Battery Vehicle Society

My question is how much voltage is needed to charge a 6v battery

Can we charge it from a dynamo?

The charge voltage depends on the battery chemistry and construction. For example, if the battery is a standard wet lead acid one, then it will like a fast charge profile that tops off at around 2.4 volts per cell, or 7.2 volts for your 6 volt battery (3 cells in series). The float charge voltage per cell should be about 2.25 volts per cell, 6.75 volts for the battery.

If the battery was a lithium polymer one, then charge control needs to be more critical and really each cell needs monitoring. The profile is similar to that used for charging lead acid though, with a constant current charge phase followed by a constant voltage stage, but unlike lead acid there's no need to drop the charge voltage for float charging.

Nickel chemistry cells are different again, as they ideally need constant current charging all the way to fully charged, with the charge current then being reduced to a low level for float charging. Detecting full charge can be done by measuring cell voltage and looking for the changes that occur at full charge, or more reliably, by measuring cell temperature and cutting the charge off as it suddenly starts to rise at full charge.

You can certainly charge any type of battery from a suitable dynamo, although the type of regulator used will depend on the battery chemistry, as it needs to match the characteristics I've briefly outlined above. Also, it's generally more efficient to use an alternator for charging.

Not as simple an answer as you might have been looking for, I suspect. If you post some more detail of what it is your charging we might be able to come up with some better focussed help.