Battery charging - is it about voltage or amps?

[sw-uk]

for instance, take a sealed lead acid battery of either 6v or 12v. If I'm to charge a 12V, I need at least 13.5V and 7.2/7.5V for a 6v.

If I provide 13.5V DC with a maximum of 500mah available does that mean a 7ah (7000mah) would take 14 hours? (14x500) but if double that of 1000mah it'll take half the time at 7 hours. Then with a 6V 7ah it would be charged at 7.2v/7.5v and time is down to available current.

 

[Diver300]

You've got the basic idea correctly.

If you are charging a 12 V, 7 Ah battery with a charger that has a maximum current of 1 A, and a maximum voltage of 13.5 V, it the current will reduce below 1 A when the battery voltage gets to 13.5 V, so it will take longer than 7 hours.

A lead-acid battery will not be fully charged as soon as it reaches 13.5 V.

 

[rjenkinsgb]

Yes, that's pretty much the principle!

In practice, starting from fully discharged, the charger would be operating in current limit initially - maximum current and with the voltage being pulled down to eg. 11V at the very start.

It would stay in current limit until the battery voltage reaches the set charge voltage, then the current starts to drop and the voltage remains the same.

The time to full charge is extended both by the time at lower current and the fact that there are inefficiencies; in reality it probably takes around 50% longer than a basic current and capacity calculation gives.

 

[crutschow]

You should probably not use a charge current higher than its Ah rating divided by 10.
Thus the maximum charge current for a 7Ah battery would be 700mA.

A good charge procedure for a 12V lead-acid batteries is:

• Charge at the maximum current-limit value until the voltage reaches about 14.4V (initial charge)
• Limit the charge voltage to 14.4V until the current drops to about 1/10th of the maximum (top-off charge)
• Drop to a voltage of about 13.5V ( float/maintenance charge)