Grant Brown wrote:
> Hi List
>
> I need some sort of battery back up switch IC that can do the following 
> things
>
> The back up battery will be two 12Vdc 220 AH lead acid battery's set up 
> in parallel in order to have a 24Vdc backup supply. I am open to other 
> suggestions so long a s the amp hour rating is maintained.
>
> The system main power source is a 24Vdc rail that supply's 20Amps.
>
> What I need is an IC or device that will do the following,
> (1) Maintain the backup battery voltage and health.
> (2) Provide automatic switch over to the backup battery on main power 
> failure.
> (3) Notify the system of the backup battery switch over event.
>
> The device can be one that uses an external MOSFET for the main load 
> switch if required.
>
> Look forward to your input.
>
> Kind Regards
> Grant Brown
>   
Its a bit tricky, because two 12V batteries cannot be charged with 24V. 
The ideal voltage would need to
be about 28-30V. If you have that, then you have something.

Having said that, I would charge a NiMH battery array instead of 
Lead-acid batteries, because the lifetime
of lead-acid batteries is limited, especially in cold weather. Again, 
30V would be needed, as 15.1V is the
ideal charging voltage for NiMH packs. I see lead-acid batteries going 
the way of high-button shoes.
Notice that NO hybrid vehicle uses lead-acid chemistries. Not one.

I would use MOSFETS to switch the power, as the switching speed could be 
instantaneous. Might need a tiny PIC
to do some housekeeping, communication, etc.

--Bob Axtell
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