# Powering circuit from 12V with stand-by battery + solar charger

I did some research but I haven't been able to find the information that I really want.

I am building a home made security system consisting of several IP cameras. Brownouts are very frequent so I have purchased a 12V 7Ah that I am intending to use to power my system during these periods.

I also have a 12V battery charger which runs off 220V AC.

So,

• Can I wire a load to my battery if it is connected in parallel with the charger?
• Are 12V devices (IP cameras) supposed to tolerate 14V coming from a fully charged battery?

I also have a 12V solar panel that I'd like to use to charge my battery during the day and have the 12V charger provide the remaining necessary power to keep up with the demand. I have a solar controller which connects to the solar panel and the battery and takes care of regulating the charge.

• How can I add a solar panel + controller to the previous circuit?

• Power consumption will be less than 3A.
• I am guessing the solar controller DOES have diodes that prevent the battery from being discharged.

I am going to get some regulators but are there any other alternatives? I have plenty of resistors, transistors and the like but no regulators like that.

I am not sure what the maximum voltage for the cameras could be ( no datasheets or anything like that available ) but would it be such a bad idea to add a series diode to cause a voltage drop of about 0.7V? Voltage would be about 13.5 if the battery is fully charged ( 12% more than 12V ).

• If your cameras can tolerate 10v you can use a simpler regulator that you could construct or a 7812 device which you may be able to get. They are only rated at 1 amp so you need to work out what current your cameras take. Commented Feb 11, 2014 at 23:16

Firstly, the feed to the cameras: I think, to be safe you should use a low drop-out regulator to feed your cameras - this takes care of slight overvoltages. The KA278R12C is a linear voltage regulator with very low drop out: -

Note that even when the input voltage is at 10V, the device is still able to ostensibly produce 10V at its ouput when delivering over an amp (6 ohm load). I suspect this device will be good enough to feed your camera system but I can't absolutely say because you haven't specified current. There are other higher power devices that would fit the bill.

Can I wire a load to my battery if it is connected in parallel with the charger?

If the battery is lead/acid and the charging current is significantly more than what the camera load takes when attached to the above regulator then yes you can. If the battery isn't lead-acid then we need to know which technology it is.

How can I add a solar panel + controller to the previous circuit?

Playing safe, you can use a relay circuit that activates the relay when the AC power is applied to the charger - the relay contact can switch the battery from solar charger to AC charger in a few milli seconds. Playing a little bit unsafe, it's likely that your solar charger will have a diode in its output that protects the battery from discharge when the sun doesn't shine.

This very same component probably can mean that you can connect the AC charger permanently to the battery (and solar charger) BUT, you may need to add a series diode$^1$ in the AC charger's output when AC is off and the solar charger is feeding juice to the battery; the AC charger's output circuits may be activated by the solar charger and it's difficult to say what will happen - worst case it might pop the output transistor in the AC charger - best case no problem.

However, the chances are likely that your AC charger (just like your solar charger) will be protected from reverse voltages when power is down (or sun is not shining). You need to check this.

$^1$The diode needs to be a low volt drop schottky type capable of taking the charge current (again, you haven't specified max charging current so it's impossible to say but there are plenty rated for 10A and 20A continuous usage).