Question for Those Smarter than Me

[Wolverine]

Okay, so I guess this is for everyone. I have 1 battery on the boat connected to a 4 way switch. Off, bat 1, bat 2, both.
A 50 watt solar panel is mounted on the cabin top leading to a charge controller and then to the battery. I just purchased a second battery and would like to wire it to the controller too. If I connect the batteries in parallel, will they both get an equal charge from the controller?

[tjspiel]

I can't speak for every set up but I know for the LifePo4 batteries I have, they recommend that you fully charge the batteries individually before connecting them in parallel and that you should connect them with bus bars.

But yes, you should get equal charge that way. And if they are LifePo4 batteries, I'm sure that they'd also recommend that you get the same brand and size so that the battery management systems will behave the same way.

[Wolverine]

Yes, they are the same brand/size LifePo4. 100ah each.

[Charles Brennan]

Wolverine, You stated:

Yes, they are the same brand/size LifePo4. 100ah each.

And also:

A 50 watt solar panel is mounted on the cabin top leading to a charge controller and then to the battery. I

So, I'm curious to know a few things:
1) What boat are these 2 LFP (the industry designation for LiFePO4 batteries, these days) 100 A/Hr going to be used on?
2) What are your energy requirements on board?
3) To have 2500 watts of battery capacity on board, are you considering electric propulsion?

For context, on Urchin, I had a 30 watt solar panel running a charge controller to replenish a 100A/Hr flooded lead-acid (FLA) battery.  I had all LED Nav and cabin lighting, a tiller pilot, a CPAP machine on an inverter and the ability to re-charge all the various USB Toys.  Things like Depth Sounder, GPS and VHF radio, had their own batteries.
This system basically ran at equilibrium, in that the solar array replenished all the energy I had consumed on board.  This meant that power-wise, I could cruise indefinitely.
I am building a sailboat that needs all the same things just listed above, PLUS a trolling motor.
VERY different math! 
I too, have two 100 A/Hr LFP batteries and a 100 watt solar panel going into a MPPT charge controller and re-charging both batteries, with all the same caveats as tjspiel has already posted.

So here is the reason for my questions:
1) The math is a little dismal for re-charging LFP batteries in these wattage ranges.
2) While we do get nearly 12 hours a day of sunlight, only about 5 or 6 of those hours, are doing the heavy lifting.
3) Panel and charger in-efficiencies (panel partially obscured on different tacks, heat, clouds), can all affect controller conversion in-efficiencies and eat up about 20% - 30% of your conversion energy.
4) You didn't state whether your charge controller is MPPT or PWM. If it's PWM, then you are closer to the 30% loss, than the 20% loss, in conversion efficiency.

To replenish two 100 A/Hr LFP batteries from flat, would require 400 to 600 watts per day, from a solar array.
That's a LONG WAYS away from 50 watts, My Man. 
However, Real Life is not quite as bleak as all that; that's predicated on worst case performance, which is the only kind of performance that Engineers believe in. 
If you are using a trolling motor, you are likely not running it until you've chewed up all 2500 watts of energy.
10 minutes to get out from the ramp and 10 minutes to get back in, is much more doable.  Factor in a 2 or 3 hour run in zero-wind to get somewhere, and you're still generally in the ball park, but those batteries WILL be down a significant amount.

Getting back to worst case performance, I would actually need between 400 to 600 watts of solar arrays to replenish two 100 A/Hr batteries!!  Pave the entire cockpit, deck, and Bimini top with solar panels, and I'd STILL have stuff hanging over the side! 
My current anticipated strategy is based on gradual resource attenuation.

Take for example, a typical FL 120 or BEER Cruise. 4 or 5 days, with a trolling motor and all the aforementioned Toys.
I won't really pull the battery down all that much, with trolling motor ramp or beaching maneuvers. And all the while I'm sailing, I am partially replenishing what I used up back at the ramp, less whatever the tiller pilot is chewing up.  At the end of the day, I'm no longer at full charge, but still have plenty of capacity.
At night, running other appliances (CPAP, cell phones, re-charging my VHF, etc.) I am chewing up watts with no way to replenish them until the next day. But!  The next day I DO get to replenish from the solar array.

No illusions that the re-charge/use-up cycle will continue to trend downward, but I fully expect to reach the end of a several day cruise, with enough energy to reach the ramp.  And if the batteries are flat by then it doesn't really matter, since they'll get re-charged off the grid when I get back home.

Hence my Main Question:
What are you doing? 
If just running the basic on-board electrical system, then a 50 watt solar panel, controller and a single 100 A/Hr battery will be fine.
If instead, you are going the trolling motor route, (like me), then 50 watts of solar array will be woefully inadequate.
Please bear in mind that I have no real-world hard numbers to give you, since my boat isn't finished, yet.
But I did run a trolling motor powered with a FLA 100 A/hr battery on Urchin for three years, so I have some experience with the challenges I expect to face.
Plus, I HAVE done the Dismal Math and if I come up short, my private contingency plan is to shoe-horn in two 100-watt solar panels, if necessary. My MPPT controller is already up-sized, for just that future contingency.

Food for thought,
Charles Brennan

[Wolverine]

I was hoping you would chime in. The 3 interior lights are led as are the steaming, running, and anchor lights. A vhf radio, depth meter, and gps. I will need to charge my phone too, but I have a separate portable solar charger for it. There are 2 cheap 12v oscillating fans that are marked ".8 amps/12 watts". Those will be used intermittently. There is also a 12 volt computer fan used to draw air into the cabin. Again, to be used intermittently. Now the biggest draw. A Coleman electric cooler. I haven't found any labels stating its power requirements.

I am leaving for the keys January 3rd for a 5 week sail. I did perform a simple test with the single battery. I left the cooler and the anchor light on for 7 days with the solar panel connected. Afterwards the meter showed the battery at 60%. I added the second battery today and am hoping it will help. The Coleman isn't necessary, and I may not bring it.

The charge controller is an mppt.

[Charles Brennan]

Wolverine, KEWL!! 
Always do better when someone gives me something approaching a FACT!! 

The 3 interior lights are led as are the steaming, running, and anchor lights. A vhf radio, depth meter, and gps.

That gives us 6 LEDs at around 200 ma each, or 1.2 Amps. But except for maybe the anchor light they're not on all the time.
A VHF radio typically draws in the .4 to .6 Amp range in standby.  Don't know if you monitor continuously (although you should) but VHF doesn't consume a lot of energy, except when transmitting.
Depth finders are around the .6 Amp range and GPS varies too much to estimate, mostly because of all the display choices.

There are 2 cheap 12v oscillating fans that are marked ".8 amps/12 watts". Those will be used intermittently. There is also a 12 volt computer fan used to draw air into the cabin. Again, to be used intermittently.

So 800 ma times two fans and probably 150 - 300 ma for the computer fan, so let's call it 2 Amps total. Again, the fans will probably only be used in the evening or overnight, for sleeping.
Power draw on all the above stuff is probably around less than 4 Amps so we're good there, since all that stuff is basically intermittent usage. 

Then we come to the thermo-electric cooler. 
They generally draw around 5 or 6 Amps.

I left the cooler and the anchor light on for 7 days with the solar panel connected. Afterwards the meter showed the battery at 60%.

OK, a single 100A/Hr battery recharged with a 50 watt solar panel into a MPPT controller left you with 60% capacity, after a week.
Sounds like 40% consumption per week, so arithmetic would tell us that 2 batteries would get you to 60% capacity after two weeks.  Except that the second battery would ALSO need 50 watts of solar replenishment.

I am leaving for the keys January 3rd for a 5 week sail.

OK, getting closer . . . . 
Sounds like 60% capacity after two weeks (for two batteries) would be 20% after four weeks. The rest of your gear would eat about half of the remainder, so assume at four weeks, you have a 10% charged battery bank.

Here are my 3 ideas:
1) Definitely upgrade to a 100 watt solar array.
It is needed to keep up with the current consumption you stated. 50 watts trying to replenish 200 A/Hr, is barely above the trickle charge range.
OK, that's an exaggeration, but not by much!! 
A mitigating factor is that you are going down to the Keys.
You cannot view this attachment.
More usable sunlight in Zone 5 than in Zone 4, where you live.

2) Cooler resource attenuation.
Does the cooler really have to run 24/7? 
Stuff will stay cool for a while anyway, due to the insulation.  The key question is: How long, for how much? 
Get a gadget like this and stick it in the cooler:
https://www.tractorsupply.com/tsc/product/la-crosse-technology-wireless-digital-thermometer-ts74955-1739426?
Monitor temps from your smartphone and see if they stay in a usable range, or not.
The ideal would be to turn off the cooler when you turn in and kick it back on, when you wake up.
At worst, you might need a middle-of-the-night alarm on your phone to kick it back on, again.
Another strategy would be to turn it on and off for several hours at a time, throughout the day and evening, then let it run all night.  The temp range drops from the thermometer would give you some idea of what you would need.
That would give you that 5th week and with some reasonable margin of battery capacity.

3) Throw away your 12 volt DC plug.
Automotive lighter plugs were originally designed for intermittent high current draw (like a cigarette lighter).
Everybody that uses those coolers, notes that the plugs get very warm, which is no surprise at 5 Amps every hour, for hours on end. I'd get an Anderson connector and wire that up, instead.
https://www.amazon.com/Anderson-Connector-Batteries-Disconnect-Connectors/dp/B0D5YF7C41/
You might also consider getting an Anderson connector to 12 volt lighter plug adapter, for the ride down in the tow vehicle, so everything arrives in the Keys, pre-chilled.

Hope this helps,
Charles Brennan

[Wolverine]

Thanks Charles, I knew you would have a detailed answer. I was able to find reviews for the cooler and they state anywhere between 40-70 watts of draw. I took into consideration your suggestion of upping the solar, but decided to leave the Coleman and go with a traditional cooler. To up the wattage I would need to take a panel off the Orion and wire it into the Compac. Because of the size, it would need to sit in the cockpit with me. My email is clumsyninja for a very good reason. I most likely would be tripping on it.

On a 2 week trip last spring, I had 1 battery and I always had plenty of power so with 2 batteries I feel good about not running down the batteries even if we have a few consecutive cloudy days.

[Wolverine]

Years ago I had another boat wired so I would use the #1 battery on odd days while the #2 battery was being charged, use the #2 battery on even days while the #1 battery was  charging.

My question is, was this more or less efficient than wiring them in parallel and using and charging them simultaneously.

[Charles Brennan]

Wolverine, In your case, it doesn't really matter.
Your 50 watt solar panel can't put out enough energy fast enough, to take advantage of the faster charging rates that the BMS (Battery Management System) of your LiFePO4 batteries could accept, if you were charging each battery singly, versus  two in parallel.
If, for example, you had a 200 watt solar panel array, then the output would be large enough that the fast-charge circuitry in the BMS could kick in, and at that point, it would be worth charging the batteries on alternating days.
If it was me, I'd just go for the simplest wiring method available to you and be done with it.

Hope this helps,
Charles Brennan

[Spot]

Good discussion.

It's nice to have a boat schematic and energy budget spreadsheet when sussing out these things.

My current budget shows a nominal draw of 17.3 amp hours and charge of 20 amp hours using a 50 watt panel Zone 2/3 assuming 8hr/day charging in summer. Biggest line item draw is the Garmin GPS/fish finder followed by overnight use of a fan. If the anchor light was incandescent, this would also be a 'big ticket' item with a total consumption of 8 amps versus .8 from the LED version.

So I might make it with one cell phone charging and playing the FM radio a little on the VHS.

I would not be able to support an electric cooler or crew charging multiple devices.

Upgrading to 100w total solar would give me some headroom.

[Wolverine]

Another question.

The charge controller has a single output marked with a battery symbol and another marked with a light bulb. Obviously the battery connects to the battery symbol, and it's my understanding the bulb output is for a light or accessory that will only run while the panel is supplying power. The question: If I connect the second battery to the "bulb" output, would it charge the battery as if it was connected to the battery output?

Does it provide less power?

I have a multimeter, but don't possess the knowledge to make use of it.

[Ziradog]

Charles Brennan, just FYI most CPPAPs actually run on DC.  If you get a DC power supply for it, it is a LOT more effecient than DC->Inverter->AC->trasnsformer PS that comes with the CPAP.  I got mine off Amazon, less than $30 several years ago & it works great.

[Spot]

Another question.

The charge controller has a single output marked with a battery symbol and another marked with a light bulb. Obviously the battery connects to the battery symbol, and it's my understanding the bulb output is for a light or accessory that will only run while the panel is supplying power. The question: If I connect the second battery to the "bulb" output, would it charge the battery as if it was connected to the battery output?

Does it provide less power?

I have a multimeter, but don't possess the knowledge to make use of it.

On my PWM charge controller, the light bulb output is to run a on-at-dusk security light. The controller's manual should provide definition for the input and output terminals of the controller. I run the light for an hour so I can see if the system is behaving normally without having to climb into the boat.

[Charles Brennan]

Wolverine,  Your statement:

If I connect the second battery to the "bulb" output, would it charge the battery as if it was connected to the battery output?

Is a difficult question to answer, since we would need to know the make and model of your MPPT solar charger, to know about its characteristics.
IOW: Gimme a FACT, man!!   

In general, the lamp indicates a load that the charger can divert solar energy to operating.
For example, if your solar panel is putting out say 3 or 4 Amps and your boat's electrical load at the time is only 2 or 3 Amps, then the charger diverts all energy to the boat's electrical load, to save depleting the battery. Any excess solar energy goes to re-charging the battery albeit, re-charging at a much slower rate.  If the boat's current load is greater than the max load of the MPPT charger, it simply switches back over to supplying current from the battery.

In highly specialized models of chargers, the output can be diverted to controlling varying loads (typically lighting) that occur between dusk and dawn, and for how long, and for how much.  For example, it might divert battery load to turn lights on brightly at dark when there is a lot of foot traffic, (that would benefit from increased brightness) and then dim during the late-night hours, when that much brightness is no longer required.
Not usually applicable to small boat chargers.

Back to your original question:

If I connect the second battery to the "bulb" output, would it charge the battery as if it was connected to the battery output?

Well . . . . depends on the particular controller.
(And why make/model is so important to answering these kinds of questions!) 
Case 1:
Some MPPT controllers only divert power to the load (lamp icon) when they sense current draw, like when the tiller pilot is running, for example. If there is a battery voltage (the 2nd battery) there, where it is expecting to see  a current draw, then the not-so-smart microprocessor assumes there has been a mis-wiring and resets and promptly turns off, until the issue is corrected.
Case 2:
The charger doesn't have that much "smarts" and simply funnels energy to the load (lamp icon) to recharge battery 2.

Here are the various possible repercussions:
MPPT chargers have various load cut-off points, some fixed, some programmable.  This is so that if a load becomes too great for the charger to handle from the solar array output, it simply reverts back to drawing directly from the battery.
As an example, let's assume you have a load setting that kicks off at more than say, 5 Amps.
A 100 A/HrLiFePO4 battery that can charge from flat to full in 5 hours, means that it can receive up to 20 Amps per hour, to replenish.  If your battery 2 is significantly depleted as you are cruising along, it will try to draw the full 20 Amps and the charger will simply divert the load back to battery 1, in effect recharging battery 2 at the expense of battery 1. 
Probably not what you are looking for.
If battery 2 is not so heavily depleted and only needs say, 2 Amps from the MPPT charger load (lamp icon), then the solar array is charging both batteries, in whatever ratio the solar output and respective battery charge state is, at that particular time.

When first reading your question, my first thought was that it ranged from dumb, to useless.
And if you were trying to deal with an electric cooler or trolling motor (my own mind-set bias) it really would be, but considering the comparatively small loads on your boat (4 -5 Amps) and the robust battery capacity (200 A/Hr) I can't see anything particularly harmful from one battery being charged by the solar array and MPPT charger and the 2nd battery being charged from the first battery through the load output connection.  While clearly not as efficient as charging both batteries tied in parallel, it would still work.

But all this leads to ANOTHER question!! 
What is the boat's electrical load attached to? 
If it's attached to the same output load as Battery 2, then it gets replenished from battery 1 which is is in turn, replenished by the solar array.  If the electrical load is attached to battery 1, then battery 2 is sitting there largely as a useless spare.  Which is why I stick by my original recommendation, to simply parallel the two batteries and be done with it.

One guy's opinion,
Charles Brennan

[Charles Brennan]

Ziradog, You are indeed, correct about inverter conversion inefficiencies.
I had other uses for an inverter anyway, so I just chalked up the conversion losses as a cost of doing business.
I had checked into that when I first got my CPAP machine and at the time, only ResMed had 12 VDC converters and none of theirs matched the connector on my machine.
I also toyed with the idea of making my own 12 VDC to 24 VDC up-converter and wiring it up, but was leery of voiding my warranty.
Thanks to your advice, it might be time for me to look around again at the market and see what's available, especially if you can find one for $30, instead of the $100 - $150 prices I saw back then.  My inverter actually costs LESS than those adapters!! 

Thanks,
Charles Brennan