4.8v Rx Battery Pack Alarm Voltage Setting

[Andy J]

Foolishly used an old Spektrum 4.8v Tx 2000mAH niMH  battery as a Rx Battery Pack which is buried deep in a new model. Now note the voltage is collapsing when the servos operate.

 

Unsure if this is due to the pack not being fully charged but just to be safe whilst I monitor the voltage in flight what voltage is suggested as a lower limit telemetry alarm.

[Shaun Walsh]

That will partly depend upon the lowest voltage at which receiver will operate. So what is the receiver make and model?

[Denis Watkins]

Apologies Andy

If the battery wasn't good enough for low level use in the transmitter, where it does very little,

Then it is far from suitable for the much higher discharge rate in a model.

I would negate the alarm with a new receiver battery.

[Shaun Walsh]

Just now, Denis Watkins said:

Apologies Andy

If the battery wasn't good enough for low level use in the transmitter, where it does very little,

Then it is far from suitable for the much higher discharge rate in a model.

I would negate the alarm with a new receiver battery.

A very good point. Cost of a model much more than the cost of a new battery.

[Andy J]

Think the battery was only pulled out of my Spektrum DX7 as I saw a much higher rated battery from HK so it was not changed for lack of performance.   Has been in my battery storage box for some years though.

 

New model is a 3ch Playboy Senior using a 6ch FRSky Rx so the electrical load will not be great but as you say better safe than sorry.

 

Giving the battery a long slow charge at 200mA to see if the voltage holds up after that.  

 

Still interested to know what other people use as the battery alarm voltages.

[Denis Watkins]

This topic has run many times Andy, and there are many opinions.

Receivers normally do a job down to about 3.5v

Voltage is not the full story, as capacity and drain come into it, from Your supply.

Your alarm would be trying its best to operate around a 1v difference.

Better to have a known good battery pack charged before flight, and checked with a load checker after 3 or 4 flights.

A good routine give us the go and no go.

You don't want an alarm at 3.5v, it is too late, so you choose.

[Frank Skilbeck]

The MPX receivers have a default 4.5v alarm which is for 4 cell packs, I have a few IC models with 4 cell Nimh packs and the alarm never seems to go off. But I do cycle them on a fairly regular basis and any that don't give a good discharge capacity (over 75% of the name plate capacity) get binned as do the ones that show a big voltage drop when on discharge.

[Andy J]

After a long slow charge the battery is still not holding its charge so have been lucky and managed to get it out without a major hack of the fuselage. New 4.8v 2400mah battery now on order so will set the alarm at 4.5v as suggested.

[Frank Skilbeck]

Don't get hung up on the mah, generally the higher the mah the higher the internal resistance, so while under a low load they will run for longer on a high load they will have a bigger voltage dip. I have some old 600 mah AA Nicds which perform better under load than some 2400 mah AA Nimhs when fully charged. 

[Engine Doctor]

I never use anything higher than 2200 mah  Nimh and always use the " ready to go " or low self discharge type. Any higher capacity cells as stated are fine at low discharge but useless under any serious load and voltage crashes. They are also more prone to failure as I once found to my cost . Use two packs if you can to double capacity or one pack of "C" cells that will handle the current needed without the volts dropping. Any used packs that have been lying around for a couple of years like your example willprobably only be fit for the recycle bin anyway. 

[Richard Acland]

Cost of a new 4.8v battery depending on capacity between ten to fifteen quid. Set against the cost of a model, peanuts.

[Cuban8]

Do yourself a favour and use a five cell RX pack. I've found many old suspect 4 cell packs with a weak cell that are apparently  fine after a full charge, but will collapse after a few flights leaving you with effectively three and a bit cells..... 4V if you're lucky, much less if the weak cell dies completely. You might get away with it for a while if you usually only have a few flights, but come the day with nice weather and a full day of longer flights or putting the battery in a more power hungry model and that could push it over the edge. 

I've not used 4.8V RX packs for years and always check my 5 cell packs for capacity regularly with my charger on NiMh discharge setting. Have found several weak cells over the years, but not so potentially disasterous  with the four remaining good cells. Amazing how many people rarely or never take the very simple precaution of performing a simple discharge test on their RX packs. Possibly the weakest component  in the R/C system along with cheapo switches.

[flight1]

I got to say i have in the past blamed a bad battery pack for low voltage drop when is use and this turned out to be a ageing switch the cause. so now regularly check the switch and routinely change them.

As for going 5 cell there is no need to with a good 4 cell pack and the down side with 5 cells it gives you higher voltage which leads to less duration for the same mA/hr capacity.

[Frank Skilbeck]

42 minutes ago, flight1 said:

 

As for going 5 cell there is no need to with a good 4 cell pack and the down side with 5 cells it gives you higher voltage which leads to less duration for the same mA/hr capacity.

While the servos will develop more torque on a higher voltage and hence pull more current, in reality the servo will only generate as much torque as required to move the control to it's desired position, as the aerodynamic load will be the same regardless of the supply voltage a higher voltage supply should be a lower current, unless the servo is stalled out.

[flight1]

38 minutes ago, Frank Skilbeck said:

While the servos will develop more torque on a higher voltage and hence pull more current, in reality the servo will only generate as much torque as required to move the control to it's desired position, as the aerodynamic load will be the same regardless of the supply voltage a higher voltage supply should be a lower current, unless the servo is stalled out.

At Higher voltage the said servos will move faster(and try to) that needs more energy to do so. As for lower current... ohm's law would very much disagree  the circuit of the servo has a designed in resistance which is fixed, each component does not change it's properties due to the load. The circuit design will be optimize for a given voltage and so be at it's most efficient at that voltage ie, more voltage _more power available and waisted heat, all to a point of something being over stressed and failing.

 

[Frank Skilbeck]

The resistance maybe fixed but the back EMF from a motor is linked to the torque developed. An electric motor will develop the torque required by the load, yes it will move quicker but for a shorter time. You are correct re ohms law, V=I*R, but power is V*I, so for a given power requirement a lower voltage will need more current not less. 

 

When specifying industrial, or electric flight electric motors we increase the voltage as the power requirement increases to keep the amps within reasonable limits.

[Nigel R]

16 hours ago, Cuban8 said:

check my 5 cell packs for capacity regularly with my charger on NiMh discharge setting

 

I do this too.

 

Even cheap battery chargers do a fine job of running a discharge/charge cycle, so you can assess the condition regularly.

[Cuban8]

17 hours ago, flight1 said:

I got to say i have in the past blamed a bad battery pack for low voltage drop when is use and this turned out to be a ageing switch the cause. so now regularly check the switch and routinely change them.

As for going 5 cell there is no need to with a good 4 cell pack and the down side with 5 cells it gives you higher voltage which leads to less duration for the same mA/hr capacity.

I honestly don't see the downside when the addional redundancy of the extra cell gives so much in the way of voltage headroom in the event of a weak or failed cell - and make no mistake individual cells, no matter what make or brand, do go weak and sometimes fail!  Fast charge or trickle charge, it make no difference. I challenge everyone whether four or five cells are your thing to do a discharge check on your RX packs - if I was a betting man I'd lay a tenner that several weak packs will turn up caused by a duff cell, especially those old packs with a decade of use behind them as "they've been fine up to now".  As for the "good four cell pack" that's ok when new, but the problem crops up without any warning after a few years of use and you won't have a clue as to the risk the model will be running if the fall in capacity isn't checked for.

A bit of a hobbyhorse of mine I'm afraid, but I've  witnessed so many perfectly good models wrecked because of lack of care with batteries. Anyway, whatever suits the individual of course.