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Old 06-08-2017, 10:41   #1
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True state of charge with solar panels connected?

We typically recharge at 12.2v.

Add solar panels. When even a trace of sun is in the sky the batteries rarely show a charge under 13v.

How do you know when to start the engine/gen set? Is there a way to show true state of charge in sunlight with the panels connected?
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Old 06-08-2017, 10:59   #2
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Re: True state of charge with solar panels connected?

Well your bank should support two full days of darkness, so running the gennie at night or before sunup (better) should be OK.

If FLA read SG with a hydrometer.

Or get an AH-counting battery monitor.

Victron BMV-702

WhizBang Jr and Bogart Trimetric also integrate with SC.
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Old 06-08-2017, 16:53   #3
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Re: True state of charge with solar panels connected?

Thanks John,

Will look into those monitors. Yesterday was the second day in a tight anchorage, last place I'd want to run the engine at night. I knew the batteries were down, but showed over 13v even though the sun was obliterated by forest fire smoke from the BC mainland. Ended up running the engine for a while. Couldn't hurt
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Old 06-08-2017, 18:34   #4
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Re: True state of charge with solar panels connected?

All running a genny or the engine does is add usable amps to the bank.

It is impossible to get it to full 100% without shore or solar power.

Best is to run dino juice charging in the AM when the bank shows SoC low, gives the lower-amp solar "a head start" on maybe getting to 100%.
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Old 06-08-2017, 18:40   #5
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Re: True state of charge with solar panels connected?

Depending on how much money and sophistication you want to put it will determine just how accurate you can determine SOC.
However you can get a decent idea by knowing your voltage first thing in the morning.
Getting to an honest true 100% SOC is difficult on a cruising boat, if you don't already have one, a Honda 2000 is likely the best and least expensive charge source available, better than running your engine cost wise, long term.
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Old 06-08-2017, 20:28   #6
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Re: True state of charge with solar panels connected?

Well if you start the engine or gen and the alt or charger is not putting out near full amps then the batteries aren't down much.
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Old 06-08-2017, 21:02   #7
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Re: True state of charge with solar panels connected?

Wow, start plugging some real numbers in and that statement is false about as much as it is true.

Max amps of charge source, bank size, bank chemistry CAR spec. . .
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Old 07-08-2017, 10:25   #8
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Re: True state of charge with solar panels connected?

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Originally Posted by john61ct View Post

Or get an AH-counting battery monitor.

Victron BMV-702
Look at the Victron BMV-700. The 702 adds 2nd bank voltage but you already have a voltmeter.
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Old 07-08-2017, 10:38   #9
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Re: True state of charge with solar panels connected?

Quote:
Originally Posted by foojin View Post
We typically recharge at 12.2v.

Add solar panels. When even a trace of sun is in the sky the batteries rarely show a charge under 13v.

How do you know when to start the engine/gen set? Is there a way to show true state of charge in sunlight with the panels connected?
Usually your controller will tell you.

I have a $35.00 Controller that shows the batteries SOC.

If all else fails, you can measure the voltage on the panel side of the controller. If it is near your panels max open circuit (VOC) output then you are very close to fully charged due to lack of current flow. Mine will read 20 volts plus if my batteries are close to fully charged

My panel's max open circuit output is a bit over 22 volts.
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Old 07-08-2017, 11:21   #10
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Re: True state of charge with solar panels connected?

If the BATTERIES were reading 13v they were charged and the genset is wasted.

If the batteries WHILE CONNECTED TO THE SOLAR PANELS were showing 13v, then you are not measuring the batteries, you are measuring the output of the solar panels as well.

So the numbers and question become moot, until you are sure that you are just measuring the batteries.
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Old 07-08-2017, 11:23   #11
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Re: True state of charge with solar panels connected?

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Originally Posted by thomm225 View Post
Usually your controller will tell you.

I have a $35.00 Controller that shows the batteries SOC.
Not SOC, just voltage.
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Old 07-08-2017, 11:25   #12
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Re: True state of charge with solar panels connected?

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Look at the Victron BMV-700. The 702 adds 2nd bank voltage but you already have a voltmeter.
If it's not much more cost, the extra input is nice to have ffr, even if you don't currently need the extra voltage display.

Measuring mid-voltage can warn or even isolate based on the battery string getting out of balance, maybe sign of a cell failure.

And measuring the battery temperature can also be useful, which the BMV-700 doesn't do.
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Old 07-08-2017, 12:08   #13
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Re: True state of charge with solar panels connected?

A couple of assumptions, first it's nearly impossible to completely charge with Solar alone, so if your Solar only, your going to short cycle the bank, that will shorten its life, but it's not the end of the world either.
A bank under load, even a small load will show a voltage that is less than its open circuit at rest voltage, but if you use the chart that is at rest no load, you will always be conservative, in other words if the chart says you are at 50% SOC at that voltage, you will actually be higher than 50% how much depends on load and bank size, big bank, tiny load, not much, big load small bank a whole lot higher.

So just for argument if your chart says 50% is 12V and you never let your voltage drop to 12V, then you are always above 50%.
You can get a good idea of SOC under charge if your bank actually gets all the way to absorption voltage by what the charge rate is, if it's small, you are pretty high SOC, if it's higher in Amps then it's not so full, and if you never get to absorption voltage, it's not so full either.

In short with some experience with your bank, it's my opinion that you can do a pretty good job of managing a bank with just a voltmeter, and if you use inexpensive batteries it may just well be that not buying all the expensive stuff to manage your bank and just using your voltmeter may actually be less expensive in the long run. It may be that you will never recover the costs of the stuff by longer bank life.
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Old 07-08-2017, 14:08   #14
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Re: True state of charge with solar panels connected?

It is going to take a bit to get a handle on how well solar is normally going to do for you. Depending on the temperature, fully charged batteries will show ~12.7-12.8 volts. Anything over that is a surface charge that dissipates quickly.

Under ideal conditions, you will go into absorb mode in the AM, probably late in the AM, and have several hours of float. Under ideal conditions, your batteries will start the day with ~12.4 volts or better.

Don't beat up your genset unless conditions warrant it's use. Solar is all you need, if everything is set up properly, the vast majority of the time. Could be a bit tricky on a boat though.
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Old 07-08-2017, 14:39   #15
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Re: True state of charge with solar panels connected?

my solar is 360 W servicing 480 AH bank which could possibly not work based on CF body of knowledge. Modest electric users but float stage is hit virtually every day, often before midday.

Reading about issues other people have, I concluded that is not the most important face wattage of solar panel, but how well does it in shade, ie in less than ideal conditions. Looks like mine do really well. I have victron panels & chargers.
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