Battery cell charger / capacity tester

M

Mad Professor

Senior Member
Good day all.

I am looking at designing and building a picaxe based battery cell charger / capacity tester.

I have a bunch of 2.00volt wet cell nicad's, and 3.7volt lipo cell's.

For the charging side I would wish to charge the cells with the constant current then constant voltage (CC/CV), with adjustable current and voltage set points.
For the capacity tester side, I would wish to be able to discharge the cell via constant current, and once again adjustable current and voltage cut off points.

The 1st design I had in my head was going to be a small portable unit with an LCD for the menu etc, but the more I have thought about it, I have then been thinking it would be better to turn it into more of a bench unit, with serial output so that I can fully log the charging and discharging the bettery cells.

So I am going to need help and advice with most of the project, I don't want to be shown eveything I need, But I would like to be point the right way.

The 1st thing I have to look into is all the hardware that I will need to be able to do this.

So I know I will need to monitor both the voltage and current when charging or discharging the bettery cells.

Any advice or tips to help this project get started would be grate.

And if anyone has example circuit's for something like this that would also be grate.

Thanks for your time.

Best Regards.
 
B

BeanieBots

Moderator
I've done a very similar project for Lead Acid 12v batteries.
Has 12v input then uses flyback transformer to drive output.
Output charges at fixed current until voltage reaches predetermined value. Time taken is noted.
Output is then reduced to float voltage and held for the same time it took to do the bulk.
Output is then cut back again to a 'safe' float voltage and held until the battery is removed.
The Ahr is calculated for bulk, float and hold charge.
Ahr is measured using two resetable integrators (removes PICAXE processing overhead)
Bulk, float & hold voltages are user selectable.
Charge current is user selectable.

There is also a feature for discharge with user setable discharge current and cut-off voltage.
There are user selectable sequences of "charge only", "discharge only", charge-discharge, discharge-charge and charge-discharge-charge.

All controlled by PICAXE 18X and just two buttons.

Thought I'd posted the project in the projects section but can't find it.
I'll have a look tonight and if not, will try to find the original (on old computer) and post later.

For NiCd you should be using constant current and terminate on delt-peak. It is very different to Pb/LiPo charge requirements.
 
S

srnet

Senior Member
I tend to shudder whenever anyone mentions Lipo charging.

Get a Lipo charger wrong and the batteries can (and do) explode, you can be injured, you can set fire to your house or shed.

Unless you are a very experienced constructor, who already understands how to charge Lipos safely, I would leave Lipo charging well alone.
 
Last edited:
fernando_g

fernando_g

Senior Member
BB;
I recall seeing your post on that charger...a very nice and elegant solution indeed!
I specifically liked a lot the ingenuity of the integrators!
Hopefully you can find and re-post the circuit.
 
B

BeanieBots

Moderator
Thanks EC, that was the thread I was looking for.
I've found the original hand drawn schematic. I'll post once scanned.
 
B

BeanieBots

Moderator
srnet said:
I tend to shudder whenever anyone mentions Lipo charging.

Get a Lipo charger wrong and the batteries can (and do) explode, you can be injured, you can set fire to your house or shed.

Unless you are a very experienced constructor, who already understands how to charge Lipos safely, I would leave Lipo charging well alone.
Click to expand...
I fully agree.
I had my first genuine exploding LiPo last week. Scared the living daylights out of me. Still not 100% sure what happened but it was a small simple single cell plugged into a commercial charger. After a few seconds I heard a little crackle and noticed the cell bulging. Picked it up quick and threw it onto the patio. A few moments later it split with a crack and started to fiz but didn't actually give off flames. For fun, I poured a little water on it. The reaction got more violent but still no flames. It was certainly a good reminder to me about NEVER LEAVE CHARGING LiPo's UNATTENDED.
 
M

mrburnette

Senior Member
Yep... the time from sizzle to smoke (or worst) is short... I have started using an extension cord and moving the charger/cells to the center of the poured concrete. Nasty little things.

Ray
 
M

Mad Professor

Senior Member
Thanks for eveyones repies so far.

Regarding lipo cells, yes I know if you don't take care I know they can catch fire and or explode.
I would only try and charge lipo cells that I could find the manufacturers data sheets, and I would follow the recommended charging specifications.
 
S

srnet

Senior Member
Mad Professor said:
I would only try and charge lipo cells that I could find the manufacturers data sheets, and I would follow the recommended charging specifications.
Click to expand...
Even if you could find the manufacturers data sheets, I think you may be missing the point here.

For a Lipo charger to be safe, everything has to be just right, is the voltage and current measurement accurate and fault free ?

Is the code accurate, fail safe and without bugs ?

Is the assembly of the electronic bits to a high, professional, standard ?

I would consider myself an experienced constructor, I would not contemplate making a Lipo charger, there is just to much to test, verify and debug. Too much to go wrong too.

A commercial Lipo charger can be got for as little as £20, up to 6 cells from a 12V supply, multiple battery technologies, cell balancing, safety features and solid robust construction;
http://www.giantshark.co.uk/imax-balance-charger-clone-p-406640.html

"Optimized operating software:
B6 features the so-called AUTO function that set the
feeding current during the process of charging or
discharging. Especially for Lithium batteries, it can prevent
the overcharging which may lead to an explosion due to
the user's fault. It can disconnect the circuit automatically
and alarm once detecting any malfunction, to achieve the
maximum safety and minimize the trouble. All the settings
can be configured by users."
 
Buzby

Buzby

Senior Member
mrburnette said:
Yep... the time from sizzle to smoke (or worst) is short...
Click to expand...
Over fifteen years ago I first got involved with Lithium cells at the company I worked for then.
Being new technology at the time I was intrigued to know how they were constructed, so I took one home to see.

If I post my procedure, and the ensuing results, here I will be lambasted for promoting dangerous activities. ( Quite rightly ! )

Suffice to say I don't take any chances with Lithium cells or batteries now.

Buy an off-the-shelf charger !.
 
You must log in or register to reply here.
Top