Data Complete Guide to Lithium Polymer Batteries and LiPo Failure Reports

[roosevelt]

Z-matrix, Thanks! So far the lowest no load voltage has been 3.5v per cell. I'm using the soft cut on a cc35 esc w' auto cell detect @ 3.0v per cell. It never actually cuts just decreases power. The other day I put almost 4400mah back into my 2s2p4000 pack/'s. The no load voltage was over 7.0v.

My 3s2p4000 pack/'s only uses 3200 before lvc and no load voltage is usually 11.2 or 11.3v. Is it getting worn out? Is there a % of usable mah point that deems these no longer safe or otherwise fit for use?

[z-matrix]

roosevelt, no problem,

others might find some of my thoughts useful too,

yes, might be a little worn out, as cycles go by the internal resistance of the li cells will increase a bit, so after an LVC cutoff, the no-load voltage will be higher.
There will be a point when you start your plane and it instantly reaches low voltage cutoff because of the increased internal resistance, i have already tried to use such a li pack, at charging, it reached 4.20V CV (constant voltage) stage in a minute so almost 0 charge was stored inside during the CC (constant current) stage.
You should decide when do you consider your li pack "worn out"
when your cell reaches 70% capacity, or 50% capacity for example.

Though i whouldn't recommend you to use a pack with 50% capacity.

[roosevelt]

Can u check the resistance w' a voltmeter? How?

[z-matrix]

Uk Rt => It
1 4.20V N/A 0
2 3.85V 10R 385mA
3 3.30V 3R3 1000mA

(U = internal voltage)
(Uk = voltage across load resistor, battery terminals)
(Rb= internal resistance of the li pack)
(Rt = load resistor in ohms for measurement)
(It = load current for measuring internal resistance)

Ohm's rule:
I = U / R

here's an example, and the formula to compute internal resistance, case (A)
only requires measurement of the resing voltage at no load conditions, and the loaded voltage across battery terminals using a load resistor.
Then compute the measuring currents using Ohm's law.
This might give a meaningful result for internal resistance in light load conditions.

Since the internal resistancce depends on load current, ambient temperature, and charge state, the measurement should take a little amount of time.
The following differential internal resistance measurement example (B) assumes that the battery internal voltage stays the same during the measurement, this might cause error in case the measurement takes too much time, more than 5-10 seconds.
Differential measurement requires that 2 different resistors be connected to the pack let's say with 1:3 to 1:10 ratio, higher currents are needed for measuring a li pack with a few milli ohms internal resistance, lower currents for higher internal resistance ones.

But the measuring time shouldn't be too small because there is also an internal capacitance of the li cell, that acts like a capacitor, so, for a short time, the voltage will decrease fast, then slower.

Btw, i remeber i saw a project on this forum measuring battery internal resistances with a microcontroller i dont have the link now, if someone knows where it is could post it here i think...

(A)
no load internal resistance(NLiR, RbnL, Rb):
2:
Rb=(Uk1-Uk2)/It2
Rb=(Uk1-Uk2)/(Uk2/Rt2)
Rb=(4.20V-3.85V)/(3.85V/10R)
Rb=0R9090909090909090909...

(B)
differential internal resistance (DiR, Rb):
2-3:
U=Uk+Ub
Uk=It*Rt
Ub=It*Rb
It=Uk/Rt
=>
U=It2*Rt2+It2*Rb
U=It3*Rt3+It3*Rb
It2*Rt2+It2*Rb=It3*Rt3+It3*Rb / -It2*Rb
It2*Rt2=It3*Rt3+(It3-It2)*Rb / -It3*Rt3
It2*Rt2-It3*Rt3=(It3-It2)*Rb / /(It3-It2)
(It2*Rt2-It3*Rt3)/(It3-It2)=Rb /

(0.385A*10R-1A*3R3)/(1A-0.385A)=0R8943

so finally, the differential internal resistance can be calculated using the following formula:
((Uk2/Rt2)*Rt2-(Uk3/Rt3)*Rt3)/((Uk3/Rt3)-(Uk2/Rt2))=Rb

[roosevelt]

Wow! that is way over my head. But thanks anyway I'll keep it for the future when I may be able to understand it. What is a load resistor?

[z-matrix]

Originally Posted by roosevelt

Wow! that is way over my head. But thanks anyway I'll keep it for the future when I may be able to understand it. What is a load resistor?

http://www.wjoe.com/resistors.htm


there are photos too, 5w ceramic should be good in most cases,
but P=I*I*R , and P=(U*U)/R
[P]=Watt

[fonixz]

Question: during charging today, I shorted my 2100mah 3-cell align battery for probably 1-2 seconds... I put it outside on the concrete for an hour to make sure it wouldn't catch fire. I'm assuming this battery is no longer good... correct? It's an expensive mistake to make, but I don't want my heli going up in flames either. The three cells measured 3.773, 3.770, and 3.772, which is good, right? :P

[z-matrix]

Originally Posted by fonixz

Question: during charging today, I shorted my 2100mah 3-cell align battery for probably 1-2 seconds... I put it outside on the concrete for an hour to make sure it wouldn't catch fire. I'm assuming this battery is no longer good... correct? It's an expensive mistake to make, but I don't want my heli going up in flames either. The three cells measured 3.773, 3.770, and 3.772, which is good, right? :P

try measuring the internal resistance of the cells, for a 20c 3s that should be around 3x15mohm charged to 4.1V/cell
if you can, measure the capacity

[bnrusso]

Originally Posted by ***Reno***

Are all balancing plugs wired the same?

ABSOLUTELY NOT!!!

I have noticed that my battery from my eflite blade cp pro has reverse polarity on the pack balancer AND on the eflite charger as compared to the "common sense RC" pack that I got from an online hobby shop.

This is a problem as it also means that chargers that charge each brand of battery from the balance port can also be reverse polarity.

This does NOT affect discharging the high cell in the pack by using a resistive load like a 12V auto bulb, with appropriate connector and meter. Again caution and care must be taken anytime you are contacting areas that are only a few thousands of an inch from creating a direct short.

There are 3 solutions to this problem.
#1 code your battery packs and chargers by some physical means to be sure that you always balance port charge with the correct balance port charger.
#2 you can re-sequence the balance port connections to put them in the same order for all your balance ports. CAUTION this is a critical operation that requires removal of each terminal from the balance port (THIS CAN CAUSE A DIRECT CELL SHORT IF YOU ALLOW ANY OF THE TERMINALS TO TOUCH EACH OTHER heat and fire can result) and putting then in the opposite order to match your charger so all your packs are the same.
#3 Charge via the output port with the correct cell count and charger setting. This way you only use the balance port for resistive discharging where polarity does not matter.

[kf2qd]

I lost a plane a week ago saturday and when I got it back last wednesday the battery was at 1 volt. this is a 1320mAh 2 cell pack from Thunder Power.

Thursday I hooked it up for a couple hours to a 150mAh charger I have. Did the same Friday. Only brought it up to 2 Volts.

Saturday I was bored so I held the cables to a 12 Volt power supply that I have and holding the connectors to the power supply for a feww seconds at first and for several minutes before I was done I brought the pack up to 6.0volts. (was only able to charge at a rate of 0.75 amps)

Hadn't been able to hook it up to my LiPo charger because the voltage was to low. Yesterday (sunday) I hooked it up to my charger and charged it fully to 8.2 volts. Was still holding this morning and also when I got home from work. Hooked my motor to the bench and ran it for 15+ minutes with some warming of the battery at full speed for 5 minutes. (motor is rates 10A full load 10A Peak) and the battery is now at 7.5 Volts.

Looks like some sanity with LiPo's is in order. I will be careful with this one but it has showed no signs of any problem so far. I also accidentally shorted it for a couple seconds (connectors got hot fast...) but made sure the battery never heated up. It would seem to me that the horro stories that everyone has heard are mostly just stories. It looks to me like using reasonable care and a good dose of common sense will prevent most of the problems with these batteries.

Also - this pack has been through about 10 discharge/charge cycles and after this overdischarge and recharge it is still in balance without a balance charger.

Pete

[Larry3215]

If you play russian roulette and dont blow your head off the first time or two around, does that make it a safe game to play?

You have been very lucky so far- twice in fact.

Good luck in the future too, but I do not recomend counting on it

Larry

[willnot]

Ei Guys, need your help my Dualsky lipo XPower 11.1 V 2250mAH 3S registers only 8.4 V and has the following reading

Red 8.4v
Blue 1.5v
Yellow 4.1v
Black 0v

does this mean my lipo pack is dead?? is der a way i could revive it??

[Larry3215]

Do NOT charge that pack.

Cell 1 is at 1.5 volts = way too low and its been damaged
Cell 2 is at 2.6 volts = too low and its probably damaged as well
cell 3 is at 4.3 volts = too HI and there is a very hi risk of igniting if you charge the pack.

Do you have a Hyperion or TP balancer you can use on the pack or a charger that can charge each cell seperately?

There is a slim chance the pack may be usable IF you can charge the low cell very slowly up to 3.7 volts each. You MUST also DIS-charge the hi cell down to a safe level.

Is the pack puffed at all?

Be very carefull with this pack or you could have a fire.

Larry

[aeajr]

Originally Posted by Larry3215

Do NOT charge that pack.

Cell 1 is at 1.5 volts = way too low and its been damaged
Cell 2 is at 2.6 volts = too low and its probably damaged as well
cell 3 is at 4.3 volts = too HI and there is a very hi risk of igniting if you charge the pack.

Is the pack puffed at all?

Be very carefull with this pack or you could have a fire.

Larry

I agree with Larry

Now, assuming you can get the high cell down safely to 3.7V, that cell could be fine.

The cell at 2.6v might be OK and could be recovered if you bring it up slowly.

The 1.5V pack is the real worry.

I don't know how talented you are at such things but you might want to consider taking this pack apart. You have 3 cells in such differnet states that one could go bad and take the others with it.

That low cell is so low it is likely lost. Better to have a good 2 cell pack than a bad 3 cell pack.

Others may see it differently. Just don't short anything!

[willnot]

hi im using propeak quattro chargers and not sure if i could use it to charge the cells 1 by 1, i think the pack is a bit bloated, ill have to try to salvage the working cell an just keep it as a backup coz i still have one pack of lipo btw, i just used the balancer plug to get those readings. thanks guys