Lipo in parallel on S800

[aerovantage]

I have had 10 flights with my s800 running 2x6000 Turnigy Nanotech running reasonable flight time of 10-15 mins every time.

Today after my 10th flight I noticed 1 Lipo has 49% left but the other 11%!. Now that is worrying. I am sure that shouldnt be the case? I wonder what could have happened if I continued flying as I have not gotten any low battery indication!

Usually they ended-up within 5-10% of each other.

Any ideas why and how to overcome this problem? The are both new, purchased at the same time and have been used as pair all the while.​

 

[nigelbrogan]

You should be leaving more than 5 or 10% in your lipos.

 

[ChrisViperM]

You should be leaving more than 5 or 10% in your lipos.

If you charger supports this, try to cycle-charge both LiPos (charge-discharge-charge). Then fly for about 5 minutes and hook them to your charger again and see if the ammount charged to each LiPo is a about the same. If not, discharge the faulty LiPo and get a new one.

Other method is to measure the internal resistance of the LiPos:

Chris

 

[aerovantage]

You should be leaving more than 5 or 10% in your lipos.

I meant the pair always end up within 5-10% of each other after each flight. 32% and 37%, 41% and 51% etc.

My other pairs are even better. Usually less than 5%.

 

[aerovantage]

Thanks Chris. I think im better off using using the 2 Lipos in single configuration now instead of in pairs. Not worth taking the risk.

If you charger supports this, try to cycle-charge both LiPos (charge-discharge-charge). Then fly for about 5 minutes and hook them to your charger again and see if the ammount charged to each LiPo is a about the same. If not, discharge the faulty LiPo and get a new one.

Other method is to measure the internal resistance of the LiPos:

Chris

 

[ChrisViperM]

If you fly them in pairs and one LiPo dies while you are flying, you still have the other LiPo supplying power....if you fly them single and the bad one dies, you have a brick falling from the sky. So your risk is becoming bigger, not smaller..... I assume you pair them parallel (Voltage stays the same, Amp's are double) and not seriel (2x 3S = 6S)...?


Chris

 

[PeterLester]

Technically the inside of each battery pack has several cells in series...... So externalizing that concept just adds one more link to the chain in terms of relative risk, i.e., two battery packs in series or parallel. Also there is a hidden risk when battery packs are in parallel....... If one battery pack develops an internal short, then the MR drops...... take your pick!! One low battery can also cause major problems....as the other tries to simultaneously run your MR and charge the other battery at the same time.

Regarding internal resistance of a battery when your charger lacks the internal resistance feature:
The internal resistance of the battery actually froms a resistive voltage divider with the external resistor you load it with.
This is what causes the difference in voltage between open circuit voltage and loaded voltage. T
he standard equation for this voltage drop is:

Vdrop = Vbatt*(Rbatt/(RL+Rbatt))
Where:
Vdrop is the difference in your two readings above
Vbatt is the open circuit voltage measured above
RL is the resistor you load the battery with
Rbatt is the battery internal resistance you're looking for

The reconfigured equation for our needs is:
Rbatt = Vdrop*(RL/(Vdrop+Vbatt))
Just plug the numbers you measured above into this equation and you'll have your answer.

Use a high wattage resistor and keep open the external resistance circuit asap once you have your current reading....
Also when measuring current with a multi-meter.... be careful..... it is easy to leave the Multi-meter in voltage mode which will burn out the internal Multimeter fuse.... or burn up your multi-meter if it has no fuse.

 

[aerovantage]

Hmm good point Chris. Thanks. Maybe I should just use the bad on for other zero risk duties like powering my monitors.

If you fly them in pairs and one LiPo dies while you are flying, you still have the other LiPo supplying power....if you fly them single and the bad one dies, you have a brick falling from the sky. So your risk is becoming bigger, not smaller..... I assume you pair them parallel (Voltage stays the same, Amp's are double) and not seriel (2x 3S = 6S)...?


Chris

 

[aerovantage]

Excellent info Peter. This will take some time for me to absorb!

Cheers mate.

Technically the inside of each battery pack has several cells in series...... So externalizing that concept just adds one more link to the chain in terms of relative risk, i.e., two battery packs in series or parallel. Also there is a hidden risk when battery packs are in parallel....... If one battery pack develops an internal short, then the MR drops...... take your pick!! One low battery can also cause major problems....as it tries to simultaneously run your MR and charge the other battery at the same time.

Regarding internal resistance of a battery when your charger lacks the internal resistance feature:
The internal resistance of the battery actually froms a resistive voltage divider with the external resistor you load it with.
This is what causes the difference in voltage between open circuit voltage and loaded voltage. T
he standard equation for this voltage drop is:

Vdrop = Vbatt*(Rbatt/(RL+Rbatt))
Where:
Vdrop is the difference in your two readings above
Vbatt is the open circuit voltage measured above
RL is the resistor you load the battery with
Rbatt is the battery internal resistance you're looking for

The reconfigured equation for our needs is:
Rbatt = Vdrop*(RL/(Vdrop+Vbatt))
Just plug the numbers you measured above into this equation and you'll have your answer.

Use a high wattage resistor and keep open the external resistance circuit asap once you have your current reading....
Also when measuring current with a multi-meter.... be careful..... it is easy to leave the Multi-meter in voltage mode which will burn out the internal Multimeter fuse.... or burn up your multi-meter if it has no fuse.