Direct connect 100 watt panel?

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Frank B

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Apr 23, 2005
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Calgary, Alberta
My son-in-law has a deep cycle 12v lead acid battery that he uses for his electric trolling motor on his little boat. When camping they want to be able to charge it. I am giving them a 100 watt Coleman 18v panel, but it has no controller.

I don't see any way the maximum 8 amp output from that panel is going to trouble that big battery unless it is left connected for months at at time. Right?
 

Frank B

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Apr 23, 2005
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Location
Calgary, Alberta
18 volt is open circuit. Doesn't take much of a load to pull it back down. It is a 36 cell panel that is commonly sold for charging 12v systems. And, it will not be connected for weeks at a time either. However, thanks for the reminder.
 

hwIII

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Jul 24, 2021
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Location
NW Florida
My son-in-law has a deep cycle 12v lead acid battery that he uses for his electric trolling motor on his little boat. When camping they want to be able to charge it. I am giving them a 100 watt Coleman 18v panel, but it has no controller.

I don't see any way the maximum 8 amp output from that panel is going to trouble that big battery unless it is left connected for months at at time. Right?
I'm a newb here on this forum so I respect my input may be taken with some skepticism, but from what I know of solar it is not a good idea to connect a panel to a battery without a controller. There's a chance of harming the battery.

One of my "go to" resources for solar information is AMSolar. (I am a customer--I don't benefit in any way, shape or form if you reference their site or use their services). A good page on controllers is here: Controllers

Another good resource is Will Prowse on YouTube or his DIY website. He has a good primer on controllers on YouTube. Search for Will Prowse there and find his video entitled MPPT vs PWM: Fast comparison for off-grid solar
 

Mark_K5LXP

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Nov 17, 2018
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Albuquerque, NM
In my experience the cheap controllers do not have correct or user adjustable setpoints, so the idea of just throwing a cheap controller at it I'm not sure is a slam dunk solution. When I use my portable solar setups, I never use a controller. True, it's possible a 100W panel left connected long term could overcharge the battery. This is not long term. It's assumed the battery would be discharged when connected, and a 100W panel over the course of even a sunny summer day doesn't have the capacity to bring back a dead ~75Ah trolling battery. The chance of unintended overcharge is small. What works for me is like this situation, the system is not unattended. In the event the battery becomes full, it's a simple matter to tilt the panel to adjust the charge rate, or disconnect the panel entirely. I would have no qualms about doing any of this, because it's what I've done literally for decades.

Mark B.
Albuquerque, NM
 
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