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LiFe low voltage alarm

Discussion about Tx, Rx, Servo's, Batteries, Chargers, and all the other things we like to talk about..
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FrankS
Posts: 308
Joined: 18 Mar 2015, 14:29
Location: Gloucestershire

LiFe low voltage alarm

Post by FrankS »

I use quite a few 2s LiFe cells for Rx power on my scale and non-scale gliders, on one small thermal soarer (2.5m) with six 9g servos, V tail, ailerons and airbrakes I'm using a 2s 700 mah 5c Turnigy LiFe cells and have a low voltage alarm which is set at 5.8v, this sounds frequently in flight. I did a test and discharging the battery at 0.8 amps had the battery volts drop to 5.6v.

Is this OK? and what would be a recommended low voltage alarm setting.

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Peter Balcombe
Posts: 1043
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: LiFe low voltage alarm

Post by Peter Balcombe »

Frank,
Wikipedia gives the following Voltage info for LiFe cells:
Minimum discharge voltage = 2.5 V
Working voltage = 3.0 ~ 3.2 V
Maximum charge voltage = 3.65 V

I understood that they Had a pretty flat discharge curve & once they reached 3V the voltage drop was pretty rapid.
Have you checked to make sure you haven’t got any high resistance joints/leads, switch etc. which could cause current surges to cause significant temporary voltage drops?

FrankS
Posts: 308
Joined: 18 Mar 2015, 14:29
Location: Gloucestershire

Re: LiFe low voltage alarm

Post by FrankS »

Peter, thanks, no switch the battery plugs into a short extension lead direct into the Rx, also on test plugged into my charger doing a discharge at 0.8 amps see a significant voltage drop. I suspect the Turnigy 5c, (3.5A), claims are to be taken with a pinch of salt.

Model has coupled aileron-rudder (V tail) so using the ailerons moves 4 servos and gives an alarm. Voltage recovers immediately.

Based on what you've noted I'll drop the alarm to 5.2v which should give me adequate warning of a low battery. Note I did one 20 min flight with the model and put back 60 mah into the battery on recharge

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Barry_Cole
Posts: 1368
Joined: 10 Mar 2015, 09:30
Location: Hampshire UK

Re: LiFe low voltage alarm

Post by Barry_Cole »

Which LiFe cells are you using and where did they come from.???

BC

FrankS
Posts: 308
Joined: 18 Mar 2015, 14:29
Location: Gloucestershire

Re: LiFe low voltage alarm

Post by FrankS »

Turnigy 700mah 2s from HK. The lowest recorded voltage in flight is 5.6v I had the alarm set at 5.8v, if cells are OK down to 2.5v per cell I'll drop the alarm down to 5.2v and see how it performs.

I've also used the larger 1100 and 1800 mah Turnigy Life cells on larger gliders and some power planes and not had a problem with a 5.8v alarm, so it's probably just the 700 mah cells. I use a couple of these in slope soarers with very narrow fuselages.

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Barry_Cole
Posts: 1368
Joined: 10 Mar 2015, 09:30
Location: Hampshire UK

Re: LiFe low voltage alarm

Post by Barry_Cole »

I have bought similar but larger capacity from Component Shop, and the voltage drops considerably before you get anywhere near the C rating.
Those you are using have a rating of 5C which gives a max current of 3.5 Amps. I would expect the voltage to easily drop down to 5.5 volts at this sort of current. However it does not seem to do them any harm.

BC

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