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Discharging 60 battery


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#1 Jonathan Bel

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Posted 26 August 2011 - 05:20 AM

I just recelled a cine 60 battery pack and this time around I'd like preserve the memory of the nicad cells for some time.

What's the easiest way to fully discharge a battery belt or pack after the frame rate dips without leaving it plugged into the motor and abusing the movement and waiting for it to fully run out of juice.

This is for a 2A camera. 16.8vlts

Edited by Jonathan Bel, 26 August 2011 - 05:24 AM.

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#2 Phil Rhodes

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Posted 26 August 2011 - 05:38 AM

Couple of car bulbs or MR16 downlighter lamps in series, or just use one if you don't mind replacing it fairly frequently. But don't over-discharge it.

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#3 Dom Jaeger

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Posted 28 August 2011 - 05:33 AM

At what voltage is the frame rate dipping?

What's the camera supply voltage meant to be?

Unless your battery voltage is too low for the camera or you're dropping voltage over the cable or connections, I would have thought by the time the frame rate dips the battery should be ready to recharge. It doesn't need to be drained completely flat, in fact that will shorten its life. The battery capacity will give you an idea of how long it should run for - if it's 8 Amp-hours for example it should supply 2 Amps for about 4 hours, before the voltage drops 10-15% below its nominal level and may start to affect the appliance (at least while the cells are relatively new).

You generally don't want to discharge a NiCad battery much more than about 15% below the nominal voltage (each 1.2V cell down to 1.0V), 10% below is roughly the recommended discharge limit for maximum longevity (down to 1.1V per cell). In your case (16.8V) you'll get the longest life out of the cells if you recharge when the battery hits 15.4V, and avoid draining it below 14V (measured under load). With modern NiCads it's not too bad if the battery is regularly being only partially discharged before charging again, as long as every month or two the cells are exercised with a full discharge/charge cycle (down to 1.0V per cell).

A very occasional deep discharge down to 0.5V per cell can be useful to rebalance and recondition the cells but it should be a very low discharge rate, otherwise you risk cell reversal and internal shorting.

All of which is to say, if you use bulbs to fully discharge your battery belt, monitor it with a voltmeter to avoid draining it below 14V.
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#4 Jonathan Bel

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Posted 28 August 2011 - 04:29 PM

I don't know exactly how to check the voltage, I just plug my battery into the motor.
It's on 16.8. The motor is a 24fps constant speed. Not sure what that draws out precisely.

It has an "overnight" charger and there's no light indicating when it's fully "charged". Plugs into the battery on a 5 pin XLR, plugs into the wall, hit a button, light turns red. I just assume a 8 hour charge will do the job and the battery will re-stabilize itself after unplugging it but I have no way of knowing when and when not to charge. Common sense tells you to recharge when the frame rate drops but i don't know how much motor time this battery "should" giving me anyway. I may have left it plugged into the wall for 10-12 hours at times which may have shortened the life span in past.

I used this camera for tests in the past so it was subject to shallow discharge and recharge. I would shoot a few rolls and soon find it sinking to 22fps, and lower and lower. Remove the magazine the and tachometer needle went back up.

So how do you know when you've reached 1.1 volts or in my case 15.4 volts? And for how long to charge at that rate? I contacted cine 60 for specifics of my charger but they weren't very helpful unless your inquiring about buying one of their products.


My pack is being re-celled now at a battery store, getting it back next week.







At what voltage is the frame rate dipping?

What's the camera supply voltage meant to be?

Unless your battery voltage is too low for the camera or you're dropping voltage over the cable or connections, I would have thought by the time the frame rate dips the battery should be ready to recharge. It doesn't need to be drained completely flat, in fact that will shorten its life. The battery capacity will give you an idea of how long it should run for - if it's 8 Amp-hours for example it should supply 2 Amps for about 4 hours, before the voltage drops 10-15% below its nominal level and may start to affect the appliance (at least while the cells are relatively new).

You generally don't want to discharge a NiCad battery much more than about 15% below the nominal voltage (each 1.2V cell down to 1.0V), 10% below is roughly the recommended discharge limit for maximum longevity (down to 1.1V per cell). In your case (16.8V) you'll get the longest life out of the cells if you recharge when the battery hits 15.4V, and avoid draining it below 14V (measured under load). With modern NiCads it's not too bad if the battery is regularly being only partially discharged before charging again, as long as every month or two the cells are exercised with a full discharge/charge cycle (down to 1.0V per cell).

A very occasional deep discharge down to 0.5V per cell can be useful to rebalance and recondition the cells but it should be a very low discharge rate, otherwise you risk cell reversal and internal shorting.

All of which is to say, if you use bulbs to fully discharge your battery belt, monitor it with a voltmeter to avoid draining it below 14V.


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#5 Dom Jaeger

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Posted 29 August 2011 - 12:33 AM

Hi Jonathan,

look you don't need to be too fastidious, those figures I gave were in the interests of accuracy and to give you an idea of how much a NiCad should be discharged, but they're pretty tough batteries. Without a low battery warning or meter it's pretty hard to monitor the voltage level for optimum battery life, my point was mainly that you don't need to keep discharging the cells after the speed dips. The danger with bulb discharging is that it will just keep draining the cells, whereas most appliances reach a cut off point where the voltage drops too low to keep working.

It would be good if the battery re-celler can check your charger as well, but if it's working OK an overnight charge should be fine.

I'll assume that the camera is an Arri 2C, which should draw 2A without a load and about 3.5A with a 400 ft load. The constant speed motor should keep speed until the voltage drops to between 12 and 13V. (That's a little past the 'recommended' discharge limit, but it won't damage the cells.) So if your new cells are 8 Amp-hour capacity the battery should run a 400 ft load for over 2 hours before the frame rate dips, at which time they'll be well ready for recharging.

If it's an old power cable, or the mag take-up tension is too stiff you'll draw more current and get less run time. If it's significantly less I'd get the charger, cable and camera checked.
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#6 Phil Rhodes

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Posted 29 August 2011 - 09:08 AM

I'd definitely invest in a multimeter. They're worth having from a lot of standpoints, but you really don't want to end up compromising the performance of your batteries (which probably cost more than even a very good meter).

There are many opinions to the effect that the whole memory effect thing is a bit of a myth anyway. Rechargeable batteries (of any type) decay in performance as they're used. It's normal.

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#7 Jonathan Bel

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Posted 29 August 2011 - 07:29 PM

thanks for clearing that up Dom, I'll print that. Thanks Phil. Found out I'll be able to get 2 extra battery belts on set which is a relief. Nothing like telling the actors to take a hike because of "NiCad failure".
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