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Importance of balancing

A modern lithium battery is an incredible invention and it has made both electric cars and multirotors possible. The sheer amount of power these batteries can deliver has completely changed and revolutionized the RC community.
But with such power comes great responsibility – the lithium (Lipo) batteries comes with a price since they can explode if not handled properly.

On Youtube you can find many videos of Lipo batteries exploding – luckily most of these fires has been deliberately provoked by the users. But still… an accident could happen – so better be safe and take some basic considerations.

A little background info about Lipos

A lithium battery is built using 1 or more cells – each cells is a separate little battery with a full voltage of <=4.2 V and you will calculate the total voltage of the battery by multiplying with the cell count. A 4s battery will have a total voltage of 4×4.2v = 16.8V.

So what happens if a battery is not balanced? Well initially nothing as it is not dangerous if two cells have a different voltage – the problems starts once the cells reaches its upper or lower limit.

Example 1:

Charging a unbalanced 3S battery that should be charged to 12.6V – before charging

  1. Cell voltage = 4.0V
  2. Cell voltage = 4.0V
  3. Cell voltage = 3.6V

Total voltage = 11.6V 

What happens next ? The battery charger will now charge the battery until it reaches the desired 12.6V and now problems start to emerge.

After charge the battery will have the following cell voltages across the cells

  1. Cell voltage = 4.4V
  2. Cell voltage = 4.4V
  3. Cell voltage = 3.8V

Total voltage = 12.6V

You can see that cell 1 and cell 2 has been charged to 4.4V and this is dangerous and will permanently destroy the battery’s internal chemistry.

Example 2:

The same can occur when you start to discharge an unbalanced battery – one cell will go below 3V and be destroyed posing a serious risk.

So how can you ensure that you don’t charge an unbalanced battery – or flying your quadcopter on a battery with a cell out of balance.

How to charge and use Lipo safely – keep an eye on the balance

First of all you should follow the standard guidelines when charging lithium batteries

  1. Use an intelligent charger that monitors the battery – even better to stay in proximity of the charging proces
    I am using a Turnigy charger accucel 6 – I will upload review soon
  2. Charge on a concrete surface and away from flammable materials

Buy an intelligent charger with balancing plug – this plug gives access to the individual cells of the battery. This allows the charger to charge each battery to its correct specifications.

Use a multimeter if you don’t have a balancing charger yet – you will have to measure the voltage across the ground (black) and the connection to each battery cell.
It is important that you have the same voltage on all cells and that you never go under 10% ~ 3.3V.

This chart shows the fully charged voltage of 1S, 2S, 3S, 4S and 5S lipo batteries.

 Balancing your lipo
Use this spreadsheet for a reference when balancing your lipo

In the field ?

In the field you should bring a Capacity controller, that allows you to monitor the your batteries before you get in trouble.
I have bought a digital battery checker from ebay – and I must say that I am pleased with its functionality.

Before I plug a battery into my Quadcopter I always check whether the cells are balanced – I have built my quadcopters myself and would really hate if they caught fire.
The battery checker can control each cell and you can see the total voltage. Another great feature is that you can see how much juice is left on the battery.

Field battery checker
4S battery – 38% power back

Conclusion

  • Buy a quality charger that supports balancing the cells
  • Never charge an unbalancing cell
  • Use a multimeter to ensure the right cell voltage
  • Buy a field checker for the LIPOs
  • Never go beyond 10% discharge voltage for any of the cells or the battery itself

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