The charging process

Charger user settings
Most modern chargers have a "settings" or "options" menu in them that will allow you to adjust various global and battery specific settings. Many have dozens of options and the majority of these settings will be just fine from the factory, but there are a few that might interest you.

More times than not the default beep settings are really annoying. I am mostly referring to the completion beeps the charger emits when the charge is finished. Either the beeps continue forever or they are very loud. You may be able to adjust these settings to be more pleasant.

Lithium battery balance mode/speed
Some chargers have the ability to change the balance modes or speeds in order to better match your needs. This includes being able to select when balancing happens and how long it will take. The usual options for balancing are always or CV phase. CV stands for Constant Voltage and is the last part of the charge when the charge current is dropping. A second option may exist where you can specify how low the charger will allow the charge current to drop before completing the charge cycle. Some chargers give the option for a lesser percentage of the capacity to be charged. For example you could set the charger to complete the charge at 95% capacity. Other chargers offer different termination current options. For example you could select (initial charge current)/5 for faster charging or (initial charge current)/40 for more accurate charging.

Capacity Cut-Off
Some chargers have an adjustable capacity cut-off. This is a safety feature that will halt the charger if it is exceeded. That is fine for small packs and maybe you might even want to match it to your packs, but for large packs or parallel charging, it can be a nuisance. So make sure it is set to a higher value than the largest pack(s) you plan to charge.

Power supply current/voltage limit
Powerful chargers (200W+) often have an option for limiting the output when using smaller power supplies. They also may have a voltage cut-off that can be handy for saving you from running your car battery down among other things.

Setting up the charger to charge a lipo
I hope to cover some of the common settings for charging lipos on most modern chargers. Since there are several styles of chargers out there, these will be fairly general. Don't be afraid to read through your manual for specific explanations concerning your charger.

Most chargers these days can charge all types of lithium currently available and that includes Lipo (lithium polymer), LiFe (A123) and Lilo (lithium-ion). Choose LiPo.

Cell count
This is where you tell the charger how many series cells are in the lipo pack you plan to charge. Some chargers do this automatically and others need to be set. On the ones that need set, some will ask for the pack's nominal voltage, like 7.4V, 11.1V or 14.8V, and others will ask for the "s" count, like 2s, 3s or 4s. This info should be located on the pack's label.

Charge rate
This is where you tell the charger how fast to charge the lipo. There are 2 popular ways chargers handle this. First, and less popular, is to ask you the size of the pack and then the charger will do the math for you and set the rate accordingly. Some of these chargers will give you additional options for faster charging and others will not. Most chargers will simply ask you for the charge rate in amps. For this setting you need to know what "C" is for the pack you are charging (see The variable C and how it applies) and know how fast you would like to charge it.

Other options
Most chargers have many more options than simply the number of cells and charge rate. These can include options to control the various stages of charging, to specify when and how the charger balances the lipo and for modifying the final pack voltage. Read through your manual for explanations of how these options change the charge cycle.

Charging a lipo
This is fairly straight forward but just in case here is the process.
  1. Power on the charger
  2. Adjust the charger's settings for the pack you plan to charge
  3. Make sure the main leads adapter and balance adapter (if needed) are connected to the charger
  4. Connect the main leads of the lipo to the charger
  5. Connect the balance lead(s) of the lipo to the charger
  6. Double check the settings and connections to make sure they are all correct
  7. Start the charge cycle
  8. Critically monitor the charger and lipo for the first 30sec to make sure all is working properly
Note: It is a very good idea to go down a mental checklist every time you charge a lipo. Here is a list of questions to ask yourself:
  • Right battery type?
  • Right cell count?
  • Right charge rate?
  • Main leads connected correctly?
  • Balance leads connected correctly?
  • Charger, battery and all cables laying properly?
  • Anything sharp, conductive, flammable, etc. nearby?

The charge process (as seen as a spectator)
When you press the start button and the charger starts its job, what can you expect over the next hour or so? Well let me give you the basics of the average charge cycle (assuming you are using a smart charger at a 1C charge rate, taking approx. 1 hour).

The ramp up
Many chargers do not immediately jump to the specified charge rate when you start the charge cycle. Instead they ramp up slowly. Some chargers will do this over a minute and others will do it over just a few seconds. Assuming the charger can output the specified charge rate, it should settle pretty closely to this chosen rate and hold steady.

The CC (constant current) phase
Once the charger has settled at the specified amperage it will hold this amperage for the entire phase, approx 45min for a 1C charge. If the charger is set to always balance then it will be working to balance all the cells during this phase. If you look at the individual cell info you should see the cells start off around 3.7-3.8V each (assuming an 80% discharged pack) and slowly raise until one cell, or all cells depending on the settings, is at 4.2V.

Note: The CC phase puts the most strain on your equipment, so this is the time to monitor how everything is handling their respective task. Many chargers will display the input voltage and charger temp, so don't be afraid to check these numbers to make sure everything is as expected. Also touch the power supply, charger, leads and lipo to make sure everything is staying cool. Don't be too worried about things being lukewarm to the touch, just make sure nothing is "too warm".

The CV (constant voltage) phase
Once the cell(s) get to 4.2V each during the CC phase, the charger will switch to the CV phase and hold the voltage at 4.2V for the remainder of the charge cycle. If the charger is set to CV Balance Only, then it will begin to balance during this phase and bring all the cells up to 4.2V. If you watch the amperage during this phase it should start off at the specified amperage and slowly drop off. The charger will stop the charge when 2 things are realized. The first is that all the cells are balanced and second is that the charge amperage has dropped to a predetermined value. This predetermined value is decided by both the charger and the user settings, and can be anywhere from .1A to 1A.

The beeping phase :)
Depending on your charger and user settings, the next phase will very likely be an annoying beeping with the screen displaying charge completed. This marks the completion of the charge cycle.

Once finished
Don't immediately unplug the pack or press any buttons after the charge cycle has completed. Instead inspect the charger to learn a few handy pieces of information. This info can include total charge time, mAh replaced and even the pack and cell internal resistances (on some chargers).

Battery/flight log
I recommend that all pilots keep a battery/flight log. This allows you to keep track of battery cycles, battery health, aircraft info, setup changes, etc. It also is a good way to keep track of which packs are charged and discharged. This may seem tedious but if you work it into your routine, it is less painful and the info it slowly collects is often very useful later on. (see Keeping a battery log for more info)