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solar battery tender
Any dashboard mounted solar panel has the potential to overvolt a battery.

it depends not only on the actual output of the panel at that time, but the state of charge of the battery at that point.

And not only that,, assuming it is 98%+ charged it depends on the battery, its health, temperature, and original design.

some maintenance free starter sealed but flooded batteries, act like AGM in that they taper to near zero amps at high absorption voltages. Even just 2 unregulated watts on such a battery can push it well over 15.5v, and that is not doing the battery any favors when it occurs several hours each day.

How much each battery self discharges changes for the worse as a battery ages, so the dash wattage might not even be able to keep up with self discharge, much less any parasitic draws, or just some overnight DC loads drawing from starter battery versus the house.

Mainsail has a video showing a 10 watt unregulated panel overvolting a pair of lifeline GC-2s, 220Ah.

The owner of a dashboard panel needs to insure their panel is not overvolting their specific battery.
The newer the battery, the more likely it is that the unregulated panel can overvolt it

Just check the voltage, it does not need to be high 13's or higher all day, day after day.
If you can remove the caps, monitor water usage.

I'm getting used to having re incorporated a separate house bank. When I want the uncycled starter bank to see some time at absorption, I turn my solar switch to both. I've been setting my phone timer for late afternoon re isolation.

Mostly my G31 Northstar AGM is being used for starting. That thing holds 13+ volts for days after a top charge.
The Dekas settle to 12.84v in 24 hours when isolated from all loads.
if I discharge these banks in parallel the Northstar provides more than half its share of amp hours consumed, and if the load is tiny the Dekas are a load on the Northstar,

So proper isolation is highly desirable, though forgetting is hardly instant demise.
I never played with solar before the 75w siemens panel , the battery maintain panels didn't seem terribly useful for my plans , even back in the 90s.(they cost quite a bit more though.)
Didn't think the tiny panel would do much at such low amp output but you say 15-16v .,,,hmmm.
A cheap pwm controller could be added . Might cost as much as the panel though LOL.

I also have a relay for sharing some solar charge with the start battery but rarely use it except when driving after the start batt calms down.. The 75 w panel is portable now and can be plugged in with the roof 300w before the controller through a 10g 30' ext in an outside compartment . OR directly to either batt through a 10w pwm .. a renogy voyager mounted on a board with fuses..
SB-50 connectors everywhere , oh my goshk !

Trees shade me from about 11:30 so can't use any of it to full potential just yet, but did have the fridge , the tool battery charger , the inverter powering the tablet charger , the fantastic fan and both phones on a usb dash lighter type outlet totaling a whopping 12a out of the 300w roof for a few hours one sunny morning last week.
Stay Tuned

Weirdo Overlord  YARC 
15 "Stinkin'Badges"  a "Full Monty Badge" 2 "Just Ignore Me" clusters  10 "Pine Cone" clusters  , one "Stinkin' Badger" and 8 of the coveted "Flying Manure Spreader"awards
(What a "Stinkin' " honor !)
The DC load output is limited. You might need a current limiter between the DC load output and a battery.
You probably have already thought of that.
The load terminals are rated @ 20a on your controller.
Stay Tuned

Weirdo Overlord  YARC 
15 "Stinkin'Badges"  a "Full Monty Badge" 2 "Just Ignore Me" clusters  10 "Pine Cone" clusters  , one "Stinkin' Badger" and 8 of the coveted "Flying Manure Spreader"awards
(What a "Stinkin' " honor !)
Yay! Found my gadget! Will be installing very soon and reporting back Smile
My body is a temple- Ancient and crumbling,  
probably cursed 

Dog Bender with 3.2 Stinkin' Badges 
"I have Faith"
$16 solar tender is pretty much a waste of money,little panel with little wires/clip,getting weird multimeter readings so not sure if it even works right

there are higher priced models that might work but definitely want some kind of charge control with it

sticking with 100w panel hooked up every week or two when not running the engine
"not of the body"

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