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.



''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: '' You slipped up a little here. ''C'' does not represent the capacity of a battery. Mah represents the capacity of the battery, which you referred to in the second sentence here when you referred to the ''capacity of the pack'', which is represented by , say, 3000Mah. ''C'' represents the discharge rate of a battery along with the mah, or capacity, of the battery. Mah is the capacity, ''C'' represents the charge or discharge rate in combination with the mah capacity of the battery. Not to make a big issue of it, but I was an electronics instructor in the military.

Thanks from this real elder "newby"

DAY, you called out the author's statement that " 'C' represents the capacity of the battery." saying, " 'C' does not represent the capacity of a battery. Mah represents the capacity of the battery..."

Actually, the author was quite correct; for purposes of determining charge and discharge rate for batteries, “C” is shorthand for capacity. MAH (milliamp hours) is simply our most common unit of measure for expressing capacity, just as ounces per square foot is our most common unit of measure for wing loading.

The author had 1000 words in which to cover a complex subject that's a source of confusion for many electric newcomers, and he needed to do it without overwhelming his audience. In “correcting” authors of online content, it’s a wise policy to make certain they were wrong to begin with.

There are several additional things that can impact charge time beyond what is expected.
Charger measurement accuracy, particularly in the balance portion of the charge.
Under charge, a Lipo will have a slightly higher voltage than at rest. How much higher is determined by the Charger and safe limits. How much does the lipo voltage "sag" after resting? How well are the cells "matched"? Poorly matched cells will take longer to balance.
What is the current value that, with the voltage of 4.2v per cell, determines the end of charge? C/10, C/20. or a fixed current value?

Thanks Greg Gimlick! Great article.

For a bunch more info. on parallel charging, I’d like to point readers to my article called “Parallel Charging Your LiPo Batteries,” here: http://electricrcaircraftguy.blogspot.com/2013/01/parallel-charging-your....

Now, a couple things I’d like to add:

To DAY (the first comment): nice comment; you’re right that C is not the capacity of a LiPo. My preference is to call the C rating the “Capacity *multiplier*” value. It is a capacity multiplier value. Depending how you look at it, it would be most correct in my opinion to give it units of 1/hr (hr^-1), so that when multiplied by Amp-hours the hours cancel and you are left with just Amps.

In regards to this quote in the article: “It doesn’t matter what you set the current limit to once the voltage reaches 4.2 volts per cell.”
-Not *quite* true: It is true that once you hit the constant-voltage (CV) portion of the charge the current you set will not go into effect, since the voltage is what matters. However, the higher the current value you set, the sooner the charger will cease charging. Most chargers I’ve seen use a 1/10 current setting as the end-of-charge determination. So, if you set the charge current to 2A, the charger will stop when the current has dropped to 0.2A, during the CV portion of the charge.

To Charlie (the 3rd comment): I’m glad you brought up that “Under charge, a Lipo will have a slightly higher voltage than at rest.” This is something that’s very confusing for most people, but you are spot on. One other note that can be made is that if prior to charging, all cells were at the same resting voltage level, but during charging, any cell is *higher* than the other cells, this is indicative of a high internal resistance in that cell, and that cell is likely dying and/or more “worn out” than the other cells in the pack. This is counter-intuitive to most, as it at first seems logical to think that the *lower* cells during charging would be the poorer cells. This, however, is erroneous, again assuming that the cells were at the same resting voltage prior to beginning the charge.

In a short space you covered the subject of battery chargers well. Thanks.

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