[time-nuts] float chargers for oscillator backup power
jimlux at earthlink.net
Sun Oct 26 14:31:55 EDT 2014
On 10/26/14, 10:59 AM, Tom Miller wrote:
> Hi Jim,
> I think these chargers are only meant for float charging the battery.
> They are not meant to supply a load (GPSDO) current.
> It is more for connecting to your motorcycle or tractor battery and
> keeping it fully charged over winter storage conditions.
This is sort of true. But consider that the application we were
discussing was "running an OCXO oscillator for hours/days off a
battery", so the load current is a tiny fraction of the battery
capacity. And hence, I suspect that the "float" charge current from the
charger would be more than sufficient to run the load (since the battery
in float charge doesn't consume much power).
If the charger puts out C/5 (5 hour recharge time) in "charge" mode and
C/20 in float mode, or something like that, and your oscillator is C/48
(2 day backup time), then that's where you are.
> I just picked up a MeanWell 24 volt switcher rated at 24 volts @ 6.5
> amps and has a fine adjustment that will go to >28 volts. Also, it has
> remote sense connections. It was $25 including shipping.
Is that in a package with a power cord, etc.? I have no doubt that one
can get an open frame power supply and adjust it to work acceptably in
this application, but I'm not sure that's always the optimum approach.
I also don't know if the charging behavior would be optimum after an
outage. Most of the chargers I've been using have a multistep profile:
higher voltage when doing the "charge up to nearly full" and then lower
voltage when in "maintenance of charge" mode.
It might well be that "maintenance of charge" voltage will recharge the
battery adequately given long enough.
One reference I have (from Powerstream) says that in float charge
applications, for sealed lead acid batteries, one should use
2.25-2.3V/cell at 25C, to prevent loss of water during the long float.
But there's a fairly significant temperature coefficient.
at 0C 2.30-2.35 volts/cell 13.8-14.1 for 6cell/12V battery
at 20C 2.26-2.31 13.56-13.86
at 40C 2.22-2.27 13.32-13.62
What you really want to do is stay below the "gassing voltage"
at 0 2.54
at 20 2.415
at 40 2.33
The challenge is that the gassing voltage at high temperatures (40-50C)
is within the "float charge" range at low temperatures.
> ----- Original Message ----- From: "Jim Lux" <jimlux at earthlink.net>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Sent: Sunday, October 26, 2014 11:29 AM
> Subject: [time-nuts] float chargers for oscillator backup power
>> There are a variety of inexpensive wall-wart packaged float chargers
>> for lead acid batteries around. Might be easier to just get something
>> off the shelf.
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