MacGregorSailors.com

I was thinking of ways to get more weight forward to compensate for extra weight in rear of boat. I liked KnotShores V Berth gas tank idea, but once it’s empty doesn’t help, and hate to be carrying around extra weight just to balance boat out. Thinking 2 group 31 deep cycle AGM batteries up there. Would have to cut opening and install hatch, remove some strofoam and run cables to rear. Would keep engine battery in current location under galley seat.

Other than really long cables, there is no downside that I can see. And since this is house power and not for engine starting, they don't have to be garden hose sized.

On my "S", I have one group 31 in the original location under the sink & I added another under the v-berth. My battery switch is by the sink so the cables for the V-berth battery are only about 6 ft long. I think the extra weight of the battery in the V-berth helps offset my personal ballast at the tiller. I use the V-berth for storage and keep everything heavy up there.

Tomfoolery wrote: ↑Mon May 10, 2021 1:32 pm Other than really long cables, there is no downside that I can see. And since this is house power and not for engine starting, they don't have to be garden hose sized.

Extra weight in the bow should improve sailing ability slightly. I considered moving my battery bank to the v-berth area but ultimately decided on keeping it lower under the starboard seat just in front of the head.

Depending on what he's planning on doing with them he may need "garden hose sized" cables. Just rough numbers: two deep cycle, say 220AH each, will ideally be charged at .2C or around 44A each. If you can put up with 3% loss, that means 4 to 6AWG depending on how far it is to the charge source. Of course, you can charge them at a lower rate, but you risk shortening the life of the batteries. On the output side it really depends on how he's going to use them. Just running the lights and a few DC appliances should be fine on something smaller, maybe as small as 10 to 12AWG. If he's planning on an inverter to run heavier AC loads he's going to need something pretty large.

It all boils down to the load and the length. It is certainly doable but there may be trade-offs between cost and efficiency.

If you decide that it makes sense electrically (and it may) be very careful with your tie-downs. The bow of the boat is going to see a lot more motion than the areas where we traditionally install our batteries. That is a potential source of acid spills. The additional pounding may also adversely battery life if material that has been shed off the plates gets stirred up causes problems.

The batteries I would use are 105 amp hour x 2 = 210 total, battery charger is 40 amps and would use 2500 watt inverter. Not sure what size cable I would need to do this, inverter could be placed close to batteries.

Does anyone know if the ballast tank extends forward of the last wall in bow? I wouldn’t like to put batteries on top of the tank with all the up and down motion up there. Would AGM type batteries survive the motion?

My 26x HAS 2 big batteries under the v berth and the outboard battery under the small seat next to the galley.
150w solar panel...never had and issue....I run a 50l fridge/freezer plus other electronic gear...led lights.

Ozphotog wrote: ↑Tue May 11, 2021 12:09 am My 26x HAS 2 big batteries under the v berth and the outboard battery under the small seat next to the galley.
150w solar panel...never had and issue....I run a 50l fridge/freezer plus other electronic gear...led lights.

Does the ballast tank extend beyond the front fiberglass wall? How did you access the area? Did you make a level floor bottom to set batteries on?

Sheppie62 wrote: ↑Mon May 10, 2021 8:33 pm The batteries I would use are 105 amp hour x 2 = 210 total, battery charger is 40 amps and would use 2500 watt inverter. Not sure what size cable I would need to do this, inverter could be placed close to batteries.

2500W at 12.5V is 200A, not counting efficiency losses. So yes, inverter close to batteries.

My van has a 3000W inverter on a 600Ah LiFePO4 battery, and it routinely draw 150A or more when running the air conditioner off battery power. But I don't know if the batteries you intend to use can support that kind of draw for any extended period without damage. My van's system is an engineered system, including inverter/charger and BMS, and will limit current draw and/or shut down if maximums are exceeded.

Sheppie62 wrote: ↑Mon May 10, 2021 8:33 pm The batteries I would use are 105 amp hour x 2 = 210 total, battery charger is 40 amps and would use 2500 watt inverter. Not sure what size cable I would need to do this, inverter could be placed close to batteries.

A good rule of thumb for 12V inverters is 1 amp of DC current for each 10W, so a 2500W inverter will draw about 250A. Depending on the distance between the inverter and the battery that can require an impressive piece of copper. If your round trip distance (both wires) is < 15 feet you can probably get by with 0AWG but 2/0AWG would not be overkill and is absolutely required past 15 feet round trip. Don't forget proper fusing at the battery.

Disclaimer: these are rough estimates. Do your own calculations or pay someone with lots of insurance. There are lots of reasons why you could need a larger wire.

I believe you mentioned these were AGM batteries. Typical AGM batteries can be discharged to 50% without significant damage. That means you have 105AH of usable battery between them. That 105AH rating is based on discharging over 20 hours or about 5.25A continuous. Your inverter, running wide open, will kill the batteries in less than 25 minutes. I can't tell you how much less than 25 minutes without a lot of technical information about your specific batteries but it's going to be noticeably less.

The next thing to consider is how the batteries are going to respond to being discharged at such a high rate. Generally, deep cycle batteries are not discharged at a rate higher than 1C (one times their amp hour rating). Your 210 AH bank would not normally be expected to be discharged at more than 210A. Again, rough numbers, rules of thumb, lots of exceptions... You are right at or just over the top end of what the bank should be expected to do with new batteries. As the batteries age their capacity will decrease so that will need to be taken into account as well.

If you are not planning on running the inverter wide-open then the batteries will last longer but that does not eliminate the need for wiring that can handle whatever the inverter asks for. If you choose to use wires that cannot handle the inverter at its full rated power then you MUST use a fuse that will not allow the wiring to be overloaded. The fuse is there to protect the wire, not the equipment attached to it.

As always, all opinions are offered for entertainment purposes only. If you happen to find any useful information in them consider yourself lucky. No numbers were intentionally harmed in the posting of this message (I think I did the math right ) Dissenting opinions respectfully requested. Trolls will be shoved back under their bridge where they belong.

Tomfoolery wrote: ↑Tue May 11, 2021 8:19 am

Sheppie62 wrote: ↑Mon May 10, 2021 8:33 pm The batteries I would use are 105 amp hour x 2 = 210 total, battery charger is 40 amps and would use 2500 watt inverter. Not sure what size cable I would need to do this, inverter could be placed close to batteries.

2500W at 12.5V is 200A, not counting efficiency losses. So yes, inverter close to batteries.

My van has a 3000W inverter on a 600Ah LiFePO4 battery, and it routinely draw 150A or more when running the air conditioner off battery power. But I don't know if the batteries you intend to use can support that kind of draw for any extended period without damage. My van's system is an engineered system, including inverter/charger and BMS, and will limit current draw and/or shut down if maximums are exceeded.

Tom,
How close is your inverter to your battery bank? What size wire does it use? I'd like to see how my theory lined up with your engineered system.
Bill

Be Free wrote: ↑Tue May 11, 2021 10:34 amTom,
How close is your inverter to your battery bank? What size wire does it use? I'd like to see how my theory lined up with your engineered system.
Bill

I'll go out there and see what I can see. The battery is only a couple of feet from the BMS, and the inverter is on the opposite side of the van, so maybe 8 ft away. I'll see what I can find for cable sizes.

Anybody seen the floor of the V berth area with the styrofoam or even better pictures? How hard would it be to make a platform for the batteries to sit on?

Sheppie62 wrote: ↑Wed May 12, 2021 7:21 am Anybody seen the floor of the V berth area with the styrofoam or even better pictures? How hard would it be to make a platform for the batteries to sit on?

My boat is at the mechanic so I don't have pictures but I placed mine in the compartment near the v-berth under the most forward bench seat. I came up with a way to secure the battery boxes snuggly enough to limit moving there is some slight weight distribution issues but I was able to somewhat compensate that with a water tank on the port side for the sink. I use 2 - 27 series AGM batteries and they have worked great the only issues has been my wiring which I still need to fleece out.


Please feel free to reach out, this site has helped me tremendously and I want to pay it forward.

Be Free wrote: ↑Tue May 11, 2021 10:34 amWhat size wire does it use?

Factory harness from Xantrex is 4/0 welding cable. This runs between the single 600Ah battery and the BMS box. Voltage drop would be about 0.5% at 150A, assuming 4 ft out and 4 ft back.

The battery has a recommended charge and discharge rate of 300A, but an allowed charge/discharge rate of 400A. I don't think I've ever seen it go over 200A from the 280A second (dedicated) alternator.

4/0 copper cable presents about 0.05Ω/1000 ft, just for reference. That's from the WM table, as I'm not in my office and therefore can't consult the sacred text (NEC).



I'll sniff out the inverter/charger wiring shortly and report back.