and a few other things besides run the minimal 12v house loads, but we use propane to cook and heat with. When it's 7 degrees F outside, we would go through 20 lbs of propane a week (not much insulation inside unfortunately), and that amount of electric heat would be insane.

As for your main problem, like was suggested before, this is a DIY forum and few here are adept in marine stuff, so hiring a marine electrical engineer sounds prudent to at least give you a working design. Working on shore is so much less complicated. If something fails, just order a new one or run to the hardware store...no such option 1800 miles out to sea.

That said, I think you're probably on the right track to run AC vs DC. There was another post here today where someone wants to generate power at their barn and send it 175 feet away to their house...it was suggested to convert it to 120v or 220v first, send it to the house and in the case of 220v, convert it down to 120v. I think you're on the right track, my guess is no one here wants to touch this because few have the marine experience.

Do keep us posted though on what you come up with, I'm curious to hear how it works out.

From my perspective as an RV owner: propane is a pro pain in the rear. It's also dangerous. In 2023 with panels and other gear being as cheap as they are, going all-electric starts to look really attractive. I just replaced my ailing ammonia-cycle propane/120VAC fridge with an all-12V compressor unit and WOW is it orders of magnitude better than what it's replacing even when the ammonia fridge was in its "prime."

 

[Ohms_Cousin]

OP here......sorry - been busy working on the boat.

I will read through the new replies and answer them a bit later.

 

[aybabtme]

I have an electric sailboat catamaran, 35ft, with 2x6kW pods from ePropulsion. I don't understand why you need two batteries on a 42ft boat split between bow and stern. Your suggested battery pack is wayyyy too big, especially if you have a diesel onboard as main propulsion. If I were you I would aim for 1h of range at full power (~12kWh) which will be plenty enough for your house loads, including electric galley. I have 14kWh myself, using 16x280Ah cells from CATL. My BMS is the REC one, with 2x 3KVA MultiPlus II in parallel for charging and house loads. I actually almost never charge from shore power, so the inverters are really just for house loads. I can run the aircon all night, never think about power usage, and cook with an induction stove top, and still wake up in the morning with 60% battery. And hot showers from an electric instant water heater. I have friends with a pure-electric monohull, who made it to Mexico from SF and cruised around everywhere. They have 21kWh of battery, and a standby diesel genset that they never used so far. But they have a large solar array. Solar is key.

If I were you I would prioritize solar power and being able to charge quickly from your diesel. Hauling 35-40kWh of battery and splitting it across bow and stern, I really don't understand. You don't need that power at the bow, if you want to power a bow thruster just run "thick" (1/0 or 2AWG) cables from your bank. Bow thrusters don't have continuous duty, they're used in short burst every once in a while, the cabling doesn't need to be oversized. Your anchor windlass is never going to need thousands of amps. Use less capacity and focus on charging from solar and from a wakespeed alternator on your diesel.

If you just want maneuverability with the 6kW pods, consider using outboards instead. They will be easier to maintain and less drag and snag under the waterline.

Personally if I need more range than my 1h of full-power autonomy, I'll install a Hatz PMDC-56-100. But in reality, I don't need 12kW of propulsion to move, I usually actually only use 1000W or 3.5kW if I really want to move more properly, and sail for everything else. 12kW is only used in emergency stops/change direction real fast, stuff like that. It makes no sense to push the boat at that power level continuously as the power consumption per mile traveled grows exponentially, and the sweet spot is around 3-4kW to go 4-4.5kts.

If you really want a super large battery bank, put it central in the boat. Don't split it. I'm really unsure what sort of load at the bow of the boat would need hundreds, yet alone thousands, of amps. In any case even if you were putting the entire battery pack in the bow, you could just run some 1/0 or 2/0 cables to each 6kW pods. The cable run on a 42ft isn't going to be a problem. My batteries are about 10ft in distance from my motors, and the wire run is ~25ft. I'm using 2AWG cables and I've never had a problem, even when stress testing at full power for an hour.