Readers you will probably recall me writing about the SkyRC iMax B6 battery conditioner purchased from China. I decided to run a trial with it attempting to restore an old and dead NiMH electric drill battery. When I started the trial the battery actual voltage was 3.7 whilst the original specifications stated 12V. During the first cycle I managed to recharge the battery to 10.8V which led me to suspect there was one defective cell in the battery pack.
The iMax has now cycled the battery charge/discharge four times and the new actual maximum voltage is back to 12V. I now need to confirm the actual capacity. At new the battery had 3Ah at 12V, which means the capacity was 2500mA (3÷12). Obviously I can’t discharge the battery to zero. 20% of the total capacity is an approximate figure. Therefore the iMax should have added 2000mA to the battery when it’s fully charged. The iMax can measure this.
Of course all of this is just an experiment and I now need to start thinking about checking the condition of the six lithium 18650 cells recovered from the netbook battery. I’ve already used the multi-meter to check the voltage. Three are at 1.92V and the remainder 1.72V. The voltage of a fully charged 18650 cell is just under 4V (say 3.7V). There are several advantages of using lithium over lead-acid or NiMH batteries.
- High specific energy and high load capabilities with Power Cells
- Long cycle and extend shelf-life; maintenance-free
- High capacity, low internal resistance
- Reasonably short charge times
- Low self-discharge
The following graph from the Battery University website shows their discharge characteristics
As you can see the voltage drop is minimal (3.7 to 3.5) until the battery reaches 20% discharged when it starts to rapidly fall. This point is known as the “knee” because the shape looks similar to the position of a seated persons leg. The horizontal portion of the line being the thigh.
Attempting to re-charge a cell that has gone past the ‘knee’ is complex and the objective is to avoid that occurring. However not all cells discharge and recharge at the same rate. This means when the lithium cells are connected together to create a battery they must be ‘balanced charged’. Effectively this means the lithium cells require two types of charging connection. A main charge to the pos (+) and neg (-) battery terminals and a balance change to individual cells.
This next diagram shows three cells connected in series to produce 11.1V (3.7x3) There is a main pos and neg connection between the cells and separate minor connections (red, green, blue, black) for the balance charging to occur.
The iMax B6 charger has the ability to simultaneously do both the main and balance charging. However I want to use it for it’s testing and cell recovery functionality rather than having it tied up charging lithium batteries.
My intention is to rebuild my old NiMH drill batteries by replacing the NiMH cells with lithium 18650 cells. The NiMH battery charger only has a pos and neg 12V connection which means it can’t balance the lithium cells. It’s at this point I should explain how the lithium cells will be connected.
Connecting three cells in series (+-+-+-) increases the voltage. This is described as 3S. If there were four batteries it would be 4S, etc. In this case my voltage would be a minimum of 11.1V
When the cells are connected pos to pos and neg to neg they are in parallel. In the photo below this would be described as 1P. When in parallel the voltage doesn’t increase but the current (amps) does.
With only six cells I plan to connect my cells 3S2P
I would prefer to have eight cells and connect them 4S2P raising the voltage to 14.8V. That would put the drill on steroids!
To over come the charging problem I have purchased a small 3S balancing charger. The original battery charger will provide the main 12V charge and this little charger will fit inside the battery case.
It will be wired the same as the diagram above. The board was purchased from China at a cost of $1.32.
By doing this I can use any 12V charger to recharge the drill battery and free up the iMax to do the more sophisticated battery conditioning and recovery.
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