SLA Charging Question

[Magen]

Hello Everybody,
I would like to use a 7AH 12v SLA battery to power a hydroponics system. I have a pump that will consume 250mA when in use. The pump will probably be used twice during the day and will run for approximately 15 mins each time. I would like to charge the battery via a solar panel. We usually experience 8-10 hours of sunlight per day. What should the minimum panel output be to keep the battery charged?
Is it correct to assume that if the load was 250mA for 1 hour, and a minimum of 8 hours sunlight - the min would be 250/8 = 31.25ma to recharge the battery?

 

[Hero999]

That won't do because the panel will only put out the rated current at mid day in mid summer.

Use a 50mA panel minimum and you'll need a regulator to prevent it from overcharging the battery.

 

[Magen]

Txs Hero,
Another question: I have seen a panel that is rated at 12v but does not give any other specs. Can I test the panel in sunlight using an ammeter across the output to test its short circuit current - If yes, how much higher than the 50mA should this current be?

 

[Hero999]

I wouldn't recommend buying a cheap panel with no specification.

 

[Magen]

Txs, I was weary as well but thought I'd take the risk. I'll look around some more.
Txs again for the help.
Regards
Magen

 

[MikeMl]

Can I test the panel in sunlight using an ammeter across the output to test its short circuit current - If yes, how much higher than the 50mA should this current be?

The short-circuit current is immaterial. You need to measure how much current it can deliver at near the voltage you will be operating it. In your case, about 14.5V, because that is the voltage to charge your SLA to. You need a panel that has an open-circuit output voltage of ~18V. I'll bet that the "12V panel" has an open circuit voltage of only 12V, meaning it wont even come close to charging a 12V SLA.

Read my post where I posted an I/V plot of a panel I am using to charge my 12V boat batteries.

 

[Hero999]

In your case, about 14.5V, because that is the voltage to charge your SLA to.

No you don't, not unless you're going to disconnect it, when the voltage reaches 14.5V, otherwise it'll gas and boil dry.

If you don't want to add a comparator circuit to disconnect it when it reaches 14.4V, you want to float charge it to 13.8V.

 

[MikeMl]

Of more than twenty 12V SLAs I have around here, they all have "Float at 13.5 to 13.8V, Charge at 14.5V to 14.8V" printed right on them. Regardless, the OP needs a panel that has an open-circuit voltage above 14V, especially considering his short days.

 

[Hero999]

Yes, there's nothing wrong with charging to 14.5V, my point it shouldn't you shouldn't leave them permanently connected to a 14.5V source or it will kill the batteries.

Unless he disconnects the battery when the voltage reaches 14.5V, he's going to need a regulator to float charge it.

Most 12V panels will output >18V open circuit which gives plenty of headroom for an LM317 set to 13.8V.

He has short days now because it's winter, in summer he'll have 14 to 16 hour days.