Create 12v Lead Acid charger with 12v adapter

Thread Starter


Joined Nov 19, 2008

I'm new to the forums, and I was searching around but couldn't find a simple explanation on if a 12v adapter can be converted to a charger.

What i'm trying to do is charge a 12v 7.5Ah/20h lead acid battery. At home I have a 12V 500mA 14W adapter which looks exactly like the 12V 1000mA charger at an electronics store. Minus the 500mA difference, what else could be different inside the charger that the adapter may not have?

From the outside they look exactly the same, i'm wondering what circuitry is inside beside the transformer?

I've seen all the more advanced schematics on this site with float charge and multiple stages. I'm just trying to create the most simplest of chargers for a 12V lead acid battery.

Reading all your informative posts, I realized that an adapter that says 12V is really around 14.7 - 15V which I guess is perfect for charging as the charger has to have more voltage over the battery.

Hopefully someone can answer this for me, I've seen posts stating that an adapter cannot charge batteries and some say it can, i'm totally confused at this point.




Joined Nov 29, 2005
A true 12VDC adapter cannot be used to charge a 12V lead-acid battery.

If it is at least 16VDC and sustains its voltage under the desired charging current, then a voltage regulator like a 78T12 tricked to deliver ~13.6V DC can be implemented to supply the battery.

It is not how they look outside; compare the printed ratings, compare voltage and current measurements. Try it.



Joined Oct 22, 2008
If you want the simplest charger that won't damage your battery then you need to limit the current.

Add a power resistor in series with your voltage supply so no more than about one tenth the batteries amp-hour rating of current could ever pass.

If you're not in a hurry, your battery will charge with only a few hundred ma of current, and you'll be in less danger of overcharging the battery.

But if you find that your "12V" adapter cannot deliver this much current at 13.8 or so volts then you will probably have to find another adapter with higher voltage.



Joined Jul 5, 2006
Miguel and David are correct.

Google the names of various gel cell battery manufacturers and read their app notes on charging. They want your use of their products to be successful and offer quite detailed information on proper charging techniques and even sample charger circuits. It's more complex than just applying a voltage and walking away.

If you accurately limit your applied voltage to the float charge voltage recommended by the battery manufacturer, you can just walk away and leave the charger connected indefinitely without damaging the battery, but your battery will be charged to less than its full capacity. I don't remember the percentage of maximum capacity, but I think it may be around 70 or 80 %.

You don't really need charge rate (current) control if you never exceed the rated float charge voltage because the acceptance of current by the battery is self-limiting and tapers off to near zero as float voltage is approached. However, if you don't need to minimize recovery time after power is drawn from the battery, it can't hurt to insert a moderate power resistor of a few tens of ohms in series with the charger because as the battery voltage recovers during recharge, the current drops to the point at which the voltage drop across the resistor is negligible. Therefore, the battery will ultimately reach float charge even with the series resistor. Some manufacturers print the float voltages and the intermittent charge voltages on the battery case, which is very handy.

If you want your battery to be charged safely to maximum capacity, you need some sophistication in your charger because you have to charge to a higher voltage but limit both the charging current and charging time. This is why a true battery charger is more costly that an ordinary wall wart - even a regulated one. But beware! Many "battery chargers" switch from an intermittent charge voltage to a "trickle charge" current at some preset voltage, but the trickle charge current can overcharge the battery, gradually driving off the water and shortening battery life. Read the specs on the charger to determine whether it is truly safe for continuous connection. If you are not sure, disconnect the charger after it switches to trickle charge.

If you really want to do it properly, you will sense battery temperature and compensate your charge voltages for cell temperature. However, virtually all chargers ignore temperature and just trim the voltage a little to be safe at all expected temperatures. Temperature compensation might be applied in a sophisticated commercial battery bank, but almost never at home.

Have fun.


Thread Starter


Joined Nov 19, 2008
I will get the digital multimeter and get some measurements done. This will definitely be alot easier for us all to figure this one out.

Thanks for all your support guys!