backchannel: killing boat batteries

Saillors/cruisers typically have a pretty good handle on power, as their life may depend on it. This thread, though, reminds us that power fails come in boat form, too.

we have 4x125w solar panels fed in pairs to 2 x mppt controllers. > >

Not related to the issue, but an explanation may be in order. Van folk would likely run one big controller, but boats have more shading issues from roof-mounted equipment, masts, etc. Multiple controllers help attenuate the negative effects of partial shading. battery bank of 470 ah (1 x 180ah, 1 x 185ah and 1 x 105ah). Yes I know I should have all the batteries the same but this is not anything to do with the issue I believe. > >

The mismatched batts in parallel are a performance/longevity issue but I agree it’s not the source of the charging problem.

Without knowing anything else, we can guess this setup is underpaneled. 500w:470Ah (1.06:1) is insufficient to charge lead-chemistries by solar alone. If we assume no shade and excellent sun solar-only it would require closer to 700w. Given the shade issues on boats I’d say 1000W. On the other hand, OP has other options and just isn’t leveraging them. Read on.

But for some reason the house bank will not go above 13.1v and then by the morning it is down to 12.1v. > >

The reason is insufficient charging.

Under excellent solar conditions charging 470Ah of lead from 50% takes about 3,320w, or about 8 hours of Full Sun Equivalent assuming there are no loads while charging, no shading, etc.

So, is it because the batteries are always drawing a load they will not go to float? > >

Reminder: dropping to Float stage is no guarantee the bank is charged. The controller may have just timed out.

On shore power the batteries get to float as that is with a 30a charger... We have the generator to use if needs be but would rather not depend on that as thought 500w solar would be more than enough for our usage. > >

There is an uncouth saying about “wish in one hand and sh*t in the other” that applies here. Power configurations don’t care what our hopes and dreams are. They care about chemistry and electrons.

500w of solar-only charging is not enough. Crank up the genny each morning. Every hour of generator charging reduces the lift required of the solar. Just getting the bank to Absorption voltage on genny power reduces solar workload by at least 60%, which means we are now talking about 3 hours of FSE instead of 8 – much more workable. Note: power “cruisers”

Excellent input from one participant: “you probably need to run the gen in the morning for a couple hours each day… upgrading or changing the bank does not solve this issue. you are using more then you are charging.”

bits and bobs

OP is in a sailboat like this, which is why s/he has a generator. Motor yachts (“cruisers”) typically use DC-DC chargers since the engines run for hours while they “cruise” around. OP’s reluctance to run the genny is more understandable if they are frequently running on wind power.

Sailboats sometimes have hydrogenerators which turn the normal hydroelectric paradigm on its head. Normally the water flows past the hydrogenerator. Here the hydro-gen flows through the water: the hydro unit is lowered into the water and the craft’s movement makes the impeller spin and make power. “5 knots of boat speed will produce around 130W… at 7.5 knots of boat speed, the Watt & Sea produces around 300W, and at 9 knots it cranks out north of 500W”. That’s a cool idea and I wish I’d thought of it. As with most marine stuff, it is $$$; the 300w model is about $4000.