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Causes of Li Poly Battery Failure

By Stan Yeo

High Discharge Currents
Discharging below the Minimum Cell Voltage

Foreward (Back to top)

Lithium Polymer batteries supplied by us, in common with other retailers / distributors, do not carry the same guarantees normally associated with other electronic products. The cell voltages of all batteries supplied by us are checked by ourselves and our suppliers before dispatch (except when bubble packed). This is to ensure that you receive the batteries in good condition as once the batteries are taken out of their packets and charged / discharged we can no longer accept warrantee claims because we have no control on charging conditions or how the battery is used. In view of this we have written the following as a guide to good practice when using Li Poly batteries.

We strongly recommend that:

1. You check individual cell voltages with a digital multi-meter within 24hrs of receipt of your battery, if possible without removing it from the packing and certainly before fitting the battery connectors. If, in the unlikely event an individual cell is below 3v contact us immediately.

2. You balance charge your battery as soon as possible after cell checking your Li Poly battery and before attempting to use it no matter how short the period. If the difference in cell voltages prevents balance charging as it sometimes can, partially charge the battery without a balancer, then using a balancer independant of the charger balance the cells before completing the charge with the balancer in circuit.

Charging (Back to top)

It is important that Lithium Polymer batteries are charged using a Li Poly compatible charger in conjunction with a suitable Li Poly cell balancer and the charger is correctly set up for the number of cells being charged and their capacity /correct charge current. This is normally 1'C' i.e. the milliAmp hour capacity of the battery. If using a separate cell balancer then we recommend the battery is partially charged, removed from the charger and the cells balanced before completing the charge.

Cell balancing is important because no two cells behave in the same way and during use a variation develops in the charge state of the individual cells. This is a double cause for concern as cells that hold more charge than others run the risk being damaged during charging by being over-charged and whilst the remaining undercharged cell(s) risk being 'killed' by being discharged below their low voltage threshold.

High Discharge Currents (Back to top)

Exceeding the recommended maximum discharge current can irreparably damage the battery as can continually operating at the maximum continuous discharge current. Very high discharge currents can cause the battery to 'balloon' whilst the latter will significantly shorten the life of the battery.

As a gauge to safe currents loads on Li Poly batteries, if the battery gets hot then it is in danger of being overloaded and sustaining damage. Our recommendation is, with adequate ventilation, the battery should only get warm i.e. be operating at 2/3rds to 3/4 of the max. continuous current. If the battery gets hot operating in this range then there is either inadequate ventilation, the battery has been damaged or the 'C' rating is incorrect. There is a correlation between operating current and the number of usable life cycles. The higher the current the fewer the life usable life cycles.

Discharging below the Minimum Cell Voltage (Back to top)

Once a Li Poly battery is discharged below a nominal 3 volts per cell it is unlikely that it can be recharged. This is the most common cause of Li Poly battery failure. There are a number of reasons for this, the most common being the speed controller failing to cut-off at the required voltage. This could be due to incorrect set-up, using the wrong type of speed controller i.e. one that is not Li Poly compatible or failing to recharge the battery between flights. The latter is particularly important if using a speed controller that cuts off at a percentage of the start voltage. If the speed controller cuts off at 75% of the start voltage this would be 12.6v x 0.75 = 9.45v for a fully charged 3 cell pack. If after an initial flight the start voltage on a subsequent flight is 11.4v then the cut-off voltage would be 8.55v i.e. 2.85 volts - below the recommended 3 volts per cell. This could be further compounded if cell voltages are not equal i.e. unbalanced, leading to damage to at least one cell.

Recommendations (Back to top)

We recommend that initial flights are timed and deliberately kept short and a note made of the energy (milliwatts) supplied by the charger to recharge the battery. This will give an indication as to how much of the batteries energy has been used. From this a usable flight time can be judged. We suggest up to 75% of what is on the label. Remember that the higher the discharge current the less that can be taken out due to internal losses and a higher voltage drop under load which means the battery will reach the cut-off / minimum cell voltage sooner.

We strongly recommend a top-up charge between flights and a charger is used that gives a readout of current supplied. We also recommend using an integrated charger / balancer system rather than balancing the cells independently after charging.

Finally, the most common cause of LiPo battery failure is failing to disconnect the battery immediately after landing from the ESC (speed controller) and UBEC (if fitted). If left connected then the speed controller / UBEC will, over a very short period of time, discharge the battery below the critical 3 volts per cell threshhold.

August, 2008

(Back to top)


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