This is the review for the Hobby King HKE4 (TE4) Lithium-Polymer (LiPo) battery charger from Hobby King. This is an inexpensive charger for 1 to 4 cells with a variable charge current from 100mA to 4.5 Amps.
Although, it makes no mention about things like pre-charge conditioning or bad cell checker, it’s still not bad for the price. Especially, if you need to charge many high capacity batteries, you don’t want to spend $30 or more, per.
These chargers can be purchased from Hobby King (formerly know as Hobby City and United Hobbies) for less than $13 USD (as of 2011).
The Function
The unit comes with a manual and it’s written in decent English. The operation is not that difficult–it only have one knob and a button.The instructions are simple. Just plug the supply (11-15V) to the charger. Connect the main battery connectors to the 4mm banana plugs. Connect the balance port connector to the appropriate port of the charger. Set the charge current dial and push thea button.
This charger charges and balances the cells at the same time. Unlike the Turnigy T-5011 charger, this one charges through the main battery connectors. The balancing is done by bleeding excess charge from each cell through a resistor. Therefore, at the end, all the cells have the same voltages, meaning they are balanced. The charger will not balance the cells by charging each cells independently. A brief reverse-engineering of the unit revealed that a 20 ohm resistors are used across each of the cells to discharge cells that have higher voltages than the others.
Each LED’s indicates the status of each of the cells corresponding to that LED. The LED’s turn red when charging; green when finished charging.
This charger can charge 1-cell to 4-cell (1S-4S) Lithium Polymer batteries. Some people wonder about the lack of balance port socket for a 1-cell battery. There is no balance port for a 1-cell battery, because the balancing concept is meant for multiple cell battery packs. The idea is to have all of the cells in the pack to have the same voltage.
Here is the manual for this charger.This manual is not the one that comes with the unit and it includes other languages besides English. Obviously, the unit is re-branded by Hobby King and the manual that comes with the unit is different, but basically says the same thing.
Lithium Ion Charging Profile
Charging Profile Explained
Without doing extensive measurements, it seems from observation that the charger function as a Constant-Current(CC) at the beginning of charging stage until about 4.2V/cell is reached. At which point, the charger goes into Contant-Voltage(CV) mode, maintaining the 4.2V/cell.The graph above is a typical charging profile for Lithium batteries. I’m not certain about the pre-charge cycle, or the check for the failure to pre-condition the cells before the main charge cycle begins.
The balancing is done by discharging each cells through the 20 ohm resistor across each of the cells. They are switched in by a FET switchs in series with each of the resistors. At maximum voltage on the cells, the discharge current through the resistors would be about 200mA. I have not done any characterization on this unit, so I don’t know when the balancing act starts. It might only happen during the CV stage, when the charge current is reduced. I’m theorizing that if a battery is charging at 4.5A, the 200mA discharging of an overcharged cell is not enough to keep up with the 4.5A of charging current. Thus, that particular cell would endup being over charged.
Another possibility is that the charging actually stops, or current reduced, if any of the cells reach max voltage and discharges it until it matches the other cells.
One thing I did notice was that in CV mode, the current keeps dropping until below 100mA and the unit abruptly shut off. The LED’s just turned all Green simultaneously, indicating the end of charge.
The Innards and the Specifics
Looking at the quality of the assembly, it actually looks quite good. It doesn’t look hand soldered. Much of the components are surface mount and all of the components are on one side of the PCB.The control is handled by the Atmel, ATTiny48 micro controller. All of the decision making is done in the micro controller.
The main power section is a Buck-Boost circuitry that can either boost up or step down the voltage, depending on the supply voltage and the battery voltage. The input voltage is stepped up or down to the battery voltage and feeds CC directly to the battery’s mains.
The balancing circuit is simply a 20 ohm resistor across each of the cells with a FET in series with the resistor to act as a switch to connect the resistor across the cell. When the resistor is connected in, the maximum discharge current comes out to about 200mA.
Power Supply Requirement
There seems to be many people who are confused about what power source to use. The voltage requirement is mentioned in the manual as 11V to 15V, but no mention about the current requirement. The current rating that your power supply needs to have is actually dependent on the current setting you’ll be using. It would be best to assume you’ll use the maximum setting which is the 4.5A setting.
Next thing you’ll need to understand is the power requirement at peak charge current/voltage of the battery. For example, if you have a 4S battery pack at about 17V and charging at 4.5A, that’s
17V x 4.5A = 76.5W
That means you also need a minimum of 76.5W at the input, assuming the efficiency of the charger is 100% (and it’s not). To get 76.5W at the input you need to calculate the input current. For a 12V power supply:
I = 76.5W/12V = 6.4A
You need 6.4A at the input from you 12V power supply. Since your charger is never 100% efficient, you need to get something with a higher current capability. Say maybe 8A or more. Having more current capability won’t hurt the charger, either. So, if you have a 12V with 100A rating, that would work.
One cheap source for a power supply can be had by modifying a power supply out of an old computer.
Afterthoughts
This charger charges without any issues. I’ve tried 3 and 4 cell Lipos.The accuracy of the balance between cells is something I’m still questioning. I’m not convinced that this unit can balance accurately. The voltages of the cell at end of charge seem to be off by about 10 or 20mV. That isn’t bad at all, but I’ve see better results with the T-5011.
From what I’ve observed, it’s possible that the balancing routine may be a bit primitive. As the battery charge capacity is reached, it goes into CV mode. Then after some time, it abruptly shuts off rather than one cell at a time. In the CV mode, the charge current starts dropping off quickly and hovers around 200mA for some 20 minutes. I’d speculate that that’s where the balancing is being done. The cells that are already high gets the 20 ohm shunt. The charger doesn’t stop charging while balancing. At this point, near the end of the cycle, the low batteries are getting charged at around 200mA. One by one each of the cells should reach the 4.2V threshold, at which time the 20 ohm resistor is placed across that cell. This repeats until all of the cells have the 20 ohm shunts across them then the unit shuts down.
Now, if I were to implement the balancing algorithm I just mentioned, I would turn on the green light when the shunt is placed across the cell. When the last cell gets the shunt, the unit stops charging and all led’s are green. Of course, that’s just what I would have done–the unit doesn’t really do that.
A question pops up once in a while about, “how to balance a 1S single cell battery” and that there is no balancing port for a 1S battery. First of all, the reason for balancing a battery pack is to make each cell in a pack the same with each other, so that they all match within the pack. Well, a 1S battery pack only has one cell–there is not another cell to match with. It’s by itself…all by its lonesome…no need to answer to no one. So, there you have it, no balancing port for a 1S. Just connect the main terminals and the charger will know that it is a single cell battery.
Some pictures…
- PCB inside…
- Charging connections…
- Charge current setting knob…