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Can I upgrade my 18v drill power pack?

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evozero

New Member
Hi All,
I have an 18 volt 1300mAH Ni-Cd drill pack no longer holding charge.
The battery has 15 x 1.2v 1300mAH cells inside.

Can I replace the cells with a higher capacity Lion or other type pack any recommendations on what to do?

I own a smart charger capable of charging Lipo, Lion, nicd etc for my RC Car.
Many thanks
Ian
 

JLNY

Active Member
It can be done, but unfortunately it is more complex than you might think. Ni-Cd cells are pretty robust and are well suited to applications with large discharge spikes and simple charging arrangements that don't mind unbalanced cell charges or overcharging the cells at low current. Li-ion cells, on the other hand, are a lot more picky about how they are charged and used. Replacing a Ni-Cd tool pack will be a fair amount more complicated than just throwing together a bunch of bare cells without protection. You will have several main concerns when designing a lithium pack:

Firstly is the easy part: in order to get the same voltage as the Ni-Cd pack using li-ion cells, you will need to calculate how many cells in series you will need. You will also need to consider the maximum and nominal charge voltages for both cell types. A Ni-Cd cell is nominally 1.2V, but can be charged to about 1.45V per cell on a full charge. This would place the voltage of the pack at 18V nominal and 21.75V maximum for 15 cells in series. Likewise, most Li-ion cells are 3.7V nominal and 4.2V at full charge, so an arrangement of 5 cells in series would come to 18.5V nominal and 21V maximum, which should be a close enough range to the NiCd pack. The number of cells in parallel will depend on the size of the cells used and the amount of space you have to work with in the battery housing.

Secondly is the charging arrangement; Li-ion cells connected in series need to have balanced charging between all the series cells, so unless you plan to disassemble the entire pack and recharge the cells/packs individually every time you want to charge it, you will likely need a 5S balanced charger of some kind to charge it "in situ". What voltage/number of cells is your RC car battery charger designed for?

Next, you will need some kind of protection circuit to prevent the battery voltage from getting too low. Lithium batteries can get damaged and start to degrade if they are discharged below about 3V or 2.7V (depending on the battery type), and the motor in your drill will happily drain the batteries down to zero without a protection circuit to cut it off. Some lithium packs or cells come with protection built-in, or you may wish to get a pre-made li-ion protection board online.

Lastly, you will need to have some kind of inrush current limiting on the output of the battery pack. The aforementioned protection circuits will also have over-current protection that disconnects the cell if the current rises above a certain level. The motor in the drill will cause a large spike in the current right as it spins up, and it might only take a few microseconds to trip to protection circuit and shut off the battery. Once the drill has started moving the current will drop to a lower level, but the current may also go up again if the drill bit gets stalled in a hard material. You will need some kind of current limiting to keep the current below the level that will trip the protection circuit, and possibly some kind of LC suppression filter to ride out the current spike at startup. Measuring the stall current and the free current of the drill, and knowing the cutoff current of your protection IC will likely be critical to doing this.
 
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JimB

Super Moderator
Most Helpful Member
In my own experience, I have replace worn out NiCd cells with newer NiCd cells which usually have a higher capacity for the same physical size, or if NiCd cells are unavailable, replace them with NiMH cells.
Again the NiMH cells will usually have a higher mAh rating than the original NiCd.
As battery technology advances, the cell capacity increases for a given physical size.

Increasing the mAh rating of the battery means that it is a good idea to increase the current supplied by the charger.

I had a situation where after two changes of cells over a number of years meant that the charging circuit for the 650mAh NiCd cells was just not charging the latest 1500mAh NiMH cells and I had to build a better charger.

As for using lithium based cells as a replacement, sounds like a bad idea to me.

JimB
 
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