What size solar panel to maintain batteries?

[Jay4200]

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?

You don't need much of anything for a panel. You'll only have to replace the ~50mA-hr that is lost due to self discharge. Assuming 8 hours of average charge per day, you'd need all of about 75mW. So, get the smallest panel that you can find and use it.

A typical 12V-system panel generally produces peak power at 14-17V, and will approach 25V when illuminated at no load. When hard-tied to a battery, the panel will look like a current source, and will force excessive current into the battery. Ultimately, the battery voltage will rise to gassing voltage (where you lose electrolyte) then further where the plates will begin to corrode.

I recently designed a charger that is great for this type of application (search LT3652).

I just did a search, and Battery tender makes a slick solution for $80 that contains a panel and an integrated controller. Buy one of these and be done with it.

5-Watt Solar Panel With Built-in Controller - Solar - Batterytender.com

JayC

 

[RalphVa]

I havr a little (think it's 50 watt) panel that I use to maintain a little utility battery on my fence charger. I've moved it to my carriage house for the winter (as I've done the past 4 or 5). The battery lasts fine this way. Without it, one of these utility batteries are typically good for only 1 year. It puts out about 18v with sun full on it. It's one of the "mule gate" panels sold by Tractor Supply.

Ralph

 

[Travelover]

I'd go with one or two of these:

1.5 Watt Solar Battery Charger

Reviews are good.

 

[MossflowerWoods]

This is a great thread!

I need a similar set up to trickle charge my 4 Harley's in the shed I will build soon.

At the last house I just used a plug in trickle charger, but now there is no juice so I gotta provide something.

Should I get 4 systems, or one larger system and wire up four connectors in parallel?

Thanks in advance!
Be well,
David

 

[greasemonkeyok]

There are much smarter folks here than I, but I believe you need four separate systems for four separate batteries.

 

[hosspuller]

I use a deep cycle battery to power a diesel fuel pump. Bought a 15 watt panel and a charge controller. The panel will recharge the battery and maintain it. Less than $100 from E-bay...

 

[MossflowerWoods]

There are much smarter folks here than I, but I believe you need four separate systems for four separate batteries.

That is both good and bad.

It is much simpler, but it seems I should be able to present the 4 motorcycle batteries as a single 12v load (as if they were one larger battery) and have a single larger photo cell.

I was going to have a spare battery to run a small 12v light (and for jump starting other stuff.

I was also considering a lean-to on the side with my mower in it and a trickle charger for that one as well...

Hmmm...

I hope on of those guys who is smarter than you and I speakup and tell me how to do this properly before I goof it up.

Be well,
David

David

 

[Redneck in training]

One charger with multiple batteries would work but I would separate the the batteries with diodes. In examle you have four batteries. All negative terminals will be connected together and the positive of the chager will be connected to four diodes. Each diode will then connect to single battery. This arrangement will prevent cross feeding from one battery to another.

 

[Soundguy]

One charger with multiple batteries would work but I would separate the the batteries with diodes. In examle you have four batteries. All negative terminals will be connected together and the positive of the chager will be connected to four diodes. Each diode will then connect to single battery. This arrangement will prevent cross feeding from one battery to another.

exactly.. that way a marginal battery does not cause cross discharge.

soundguy

 

[MossflowerWoods]

One charger with multiple batteries would work but I would separate the the batteries with diodes. In examle you have four batteries. All negative terminals will be connected together and the positive of the chager will be connected to four diodes. Each diode will then connect to single battery. This arrangement will prevent cross feeding from one battery to another.

Thank You!!!!

I knew there was somebody ahead of me on this type of a path!

Much Obliged!

Be well,
David