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Old Jul 09, 2013, 03:16 AM
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As slipstick mentioned for any given voltage, the motor /prop combination determines the current flow in the system.

But don't ever believe that the calculated maximum current from a battery is the limit, i.e. the max current of a 2200mah 20C calculates to 44 amps. If you load that battery enough it will produce much more current than that. It could produce 100 amps or more. However it won't produce it for long and the battery will be shot after doing it.

The bottom line is everyone should have a wattmeter in their toolbox and use it every time you change a motor or prop.

Glen
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Old Jul 09, 2013, 04:40 AM
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The question is asked because we are taught in this hobby compartmentalized. You get bits and pieces only to find out, Oh that's why when I was looking at my voltage, my battery still puffed.

Think of it this way. Why the question. Because you don't want to damage the LiPo. If you look at a LiPo from only voltage will you damage your battery. Absolutely.

If you look at the battery tests they reflect this. Some have good sustain voltage but have high temperature rise producing shorter life spans or fails.
Others have a low IR which tracks with a C rating and have a consistent higher delivered volts. LiPo pastes are not a power plant, they are a chemical reaction that is partly dependent on a better IR and C rating. We've demonstrated this going from a 40C battery to a 65C battery which delivered .4+V per cell which amounts to 1.2 volts more for a 4S and thus a higher rpm applied to the same KV setup. The result was the voltage sag was generally less. How much varies from brand to brand but the combined relationship of all of these is there.

With that higher voltage being fed to the motor the amp draw increases. Just as it would by adding an entire cell. But if the battery can't maintain that demand as you just pointed out it won't deliver for long because it is then in the batteries peak performance rating which is again the short period that it takes to over heat the cell. Think of it. You never hear of a PEAK VOLTAGE rating from a battery. Its always e.g. 40C Peak 60C or what have you.
The result on EDFs, one of the more demanding applications, was a significant efflux increase some increased from 175 mph to 220 mph with just a battery rated with a higher C rating. And in doing this the setup was further away from being concerned that you are getting to some critical minimal voltage number that varies from brand to brand and batch to batch.

Now apply that to the question. Do I get a battery based only on the fact that it is the right cell count or does the C rating and capacity matter as well? Of course all of these other considerations factor in as to weather you are reaching minimal values for long term use and healthy battery life.
We all know if I get lower C ratings with a high demanding setup, the volts will sag much more quickly but will still run but sustained in peak IR loads. And we also know voltage sag is always less at the beginning of a fresh battery. Many people have done this thinking I haven't reach a 3.2 or 3.3V alarm but the cells in such are already running in peak amp from almost the start but because of it being a fresh cell it initially hold up longer but the cells has then already reached temperature and fail parameters much sooner.

Looking at it another simple way, why would you need amps sensors with telemetry if Voltage gives you everything you need to know to manage your batteries correctly. Why is it a concern with electric cars where its not just voltage alone that is monitored. It all factors in. How do you determine watts without amps? you can't! So you have to monitor/manage both your voltage and your amps because they can be drawn from the battery for disproportionate reasons. Amps can get lost to wiring and connectors via heat. That doesn't show up in the voltage but it does in the amps. And the battery feels that too.
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Old Jul 09, 2013, 03:08 PM
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Battery Comparison Test: X amp Nominal Constand load
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Old Jul 09, 2013, 04:17 PM
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There is no simple answer to the question about voltage in flight. If you are flying at a full load, the pack voltage will drop a lot more than if you are cruising around at a low throttle. Actually you can draw the pack down a lot more at low throttle because you don't get a big drop and as a result can drain the battery pretty dry before hitting 3.2 or 3.3 V.

The most obvious thing is to fly to time. You just need to figure that time out, for the type of flying you do. If you know the nominal time to reach an 80% drain, then you will be in good shape. Small changes about 80% aren't going to matter very much, because the pack is still holding a good reserve of charge. Occasionally hitting 17% isn't going to puff the pack. However if you are regularly beginning to hit that value because your flying is getting more aggressive, then you will need to backoff the time a bit.

The object isn't to see who can hit exactly 20% charge left. The object is to treat the pack in a way that it has a good chance to keep you happy over a longer time.
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Old Jul 13, 2013, 10:13 PM
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14.4v 4s 3600mah lowest safe voltage per cell?

Quote:
Originally Posted by Alan Hahn View Post
There is no simple answer to the question about voltage in flight. If you are flying at a full load, the pack voltage will drop a lot more than if you are cruising around at a low throttle. Actually you can draw the pack down a lot more at low throttle because you don't get a big drop and as a result can drain the battery pretty dry before hitting 3.2 or 3.3 V.

The most obvious thing is to fly to time. You just need to figure that time out, for the type of flying you do. If you know the nominal time to reach an 80% drain, then you will be in good shape. Small changes about 80% aren't going to matter very much, because the pack is still holding a good reserve of charge. Occasionally hitting 17% isn't going to puff the pack. However if you are regularly beginning to hit that value because your flying is getting more aggressive, then you will need to backoff the time a bit.

The object isn't to see who can hit exactly 20% charge left. The object is to treat the pack in a way that it has a good chance to keep you happy over a longer time.

Alan, I totally agree with your thoughts. I use telemetry stuff on each and every model I fly; it helps a lot in extending my flying times, but if you fly different models requiring different flying styles, it's really hard to trust the telemetry readings. For instance, an acrobatic plane and a 3D helicopter can both be flown the hard and soft way and there is no way we can pre-program the telemetry parameters for each flight.

Having said that, I will continue to put telemetry in my new models, but until Spektrum doesn't come up with an amp sensor that shows how much current was actually consumed, timing the flight like we always did in the past is the way to go.
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Old Jul 14, 2013, 06:23 AM
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Jeti actually does offer mAh through from their telem either built into their ESC or a through sensor.

Unlike an electric car that reads the charge into the pack, you have to take the mAh figure charged to the pack when you charged it and set that as your start point for the telem capacity to count from and to.

Glade they are coming up with better ways to take the guess work out of stuff.
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