15Wp panel to battery without controller

[fivewin]

Ive read that using small panels up to 20Wp you can connect directly to a AGM or VRLA battery without using a chargecontroller. BUT, for a 15Wp panel which diode is then recommended?
.

[Mike90250]

It's a delicate balance between the size of the battery and the size of the panel. 18W panel and 50Ah battery is not a good match, too much charge for a small battery,
The diode is to prevent the battery from back feeding the panel at night. A simple 50V, 5A Schottky Diode would be fine. I've seen LED's used both as a charge indicator and blocking.

https://en.wikipedia.org/wiki/Schottky_diode

[fivewin]

The intention is to connect a 15Wp panel with a diode directly to a 7.2Ah vrla for two 5 watt 12volt led bulb. What happend if the battery is fully charged and no loads for lets say 1-2 weeks?? Panel try to charge continiously. Will it damage the battery??

[Mike90250]

Do the math,
10W @ 12V = 0.83A Load
15W @ 18V = 0.83A Charging source.
How many hours load ? Solar charging hours ?

More than 5 hours of powering the light, the battery will be quickly damaged by deep discharge.
After that, more than 8 hours of charging will overcharge the battery and vent electrolyte (you pay all that extra $$ for the fancy Valve to release the magic juice)

[fivewin]

So it is not wise then to connect directly to the battery although the panel is less than 20Wp.

[SunEagle]

The cost of even a 10A cheap solar charge controller is probably a small price to pay to protect your battery.

[fivewin]

You r right, can order one for just 8 usd. Thanks.

[Sunking]

The intention is to connect a 15Wp panel with a diode directly to a 7.2Ah vrla for two 5 watt 12volt led bulb. What happend if the battery is fully charged and no loads for lets say 1-2 weeks?? Panel try to charge continiously. Will it damage the battery??

A 15 watt panel provides roughly 1 amp. The maximum charge current your battery can accept is about 1 amp.

A 10 watt load with your two LEDs, the battery is destroyed in 8 hours.

Put a 15 watt panel on 12 volt 7.2 AH battery with no load your battery is destroyed in 3 days max if not the first day.

Having said that your plan is perfect, Gofferit.

[Amy@altE]

A rule of thumb I've seen around is you can use a solar panel that provides 2 to 3% of the amp hour without a charge controller. It's just a trickle charge at that point, certainly won't keep up with a load, but helps replace self-discharge. So for your 7.2Ah battery, no more than 3W of solar without a charge controller. Get a charge controller.

[fivewin]

Thanks for all replies

[PNjunction]

Amy - that rule of thumb has ruined many batteries due to leaving out some important information.

The reason it works for 99% of the people with large batteries is because they don't fully charge large batteries in the first place, or use it often enough that the little panel combo is just maintaining the battery at the level it found it in.

HOWEVER - for those who actually DO get a large battery charged properly, and then think about putting this little kludge on their expensive battery to keep it topped off - guess what happens. An endless high-voltage EQ.

You can prove it yourself, or watch this video of what a 12 watt panel will do to a 220ah bank that has already been dutifully charged (the catch 22!):

https://www.youtube.com/watch?v=z5RtZe9AW2E

Of course the small panel will easily charge a small battery, but of course will put it into an endless high voltage eq. Yep - use a charge controller, and forget about that "rule of thumb" so you can cover ALL bases, and not get returns from those who TRY to do the right thing.

[Amy@altE]

Excellent point, thanks for that. I need to be careful with those darn rules of thumb. While they work for some, they don't work for all. My point was that his panel was too big for his battery.

[SunEagle]

Excellent point, thanks for that. I need to be careful with those darn rules of thumb. While they work for some, they don't work for all. My point was that his panel was too big for his battery.

Most people come in thinking they can save a few dollars charging a battery directly from a solar pv.

IMO anything larger than a 5w panel can hurt a battery and the cost of a PWM CC is so cheap why not use one.

[kdaddy1980]

what about this scenario..youre off grid.its cloudy all day and your batteries are low. Your one 12v /25 watt panel usually produces 22v more or less, its cloudy and only producing 15v all day. Can you safely hook up that one panel directly just to get your battery bank charged up fully?

[SunEagle]

what about this scenario..youre off grid.its cloudy all day and your batteries are low. Your one 12v /25 watt panel usually produces 22v more or less, its cloudy and only producing 15v all day. Can you safely hook up that one panel directly just to get your battery bank charged up fully?

Unfortunately it is not the voltage that needs to be high enough to charge a battery. You also need current and a panel in little to no sunlight may show some voltage but will not provide enough current to charge a battery.