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GLBCamper View Drop Down
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Posts: 274 		Direct Link To This Post Topic: Solar charging mystery
    Posted: 02 Sep 2017 at 7:09pm
this definitely has me stumped and frankly it doesn't make any sense to me.

Two 6v golf cart batteries (yes, they're good) charged with shore power through the converter to at least 12.9 then disconnected takes 5-7 days for the parasites to drain to 12.4 (as expected.)

BUT

charged to at least 12.9 using the Renogy 100w solar suitcase drains to 12.4 in less than 24 hours.

Absolutely NO change in amp draw between tests.

I am taking all of my readings 3-4 hours after disconnect to allow for decay.

Ideas??
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Tars Tarkas View Drop Down
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Posts: 1447 		Direct Link To This Post Posted: 02 Sep 2017 at 7:29pm
Strange.  Maybe it was cloudy when you did the solar charging.  (Joke!)

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Posts: 2654 		Direct Link To This Post Posted: 02 Sep 2017 at 7:32pm
Just because the voltage reached 12.9 volts does not indicate the batteries are charged. Solar controllers will do a standard Bulk->Absorb->Float cycle, and will "pass through" intermediate voltages on their way to fully charged. 

You should actually watch the voltage over time. During the first stage (Bulk), the battery voltage will reach 12.9 volts, and keep on going until the current drops to a preset level. At that point, the charger will go into Absorb mode, and the voltage will be in the 14-14.5 volt region. Absorb is voltage limited, and when the current drops below another preset point, the charger will go into float mode. In float, the batteries should actually be above 13 volts; maybe as high as 13.5. Once the charger is in float mode, then you know the batteries are "full". The charger should maintain float until there is no more sun (solar), or you turn the charger off (shore power).
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GLBCamper View Drop Down
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Posts: 274 		Direct Link To This Post Posted: 02 Sep 2017 at 8:17pm
I'm definitely going over 13v, but it does tend to decay in a few hours after I disconnect down to about 12.9.  Maybe I'm NOT giving it enough time. It was on the batteries for 2.5 hours of direct sunlight to go from 12.4 to 13+ at the time I disconnected it. I'll give it more time and see if that makes a difference (that is if I get another window without wildfire smoke to even get direct sunlight!)
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JandL View Drop Down
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Posts: 364 		Direct Link To This Post Posted: 02 Sep 2017 at 9:16pm
I can can get about 4.25-4.5 amps out of my 100 watt panel in California. If it's in the sun all day it charges my single group 31 battery to full. You had it in the sun for 2.5 hours which is not enough time to charge your golf cart batteries to full. You have about 220 amp hours in your batteries, my group 31 has only about 100 amp hours so it charges to full sooner
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StephenH View Drop Down
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Posts: 6326 		Direct Link To This Post Posted: 02 Sep 2017 at 10:03pm
You need to note that a reading right after charging is a surface charge reading. You would need to wait a couple of hours or run a load to drain 1% of charge to eliminate the surface charge to get a true voltage reading according to batteryuniversity.com. Follow the link for some interesting information.
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GLBCamper View Drop Down
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Posts: 274 		Direct Link To This Post Posted: 02 Sep 2017 at 10:20pm
Originally posted by StephenH

You need to note that a reading right after charging is a surface charge reading. You would need to wait a couple of hours or run a load to drain 1% of charge to eliminate the surface charge to get a true voltage reading according to batteryuniversity.com. Follow the link for some interesting information.

Definitely did that (as mentioned in original post.)
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Posts: 281 		Direct Link To This Post Posted: 03 Sep 2017 at 7:21am
JandL is right.  2.5 hours with 100 watt panel does not provide a lot of power.  Another thing, voltage is a decent indicator of state of charge, but doesn't tell the full story.  A good battery monitor will keep track of amps in and amps out.  Further, using a hydrometer to measure specific gravity of each cell will give a good measure of state of charge.
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GLBCamper View Drop Down
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Posts: 274 		Direct Link To This Post Posted: 03 Sep 2017 at 11:03am
It sounds like my problem was assuming that a charge to 13+, regardless of the source or duration, was sufficient. I'll give the solar another good long run and see if the batteries hold their charge longer. Thanks.
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GlueGuy View Drop Down
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Posts: 2654 		Direct Link To This Post Posted: 03 Sep 2017 at 11:13am
Originally posted by GLBCamper

I'm definitely going over 13v, but it does tend to decay in a few hours after I disconnect down to about 12.9.  Maybe I'm NOT giving it enough time. It was on the batteries for 2.5 hours of direct sunlight to go from 12.4 to 13+ at the time I disconnected it. I'll give it more time and see if that makes a difference (that is if I get another window without wildfire smoke to even get direct sunlight!)
The voltage should get to > 14V in order to get to absorption mode. If it didn't get up there, then your batteries are not fully charged.
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