You don't know til you try . . .


Multi Cell Alkaline charger

, posted: 7-Dec-2009 09:20

Most "Non rechargeable" Alkaline batteries can be recharged if done right.
They key is not to let them get dead flat, than less than 0.9v.

I had a Alkaline charger bought from The Warehouse years ago.
A "ReZap Battery Doctor", Silver in colour, 4 cells. AAA-D-9v.
It didn't do a very good job, cut off too early, made some batteries leak.

After a week of testing the ReZap vs trickle charging (10ma) I had the numbers see how bad the RZ was.

For discharging I used a Turnigy Accucel-6
http://hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=7028


So I went about building my own trickle charger.

My initial design included a temp sensor for each cell.
But after some tests I found it was not required due to the low currents they were being feed.
0.03w (1.7v*0.018a) dosen't make a lot of heat, assuming there was aproblem and the battery was not taking charge.

I made the first prototype using my current  favorite micro AVR MEGA48.

Very few parts required.
The IC can source/sink 200ma, 25ma per pin.
It has an internal 10bit ADC with 8 way switch.

Operation is pretty simple.
Insert battery, remove when done/ready.

The charger will detect the battery presence and start charging.
It's just a simple voltage cut off at 1.7v
If the voltage drops below this it will keep topping it up until you remove the battery.
The display shows
Current voltage - Voltage change in the last 60 mins - change since insertion - total time in charger

This is definitly not a fast charger.
1000mAh would take 56 hours (1000/18).
For my application it is fine as I only need them at most once every 2 weeks.

By adjusting the cut off voltage it could charge anything.
NiCad, NiMh, Lithium Ion/Phosphate/Polymer


Improvements:
-self calibrating voltage detect
-battery capacity estimation

To test, self discharge.
If it's low enough it may beat some NiMh's in applications like remotes.

Charge cycles.
I have read it can be anything from 50-200.

Now to make a 16 cell version.


Results?
This is my first week using it so still testing.

Here is what I found so far

Your typical AA will have up to 1000mAh recharge capacity.
Discharging to -> 0.90v at 300mA
AAA, 400mAh.
Discharging to -> 0.90v at 200mA

Current AA NiMh rechargables are up to 3200mAh
2500mAh commonly available locally at $4-7 ea.
Alkaline AA's $0.6 - $3.

Does it make ecnomic sense?
NiMh 625mAh per dollar. (2500/4)
Alk 1666mAh per dollar. (1000/0.6)

In my case, yes.
I don't need fast charging or the capacity offered due to type of use.
I do need lots of them that may be lost by the users.

I have just had my first (AAA) leak in the charger, might have to drop the cut off to 1.6v
Photobucket Photobucket

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Comment by Kezza, on 9-Dec-2009 20:16

Cool read man. Thanks for taking the time to answer something so many would have thought about. Keep us posted on your progress. lol, don't forget to pay your insurance premium too. ;-)


Comment by Joe Yu, on 8-Sep-2010 17:25

If anyone is interested, we have a battery charger that charges disposable alkaline batteries. To good results and is absolutely safe.


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