DIY alkaline 1.5 to 9v battery charger Question. AC or DC supply?

[John Lyons]

 
I realize that this is frowned upon but I would like to try it anyway.

In this article there is a schematic But I'm unsure about whether the 12 supply voltage is AC or DC.
I assume it's DC but though I'd ask first...

http://www.afrotechmods.com/reallycheap/batteries/batts.htm

John

[Mark Hammer]

I'll start off with the obligatory frown ....BUT...

I've been charging alkaline batteries with a standard NiCd charger for years, and have kept some alkaline 9v alive for quite a while.  Thye thought of all that battery garbage piling up out there gives me the creeps, so I try to do what I can to extend the life of ALL batteries.  Several caveats:

1) You CANNOT leave non-rechargeable batteries sitting unattended in a charger the way you can with conventional rechargeables.  I only recharge mine if I am nearby.  I check them intermittently to feel if they or the charger are getting warm.  If either of those is happening, I take the battery/ies out and wait for things to cool down before starting up again.  I would not personally leave a nonrechargeable in a charger unattended for anything longer than 45min-1hr, and certainly not for a total of more than 2-3hrs, even IF I'm checking up on thetemperature.

2) While true rechargeables benefit from a close-to-full discharge before recharging, nonrechargeable alkalines (and carbon zincs) have a "window of rehabilitation" below which you are wasting your time if you are trying to bring them back to life.  Nine-volt batteries can be reborn surprisingly zippy if they have not sunk below more than maybe 7.8vDC.  Once the internal resistance or whatever has sunk below a certain point, all the recharging in the world will do them no good.  As well, generally speaking, the better the battery, the more amenable to a recharge.  Duracells are terrific.

3) The recharge will not carry the battery quite as long as the original charge.  If your original 9v took 3 months of normal intermittent (nongigging) usage in a fuzz to drop down to 7.8v, a recharge back up to, say 9.2v, will only take you for maybe another 5 weeks of use.  That's no miracle of science, but it can be repeated and save the earth from yet more battery-garbage.  Gigging musicians are certainly recommended to use wallwats, but in my case, I'm a nongigging dude who has a pile of pedals sitting around and a need to have a pedal but amenable to audition without fishing around for an adaptor.  I have lots and lots of 9v batteries on light intermittent duty, so this suits me just fine.  YMMV.

4) When it comes to 1.5V batteries, they too can be recharged but some attention needs to be paid to making sure all the batteries placed in the charger start out at roughly the same point.  For instance, do NOT stick two AAs measuring 1.12v in a four cell charger along with one measuring 1.34v and another measuring 1.40. There may be some instances where this CAN be done, but you'd need to know a lot about the internal working of your charger to pull it off safely.  When more current is being pulled through (and into) some cells vs others, you can get overheating, internal gas buildup, and......KAPLOOEY!! (like they used to say in Donald Duck comics).  It is worth noting that in some medium-quality 9v batteries that can happen too.  The ultra-cheap carbon zinc using slugs are not enclosed cells internally.  The better quality units that use sealed cells (that look like tiny AAAs crammed into the 9V package) can sometimes pop as the gas buildup expands the cell.  The entire battery will not explode, just the weakest of the 6 cells.  The pop you hear is produced by the shrink wrap around the individual cell breaking.  Obviously once one cell is done for the electrical path is broken and current stops moving.  Incidentally, this explosion-by-discrepant-current-draw phenomenon is NOT unique to recharging nonrechargeables.  When my older son had his original Game Boy that used 4 AA batteries, the instructions cautioned strongly against using 4 batteries of different age, and they were right.  When weaker and stronger batteries were mixed, we experienced several internal battery explosions while the Game Boy was running.  Nothing was being recharged, just normal use.

So, a summary. 

• It CAN be done, but entails risk that must be thoughtfully and conscientiously managed.
• Just about ANY charger can be used, though some will require much greater vigilance and "checking in" than others.
• There are finite limits to what you can expect from attempting to recharge nonrechargeable batteries, and the circumstances under which it is possible

And again, BE SAFE.

[John Lyons]

Mark

Very nice overview, I'll keep these things in mind. I too cringe at the landfill issue!

Still need to know if I would use a 12v DC (or AC)  voltage in the schematic linked above.

Thanks

John

[mac]

I guess a you need a 12V DC wall wart that can handle 200mA or more.

I recently bought a six AA battery holder that I expect to last for a long long time.

Mark, thx for the info


mac

[John Lyons]

Ok thanks Mac, I'll go with that DC supply.
I'll report back with results after I test this thing out.

John

[mac]

While I was wirting this I breadborded the circuit with a 10.5 DC supply (loaded). I put a 8.7V duracell batt and in less than hafl an hour it is 9.7V!!! Only the 10R resistor was a little hot. Cool.
I am going to buy the parts, maybe a bd139 instead of the 2n3904 and a 47R resistor intead of the 10R to decrease the current. If it works like a car batt the less charging current the better.

Mark, when you say don't mix AA batt, for example one with 1.2V and the other with 1.3V, do you mean not to recharge them in paralell? What about putting them in series?


mac

[Mark Hammer]

Mark, when you say don't mix AA batt, for example one with 1.2V and the other with 1.3V, do you mean not to recharge them in paralell? What about putting them in series?
mac

Quite frankly, I'm not sure which is the riskier case.  I know that when batteries have popped under use, they are unequal cells in series. But of course, using and charging are two different things.  I also have no idea how my charger is wired internally, though I think it's parallel.  Since I don't know, I just don't do it, and measure my batteries before I stick more than one in the charger.

Somebody else care to offer a reasoned theory-driven opinion?

[mac]

In the lack of a theory I'm going to experiment with a couple of AA batt in series... I like fireworks, you know...

Also I'm planning to use a LM317 to set the max voltage across the circuit. 10v or 10.5v max if charging a 9v batt, 1.8v or 2.0v if using a single AA or 3.5v for a couple of AA in series.


mac

[Mark Hammer]

In the lack of a theory I'm going to experiment with a couple of AA batt in series... I like fireworks, you know...

Fireworks is one thing, battery contents spattered about and corroding what they come into contact with (or worse) is another.  Ideally, you'd like to be able to use that charger a second time.

[mac]

Fireworks is one thing, battery contents spattered about and corroding what they come into contact with (or worse) is another.  Ideally, you'd like to be able to use that charger a second time.

Well, I put the circuit on the backyard. When the cat saw me with the breadboard it ran away. Animals feel incoming danger better than we do so I'm going to trust mother nature, and you.
....

I was looking at the National LM317 data sheet and it has a 6v current limited battery charger. It can easily be tweaked to deliver less current replacing the 1R with a 10R or higher, and with a 2K pot the output voltage can be adjusted to any value.


mac

[John Lyons]

Short of using a regulator on the voltage this might be a good place for one of those $3 multimeters form harbor freight!
In the artical he mentions using a meter if you are unsure. Once you get a handle on the charging currents and safe charging times then you can just clip in a battery and let it go. Wouldn't be a bad idea to house this thing in a box incase batteries want to explode. Some kind of splash screen ...

John