It’s an interesting question and I’ve found hardly anybody who knows or understands the answer. It’s really very simple, though not straightforward. What is your answer? I submit to you that your charger does not display the voltage of your battery. There really is a simple point of logic here. Certainly, nearly everyone reading this will look back on their experience charging batteries and think to themselves that I’ve gone nuts. But, I submit to you that I have not, and if you’ll participate in a little test you will soon be a “crazy” like me.
Let’s gather the parts of our test. We need a charger with a digital readout that accepts banana ended charge cords. To make this test simpler, let’s use a Li-poly or “A123” pack. I want you to use one of these kinds of battery packs because it has multiple connections to the cells inside and will make the test easier than if we use a NiCad or NiMH pack. We will also need a digital voltmeter with probes.
We can do this test under charge or discharge, you decide. I want you to apply a 2-amp charge or discharge load to the pack. If it’s full, you’ll be discharging, if it’s empty, you’ll be charging. Charging or discharging makes no difference. The results will be madness, I promise.
First, let’s take a resting no-loaded volt reading of the pack. Measure the voltage out of the output wires first(what you’d be connecting to your speed control) . Jot down this number. Now, measure it from the red and black (or outer most 2 wires on the balance plug). Jot this number down. These two numbers should be identical.
Next, plug a charge cord into your charger and then into the output plug of the pack. If your charger is on, you should be able to read out the voltage on its screen and it should agree very closely with your digital voltmeter measurement. Jot this number down. There may be a slight difference here and we can explain that because there may be some calibration difference between your meter and the charger. Also, most chargers do not read out to 1/1000th of a volt like your meter may read. Some chargers drop the digit, some round it. It’s hard to figure out what your charger does here but let me promise you this. For the purpose of this test it is of no consequence.
Next it’s time for another measurement. Slightly pull out the banana ends of your charge cord from the charger so you can easily probe them with your meter. Take a measurement at the partially retracted banana plugs. Now, you have made 4 measurements and jotted down each of them. All are essentially the same voltage. Now, apply your charge or discharge current of 2 amps to the pack. Connect your meter to the balance port of the battery. If you’re charging, you will see the voltage is lower in the battery (measured from the balance port) than on the chargers digital readout. If your discharging, you’ll see the voltage is lower on the chargers readout than at the balance port. How can this be? Are they are connected to the same thing?
Another test is to move the meter back and forth between the balance port and the partially retracted banana plugs. You’ll get a similar spread in your readings when you do this (while the pack is being charged/discharged). How can it be that essentially at both ends of the same connections you get two different readings?
If you repeat this test with a smaller weight charge cord, you’ll get an even bigger disparity between the charger and voltmeter. Poor quality connectors (like Kyosho/Tamiya/BEC/etc) will add to the voltage difference as well. We’ve performed this test in shop and have seen several volts difference before. What you are seeing is the voltage drop across the wires/connectors/solder joints, etc. between the charger and the battery pack. Now that you’ve run the tests, you’ve seen the voltage in the balance port is not the same as the voltage at the charger. What I’m pointing out here is the simple logic of understanding that the charger can only measure the voltage on its circuit board where the banana sockets are soldered on. It’s not measuring the pack voltage but the voltage as delivered by your connections to its internal circuit board.
Repeating the test at lower charge/discharge rates will show lower differences in voltage readings. Higher discharge rates will produce higher differences.
Higher resistance charge cords and connectors cause all kinds of problems. The higher the resistance, the more of your capacity is being wasted making heat (warming up the wires and connectors) rather than spinning your prop or being measured when you test capacity. The lighter the wire or more worn (or poor quality) the connections between the charger and the battery, the less accurately it can do its job and the less accurate the information it will provide you.
I’ve had instances of customers replacing battery packs with new ones which they tested to be just as bad as the ones they replaced because the charge cord was faulty or too cheaply made. The discarded packs were good when tested with a better quality cord and/or connectors. I’ve even had a customer with a 50cc gas model lose it in a dead-pack crash because he was using a “Quick” charge mode on an “A123” pack. In this mode the charger pumps the pack up to 80% where it’s safe to fly again. He flew and crashed because the resistance between the charger and the charged pack was so high that the charger was reading out a couple volts higher than the pack really was, under the condition of being charged. This condition was aggravated due to the high charge rate and small charge cord size. After the crash the pack was empty yet cycled good.
The point I’m trying to drive home here is don’t assume anything. There are many facets to doing accurate charging and battery testing which are overlooked by most. Certainly this short article will have confused some and enlightened others. One could go on to explain Ohm’s law and use these test measurements to calculate the resistance of your charging harness and thereby infer the error (believe me there is one) in your discharge readings when testing battery packs. But, I don’t think we need to take it that far in order to get the idea across. Are you crazy like a fox yet?