Spurred by the release of a new charger, a popular online retailer has published several pieces incorrect technical information. I have contacted them and pointed it out, but they have ignored me. Now I think it's time to lay it out here.
1) For nearly 2 months the following statement was printed on a charger's product page. It was no doubt added to the page in an attempt to sell more chargers but it is incorrect and misleading.
"When using two Voltz Power Supplies wired in Series, the Voltz 101K is easily capable of charging a 6S 5100mAh LiPoly at 30A continuously. That's an 8-10 minute charge time!"
I have 3 problems with this statement. First suggesting the average consumer use the stated power supply units in a series configuration, a configuration not intended or supported by the manufacturer, is just not cool. Next comes the use of the word "easily". Not only is charging in this manor not "easy" for the equipment involved but because it surpasses the rated output of the power supply units by roughly 15%, it is bound to cause premature failure of the units. Lastly by publishing both the above statement and a supporting video, many people are now trying to use this power supply configuration on other chargers that only support 28V, instead of the 30V it creates. This last problem is not the retailer's fault exactly, but they should take responsibility for their statement and explain that not all chargers support the configuration.
I stated "2 months" above because they did finally modify the statement to state 25A instead of 30A. Of course it did take them 2 months to do so. So one of the three problems I have stated have been resolved.
2) Recently the retailer published what they refer to as a "Charger Data Sheet". In the document they attempt to clear some of the fog surrounding many of the charger limits and requirements. Good idea but not well implemented. Most of my beef comes down to a "formula" they publish in the document. Here is an excerpt
"To calculate your maximum charge rate per lithium polymer pack, simply use this formula:
(CHARGER WATTS / MAX CELL VOLTAGE) - PS EFFICIENCY LOSS
Basically, you take the maximum advertised wattage for your charger, divide that number by the maximum voltage of your fully charged lithium polymer pack, then substract approximately 15 to 25% for efficiency loss. For example, if you are using a charger that can deliver 500 watts, and want to charge a 10s lithium polymer pack, you would apply the following formula:
500 / 42 = 11.90 - 1.79 = 10.1 Amps
In the formula above, 500 watts are being divided by 42 volts (maximum voltage of 10s pack), then the result is subtracted by its own 15%. As you can see, you can charge a 10s pack at 10.1 amps safely. Of course, you always need to make sure that you are using a power supply that is rated equal or greater to the power output of the charger."
In short the above "formula" is incorrect. The efficiency of the power supply effects how much AC input power the power supply draws. It has nothing to do with the output of the power supply itself or the charger. Again the power supply efficiency has NOTHING to do with what the charge rate will be on a specific lipo. Assuming a properly sized power supply is used, the charger will output its full ratings, no ifs or buts. In the above situation, charging a 10s lipo with a 500W charger, the charge rate will start out at roughly 500W / 37V = 13.5A and taper off to 500W / 42V = 11.9A at the end of the charge cycle. There is no efficiency to take into account in this calculation. The proper equation for calculating the max charge rate is
Max charger wattage / Max pack voltage = Max charge amps
Basically I am appalled by the lack of technical knowledge shown by such a large online supplier of rc batteries and chargers. They suggest the use of a power supply in a way not supported by the manufacturer and mislead customers on the capabilities of the units. Secondly they have published several statements that are absolutely wrong. Retailers have a responsibility to provide accurate information to their customers and in this case that has not happened.