Hi
Is it OK to charge a 12V car battery with constant voltage say 12.5 volts ?
Thanks
Elico
Generally the charging voltage for a battery needs to be higher than the voltage rating for the battery itself like the 3.3v batteries in our phones have 5v charger, this is with almost every battery.
However to pricesly know that how much voltage your car battery needs to ger charged you should check the alternator output of your car.
Basically yes, but it will not be capable of fully charging the battery. You need somewhere between 13.8 and 14.4 to fully charge - depends upon battery chemistry.
Apart from that, it's improbable that your source is a true constant voltage - it will have an output impedance which will cause some voltage droop as the load increases.
When you charge a battery with a constant voltage you'll be feeding it a lot of current when the battery's charge is low and less current when the battery's charge is high. For example charging it at 14.4V when the battery is at 11V has 3.4V of potential difference (high rate of charge) and when the battery is 13.2V there's only 1.2V of potential difference (low rate of charge). Ideally you want to charge batteries with a consistent amperage around 1/10th the battery's AH rating.
Chagrin:
Ideally you want to charge batteries with a consistent amperage around 1/10th the battery's AH rating.
generally yes but 1/10 of the AH rating isn't really that much of a guideline. The battery's data sheet (assuming you have one) will tell you what the max charge is [this is very important for LiPo and LiIon batteries, so they don't catch fire].
Car battery chargers typically run 5A, some lower [I have a 2A charger that plugs into the 'accessory' plug]. So if you are charging a typical car battery, it's probably best to have a voltage that is significantly higher, with a constant current output. A switching regulator will do this efficiently, so long as you filter it and measure current going directly into the battery in your current feedback loop.
The ideal way to charge a battery is to use a 'charge curve'. For Pb+acid batteries, that would involve bringing up voltage with a constant current to some pre-determined value that's around 10% above the normal volts (let's say ~14.5V), then holding that voltage until current drops below a pre-determined value, at which point it is "charged". Detecting 'bad cells' and other issues would require extra effort, maybe measuring the rate of voltage increase when you're below 12V or similar.
Anyway, it's worth an online search, to find out what the ideal charge curves are for each type of battery.
Here's some relevant information -- BU-403: Charging Lead Acid - Battery University