Too Much Solar For Too Little Draw?

[SURV69]

I have 4 30 watt, 12v(about 19v), solar panels to charge my two size 24 flooded deep cycle batteries.

This setup was intended to recharge my batteries, which would be under load when I sail for a few days ... or more.

But, I'm generally in a marina with shore power and the only "usage", I have is a light(1.2 watt LED) that I sometimes turn on for an hour or so. I have small dedicated solar panel for my cabin light(not on any circuits)

This has me wondering about the "matching", of solar panels and actual power usage since I now "unplug" the solar panels for about 5 days at a time, for the batteries to get to 12.7 volts, to recharge them.

I use a PMW(?) 20 amp controller, which has NEVER indicated ... even when the batteries read 13 volts ... that the battery was charged or indicated that the controller has stopped charging.

I wonder, at times with so very little draw, if I'm right to unplug the solar panels til the batteries get to 12.7(which I think is actually still fully charged), or should I just let the solar panels/controller to continuously charge(low trickle ... I hope)?

Any thoughts on this would be appreciated.

[sailorboy1]

I turned the absorption time down to only 30 minutes because I'm in a marina currently and not drawing the batteries. I could tell that since the batteries were charged my normal settings were causing gasing.

This morning I turned off the battery charger to let the batteries cycle some instead of sitting around on float. I don't know it this helps and have read it both ways, but I don't see how letting the batteries discharge a little now and then can hurt.

[wyb2]

Originally Posted by SURV69

...

I use a PMW(?) 20 amp controller, which has NEVER indicated ... even when the batteries read 13 volts ... that the battery was charged or indicated that the controller has stopped charging.

I wonder, at times with so very little draw, if I'm right to unplug the solar panels til the batteries get to 12.7(which I think is actually still fully charged), or should I just let the solar panels/controller to continuously charge(low trickle ... I hope)?

Any thoughts on this would be appreciated.

Voltages under charge are higher than open circuit "battery is full" voltages. Your charger is probably looking for something closer to 14 volts before it switches to the absorption stage, then will regulate back down to something lower (13.5-ish) for the float stage.

If you are disconnecting the panels at 13 volts, they are never seeing the battery as "charged"

https://www.powerstream.com/SLA.htm

Charging Information For Lead Acid Batteries – Battery University

[john61ct]

Yes better to have automated charge sources you trust.

Then change their setpoints to match the current context.

Way you're doing it now will murder the bank long before its time.

[SURV69]

of course ..... I see my earlier post didn't get past the poor internet here...

The charge voltage is about 14+ volts, amps vary from .5 or so to as much as 6(noon). 14 volts(actually 14.2-14.4), sounds right for charging a 12 volt flooded battery.

I draw off about 1/2 watt a day for a single LED light(a 1.5 watt bulb). I probably lose more just by letting the battery sit.

the controller voltage appears to be from the solar panel, not an indication of the battery's charge) ... voltmeter agrees when check with solar panels directly(disconnected). When I disconnect the panel, the controller then indicates the battery's charge ... voltmeter agrees with that too.


Typical sunny Solar day:

SOLAR READINGS
13+ volts beginning about 8:30 am
14+ volts from about 9:30 til about 6 pm

I have not seen any indication of
a cessation of charging all day long
battery immediately after disconnect 13.1-13.2 volts
after about an hour or so bat reads 13.0(over charged?)
a couple hours later still bat reads 12.8
after the night to next day(before
connecting the panels 12.7 fully charged

at this point, if I were to connect the panels, the voltage would show 14+ volts from the panel correctly, but seems to indicate that the bat is still being charged .... or I am reading the controller incorrectly ... very possible .... the instructions aren't the best.

I just realized, I haven't taken a reading on the controller's bat posts, will do that tomorrow.

But still 13.2 volts and 13 volts after an hour or so?

[wyb2]

Sounds like you already understand what I posted before. Only further thoughts, see if you can check/change the "max absorption time" as sailorboy1 mentioned

[john61ct]

Sounds to me like your Absorb timeout is set to long, I assume your SC doesn't do endAmps?

When you know your bank isn't actually being cycled, you should set your Absorption time to the minimum, say 30 minutes.

Or even just set Absorb voltage same as whatever Float is supposed to be.

When you're back into normal cycling, say over 5% DoD then re-set the SC appropriately.

If your SC doesn't allow customizing the charging algorithm, replace it with a better unit.

[Tessellate]

Your batteries aren't overcharged because they read 13.0 after taking them off charging sources. It takes time for batteries to reach their "resting voltage". Check out this site for a good explanation of this topic: Measuring A Lead Acid Battery State of Charge Photo Gallery by Compass Marine How To at pbase.com

My understanding is keeping batteries on a float voltage (usually 13.5-13.8) indefinitely is perfectly fine, and in fact healthy for them. Unplugging your solar panels for five days to let the batteries "rest" to 12.7 is pointless, and may actually decrease battery life slightly (but probably not by much - after all, disconnecting charging sources is what you do each time you go sailing).

It sounds like you're connected to shore power but aren't using a shore power *charger*? (since you said you disconnect only the solar panels and the batteries eventually go to 12.7). That's a bit unusual but probably fine since your DC usage is so low.

I asked a similar question to your's a couple months ago: http://www.cruisersforum.com/forums/...rd-186947.html

I didn't get a ton of responses, but my question was intended to figure out what the optimum shore power charging + solar charging choices were for liveaboard life. The conclusion I got was keep shore charger on (if you have it), keep solar on, and the shore charger will keep the batteries at float (13.5) at night and the solar + shore will keep it at float during the day.

And my concern about going into absorption an hour each day (1 hr = min absorption time on a Victron MPPT) while the batteries are already full is probably unfounded, as I gathered from a couple responses, because flooded lead acid batteries have no problem with an hour of higher voltage. At most it might mean I need to check / fill the water more often.

Note absorbtion period on the Victron actually can end sooner than 1 hr, if the batteries are full and accepting less than 1 amp of charge. Quoting from the manual:

"The absorption period also ends when the output current of the solar charger drops to less than 1Amp, not because of low solar array output but because the battery is fully charged(tail current cut off).
This algorithm prevents over charge of the battery due to daily absorption charging when the system operates without load or with a small load."

Check the manual for your PWM. If it doesn't have absorb config or tail current cutoff, it might be worth upgrading. Although in the grand scheme of things I wouldn't worry too much because 2 group 24 batteries aren't that expensive as boat costs go.

[a64pilot]

Whenever I am plugged into shorepower like now, I set absorption voltage to float voltage, likely doesn't hurt the bank going through a short absorption charge daily, but what is the point?
Ideal life for a bank is be fully charged, kept cool and left at float voltage indefinitely.