Solar charger switching

I have a project powered by a small 12V SLA battery that I would like to keep charged by using a solar charger. My plan is to monitor battery voltage and connect/disconnect the charger as needed by toggling a digital output pin connected to the base of the NPN transistor. Should this work?

charger.jpg

Why not let the charger and the battory handle the charging without intervention from external logic?
External logic can cut off charging but can't arrange charging during nigth, sun free days etc.

The battery is just 5AH, and the solar panel puts out about 7A in direct sun. That's going to over-charge the battery unless the panel is disconnected when appropriate.

Okey. So You have no intelligence in the charger circuitry that would cut down when the battory is fully charged?
I don't know the technology used in Your SLA battory. Cut off criteria has changed during the years because of different battory technology. For some of the old accs the cut off criteria was either current depending or voltage dependant. To cut off at the proper level rather precise measuring was needed. Is the resolution of a 10 bit Arduino ADC good enough to meet the requirements for You SLA? Check that!

You make a good point. There is no protective circuitry in the battery or the charger. My goal is to avoid grossly overcharging the battery. Do you think the setup will properly disconnect the charger when the pin is LOW, and connect when it is HIGH?

No, that circuit won't work. The transistor is backwards, no base resistor, etc.

Most people use a high side switch, along with a series diode to prevent the panel from discharging the battery in the dark.

Better, use an inexpensive, tried and true commercial charge controller like this one.

What is the open-circuit voltage of the solar panel? If left connected to the battery it will tend towards that voltage.

I suspect that if you arrange a cut off as soon as the voltage reaches about 13.5v (check the battery university website for the fine details) you may find that you are under-charging because the battery won't have been at that voltage for very long. A proper charge controller will give several hours at a more-or-less constant voltage, but I suspect the solar panel will keep increasing the voltage as the charge current drops.

Rather than cut off the solar power a better solution would be to have a dump load (several lamps, for example) that can absorb the unwanted solar energy. Use the Arduino to switch on the dump load when the voltage rises too high and that way the battery can continue to receive solar energy at a suitable voltage.

...R

You have got it right, overcharging damages the battory. I would say that overcharging kills all kinds of battories unless the battory has a built in intelligence, if that exist.
As jremington says, buy a ready off the shelf charger to make Your project run safely and well. Charging the best way might be sophisticated and sencitive. If experimenting with the technic of charging in order to learn, go ahead.

As Robin2 tells, the best charging technic is program, a cycle, and a bit more advanced then just cutting off.

What charger are you using? I have not come across any solar charge controler that does not regulate this for you.

Here's the panel: http://www.naturepowerproducts.com/product-details.php?id=201

7.5 Watt I read, not 7 Amps….. Don't make such mistakes again!
That charger is told to use "intelligent charging". Just Go I would say.

12 volt, 7,5 Watts gives a charging current of 600 mA. Quite a difference. However, pushing 600 mA into a fulley charged 12 volt battory would be bad. The battory is of 5 AH. 10 hour current is 500 mA, less than the charger capacity. For curiosity You can check the data for the battory regarding "maintaining current". (I know the Swedish term only.)

I sum it to: That charger is safe.

@couldabin
Credits to You.

What is the project, and how much current does it draw?
You also need some mechanism to prevent the battery being completely discharged if there is insufficient solar power available.

@Mauried -- the project involves using an Arduino Uno to operate a small 12V motor (~250 mA without load) for about 3 minutes each day. It also monitors a photocell to decide when to operate the motor. I think I read that the Arduino draws around 45 mA. I expect the total consumption will be roughly 1.4 AH per day.

How much current does the motor draw when its loaded?
No load current isnt very useful unless the motor isnt intended to do anything.

My DMM shows it draws between 300 mA and 360 mA under the load. It runs twice per day, for a duration of about 70 seconds each time.