I know this may be an incredibly stupid question, but I’d rather be safe than risk blowing up a battery. I have a 12V lead-acid battery I need to charge. Can I take a normal 12V power supply, cut off the tip, and hook up the + and - wires to the + and - poles of the battery to charge it up? The power supply is 12V 1A from an old router I used to have.
And if it’s possible, how would I be safe against overcharging? Or is that even a risk?
And one last question. I’m going to hook up a cigarette lighter socket to the 12V battery. Can I have the charger plugged in and connected to the battery poles at the same time I’m using the cigarette lighter socket? Or do I need to charge the battery and disconnect the charger before using the socket.
(If you’re wondering why I’m doing this, it’s so I can use some automotive devices in the house. I also need to charge the battery in a kid’s car. I know I can buy standalone converters that do this, but I already have the charger and battery and thought this would be a fun project.)
You’ll need at least 2.25 V/cell (13.5 V) to charge it. And that’s just a float voltage, which means it would take a long time to charge at that voltage. (A good lead-acid charger starts with an even higher voltage, and then decreases it to around 2.25 V/cell when the battery is fully charged.)
Assuming your other battery is fully charged, it has an open circuit voltage of around 12.6 V. This is almost 1 V less than the minimum required voltage of 13.5 V. To make matters worse, your “good” battery is not regulated, which means its voltage will be less than 12.6 V when you hook it to the depleted battery.
If you want to charge a battery, use a battery charger.
If you want to run 12vdc automotive accessories inside, connect the 12vdc power supply directly to the lighter socket. After making sure that the devices don’t draw more than the power brick can supply.
Also, when the battery is not fully charged, it will be below 12 V. If you connect a 12V 1A power supply to it, the supply will force 1A of current into the battery. That may be OK for an automotive battery, but not for something smaller.
Confession: Once, when I was too young to know better, I tried to charge a 12V SLA (probably only a few amp-hours) battery by connecting it to a 12V 2A supply. I used a rather thin wire, and the insulation immediately melted, thus tipping me off to the fact that I was doing something wrong. I was probably lucky there, if I’d used a thicker wire I probably wouldn’t have known until the battery failed in a catastrophic and messy way.
Well, no, although your larger point is correct. The supply will–or rather, will attempt to–push through whatever current it can, limited in theory only by the difference in voltage between the battery and the supply divided by the battery internal resistance (which in lead-acid cells is quite low). In practice, most supplies have some sort of fusing or thermal protection which will cut off if the demand greatly exceeds the rated current, but even then it can be a lot more than rated for a short time. Likely worse for the supply than the battery, though.
I think scr4’s example included a constant voltage power supply with a regulated output voltage of 12.0 VDC (V[sub]ps[/sub] = 12) and output current range of 0 to 1 ADC (0 ≤ I ≤ 1). If you were to connect this supply to a battery that was less than 12 VDC, the result will depend on the internal resistance of the battery (R[sub]int[/sub]) and the open circuit voltage of the battery (V[sub]oc[/sub]). If the initial current “wants” to be greater than 1 A (i.e. [(12 - V[sub]oc[/sub])/R[sub]int[/sub]] > 1), the power supply will (or at least should) go into constant current mode (I = 1 A), and the power supply’s output voltage will decrease to a voltage that is slightly lower than the battery voltage (V[sub]ps[/sub] = V[sub]oc[/sub] - 1*R[sub]int[/sub] to be exact). But not all supplies are this robust or graceful. Some will freak out and shut down if you do this. Some might even smoke. All depends on how it was designed.
I see that my post is gone. The boards were having problems this morning and even though my post was here, now it’s not. Oh well there are plenty of posts now so I won’t repost my answer.