Electric Tug Battery Life

[Chris Hurst]

Thoughts would be appreciated.....

Our group have a Wot 4 XL (i.e a lazy man's Ampley!!) tug running on 10S (2 x 5S 5000 25-35C LiPos). It has a clone 230KV motor and we have a throttle curve set so that it draws 57 amps static on freshly charged batteries. Under that load the voltage measures c. 38V so 2166 watts. I assume this will unload slightly in the air so we are towing at around 2kW. There is good flow of cooling air around the batteries and a large air outlet just behind the wing. Flying weight 5.6 kg.

It does the job nicely and we limit each tow to 1 minute at full throttle. Four tows per set of batteries before recharging which typically takes 3600 to 3900 mAh so an average of 900 mAh per tow.

Despite all this care our batteries seem to be ageing quite rapidly. Around 12 to 15 cycles (maybe fifty tows maximum) there is a feeling that the performance is noticeably down.

Should we be disappointed?

Thanks

Chris

[Peter Balcombe]

Chris,
My initial thoughts are that as you are discharging the batteries to only 20-25% capacity between charges, that will tend to reduce their useful life more than say only discharging to nearer 50% between charges.
I know that this would mean having more battery packs for the same number of tows

The biggest imponderable though is likely to be down to the quality of the cells themselves as despite different manufacturer labelled batteries having similar capacities & C ratings, there can be quite a variation in actual performance over time.
I understand that in the battery world, price isn't necessarily a good indicator of quality/performance.

Hopefully, there are others out there with some indications from practical experience with electric tow usage.

[GordonT]

Chris - first question - what make are the batteries and their 'C' rating? Second question - how do you store the batteries when not in use? Fully charged? Part charged? In a fridge or other cool place? Third question - what age are the batteries?
In my extensive experience of all things electric If a battery lasts for 2 years, you're doing well, whether or not the batteries are being used. Some makes do have a very low cycle life, particularly if the batteries are being even slightly punished. When you get time to reply to the questions, I can probably advise you more.

Gordon

[Chris Hurst]

Thanks for the replies.

Batteries are Turnigy 25 to 35 C

Two sets are around 18 months old, third set 12 months.

Kept charged if likely to be used within a couple of weeks (not ideal I know but the practicalities of charging just before use are not easy). Storage charge for winter layoff.

Kept in steel cupboard in unheated garage.

They never seem more than tepid when removed from the model so I think the cooling is about as good as I can make it.

[Simon WS]

The bit that interests me is when you say "we have a throttle curve set" - do you mean that you have limited the throttle end point? If so, that might be part of your problem because the way brushless motors work is that they will draw the max current in bursts based on the kV of the motor and the prop and battery voltage regardless of where you set the throttle end point - the ESC switches this max current on and off to give the desired overall output - hence an ESC is working harder at low throttle because it is switching the current on and off more frequently.
How did you decide on the 10s motor and prop configuration - did you use one of the online programmes or was it recommended? It may be that it is fine and not part of the problem but it is worth running your configuration through an online programme if you haven't already just in case you are closer to the C rating of the batteries than you think.


Simon

[Peter Balcombe]

Simon,
I don't think that the discharge rate should be an issue as taking a static 57A current (see earlier post) from a 5000mAH pack is not pushing the 25-35C rated batteries very hard as this represents roughly a 12C discharge rate.

To be picky, speed controllers do not switch current - they switch voltage.
It is far more efficient to switch voltages on/off very quickly at variable on/off ratios, known as Pulse Width Modulation (PWM) than it is to linearly control the voltage. (PWM greatly reduces waste heat generated in the transistors, enabling far greater currents to be handled without overheating).
I think you will find that the controller operates at a fixed switching rate (may be a selectable rate in higher end units) and the switches connect the full battery voltage for a variable period, determined by the throttle input, at the fixed switching rate. The on/off ratio determines what is effectively the average voltage (variable between 0 & 100%) applied across the load impedance presented by the motor windings. The motor current will vary in proportion to the applied voltage and will average out to what you read on your wattmeter.

I wonder if the batteries were 'run in' at all, as I understand that higher power LiPo batteries need a few initial gentle charge/discharge cycles before they learn what their real capacity is. Maybe, a failure to do this can shorten their effective life.

[Simon WS]

It was my understanding (but it sounds like you may be a lot more expert!) that the 57 amps "peak" is meaningless if the throttle end point has been artifially reduced - but I accept that it would have had to have been reduced a hell of a lot to go over the batteries C rating!
As a matter of interest (to the original poster) what prop size are you using?

[john greenfield]

I think the problem is the batteries.
we gave up using cheap lipo's some time ago for exactly the reason mentioned above. In discussing this issue with others who have far more experience it seems that the cheaper (lower grade) lipo's are not really happy supplying more than about 30 amps and asking for more severely shortens their life.
We have now changed to using Ombo 5200mha packs and since then have done lots of cycles in various models with no sign of falling performance.
I get my Ombo batteries from Steve Spittle (PM him via SSUK) and as they come direct they are far more reasonable price wise that you would expect.

AEB

[Chris Hurst]

The bit that interests me is when you say "we have a throttle curve set" - do you mean that you have limited the throttle end point? If so, that might be part of your problem because the way brushless motors work is that they will draw the max current in bursts based on the kV of the motor and the prop and battery voltage regardless of where you set the throttle end point - the ESC switches this max current on and off to give the desired overall output - hence an ESC is working harder at low throttle because it is switching the current on and off more frequently.
How did you decide on the 10s motor and prop configuration - did you use one of the online programmes or was it recommended? It may be that it is fine and not part of the problem but it is worth running your configuration through an online programme if you haven't already just in case you are closer to the C rating of the batteries than you think.


Simon

Thanks Simon

No, it is actually a throttle curve. I am aware of the end point issues you mention. The 57 amps is the measured current draw at full throttle with the model held static on the ground so will be slightly less in the air.

I am using the generally recommended prop for the 230 kV motor (19 x 10 from memory but I will double check) but without the throttle curve the maximum draw was nearer to 70 amps. Probably fine for occasional bursts in an aerobatic model but I wanted to remove the temptation of towing for a minute or so at that sort of load.

Obviously I could experiment with smaller props and take your point that it may reduce the speed controller's work load.

[Chris Hurst]

I think the problem is the batteries.
we gave up using cheap lipo's some time ago for exactly the reason mentioned above. In discussing this issue with others who have far more experience it seems that the cheaper (lower grade) lipo's are not really happy supplying more than about 30 amps and asking for more severely shortens their life.
We have now changed to using Ombo 5200mha packs and since then have done lots of cycles in various models with no sign of falling performance.
I get my Ombo batteries from Steve Spittle (PM him via SSUK) and as they come direct they are far more reasonable price wise that you would expect.

AEB

Thanks John

Some local feedback has suggested much the same thing. I will investigate.

Unfortunately the length of the batteries is critical for out setup. Any longer at all will need significant structural modifications!