Battery charger project

Projects General Guidance
1

I have had many electric drills which i have skipped due to the non availability of replacement battery packs.

Many of my aquaintence have the same problem and im sure many would like a solution.
Please do not suggest dewalt as they are just too expensive for most of us.

I can get replacement cells of the lithium phosphate type but they require spot welding.

I can get lipo cells with solder tags however.

Some packs i have dismantled have very poor proection circuits, for instance one five cel pack had two in series and 3 in series with some sort of protection ccircuit.

Both lithium phosphate and lipo cells should ideally have indvidual protection and individual cell balancing when charging.

Without this cell longevity is drasticly reduced and in the case of lipo can be dangerous.

Individual protection circuits and individual protection and charging circuits can be had but are expensive when considering multicell packs.

I would like to build a micro based solution as nanos can be had for about 3 bucks so a custom board may do it.

However unit will need to be permanently connected to battery and needss to be put to sleep when not in use, battery would be about 1200 mA H.

How do i wake up and put to sleep though ?

Any other thoughts ?
Am i missing something blindingly obvious ?

2

How do i wake up and put to sleep though ?

Power savings and sleeping made straightforward by codingbadly, nick gammon, others....

My t85: http://www.hackster.io/rayburne/hot-yet

Nick's site: http://www.gammon.com.au

Jack: AVR microcontroller sleep demonstrations · GitHub

3

The arduino is powered by the battery.

The drill has a switch that connects the battery to the motor.

i need a sensing circuit i think that does not consume power.

Not sure how to implement that though, considering that in operation the motor will generate considerrable power.

Edit
I mean noise

4

Surely you only need the nano only needs to be awake when the battery is being charged? If so then you can simply power it from the incoming power source.

5

No
For protection i need the nano to shut off the power to prevent the cells from droping below minimm voltage as this will destroy them.

Two problems

Cell protection from minimum voltage.

Charging requires idvidual cell balancing.

Both can be solved by application specific ic,s but is expensive for multicell.
Circuitry costs more than batteries at hobbyking prices.