My universal Li-ion Battery

Safety first!

Li-ion batteries pack a large amount of energy in a small amount of space. When li-ion batteries overheat, are used the wrong way or are defective, they are prone to explosions which can result in injuries and fires.

Never work with these batteries unless you are educated and know what you are doing.

18650 Li-ion Battery

Why did I build it?

It all started when I was taking long exposure photographs with my Canon Eos M camera. This camera has a small 875mAh battery which does not last very long so I needed to find a way to extend battery life.

After a bit of googling on the net, I found an adapter that you put in the camera’s battery compartment instead of the battery. The adapter has a connection where you can connect a battery eliminator and power the camera from the mains.

I don’t want to be dependent on the mains but the god thing is that you can also connect an external battery to the adapter.

Battery adapter for Nikon D810

My design

My universal battery consists of dual li-ion cells to obtain an 8.4 volt battery, the same voltage used by most cameras.

Since I have always lacked a quick way to check the charged status of my camera batteries, I have included a small and cheap voltmeter module which can be activated using a small switch to see the current voltage in the battery.

For safety, I have also included a 5 Ampere fuse in the event of a short circuit.

I used a sturdy shrink tubing with glue to hold the whole unit together. On the outside of the shrink tubing, I glued a piece of velcro tape to have the ability to easily attach it to my camera stand or other surfaces nearby where the battery is used.


Some parts for the battery

Finding batteries

Li-ion 18650 cells are common in laptops and powerbanks. I had a broken laptop and opened the batterypack to check. There were nine Li-ion 186502 cells. (See picture above)

18650 cells in good condition present a voltage between 2.7 volts and 4.1 volt, bad cells measure less than 2.7 Volt. Using a digital multimeter, it was simple to identify if the cells were still in decent shape.

Schematic diagram


How to charge it?

I googled chargers for 8.4 Volt Li-ion battery and got a number of hits. This charger had good reviews so I ordered one for 4 EUR including shipping. The charger came with a DC connector which I cut off and replaced with a 4-pole female that fits my 4-pole male that I have on my battery.

A two-color LED on the charger indicates whether the battery is charging or fully charged. I measured the charging current from the charger. It showed 930mAh, not far from the specification of the charger.

1000 mAh Li-ion charger

Another way to charge

I also found that it works great to charge the battery with the battery charger that comes with the camera. If I connect the battery to the battery adapter as mentioned above, insert the adapter in the original charger, the battery will be charged.

Charging the battery with my original Canon charger

Close-ups of the battery



The pushbutton

Checking voltage