What size solar panel to maintain batteries?

[Cord]

I'm looking to purchase a solar panel to keep by batteries at full charge for a piece of equipment which is stored in a remote location. This piece of machinery has a diesel 12v engine and two batteries. Typically, I'd use a 3amp battery maintainer. The solar panels are not rated in amps, but watts. How many watts do I need to maintain these batteries? In the winter, this panel will be covered with snow, so please take that into consideration. I'm currently thinking about a 15watt panel, but it says that a controller is required. When do I need a controller and why?

 

[GaryM]

Your 3 A 12V battery maintainer would be 36 watt max output. So if it had been sufficient in the past, I'd look for a 36 W panel. That doesn't take snow cover into account.

 

[greasemonkeyok]

A straight solar panel with no controller will continue to charge the battery even when it is at full charge, and end up overcharging it.

 

[Soundguy]

in the end.. just about ANY positive current flow will keep a battery topped off. all you need to do is overcome quiescent discharge, which is typically in the milliamps at best.

I used to use one of those 9$ harbor freight panels that has a cig lighter adapter on it. thin is the size of a shoe box lid.. it kept my tractor batteries topped all year with -0- use in some cases.

snow will block it till it melts off.. it won't snow forever though...

soundguy

 

[Monster5601]

A straight solar panel with no controller will continue to charge the battery even when it is at full charge, and end up overcharging it.

That depends on the output voltage of the panel. If the panel's voltage is below the battery voltage, there would not be a current flow.

 

[Monster5601]

I'm looking to purchase a solar panel to keep by batteries at full charge for a piece of equipment which is stored in a remote location. This piece of machinery has a diesel 12v engine and two batteries. Typically, I'd use a 3amp battery maintainer. The solar panels are not rated in amps, but watts. How many watts do I need to maintain these batteries? In the winter, this panel will be covered with snow, so please take that into consideration. I'm currently thinking about a 15watt panel, but it says that a controller is required. When do I need a controller and why?

Look for a panel that can supply at least 12.6 volts DC, this is the threshold voltage of a fully charged lead acid battery. As far as current needs, parasitic loss of a typical lead acid battery depends on the type of battery and temperature, lead acid batteries have a parasitic loss somewhere between 1% to 60% rate per month. The warmer the temperature the battery is stored at increases the parasitic loss rate.

A panel that can produce between 12.6 and 14 volts DC with a current of a few hundred milli-amps will keep a lead acid battery topped off, provided the panel has adequate time exposure to the of light needed to produce power and there is no other current draw on the battery.

 

[Farmerford]

I have two 15watt Battery Minder solar chargers that I got from RVupgrades online. One is attached to a John Deere 455D with two 12 volt batteries and one is attached to a Caterpillar D3B with two 12 volt batteries. Both work well, and the price is the best I could find for a panel and controller. I think one system would work for both machines, but they are located in different places so I got one for each.

 

[Redneck in training]

I'm looking to purchase a solar panel to keep by batteries at full charge for a piece of equipment which is stored in a remote location. This piece of machinery has a diesel 12v engine and two batteries. Typically, I'd use a 3amp battery maintainer. The solar panels are not rated in amps, but watts. How many watts do I need to maintain these batteries? In the winter, this panel will be covered with snow, so please take that into consideration. I'm currently thinking about a 15watt panel, but it says that a controller is required. When do I need a controller and why?

http://www.amazon.com/Sunforce-50032-Solar-Battery-Charger/dp/B0006JO0X8
Here is one.

 

[RPW]

You can get an inexpensive 5w panel and keep the battery topped off. I use one on my flatbed trailer to keep the winch battery (deep cycle) charged. Works great and does not require a controller. And if you orient it to the south equal to your latatude (sp.) the snow shouldn't be a problem.

 

[Gary Fowler]

I had a couple of 12 volt 15 watt panels and lent one to my brother in law to charge the battery on his Mule gate opener. It is patched in direct to the 12volt gate opener battery without a controller and has been in service for over 2 years and still works as required. I dont think these low wattage panels will overcharge a 12 volt battery.