Battery switching without tears

On 4/7/20 10:07 PM, Andy Blackburn wrote:
On Tuesday, April 7, 2020 at 8:58:53 PM UTC-7, 2G wrote:
On Tuesday, April 7, 2020 at 8:06:46 PM UTC-7, Andy Blackburn wrote:
I put a power resistor in the circuit to keep the current surge down. I undersized the capacitor so if I mess up on the with rotation I can lose power. Typically I'll shut off some non-essential equipment to lower the draw if I really don't want a computer reset.

Andy

On Sunday, April 5, 2020 at 8:45:30 AM UTC-7, jfitch wrote:

What is the inrush current when you first switch the power on? Must not be enough to blow the fuse, but that'd be something I'd want to O'scope with a current probe.

You didn't say what the resistor size was. There is already a resistor in the circuit - it is called the internal resistance of the battery,wiring resistance, switch contact resistance and the equivalent series resistance (ESR) of the capacitor.

I think it was only a few Ohms, but it was a big sucker so it could take the current. I figured bet to play it safe so you know where you are dissipating the energy. Probably unnecessary, but I am a belt + suspenders kind of guy.

Andy


For people who didn't sleep through their EE101 class, they can figure
out how to parallel a diode across the resistor to allow very slow
charging of the capacitor, yet rapid discharge to power the panel
temporarily. Charging the cap in seconds instead of milliseconds would
give oh about a factor of 1000 reduction in the inrush current. There's
better ways of doing it, but this would be an adequate solution.

The batteries Tom uses are both capable of delivering several hundreds
of amps at a reasonable voltage to a starter motor, so that should give
an idea of how much current limiting they provide.

The really funny thing is, most people reading this have no need at all
to ever switch batteries. Instead of two small batteries, one big one
is so much easier to manage. If they have to be broken into multiple
units, then just wire them in parallel and let them all provide power
until depleted. If you switch them, then you risk switching too soon
and wasting capacity in the first one, or switching too late and ruining
a flight log or messing up a flight computer when you key the mike and
don't realize how weak the battery was.

-Dave