I've been searching for days a way to charge a 12v acid battery so i can use it instantly just in case my power goes off.I found a circuit to charge using lm317, so far so good, ,but i can't use it as an alternative because the charger provides more than 12v, as i need exact 12v to put into an arduino pro mini. I will power my board from external 15vdc supply so i thought about using a relay to change what source of supply my circuit will use.ralay pin Common is the voltage that goes to my pro mini and other 12v stuff. problem is that 12vbat is not really 12v, as it's output charger that goes to battery. what can i do to use 12v from battery instead of 13.X charging circuit?

I appreciateView attachment 113638 View attachment 113638

 

You need a voltage regulator if you want accurate voltage control. I don't think you can get regulated 12V from a nominal 12V battery using even a LDO low dropout IC. You may need a DC-DC converter with a regulated output.

How accurate a voltage is required and how much current? Maybe all you need is a 12V zener.

 

I will need at max 500mA

 

OK, well that rules out a simple zener. This is not an endorsement, but maybe something like this:
http://www.ebay.com/itm/2PCS-DC-DC-...005778?hash=item5d632f5892:g:laYAAOSw9N1VlfoK

 

thankss buddy, I'll use a 12v zener..

 

There are two ways to go between line power and battery power.

1. Have a monitor and switching circuit, like a comparator driving power MOSFETs, or the relay circuit you posted, etc. The problem here is how fast the switching occurs vs. how long a dropout the load device can tolerate before it goes into a power cycle reset.

2. Don't switch. Run the load off of the battery all the time, and have the battery on charge all the time. If the charger circuit dies, or the power line goes out, or whatever, the load never sees it. This is the better method.

To get the most holdup time out of the battery, you need a buck/boost regulator between the battery and the load. This is because the load needs 12 V, but the battery can go from 14 V to 9 V when discharging. Buck/boost regulators are cheap on ebay. Buy one.

Charging a battery to 14 V or 14.4 V with a 15 V source it doable, but tricky. And, it cannot be done with an LM317 because that chip requires around 2 V minimum between its input and output. Linear charger circuits are easy and reliable, so I recommend sticking with what you have and getting an 18 V wall wart.

ak

 

thankss buddy, I'll use a 12v zener..

Huh? I just said that won't work for that much load current.

 

you should use a modern electronic charger + a deep discharge protection module.

If not your battery life will be reduced to a couple of months.

 

A 12v battery will reach 13.6v when charging and you need at least 15v to charge it.