800w of solar, 200ah of LiFePO4?

[earthbm]

Power sources and uses. The numbers in Amps per day (assuming 12v).
8x100w solar panels, so 8A max at 12v.
Min/Avg/Max assumes 30%, 75%, 100% output for 6h, 8h, 10h:


The 640 amp-hour max intake suggests a third 100AH battery might be needed, but consumption doesn’t seem to warrant it. I may be under-estimating consumption.

Any suggestion from the hive mind?

[HeywoodJ]

I'll take a crack at this:

1. Typical LiFePO4 are at ~13.5V when charging, which drops your 8A down to ~7.4A from each 100W panel. That's an 8% reduction off the top.

2. The "rule of thumb" is for good days you'll get the equivalent of 4-5 hours at nameplate unless you have tracking panels. Depends on where you expect to cruise. If you go with 5 hours then 5x8x100/13.5 = 300Ah (I would consider this close to "max" in your categories - the times when you get much more will be few).

Using those two considerations suggests to me that either you have too much solar, or not enough batteries, to effectively use your power. If you have heavy consumers that you only use during the day then that may change the picture, but otherwise add a battery or waste a lot of solar potential.

That's the short version.

[earthbm]

Originally Posted by HeywoodJ

I'll take a crack at this:

2. The "rule of thumb" is for good days you'll get the equivalent of 4-5 hours at nameplate unless you have tracking panels. Depends on where you expect to cruise. If you go with 5 hours then 5x8x100/13.5 = 300Ah (I would consider this close to "max" in your categories - the times when you get much more will be few).

Using those two considerations suggests to me that either you have too much solar, or not enough batteries, to effectively use your power. If you have heavy consumers that you only use during the day then that may change the picture, but otherwise add a battery or waste a lot of solar potential.

That's the short version.

Thanks for your answer. Let’s say I consume 300ah a day, evenly spread. If the battery needs to cover 18hours of that, I need 300ah*18/24=225ah. Assuming I don’t want to drain LiFePO4 below 20% then I need 281ah, or 300ah, rounded up.

If the consumption number is as I assume 216ah max, I need 162ah of batteries and my 200ah is enough.

But the issue then is that I have too much solar, as you say. On balance, since I already have the panels, adding a 100ah battery makes sense. Especially since I also have a D400 wind gen, fwiw.

Originally Posted by HeywoodJ

1. Typical LiFePO4 are at ~13.5V when charging, which drops your 8A down to ~7.4A from each 100W panel. That's an 8% reduction off the top.

It’s on a little tangent, but does that mean that my 100ah batteries are really 89ah batteries at 13.5v?

[PaulCrawhorn]

No such thing as too much solar.

Need to plan for worst case insolation conditions

So on clear sunny days there will be "excess" nature of the beast.

Put into a load dump, heating HWS, or freezing blocks of ice, instapot, dehumidifier, watermaker...

[earthbm]

Originally Posted by PaulCrawhorn

No such thing as too much solar.

Need to plan for worst case insolation conditions

So on clear sunny days there will be "excess" nature of the beast.

Put into a load dump, heating HWS, or freezing blocks of ice, instapot, dehumidifier, watermaker...

Awesome! Water maker as the power sink!
Thank you!

And fwiw, I’m selling my D400

[Adelie]

Rule of thumb for a mostly sunny day in tropics or near tropics, panel mounted flat:
PWM controller: 1/4 panel nameplate wattage = average daily output in Amp-hr. So 1/4 x 800W = 200Ahr
MPPT controller: 1/3 nameplate = Ahr. So. 1/3 x 800 = 264Ahr.

Using a small controller on each panel or pairs of panels ON THE SAME SIDE OF THE BOAT helps alleviate shading on individual panels.
Do not put panels on opposite sides of the boat on the same controller. Panels on one side will be fully shaded by the sails and massively drag production down for all panels.

[earthbm]

Thanks, looks like a reasonable rule.

I have 2 MPPTs, one for port and one for starboard. The 4 panels on each side are 2s2p - hope this doesn’t screw things up.

[LeeV]

Originally Posted by earthbm

Awesome! Water maker as the power sink!
Thank you!

And fwiw, I’m selling my D400

Not worth it, or too noisy?

[earthbm]

Originally Posted by LeeV

Not worth it, or too noisy?

1. Don’t have the skills to integrate it with my new LiFePO4 (another thread here).
2. Got more solar than I need
3. The final destination is So Cal, with 10-15kt winds at best

[LeeV]

Originally Posted by earthbm

1. Don’t have the skills to integrate it with my new LiFePO4 (another thread here).
2. Got more solar than I need
3. The final destination is So Cal, with 10-15kt winds at best

Makes sense. I’ve considered them, but the one I observed from sitting nearby was loud with good winds (especially at night!), and with the progress in Lithium Ion and cheaper solar, I wonder if they’re becoming a thing of the past?

[earthbm]

Originally Posted by LeeV

Makes sense. I’ve considered them, but the one I observed from sitting nearby was loud with good winds (especially at night!), and with the progress in Lithium Ion and cheaper solar, I wonder if they’re becoming a thing of the past?

Well I don’t know. If you remove just one of my three reasons I’d keep it. Belt and suspenders. No Sun at night, etc.

[LeeV]

Originally Posted by earthbm

Well I don’t know. If you remove just one of my three reasons I’d keep it. Belt and suspenders. No Sun at night, etc.

I get it; to each his own. I was always interested but didn’t want the pole, etc on any of my trimarans, so I wound up going in the solar/lithium direction, always self sufficient, except on one I had a Honda 2000.

[hlev00]

A data point. My boat is a Beneteau 423. I have 380W of Solbian flexible panels on my bimini split port and starboard between two Victron controllers and 480Ah of FLA batteries. Cruising Florida and the Bahamas we can generate about 1800 Wh on sunny days, which is most. Our biggest consumer is frig at approx 900 Wh so at anchor the batteries top off closer to 1400Wh. Underway during the day the MFD and AP can take a lot but the panels are normally sufficient. In the summer in New England, generally day sailing, the panels are still fine for most of the season. 30 minutes under power to get out of a harbor in the morning does a lot. (120A alternator). On our cruise south along the east coast this last fall there were days we needed to run the engine even with reasonable sun but that wasn’t a big problem. For FLA batts if I am down in the morning due to low sun the day before, I will run the engine for 30-40 minutes to help with the bulk charge. Of course LiFePo will be different.

[HeywoodJ]

Originally Posted by earthbm

...It’s on a little tangent, but does that mean that my 100ah batteries are really 89ah batteries at 13.5v?

Nope, an Ah is more or less an Ah (disregarding Peukert). Where it gets tricky is when you figure out how much energy is in that Ah.

Much easier to demonstrate in lead-acid because they have a much broader voltage range. Putting 1A into a battery at 14.4V for one hour is one Ah and takes 14.4Wh of energy. When you take that same 1A out of the battery it is likely at 12.6-12.8 volts. You took out 1Ah, but you only got 12.7Wh when you did that. The difference is the round-trip inefficiency of the battery. You lost about 12% of your energy in the transaction.

Lithium is a lot better in this regard, the voltage difference between charge and discharge is much less and the efficiency is much better.

When calculating from W to Ah you have to be careful to use the actual voltages rather than nominal, as the difference between (as an example) 12V and 14.4V is 20%. That can throw a big error in your overall calculations.

[Rumrace]

I think this thread will pop up monthly cause there are so many factors. There is an active one right now and another on isolating you bow batteries from the house or not.
I think we pegged a fridge at 2amps per hour and a plotter at 5! Electric winches other instruments. A couple engineering types was good information to consider.

I have 3 105watt panels flexible in the Bimini. I’m adding a forth. I’m also going to add a resistor to the engine alternator and generator. We saw another boat with a zener diode to dissipate excess energy.

I have 4 lipo 100 which can peak at 200 House
2 lipo Bow 100
1 90 Lead Acid crank.
1 1200mA 3.5V


I understand the fridges can be tweaked to run with less power.
I had a power boat previously with two fridges and had 4 house batteries 100. One solar panel on the coach roof not sure the wattage. The boat ate 3 batteries a day and the solar panel did very little.