Battery failing. Oscilloscope?

Question

The question is - is there an inexpensive oscilloscope I can use to measure DC voltage and current to see why my battery pack is failing to power a DSLR?

In order to take long time lapse photos with long (night time) exposures in cold weather, I have built an external camera battery pack.  However, after a period of time (15 minutes to a couple hours) the camera stops.  If for even an instant the camera does not get sufficient power, it stops the series and once stopped, even if it gets restarted manually, the time lapse cannot because of the gap that gets created.

The battery pack initially was a 12v 12Ah AGM battery connected to a buck converter to supply the camera with 8.4v which is the voltage of a fully charged Nikon EN-EL15 battery.  When even this failed, I tried connecting two smaller batteries in case one could not supply the needed current.  (I tried the two in both series and parallel even though creating a 24v battery and then down converting to 8.4v seemed futile.  It was.)  I have also added two small 5W 12V heating pads to keep the battery from getting too cold.

The camera is a Nikon D750. There is not much I can find about power consumption although 2.5A is written on it somewhere or other. Long exposures are the highest energy drain on a camera as the sensor needs to stay on and the mirror needs to be held open.  (I have not yet tested current usage but now that I think of it, I should with a multimeter.)

I was hoping an oscilloscope could be used to determine if it is the voltage that is dropping or the current is insufficient.  My guess is the current is briefly inadequate as the bucks should keep it at 8.4v as even a discharged 12v is around 11v.

Mostly I want to understand the issue.  But if not and my guess it is the current, what about a capacitor in the circuit?  How would I choose the right size capacitor for this?  (I do realize the capacitor would drain the battery when not in use but could be easily disconnected from the power until needed.)

Eugene K · Answer

I agree, that the reason of malfunnction is in voltage drop due to high current bursts made by camera. Possible reasons:

converter is not capable to supply needed burst current (what current it rated for?, what is minimum input voltage, at which it rated for that current?)
poor connection between converter and camera: poor contacts, high resistance of wires (too long? too thin?)

Large capacitor may help to supply high currents bursts (try 4700uF and more). Place the capacitor as close to the camera as possible.
But having enough power of converter is more important.

WhatRoughBeast · Answer

The standard EN EL15 battery is rated for roughly 2.5 Ahr, and is good for about 1200 shots. That a 12 Ahr battery will show problems after 15 minutes (although you don't say what the exposure rate is), suggests something else is going on.

I'd be inclined to look very closely at your external battery wiring, and particularly at the connector you're using. Removable connectors can do odd things, particularly when you are using them well outside of rated conditions.

Battery failing. Oscilloscope?

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