This is the review for the Turnigy-5011 (TR-5011) Lithium-Polymer (LiPo) battery charger. This is a very inexpensive charger, but it does have it’s shortcomings.
This is a very simple, no frills charger. Just power it up and plug in the battery.
We’ll explore what it can and can’t do as a charger.
These chargers were purchased from HobbyKing (formerly know as Hobby City and United Hobbies) for $6 USD. They seem to be discontinued now.
NOTE: This review is for the old(new?) version of this model. There seems to be more than one versions around. This is the one where the three LED’s indicate the status of each of the cells separately.
The Function
Although, the TR-5011 instruction manual that comes with the unit tries to explain few things, but it’s very difficult to make sense of it all. It’s quite useless.The instructions are simple. Just plug the 12V supply to the charger, then connect the battery balance connector to the output of the charger. The LED indicators are explained below.This charger charges and balances the cells at the same time. It balances by charging each cells independently to the same voltages. Therefore, at the end, all the cells have the same voltages, meaning they are balanced. The charger will not balance the cells by discharging cells to match the cell with the lowest voltage like some balancers do.
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. You’ll notice that LED’s will turn green at different times as each of the cells finish charging. The charge process is independent between cells. LED’s are labeled A, B and C. LED-A corresponds to the cell at the positive side and LED-C is for the cell at the negative side.
Charging Profile Diagram
Charging Profile Explained
The charging stages goes like this: Looking at the diagram above, first, the charger checks the cells for minimun cell voltages (Vmin) of 3V nominal. If it is below, then the charger goes into “Precharge” phase where it tries to charge at some lower current. The charger remains in precharge until the battery voltage goes above the Vmin. If the voltage never reaches Vmin within 15 minutes, then the charger shuts down, assuming that the battery is bad.When the battery voltage is greater than Vmin, the charger goes into constant current (CC) mode (the main charging stage) and stay there until the battery voltage is at “Regulation Voltage” (Vreg) of about 4.2V, which is the final voltage. Once it reaches Vreg, then the charger switches to constant voltage (CV) mode, where the charge current is reduced to maintain Vreg. It keeps reducing the charge current until it reaches termination current (Iterm) of about 50mA. This termination current is derived from the current limit setting. At termination current, the charge cycle ends.
The Innards and the Specifics
It uses the VM7502 Battery Charger controller for the charging circuitry. There are three of these circuits for each of the battery cells, all separate and independent of each other. At the front end, there is what appears to be a Flyback Converter (DC-DC regulator) to step down the 12V input to about 5V. There are three separate secondary windings in the converter transformer that independently powers the three separate charger circuits.The unit charges at 500mA, nominal, during the main “current-mode” phase. The charge current variations between the cells are only due to the component tolerance and the VM7205’s refernce voltage used for the current limit.The unit has no circuitry to control charge current based on number of cells. Each of the charge circuits uses one resistor to program the charge current for the Constant Current mode stage. The same resistor is used for the termination current, at which the charge cycle ends.
The only other thing that may affect the charging current might be if the supply to the charger is not capable of the required power, or that the frontend converter is weak such that it hits the max current when 3 cells are connected. However, if that did happen, I’d think that that would be a poor design issue rather than a feature.
The resistor used for the current limit setting is a 0.3 ohms and the controller senses the voltage across this resistor to keep a constant charge current. The charge current is calculated as voltage across the resistor divided by 0.3. I measured 150mV across the resistor, so this gives us 500mA for the charge current.
Voltage Variations between the battery cells after charging are due to the VM7502’s Battery voltage sense threshold of about 4.2V nom. The range can be from 4.168V to 4.232V, so the final battery voltage per cell can be as low as 4.168V. However, this voltage will be even lower when the battery is no longer being charged. It depends on the condition of the battery. Newer batteries may hold closer to the voltage that it was charged at.
Afterthoughts
This charger work fine for small capacity batteries. Since it only charges at 500mA, the bigger capacity you have, the longer it would take to charge. Also, avoid charging batteries smaller than 500mAhr capacity. Any smaller, you’ll be charging at more than 1C. This was the “rule of thumb,” but in the recent years (of this writing), charge rates of upwards of 5C are coming out. I’m personnally not yet comfortable charging LiPos at these higher rates.As far as I can tell, the three outputs are floating and they are stacked one on top of the other and the input to output is isolated. So, I don’t see why you can’t stack multiple of these units to charge 6-cell batteries (or any number of cells, for that matter). You just need the make an appropriate splitter/adapter ie: 6-pin connector to two 3-pin connectors. One caveat here is that I have not verified this, so attempt this only at your own risk.Although, not mentioned in the manual, if you connect the battery first, then the power to the charger, the charger will go into charge cycle immediately if the Vbat is lower than Vreg. I use this method to “top off” a battery that was slightly discharged. Otherwise, in normal operation, the charger will not charge a battery that is only slightly discharged. Vbat has to go below recharge voltage of about 4.075V for the charger to start charging again.
Also not mentioned in the manual, you can charge a single cell if you make an adapter. IMPORTANT: Just make sure you have the polarity correct.
One thing to remember is that this unit does not have protections for catastophic failures. The unit uses a small SOT-23 MOSFET for the voltage dropping element that passes the 500mA current to be fed into the battery cell. If this FET shorted out, the full voltage of about 5V is applied to the battery and an unregulated amount of current will be forced into the battery. The amount of the current depends on the frontend voltage regulator and/or the 0.3ohm current limiting resistor.
