Lipo cell help!

[Trex3940]

Hi everyone,

Just been flying the Trex 700 Dominator. I have a question regarding the individual cell voltages on my 6s 5000mah 50c lipo. The helicopter is 12s so two lipos is required.

After my first flight today, lipo 1 & 2 both were indicating 14% fuel level using my digital capacity checker. Each cell voltage (1-6) were between 28-29%.

After the second flight, lipo 1 indicated 14% fuel level with cells 1-6 shown as:

1= 14%
2= 14%
3= 28%
4= 14%
5= 14%
6= 28%

lipo 2 indicated 28% ful level with cells 1-6 shown as:

1= 28%
2= 14%
3= 29%
4= 32%
5= 14%
6= 30%



Im a bit concerned as to why the fuel level between the second lipos are at a discrepancy and why the individual voltage percentages are a lot more fluctuated. The lipos used in them second flight are much older and one of the lipo's balance lead connectors burnt out today when connecting into the balance lead charger ports, whether this is a reason or not? All the lipos charged perfectly and evenly.

Can any whizzes help me out in this irregularity in cell voltages and fuel level???

Thanks

[waveydavey]

The only way to really assess whether a cell is weak is to check the IR. Percentage capacities are okay for a rough check but voltage is a better one, have you got a charger that can check the IR.

[Mark_T]

As Dave suggests, you need to check the IR value for each cell rather than trying to interpret the data from your 'Capacity checker'.

In reality your checker is just a volt meter that tries to estimate capacity based on the pack voltage so if the voltage reading from the balance wires is faulty then the capacity estimate will be wrong as well.

If the balance wires subsequently burnt out on one of the packs that is a good sign that you may have been a problem with loose or even short circuited balance wires that may have been giving you the voltage across two cells perhaps instead of just the one it should have been reading which would explain the 28/14 numbers you were seeing.

I would treat both packs with caution until you figure out what is going on.

[Trex3940]

This was the internal resistance measured by my charger (milli-ohms):

Lipo 1 (14% fuel level)

1= 1.7
2= 1.7
3= 1.4
4= 1.6
5= 1.7
6= 1.9

lipo 2 (28% fuel level)

1= 2.4
2= 1.6
3= 1.6
4= 2.3
5= 2.2
6= 1.6

Could any of these figures indicate why I'm having these cell discrepancies and how do I know when a Lipo is coming to the end of its days?

[waveydavey]

No they look okay, obviously 1 pack is different to the other but quite normal. When on charge do they balance within the time you're expecting IE if you're charging at 1C do you get done on around 1 hour. Or does the balancing take longer

[Trex3940]

I'll have a look. What indications would I expect to see from the internal resistance when a pack is nearing the end of its life?

[waveydavey]

They tend to rise so your packs look okay, the highest IR dictates the rate at which it can release and be charged so I would just see how you go.

[Trex3940]

What do you mean release? What milliohm readouts will I see when the lipos are near dead? Will it be up at 4.0 or so

[waveydavey]

What do you mean release? What milliohm readouts will I see when the lipos are near dead? Will it be up at 4.0 or so

The IR of a cell dictates how quickly that cell can release it's current. Consider a tap, turn it on a little and no matter the pressure behind it you'll only fill the sink as much as the tap will allow. Turn it all the way so very little resistance and you'll fill the sink in no time

As for readings that say the pack is passed it, I'm not sure as long as you're not taking it past what it can give you then you're not damaging it so it'll go on for a while. Some on here report over 200 cycles from their packs, I'm sure the IR wold be quite high but if you're not expecting much then they'll carry on. On the other hand I've seen packs 20 hard flights in start to show signs of giving up.

I would just carry on and see what happens

[nkma]

There is a document about relation between internal resistance (Ri) and load for LiPos.

Edward Sadok:
LiPo Batteries:
Internal Resistance, Max. Current Draw, true C-Rating

A rule of thumb recommended in this document is to stay below 6 watts per amp hour per cell,
as the loss in the cell caused by Ri * I**2.

Example:
If you pull 50 amps from a 5 Ah Lipo with Ri = 3 milliohms you have 7,5 watt vs 5*6 = 30 allowed.
100 amps gives 30 watt - just on the limit!

Here is a cheat sheet (hope it's readable...):


Regards,
Bo

[nkma]

Two more things:
– Pay attention to the cell with the highest internal resistance.
– Don't discharge to much. To not get down to 20%, or lower, I try to quit before I get to 25%.

Regards,
Bo

[Mark_T]

Also factor in that most chargers are overly optimistic with the IR values they show so don't push things too far

[Trex3940]

Bo,

sorry can you explain how you derived 7.5 watts using a 5ah Lipo at 50amps draw? What do you mean RI*I**2??

[Grumpy]

Bo,

sorry can you explain how you derived 7.5 watts using a 5ah Lipo at 50amps draw? What do you mean RI*I**2??

Ohms law:

Power = I^2 * R

So using the data for the 5000mAh battery frem the previous table: I= 50A and R = 3 milliohm

Power = 50^2 x 0.003 = 7.5W