battery capacity and discharge

[andy watson]

I hope someone is up late/early!

I managed to drop my hack into a tree after a deadstick, so decided to replace it with an electric model. This is my first foray into electrics! I am running a 4S system which pulls 55A max. I doubt it will need to be on full throttle very often since I have about 750W for a 2 and a bit kg plane.

Anyway, the batteries are 4S 4000mAh capacity- obviously 16.8V fully charged. By my reckoning if I fly WOT non stop then I will drain 4000mAh in about 4 1/2 minutes. However I also understand that LiPos should not be run to empty.

My questions are:

1) How far should I discharge my packs? I have a watt meter & battery monitor to check cells, but I don't think it tells me capacity- just voltage (not under load).

2) Given I will be using the plane as a hack not prophanging/racing for the full flight, what would be a reasonable time setting to use for my initial flights (presumeably I can check the batteries at the end of the flight and modify the timings based on these results).

Many thanks in advance

Andy

[Biggles' Elder Brother - Moderator]

HI Andy,

yes some of us are up late!

Personally I try not to go below about 20% capacity - that is fairly conservative. But I am rewarded I think by the fact that I have lots of Lipos I have had quite a while and they all still seem to be giving me full performance. So it pays I think to treat them gently.

If you get 4.5 mins at WOT, and given you have about 150W/lb (excuse the mixed units!) which is plenty, you should get about 6-7mins "normal" flying - assuming you don't blast constantly round the sky and manage the energy a bit in between moves!

I'd set my timer for 5.5 mins to start with and then see how much capacity is left after the flight. If its significantly more than 20% then up the timer to 6 mins and so on.

A battery tester is a useful investment - something like this. It will give you a capacity remaining reading by just pluging the battery's balance lead into it. Money well spent I found.

BEB

[Allan Bennett]

I time my flights to aim for 20% remaining capacity, like BEB. Apart from being nice to the battery, it also gives you a bit in reserve in case you need to make a go-around.

Battery voltage should ideally never fall below 3.0v per cell -- some people say 3.2v or 3.3v. The problem is that it may fall to that voltage while amps are being drawn, but will recover to some higher voltage when you stop the motor to measure the voltage. So, with no load being applied, you probably don't want to see any voltage less than about 3.7v per cell. The lvc in your ESC should be set at between 3.0 to 3.2v per cell, for it's checking the voltage under load.

If you don't have a capacity meter, LiPos exhibit quite a constant relationship between voltage and percentage charge, so can be checked with a voltmeter: 4.2v is fully charged, 3.85v is about 50% charged, and 3.7v is about 10% remaining.

[Frank Skilbeck]

Also if your charger tells you how much it's put back in, you can start with short flights like BEB stated, check how much goes back in the battery and then adjust your timer accordingly, but don't cut it too fine, you'll find you might use a bit more power on a windy day.

Using a telemetry current sensor is another alternative, but this would depend on whether your radio supports this.

[andy watson]

Sorry guys- remiss of me not to say thanks earlier!

Turns out the wind stopped me flying. Current plan is to set the timer at 7 minutes and then see what the batteries are like after that. It sounds like they can go lower than I feared.