The thing is, when a SLA is being charged at the float charge voltage
range... 13.8V for a 12V SLA, it isn't all that low impedance.  Remove the
charger, and it will almost instantly drop to 13.2V.  At best, the SLA
is behaving as a very ugly high ESR electrolytic capacitor.

If you want low noise, and drift, you are better off building a good linear
supply.

-Chuck Harris

Gary wrote:
The instruments I have used that employed an internal battery supply were meant 
to
run off the batteries only intermittently. But that isn't the case for the 
OCXO. So
the comparison has to be made with trickled charged batteries versus a power 
supply.
But once we get to trickle charging, NiCads are not suitable. That might be why 
lead
acid was used. [I'm thinking of the old Quantum noise measuring gear.] 
Technically
SLA since we are using the batteries indoors around electronic equipment.

Incidentally, I favor shunt regulators if noise is the primary concern. They are
easier to design with ginormous cap filtering, while LDOs can have issues if 
the cap
ESR is too low. Of course, shunt regulators aren't green.


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