Disclaimer: always be careful messing around with Nicad or Metal hydride
cells etc They have a lot of short circuit capacity and will cause fires
if you inadvertently cause a short!
I recently did a few bench tests of some battery packs
just to find out what the voltage cutoff point was for my Pentax DSLR.
This may be of some use for Canon/ other types of DSLR users.
One of the MOST frustrating things doing time exposures all night is your packs failing.
Several early mornings doing Lovejoy stuff I had to
replace the internal NiMH cells and external packs a few times over
the 3 hour sessions of a typical night. I don't use Lithiums BTW.
Too expensive...and I don't have a mains adaptor.
That's what comes about mainly by not being prepared.
What a time to realise your batteries are not up to scratch eh
That's me all over
So I made up two external packs out of old (but still good) emergency
exit light cell packs.
These are called 'sub C' cells and are used in typical older model
battery drills.
They have a 4000mA/H rating...a lot more than my internal cells
which are typically 2500mA/H.
These two sets, subsequently, easily got me through the 3 hour
sessions.
My camera spec says it needs 6.5V DC in (using the ext power socket)
with a maximum input never exceeding 7.5v.
5 x cells (NiMh / NiCad) should be roughly 7.0V if fully charged (5 x 1.4v)
A set of internals used to run for about an hour doing time exposure
BUT that was doing 5-10sec subs...lots of them...
Lovejoy needed 30 sec subs...and about 100 to make a good stack.
That's another factor that I hadn't thought of....what is the current
draw while doing time exposure?
So I measured that.
Typical standby (remote plugged in) and timing between frames: 180 mA
Typical shutter held open, exposing: 410mA
WOW!! almost half an amp.......worth remembering!
So naturally, if you take LOTS of 30 sec exposures, that is a lot
more minutes pulling 410mA than , say 10 sec exposures (with a 5 sec
interval between frames)
Another reason I wanted to test my external battery life was also to
find the cutoff point of the Pentax...when it switches itself off.
The Pentax has a menu option of choosing external and internal packs
BUT it will not jump between them when one is exhausted...another
thing I discovered and worth remembering.
The cutoff point is very close to when an external pack reaches
approx 1.200v per cell....or for a 5 x cell pack, that is 6.000V
That seems to make sense as that is the point where NiMh and NiCads usually start to 'plummet'
in an amps vs time load test.
The linear part from between full charge (usually 1.4v) and flat(1.2v) is
where the cells are reliable....model RC pilots know these values well
and never fly when the RX or TX is getting close to 1.2v / cell
(Those still on NiMh/Nicads and not LiPo)
So here are some pics:
Pic 1 is the pack under test.
Pic 2 is the discharge plot of the packs I made up...volts over time.
Nice and linear...ie reliable part of the discharge.
At the extreme right end of the line (6.0V and 1.2v/cell the Pentax
switched off)
Pic 3 is the bench test. One digital multimeter read amps, while the other
read pack voltage.
Pic 4 is the display on the Pentax. Interestingly, when I had NO internal
batteries in the camera, the LCD display showed no battery indicator.
When there WAS a set present in the camera and also an external pack
fitted, the battery indicator WAS present.
Steve
