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Author Topic: Whats the lowest safe volatage for RX Battery ?  (Read 1679 times)

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Offline MThemadhatter

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Whats the lowest safe volatage for RX Battery ?
« on: October 09, 2010, 15:39:11 PM »
I have a Hitec Aurora 9 radio and it has a function which will sound an alarm if the on-board RX battery goes  below a certain voltage. However I am not sure what voltage to have the alarm go off at. I am using one of these 2/3 A 6v batteries 6v 1600mAh

Can anyone enlighten me ? all I can find is info for AA batteries.
just one more flight...


Offline dickw

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #1 on: October 09, 2010, 16:21:18 PM »
Probably made up of cells similar to this one:-
http://www.all-battery.com/Intellect23a1600mah.aspx
If you scroll down the page there is a link to a data sheet.

The battery will be almost flat at 5.5v so I would set it at 5.75v to give reasonable margin of safety.
A lot depends on the load you are putting on it, so I suggets a test run to see how long you get after the alarm.

Dick
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Offline grahamstanley

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #2 on: October 09, 2010, 18:55:49 PM »
I would set the alarm voltage even higher, and after the alarm has gone off put the battery on your tester and see how much capacity is left.  Also make allowances for the fact that temperature will also effect the available power from the cells.

I would do this test at home, when there is no rush to land, perhaps while watching the telly.

With my Jeti Tx / Rx set up I get warnings of low voltage before the cells are too low due to the peaks as I use the servos harder causing temporary drops with small LiPo / ESC set ups.   Your cells might have less internal resistance, and with no ESC/BEC inbetween may give less warning.
Graham

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Offline dickw

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #3 on: October 11, 2010, 12:48:37 PM »
.........  Also make allowances for the fact that temperature will also effect the available power from the cells...........

Agreed - this is a very important point.

Lower temperatures cause lower volts, which effectively means a lower useable capacity.
This has probably caught a few people out on a cold winters day.

Good idea to do your testing while watching the telly, the warm room will probably result in less capacity after the alarm is reached - i.e. test the worst case scenario  :) .

Dick
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Offline Sizzling

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #4 on: October 11, 2010, 12:54:42 PM »
If it was me the lowest I would want a 5 cell pack is 6.0v or 1.2v per cell  :af


Offline Duncan

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #5 on: October 11, 2010, 13:25:08 PM »
Warm room??? Nimh's suffer in the cold and can suddenly suffer from a voltage drop. I've never heard of them doing the same at room temperature.

I've seen gliders being stuffed in on landing, when crow brakes were deployed, as the sudden drain from 4 servo's being put under load in cold (freezing) weather, killed the nimh temporally
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Offline grahamstanley

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #6 on: October 11, 2010, 14:43:36 PM »
The voltage drop will be less in a warm room, so will give worst case due to the reduced capacity remaining.

However, in the cold many types of cells are less good at converting the chemical energy back into electricity.  The voltage will drop with temperature, especially under load.  It is unlikely to be a sudden drop in the cold, but bear in mind that the internal impedance of the cells will be worse in the cold.  This might appear to be a sudden volt drop if (for example) crow flap is requested and a number of servos call for amps...

In flight it will be be unusual to get "false" voltage warnings from the telemetry when larger than usual loads are applied.  These early warnings will become more frequent as the voltage under quiescent load gets nearer to the warning level.  Applying sudden loads at a safe height is one way to see how near you are to the warning, with 5 cells you should be well away from any chance of a Rx low votage reset. (brownout)

Graham

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Offline dickw

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #7 on: October 11, 2010, 14:54:06 PM »
Warm room??? Nimh's suffer in the cold and can suddenly suffer from a voltage drop. I've never heard of them doing the same at room temperature.

I've seen gliders being stuffed in on landing, when crow brakes were deployed, as the sudden drain from 4 servo's being put under load in cold (freezing) weather, killed the nimh temporally

Exactly, and that is the point I was making about people being caught out on a cold day. However, if we are talking about setting an alarm voltge we need to look at it slightly differently.

One thing to avoid on a battery is the sudden drop at the end of the battery discharge curve. At high temperatures the whole curve moves up in voltage and at low temperatures it moves down.
So if we set the alarm at a level that gives a further 10 minutes to the knee point at 15 degrees C, then at 5 degrees the alarm will come earlier on the curve and give a longer run to the knee point. BUT on a  hot summers day at 30 degrees the alarm voltage may already be in the "drop zone.

On a cold day the alarm will just warn you a lot earlier.

Dick
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Offline MThemadhatter

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #8 on: October 11, 2010, 22:55:38 PM »
Thanks for the replies. I am going to set it at 6v and see how that works. One handy feature of the TX is that it records the highest and lowest voltage for the RX battery. As for temprature, the battery is sharing the cowl with a Saito 62 and is cocooned in a layer of foam and electrical tape to protect it.
just one more flight...


Offline grahamstanley

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Re: Whats the lowest safe volatage for RX Battery ?
« Reply #9 on: October 11, 2010, 23:07:03 PM »
There's nothing quite like getting to know your in flight voltages if your telemetry lets you.  The odd glance at the voltage (or the voice from the vario) can be reassuring or provide early warning of failure.  Get to know the trend of your batteries the way you fly and the amount you put back in after a days flying.

The highest and lowest might be misleading, some of mine certainly are as it records the voltage during start up...  I haven't found a way to reset the max/min in flight yet on my Jeti kit.

For a battery in the cowl, can you arrange for the cool air only to be on it by the use of baffles (as in full size aviation and automotive practice)?  On very post cars and I would expect on aircraft the battery temperature is controlled so as not to be to extreme either way.
Graham

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