8.4 Volt Battery Charger  

1/9/21

Hello my friends and RC enthusiasts from around the world. I hope you are all well.

As I promised in my last post, the next one would be a 2S (Li-Ion) and 7.2V NiMH battery charger.  Again I needed another charger in a hurry so I whipped this up to get me going temporarily. However, it turned out so well I have not yet bought another "off the shelf" charger.

 I DO NOT recommend you copy my designs, nor will I be held responsible for any  losses, damages or injuries incurred whilst undertaking this task. This is written for guidance purposes only. If you follow these instructions, you do so at your own risk. This charger is powered from a 240VAC supply, which WILL KILL YOU.

The charger I have built, is capable of charging both Li-Ion batteries and NiMH batteries up to 8.0VDC. It was mostly made from reclaimed items from different projects I have built over the years. The enclosure (wood) has been reclaimed twice. This is it's third incarnation. The display is also reclaimed and is an LCD unit capable of reading volts, amps, power, time and temperature.  I bought it from EBay and was £8.00 and is still widely available. The front panel sockets are new and were £6 for a pack of 4. The sockets are perfect for this situation and are very safe. The balance lead is also new and was made from components I also bought from EBay.  To charge Li-Ion batteries, I needed a 2S balance board which was £4.00. The main power source came from my old rechargeable bike light power supply (see below), which was surplus as the batteries are now kaput.

 As this was a charger in a previous life, it has all of the charging circuitry built in. 

You can see the output (above) of this charger indicated by the red pointer. This is perfect for my needs. Time to open it up.

With the power supply now opened up we can see the charging status LED arrowed with a red marker (above). When the LED is red, the battery is charging. When the LED is green, charging is complete and current flow will stop. The charger is also protected with a quick blow fuse on the charge side of the circuit. 

I cannot yet use the above power supply to charge a Li-Ion battery. In order to do this I need to employ a 2S Li-Ion battery management board or also know as a balance board.

Above is a picture of the same type of balance board I am using in this design. It has, so far proven to be ultra reliable. As you can see this board has been used yet, is still functional so I'll put it away for a future project. 

Above is a picture of the multi function LCD display. It is very small but has a lot of useful functions built in. As you can see, it has a button on the right marked "Key" and is simply a reset key for all of the functions. 

A rear view of the display showing the connections. The mini pins on the right are the display power inputs and the mini pins on the left are the thermometer connections.

I will not be giving you enclosure dimensions nor show you how I put it together. However I have provided a circuit diagram for your information only. See below.

As you can see it is a very simple drawing and an easy circuit to follow. The circuit board right in the middle is the 2S balance board. Note the connections of the display module, the left are negative connections and the right are positive. Also the top connections are outputs and the bottom pair inputs. The charge outputs are at the bottom right and connect to the battery to be charged. At the middle bottom is the 2S balance lead, which MUST be connected when charging a Li-Ion battery. Failure to do so will result in damage to the battery and a fire. I strongly recommend using a fire retardant bag during charging.  When charging a NiMH or NiCad, the balance lead is not required and will switch of the current automatically.

After wiring it all up, it looks like this,

It is a very small and neat charger. Yes it does look rough and dodgy, but it does what I need of it. 

The red arrow in the picture above shows the position of the charge indicator LED. As you can see it is glowing red and is therefore in charge mode. The display is indicating 7.50V with 0.95A of current. The picture also shows the balance lead connected as I am charging a 2S Li-Ion. The on/off switch is also positioned on the top near the LED indicator.

In the above picture, the display is reading 7.51Volts, 0.96Amps, 7.2Watts, 1730mAh charge, 14.25Wh on the battery, 7.81Ohms on the battery and a charge time of 1h 58m 34sec. Internal temperature is 25 degree centigrade. The display is a little difficult to read as there is still a protective film covering it that needs removing.

I also made up a set of plug type charging leads using 14AWG silicone cable, fitted with a Tamiya connector. The plugs are also stackable so you can connect other test equipment to check the output when under load conditions.  

Another picture pointing to the charge status indicator. 

In the picture above I have placed a warning sign to test balance leads before connection as incorrect wiring will result in fire, see below. 

The picture above is a result of a wrongly wired balance cable. Notice the melted wires.

Burned to a crisp within a second and almost caused a fire. Not Good. 

Other than a fault of my own, this little charger has been ideal. When I was testing it, I needed a dead battery to test the charge on/off function. I borrowed an old 7.2V NiMH battery from a friend. He said it was completely dead and initially, so did I. It was a beat up 3000mAh Voltz brand battery that had seen better days. However, I popped it on the charger to indeed prove it was kaput. As I had thought, the charger would think the dead battery was charged after a short period and switch off, which it did. No current was reading on the display, so it was working as it should. I then saw the voltage on the display dropping slowly, again as I expected. Again, as expected the charge circuit became active and current was flowing. After each cycle, I wrote down the results from the display and noticed the capacity was gaining after each cycle. The battery got that strong, I had to discharge it on my load rig. The first cycle yielded a capacity of 50mAh, yes 50. However, after many, many charge/discharge cycles, I slowly regained a total of 1750mAh. Yes I know it's only half the capacity but, will still be useful for low drain purposes. It also saves it (for now) from the landfill. My friend was also more than happy as he now uses it as a starter pack for his nitro cars. 

The picture above shows the charger stacked on top of the volt/amp/power/temperature tester I described in my last post. Being able to stack the unit together keeps the work area tidy, and a tidy work area is a safer area. 

If you do take it upon yourselves to ignore my device and build this device, please be careful and test at each stage of the build. Never leave any battery charging, unattended even for the shortest of periods. Failure to do so could end with tragic circumstances. 

I hope you have found this useful in some way and until my next post, take care and thanks for reading.

1/9/21

catxls.com