I built several LiPo dischargers in the past but all are designed to fully discharge batteries, and none have the ability to stop at a certain voltage. The URUAV LiPo Killer is a cheap and smart battery discharger which can do that.
Here are the DIY dischargers I built in the past:
Where to Buy LiPo Killer?
You can use it for 1S, 2S, 3S, 4S, 5S and 6S LiPo batteries.
How to Use URUAV LiPo Killer?
To completely drain a Lipo battery, connect the battery to the XT60 connector on the LiPo Killer. A single LED will light up to indicate the discharging is ongoing. It will turn off when it finishes.
For example, for an 4S 1300mah battery, to discharge it completely from 4.2V/cell will take slightly over 24 hours.
Why discharge LiPo battery completely? This is the first step to disposing LiPo safely.
To discharge a LiPo down to a certain voltage, you simply connect the battery balance lead to the LiPo Killer. You can select the voltage you want it to stop by pressing the button and it’s shown by the LED panel. Available voltages are: 3.0V, 3.2V, 3.4V, 3.6V, 3.8V and 4.0V.
To discharge a 4S 1300mAh from full charge to 3.8V (storage charge) takes about 5 hours.
Why discharge LiPo to 3.8V per cell? 3.8V is the storage voltage for a LiPo battery cell. Leaving your battery fully charged or empty for too long can accelerate performance degradation. When you are not using the LiPo over a long period of time (i.e. a month) you should put it in storage voltage (i.e. 3.8V per cell).
Beware that it can get very hot during discharging, do not touch the board to avoid burning your fingers. Luckily the LiPo Killer comes with a plastic shield you can install (not shown in this review).
Discharging Takes Time
As you can see, the URUAV LiPo Killer uses LED and tiny little resistors to drain the battery, which is extremely slow.
There are two 150 ohm resistors in series (total 300 ohm) and an LED for discharging a single cell, or two 471 ohm resistors in parallel (total 235.5 ohm) and an LED for a whole battery. The discharge current depends on the voltage, the lower the voltage the lower the current – hence the discharge will slow down towards the end too.
While it’s not a problem because you can just leave it outdoor somewhere fireproof, there are ways to speed up the discharge by adding additional loads.
DOES NOT Discharge Completely
There are some downside to this thing though, when you connect the XT60 lead wanting to discharge it completely, it doesn’t hit 0V. In fact, the discharge stops at around 1V to 2V. It’s not really a huge problem for me, because the voltage is low enough and safe to just short the positive and negative leads, this will discharge the battery completely. But this is just me, do this at your own risk at somewhere won’t cause a fire :)
The LED turns off before 0V too, because it needs a minimal voltage to work (somewhere around 0.7V). So it’s not the ideal indicator for indicating complete battery discharge.
How to Speed Up Discharging
You can add extra loads to the LiPo Killer to make it discharge faster. Simply connect extra loads in parallel to the resistors, such as power resistors or halogen light bulbs.
Here is how to add extra load to the XT60 side. The resistor shown in the image is good for 16V and lower voltage. For 25V (6S) you should use 125 ohm or higher to be safe. Higher resistance is safer (lower current thus less heat) but takes longer to discharge.
Here is how to add extra loads to the balance lead side. The loads should be identical so the cells are discharged at the same pace.
Similarly you can use light bulb instead of resistors. Here is a tutorial how to add light bulbs to the balance lead discharger: https://intofpv.com/t-the-perfect-lipo-discharge-tool-uruav-lipo-discharger-how-to-hack-diy
This is truly a cool gadget to have. It saves you a lot of time putting each fully charged batteries back to storage from a cancelled flight.
Hope this review helps!