combining old 9V batteries but only using 9V max?

bernm

Member
Messages
557
Just a quick technical question that I'm curious about.

Is there a circuit that would let me take a couple of my old used 9V batteries and put them together but only draw 9V from them, whatever their combined voltage is?

For example, let's say I have two old used 9V batteries that have dropped to about 6V each. Is there any way to put them together to make 12V, but draw only 9V from them? I'm wondering if there's a way to make some sort of external battery to use what's left in old batteries. Of course, this would only be practical if the 9V wasn't dependent on the overall voltage of the combined batteries, whether that was 9.1V or 18V or anything in between, if that makes sense.

Is there any way to do this?
 

Sweetfinger

Silver Supporting Member
Messages
12,110
Not wise …… Series wiring would technicality Double the Voltage but could overheat the cell
Nope. If hooking cells in series overheated them, we wouldn't be using "batteries".
I'm not sure what the OP is trying to do. You could build some kind of simple circuit with a voltage regulator, that will take whatever you stuff in one end and give you 9V out, at the expense of current. You could stack a bunch of "dead" batteries and plop a zener diode on the supply to keep it at 9V but you'd likely have to parallel a bunch just to get some current.
Why?
9V batteries are two for a buck at your finer dollar stores. Why stuck on "exactly" nine volts? A new 9V battery out of the pack almost never shows 9 on the meter. It's usually 9.7, even 10 volts.
 

pipedwho

Member
Messages
1,685
Once a 9V alkaline battery drops below around 7 volts, it loses compliance - the ability to provide current. So it will still be able to power a super low power circuit for a short while, but any normal circuit that draws even moderate current would load the battery causing it to drop even lower in a very short time.

You could series a couple of batteries together, but this only exacerbates the problem as the circuit will draw more current and cause the batteries to cave to zero volts even faster.

Basically, if your 9V alkaline battery has dropped below 6.5V you have already sucked out 90%+ of its available power. Even the most clever switchmode or linear regulator design can't recover much more than another 5% of the battery's energy to a typical guitar effect circuit.

In the end the effort of trying to recover the last few drops of energy is better served saving up and buying a couple of '9V' LiPo rechargables and a charger. At least you can keep recharging those and they hover around the 8V mark for 90% of their life - which is way better than Alkalines that spend 70% of their lifetime _below_ 8V. I get slightly more life out of my 9V LiPo in my wireless kit than I did with Duracell/Energizer 9V batteries - the only problem is the battery gauge is useless as the discharge voltage profile of LiPo is very different to Alkaline.
 

andrekp

Member
Messages
5,635
As someone said, yes you can make a circuit that would take the higher series voltage and regulate it to 9v or whatever, but as someone else said, you won’t necessarily gain real power from that.

As a side note that might be illustrative: most acoustic electrics with a preamp use a 9v battery for power. And they seem to tend to start getting quieter until they finally quit when the battery reaches some low level.

My Yamaha A3R uses two AA cells and uses some sort of internal charge pump to bring it up and then regulate it to whatever voltage that preamp wants internally. Works great and the sound does not weaken as the batteries die. However, the quirk is that it has is that when it finally fails, it is sudden. You do get a warning telling you the time is nigh, but if you don’t act on it, suddenly the sound will just start cutting completely in and out, like a bad cable.

I suspect this is about the same effect described above.
 

lefort_1

Nuzzled Firmly Betwixt
Gold Supporting Member
Messages
14,849
No need to bleed that turnip. The last few joules are kinda rancid and lame and watery.

Either get new batts, a rechargeable setup, or a PSU of some sort.
Recycle your old batts if possible, especially the lithium ones.
 

coltonius

Señor Member
Silver Supporting Member
Messages
13,277
Not wise …… Series wiring would technicality Double the Voltage but could overheat the cell


Not to whip a dead horse, but.. EHX did exactly this with the old Electric Mistress. Plenty of other electronics do as well (tv remotes, kids’ toys, etc...).
 

slayerbear17

Member
Messages
2,920
I built a circuit project from silicon chip ( australia) which takes 2AA batteries and pumps them up to 9V. Neat little project, has come in handy many many times.
 

bernm

Member
Messages
557
Thanks for the replies everyone. I probably should clarify what I was asking here. It was a genuine question out of curiosity.

I already have a few different power sources including a rechargeable system and also a PP2+. I run some of my pedals from power supplies and others from batteries, depending on context and effect type. When I need carbon 9V, I already get them from the dollar store, and I also buy my alkaline 9Vs at a reasonable price. So I'm not looking for a way to save money.

I also already send my old batteries to recycling too, so I'm not wondering what to do with all these batteries kicking around. It's more that I was wondering if I can do anything to get the most mileage out of them before they disappear to battery heaven. That's all. Plus, I've been experimenting with some pedal kits etc and so since I'm already playing with basic circuits a little bit, I was also curious from that perspective, to see if I can build something useful with the bits and pieces I have. I also like to re-use and fix things where possible and practical and wise - why waste more than needed?

I'm still a bit confused about the potential overheating - I had been thinking about the same thing that @coltonius posted. But I had not known about the "quality" of the remaining charge - that's a helpful bit of info.

Anyway, thanks again for the replies. So far it's looking like this isn't practically possible.
 

spentron

Member
Messages
1,676
It doesn't get you much. However, if you wanted battery to avoid ground loops, and no switching noise from a boost or switching regulator, and always almost exactly 9V (or some voltage too high to do long from 1 9V), it would be best to linearly regulate it down from 2 9V's in series. There are several ways to do it, best is an IC. Or if you simply want at least 9V, and the device can handle up to 20V, use 2 batteries with no regulator.
 

analogmike

Gold Supporting Member
Vendor
Messages
6,811
Dave Fox made something like that decades ago, it DOES allow getting a lot more life from your batteries. I think you just need a simple voltage regulator circuit.
 




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