Greenpower,

You seem to be knowing the answer. Yes, you are right, you need more than 24Volts supply to charge a 24V lead acid battery. To maitain the battery in fully charged condition (by making up the losses), it is recommended to maintain a voltage of 2.23 to 2.25 volts per cell at the battery terminals.

For lead acid batteries, you need a "float charger", not a power supply.  A "float charger" is one that reduces the current to the battery as the voltage in the battery goes up.  Normally, the "float" voltage for 12V lead acid is 13.7V.  The "float" voltage is the votage were the current is very small or zero.  If you just connect a power supply you will "cook" the battery.  Do not use this method on other types of batteries, viz. lithium, Nicad, etc, they require a different type of charing method.

Steve Owens
Finish Line Product Development Services
www.FinishLinePDS.com

Fawlty is correct; you can provide a 'maintenance' charge of 27 to 27.5 VDC indefinitely without any harm to the batteries.  If your power supply is regulated at 24 volts it will not keep your batteries 'topped up'.  You may be able to adjust it up to the voltage required if other equipment powered by it is rated for the slightly higher voltage.

Steve Owens is correct in that if you want a faster charge on your batteries you need a dedicated charger.  The "float charger" he mentions is faster than a 'trickle charger' (basically the 'maintenance' method described in the first paragraph) but slower than a fast charger.

I hope this isn't too confusing, but what it boils down to is that if the demands on the batteries are 'light' (few operations with long intervals between) the 'maintenance' or 'trickle' charge will do.  If this can't keep the batteries 'topped up' then you need to go with a 'float charger' or possibly even a fast charger.

Wow guys, thanks for the fast and detailed response, this is shedding new light on the lead acid battery charging scenario.   The reason I was wondering if a power supply would work is because the old charger on the generator was nothing more than a transformer 2wires going into the primary “120ac” with 4wires coming out of the secondary, two of the wires of course went to the negative of the battery, and the other ones each had their own diode mounted to a plate and a wire coming off the plate going to the positive side of the battery.   All I saw was a half-wave rectifier charging the battery, no current control, etc.   So since you can charge or maintain a battery with a half-bridge rectifier I figured a standard power supply taking the place of the transformer and using the existing rectifier in the charger would serve the same purpose.   The batteries on the generator stay pretty much charged, because we run it once a week, what makes the battery run down is the control circuit that starts the generator in case of power fail.   This seems to be the main purpose of the battery charger on the generator, not necessarily charging but maintaining.

It sounds as if the original was about as basic as you can get in chargers - two secondary windings with diodes to give a full-wave output. The problem with this is to get a reasonable charging current without ending up with too high an open circuit voltage, resulting in excessive 'trickle' current when the battery is charged - although it occurs to me that one could possibly play tricks using two secondaries of different voltage to improve the characteristics slightly.
If you're only concerned with trickle charging then you could use a supply of suitable voltage - the required voltage does vary with temperature but unless the batteries are exposed to large variations t should be possible to find a reasonable compromise. (there's a table of volts against temperature (for sealed lead-acid batteries)in http://www.powerstream.com/SLA.htm.)
If you could adjust the power supply to a rather higher voltage (say 32V or more) there are more possibilities, depending on how much trouble you want to go to - I've seen circuits for chargers that will sense battery current/voltage and control the charging rate appropriately - for example I came across one at http://www.uoguelph.ca/~antoon/gadgets/labc2.htm (though like most of the ones I've seen it's not intended for 24V so would need modification.
Good luck!
Mike

If there is a way to add it, I think the way I would go is -  go to big truck salvage yard and get a 24 volt alternator with built-in regulator.  Everything is done for you.  And it is designed for lead-acid batteries.