Why is my charger so slow?

The industry offers plenty of chargers, but not all of them do the same thing the same way. Article and photos by Greg Gimlick. Featured in the Fall 2011 issue of Park Pilot.

Questions about chargers and charging make up the majority of the email I get, so I knew it was time to answer the most frequently asked questions for you right here. The biggies are: 1. Why does charging at the normal 1C rate take longer than one hour? 2. Why doesn’t my 3C rate charger get my packs charged in 20 minutes? 3. What is parallel charging? 4. My high rate charger isn’t charging at its full capacity, even though I set the values correctly. 5. I have two chargers of the same brand, but one takes longer to charge than the other. For those of you who may not know, “C” represents the capacity of a battery. If you have a 3000mAh LiPo pack and it’s rated at 20C, it means it can be discharged at 20 times the capacity of the pack: 60 amps (3000mAh = 3 amps, so 20 X 3 = 60 amps). Since LiPo packs have gone beyond the standard 1C rating, many now come with charge ratings. Look for both charge and discharge ratings before you purchase or use a pack. The first two questions share the same answer. Theoretically, a 1C rate should take one hour and a 3C rate should do it in one-third of the time, or 20 minutes. The problem comes when theory runs into reality, and when that happens, performance is all up to the programmer/designer of the charger. Most LiPo chargers are of the CC-CV (Constant Current-Constant Voltage) type. When charging, they limit the current to some preset until the battery voltage reaches a preset voltage. Then the current is reduced to keep the voltage from going over that preset. The charge is considered complete when the current has been reduced to a threshold. In the case of a 3S pack the voltage will most likely be close to 12.6 volts. The current is limited by what you tell the charger to do.

Left: This TME Xtrema is set up to parallel charge and balance two large packs. Right: Progressive RC offers parallel boards available to expand your charging capabilities.

Connected to a Triton2 EQ, this parallel board by Progressive RC is charging and balancing three packs.

The charge rate is set to 2.4 amps because there are three 800mAh packs being parallel charged.

The charge termination current threshold is the unknown, and is set by the engineer who designed the charger. The variables are the actual charge current, the actual constant voltage setting, and the threshold current. It is fairly easy to measure these values with an accurate DVM. With a depleted pack, measure the charge current. This current is where the charger will spend roughly 80 percent of the charge cycle if the pack was completely discharged. At approximately 80-90-percent state of charge, the pack voltage will reach 4.2 volts per cell and the current will taper off to prevent an over-voltage condition that will damage the pack. You can measure this point by watching the pack voltage with a DVM, and when the voltage stops rising during the charge is what the charger thinks is that preset voltage. Threshold current is the most difficult to measure because you would have to be looking at the meter just before it signals charge complete. An instrument that logs would be beneficial for this. All other things being equal, the constant-current charge value will control how long it takes to get to approximately 80-90-percent state of charge value. If you have a 2Ah cell and you charge it at 2 amps, you will reach that 80-percent charged value in 48 minutes. If your cells can accept a 2C charge rate, it will take 24 minutes. A 3C rate would take 16 minutes. The last 20 percent of the charge will take the same amount of time on any constant-voltage charger because the current is reduced according to what will prevent the battery from self-destructing. It doesn’t matter what you set the current limit to once the voltage reaches 4.2 volts per cell. The big variable once you reach the constant-voltage portion of the charge is the low-current, end-of-charge point. A charger with a lower cutoff threshold will take longer to decide the pack is charged. It will put a little more charge in the pack than one with a higher threshold, and this could be minutes to hours of difference. Say you set the threshold to zero amps; you will never get an end-of-charge indication. The closer you get to fully charged, the lower the current, and with an infinite amount of time you will get there, barely. It’s all up to the charger’s designer to determine how good is good enough.

Zeus LiPo packs show charge and discharge rates on the label. Although it’s capable of 5C charge rates, the label suggests that 2C or less is best.

The CellPro 10S charger is set for a 2C charge on the charger. It determines what that is by reading the cells and evaluating the data.

Left: The TME Xtrema can service up to four parallel charged packs — and balance them, too. Right: Check charge limits, which are sometimes printed on a warning label.

You can charge really fast if you only charge to 80 or 90 percent, but most pilots won’t give up 20 percent of their flight time. Once a charger nears the end of the charge, it slows things down. Even if it isn’t balance charging, the charger tapers off at the cycle’s end. Parallel charging is a method of charging multiple LiPo packs on one charger at the same time. The cell count must be the same, but capacity can differ. My TME Xtrema will charge and balance up to four packs, and my Triton2 EQ will do up to six if I use the Progressive RC board. The answer to the fourth question is often just asking too much of the charger or power supply. Some chargers automatically adjust the charge rate if they sense that the power supply is lagging. If your high-rate charger is set for 3C and charging at a lower rate, it’s most likely simply adapting. Question five? One unit might be earlier-version firmware than the other. Many are upgradable online, so check to see if you can update the earlier version of firmware. It’s always helpful to separate charging, engineering and marketing voodoo.