12v vs 5v video transmitter power draw

[Networkit]

I have a project for RC plane application where I have 6v-12v video transmitter that has a built in 5v output to power a gopro camera. The video tx draws about 400mah and the camera about 300mah. The battery options are 7.4v 2s lipo or a 11.1v 3s lipo. Both capacities would be 1000mah. My goal is to power the video tx and gopro as long as possible. My question is if it is better to go with the 2s or 3s battery?

Thx!
Dan

 

[(*steve*)]

I presume you mean that the video transmitter draws 400mA from the LIPos?

And the camera draws about 300mA from the 5V output of the transmitter?

Have you measured these? What is the total draw from the batteries (it will be between 0mA and 700mA, probably between 400mA and 700mA while transmitting)

I assume 2s and 3s means 2 cell and 3 cell?

Do you know the rating of these batteries in mAh?

The battery rating in mAh / load in mA gives time in h (hours)

 

[Networkit]

Yes, the vtx draws @400mA direct from Li-Po and camera @300mA from 5v out from vtx. I have not measured them but collected the info off several web posts.

I assume the total draw max is @700mA. Yes, the 2s/3s is 2 cell at 7.4V and 3 cell at 11.1V.. and the battery capacity is 1000mAh.

Technically this will provide @1.5 hrs, but my main question is it more efficient to run with 7.4V or 11.1V.. also, I am curious if the vtx outputs stronger signal at 11.1V vs 7.4V? (not sure if this info is known other than by vendor?)

Thx
Danny

I presume you mean that the video transmitter draws 400mA from the LIPos?

And the camera draws about 300mA from the 5V output of the transmitter?

Have you measured these? What is the total draw from the batteries (it will be between 0mA and 700mA, probably between 400mA and 700mA while transmitting)

I assume 2s and 3s means 2 cell and 3 cell?

Do you know the rating of these batteries in mAh?

The battery rating in mAh / load in mA gives time in h (hours)

 

[(*steve*)]

Yes, the vtx draws @400mA direct from Li-Po and camera @300mA from 5v out from vtx. I have not measured them but collected the info off several web posts.

I assume the total draw max is @700mA.

You have read, you assume... Fine, but if things don't turn out right, it's not my fault.

In particular, the numbers might not add to 700mA. If the 5V is generated using a SMPS, the 300mA might only contribute to around 150mA from the 12V input. And I don't know if the 400mA draw from the LiPo includes the 150mA or not.

You're assuming that the regulator is linear and that the 400mA does not include the draw for the 5V.

At worst, your actual drain will be lower, your batteries larger, but your lifetime longer.

If you're happy with this, we can continue.

Yes, the 2s/3s is 2 cell at 7.4V and 3 cell at 11.1V.. and the battery capacity is 1000mAh.

Note that a three cell pack will contain 50% more energy than a 2 cell pack assuming they are built using the same cells.

Technically this will provide @1.5 hrs, but my main question is it more efficient to run with 7.4V or 11.1V.. also, I am curious if the vtx outputs stronger signal at 11.1V vs 7.4V? (not sure if this info is known other than by vendor?)

The best way to determine this is to measure the current draw at various voltages.

Once we know how current caries with voltage we can determine whether it is better to use a higher voltage pack or not.

If the regulators are linear (very unlikely) then the performance will be the same on both batteries. However things will get hotter using the higher voltage pack.

If the regulators are switchmode, the 3 cell pack will provide a higher voltage, therefore the current draw is less and the lifetime of the pack will be longer.

 

[Networkit]

Thanks for the info - this is great.. I realize now just how much I was not aware of.

The 5v out does appear to be switching:
http://www.hobbyking.com/hobbyking/...o_transmitter_fatshark_compatible_600mw_.html


Features.
600mW (28dBm, +/- 1dB) of clean output power
On-board quiet switching regulator, powers 5v cameras (including HD versions)

Specs:
Weight: 18 grams
Size: 50 x 23mm
Input voltage range: 7~25v
Camera Power Supply: 300mA / 5v
Video Input: 1Vpp, 75 Ohms

Below is a list of measured maximum currents at 5.00 Volt for my Hero3 Black Edition :

GoPro Battery Current
Off Empty 0.36A
On Empty 0.47A
Off Charged 0.00A
On Charged 0.40A

I assume it is safe to say max draw is @.47A?

I have a lower end multi-meter I can use to verify vtx power draw - I will try this tonight.

 

[(*steve*)]

Great. If you actually have a 2 and a 3 cell pack and you can measure the current draw using both then it will tell us a great deal.

Do you know how to use a multimeter to measure current?

 

[Networkit]

I would like to think know how to measure, but to be honest it's been since my high-school electronics class over 25 years ago..

 

[(*steve*)]

Hahaha, no problems. And it's better to be safe than sorry -- especially with current measurements.

The best thing to do is to set your meter onto the maximum current range. This is often 10A. Note that you often also have to plug one of the probes into a different socket.

Then you need to make current measurements in series with the battery. This means finding a way to break the circuit and reconnect it through the meter. This can be tricky when the battery connects to the device using a mating connector.

One way is to create an "extension" using another pair of plugs and sockets, wiring one connection straight through and having the other one go to two wires (one from each plug) that you use to measure the load current. If you take this approach be careful not to wire the battery in backwards as that will make this a really bad day.

I recommend that you use the highest current range for several reasons:

1) if you make a mistake it is least likely to be expensive.
2) you can always try a lower current range later
3) I'm not interested in ultimate accuracy. Knowing that it's drawing 0.35A (for example) is plenty of accuracy.

 

[Networkit]

Excuse late reply - just got back from work trip.

Great info - thank you! I am planning on doing this tomorrow and will report the findings ASAP.

 

[Networkit]

1. Just measured and video tx is coming in at 220mA.. Gopro pulls about 380 so total of @600mA. I had planned to use the 2S (7.4v) lipo rated at 1050mAh... if my calcs are right this only gives me @1 hr of power..?


2. A friend mentioned a possible 1S (3.7v) lipo option in conjunction with a booster.. he advised this would obviously eliminate a cell (the associated tabs connecting the cells, etc) and may be better for this application.. not sure how this affects volts/amps/wattage?

 

[Networkit]

1. Just measured and video tx is coming in at 220mA.. Gopro pulls about 380 so total of @600mA. I had planned to use the 2S (7.4v) lipo rated at 1050mAh... if my calcs are right this only gives me @1 hr of power..?


2. A friend mentioned a possible 1S (3.7v) lipo option in conjunction with a booster.. he advised this would obviously eliminate a cell (the associated tabs connecting the cells, etc) and may be better for this application.. not sure how this affects volts/amps/wattage?