The 'C' Rating & Time to Charge | Back to Article Index
By Stan Yeo
The maximum recommended continuous discharge current of a Lithium Polymer battery is the Battery Capacity (mAhr) x the 'C' Rating, i.e. a 1000mAhr pack with a 25C rating would have a maximum continuous discharge rating of 1000 x 25 = 25,000ma (25Amps). The 'C' rating is a reflection of the internal resistance of the battery and its ability to satisfy high current demands. Unfortunately not all batteries live up to the 'C' rating on the label. An indication of this is how hot the battery gets at high currents loads but within the maximum current recommended on the label. Remember the harder (hotter) the battery works the less life cycles! The 'C' rating also has an impact on how long it takes to charge the battery particularly with auto detect chargers. There are four main factors affecting the charge time other than the residual charge in the battery.
1. Internal Resistance
Just as the internal resistance of a cell determines it's current delivery capability it also affects it's charge performance in that a higher voltage will be required to deliver the same charge current (Ohms Law). Where this becomes apparent is when using chargers that automatically detect the battery capacity and number of cells. The smaller the physical size of a battery the higher the internal resistance is likely to be so a 'large' capacity battery with a low 'C' rating can mimic a smaller capacity battery with a high 'C' rating. This can result in the 'large' battery taking longer to charge and the smaller battery being charged at too higher rate. Even in cases where the batteries are of the same capacity but different 'C' ratings there is often a noticeable difference in the charge times and power output (lower propellor / rotor head RPM). Customers sometimes comment on this when discussing the performance of different battery brands.
2. Cell Imbalance
If the individual cells are out of balance with each other this will effect the charge time due to cell balancing bleeding off charge from cells in a higher charge state. If the imbalance is too great then the pack may have to be partially charged independent of the balancer, cell balanced and then balanced charged. If this does not work then try charging each cell individually.
3. Charger Output Limitations
All chargers have limits on the output current (amps), output voltage (number of cells to be charged) and output power (watts). Operating at any of these limits can affect the time it takes to charge the 'pack'. As an example a 3 cell 4000mAhr pack charging at 4 Amps would require at least 48 watts (12v x 4A = 48W) of power. A similar 6 cell pack would require double this i.e. 96W (2 x 48W) but if the charger could only deliver 72W then the charge current would only be 3A (72w/24v) extending the charge time by 25%. Unfortunately high output chargers are quite expensive. A less costly alternative would be to use 2 x 3 cell packs in series and have two less expensive chargers. The 2 packs in series option also has other benefits i.e. if a single cell goes down then only 3 cells are scrapped instead of 6 and a single 3 cell pack can be used in other applications such as a small helicopter or small fixed wing model.
4. Charger Set-up
The recommended charge current for most Lithium Polymer batteries is 1C i.e. the capacity of the pack in Amps (3200mA = 3.2A). Obviously selecting a lower charge current will result in a longer charge time. Setting a higher charge current will charge the battery quicker but does run the risk of permanently damaging the pack. Our advice is stick to 1C.
I hope you have found this article useful along with the other related articles on this site (Simple Electrics and Causes of Lipo Battery Failure). I try to present technical information in an easily digestible form so sometimes a little licence is used in the terminology and 'maths' but it is the concepts that are important rather than precise detail. If you can understand the concepts then it is easy to transfer the knowledge from set of circumstances to another.
One final point, I have just checked the LiPo batteries I am going flying with today. Both batteries were fully charged after the last flying session as per recommendations. One would not accept a charge because it was still fully charged whilst the other needed topping up. LiPos are not supposed to self discharge but they do, albeit no where near as rapidly as Nicads or /NimHs so our advice is that if the batteries have not been used for a number of weeks give them a top-up charge to avoid them discharging below the dreaded three volts per cell.