Battery discharge rate

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Does anybody know what the voltage you see left on your tentacle when you plug in the app means in relation to battery hours left?

e.g. if it reads 3.5v, what roughly does this mean? 20 hours left? 10?

Usually these batteries stay fairly high and then quickly drop off near the end of their cycle, so I am wondering how to interpret the volts as useable time left on the unit.


Hi Ian,

Peter here, another user.

I had this problem with my old v_mount batteries and built my own discharge unit to find out exactly how long they would hold for at a certain discharge rate. I obviously didn’t want to waste camera hours for this and wanted a stand alone unit , which could run over night. I will demonstrate this crazy bit of kit on youtube soon.

However with the tentacle it’s awkward ( if it’s the old one, like mine) because when you plug it in via usb, it’s immediately charging, which messes up any readings. I might do series of readings myself soon however and will try the following.

Charge to full.Turn on to green and get timer going. Come back at hourly intervals , plug in usb, note voltage, immediately  unplug usb again. At some point the unit  will be dead. Then repeat process, but  in the hour that it died, check at 10 min intervals. This way you could  at least check the total running time  fairly accurately. You could also make a table of results and read off voltage vs running time left. Crude, but functional. Because the unit is always run with the same load, it should behave in a similar fashion each time– at least until the batterie begins to wear out.

My gismo for discharging v-mounts displays very constant results for the batteries placed under a constant load. I then put stickers on the batteries so colleagues can chose which ones to take for endless interviews and which ones for b-roll and loose ends. Obviously always writing the “bought-on” dates on all batteries and units with built-in batteries is helpful to keep track of which units might be more suitable for long sessions.


Peter Dix


Thanks, Peter. I kind of thought the makers of these units would have an idea, or even a graph plotting voltage reading against time left. What i don’t know is if it stays high, then abruptly plunges, like many lithium cells, or declines gently at an even rate. Which makes the reading the voltage very difficult to know if you have got a full day left or what.

edited answer

Yes, it’s frustrating isn’t it.  That’s why i’ve ended up making usb charging adapters, so i can charge or run various units via d-tap or 4 pol hirose. Like the wretched DR -70 which only takes AAs or a micro usb input. You could make an adapter like in pic.