Lipos

[Jeff-C]

I have a query about MINIMUM voltage of a cell.

I know that it is 3v before damage, but is that 3volts loaded or unloaded.

 

Here is my example.

 

1000mah battery operating at 20C.

 

1 amp per hour x 20C = 20 amps max

1 amp per hour / 20amp output = 3 mins

 

So in theory, if I am running my battery pack at full power, for 3 mins at 20 amps, I should have exhausted the pack. Obviously I don't want to do that.

I have been running my pack in my parkjet, and which pulls 15amps at full power. So I guessed that if i fly generally, I can achieve 5 mins happily with no damage to the cells.

 

After 6 mins mixed flying, but never over 3/4 throttle, I landed the plane and had 4v left in all cells, (having charged them to 4.2v full).

My point is, when I checked the voltage at that point under load, the pack came down to 3.5v per cell.

 

Am I working things out wrong, or am i just flying my parkjet like a girl and not using as much juice as I think I am??

 

 

 

[Kozmyk]

Posted by Jeffrey Croot on 06/03/2011 19:35:31:

I have a query about MINIMUM voltage of a cell.

I know that it is 3v before damage, but is that 3volts loaded or unloaded.

 

 

That'll be 3V loaded. There's still a margin of safety there but there's no point in going further as the voltage is already dropping quickly by the time you get that far.

Trying to calculate your battery discharge is only going to give you a rough idea.

Fly with a full pack for a set time, say 3 mins to start with, then see how many mAh your charger puts back in.

Then you can adjust your flight times until your taking 80% capacity out per flight and you'll know you're safe within limits.

[Jeff-C]

Thank you Kozmyk,

 

I think I put this in the wrong thread, but I can bring it back!!

So basically, I am looking to be replacing 800ma into a 1000mah pack. That should be pulling the pack down to around 3v per cell whilst under load.

 

Do you mind me asking where abouts you gained this information. I hate not knowing things and would love to read up about it all!!

[Kozmyk]

Although you CAN go down to 3V without damage you should aim no lower than 3.3V if you're going to stay within the 80% rule. I.e. not using more than 80% of a pack's capacity.

Stopping at 3.3V will keep you away from the steep part of the discharge curve.

 

Apart from the timing method there are are low voltage alarm devices which are low costa solutions and then there's telemetry which will transmit back to you the state of your battery and other data.

 

Where did I learn this stuff?

From reading a LOT of posts on various forums and putting some of it into practise.

I also have some electronics experience which made things easier to pick up and understand.

The Battery University is a good source on battery technology. Beyond that you just have to surf for it and get involved with forum discussions ...

[nigel sheffield]

had my loongmax tipples 800mah 2 cell a year now, and although they still deliver enough punch to fly my glider and helicopter almost as well as they did when they were new, the capacity has dropped significantly, I am now putting back around 260mah fast charge from 3.8v start.

just dropped one down to 3.70v and charged it took 370mah.

I think this is ok for price of battery and to still be flying ok on them meens they are still useful, however I really like a123 batts, had them over a year and really abused them and still putting in 800mah in, these started at 850 from new!

[nigel sheffield]

update to above, thought someone might find it interesting, my tipples are now only just flying my heli for around 3 to 4 mins and are very weak on power only just enough to fly slow scale, and are definatly dying, so 2 years lifespan then, not bad for a cheap lipo.

My a123 lifpo4 1100mah packs I made from £1.50 cells from ebay are still putting out 800mah on the abused packs for rc car which get cycled hard right down to empty and the packs I use for flying which get a slightly easier life still have 850mah and are still as punchy as a new pack.

[Garry Pollard]

Hi Nigel

I use mainly 4s 5000mah lipos flying twice a week throughout the year, and my oldest are coming up 4 yrs old, and they are from the big fish, so not the highest priced ones, but still give me a good 8 mins flight on full bore, and longer if throtle is managed senceably

 

Garry

Edited By Garry Pollard on 17/12/2011 11:44:10