9V battery re-use/power supply

[Stratomaster]

I just had an idea the other day. 

My wireless system eats 9v batteries like no tomorrow, and spits them out with about 7.25V left on them--so I've got a good bit of semi-depleted batteries in a box.  I was thinking about putting 3 in series (~25 v) in a box with a regulator circuit featuring this beast: (http://store.gravitech.us/312v1aswvore.html) and outputting to a daisy chain to power a small, portable pedalboard (current draw: 80mA max).  Aside from making sure my pedals are up to running at 12V (which I have already confirmed), is there any reason why I shouldn't do this?

[JKowalski]

The batteries at 7.25V means that their charge is almost totally gone. They won't be able to output barely any current, I think, especially for any usable length of time.

The voltage on the output is not really that much related to how gone the batteries are. The cell reactions that make up batteries have a fixed voltage that does not change (at least ideally) based on the size of the cell or the charge left in them. You could have a cell at 9 volts that can only supply 1 ma for an hour, and one 9v that can supply 1A for days.

I lack the knowledge of what the causes of the end-of-life voltage drop (anyone want to fix that), but I know that you can't assume that since a battery has voltage on it that it is still usable.

I hope I got that down right! 

[theehman]

You should go to rechargeables for your wireless.  Put a charger on your pedalboard and you're set.

[trixdropd]

You should go to rechargeables for your wireless.  Put a charger on your pedalboard and you're set.

Winner!!

[Stratomaster]

I don't mind using batteries, and I've tried rechargables.  They're more hassle than they're worth--especially if you buy Procell's in bulk.  I was just wondering if I could put these semi-drained ones to good use before I toss them.

[bobp1339]

<soapbox>

Not to get off on a rant here, but yes, rechargeables are somewhat more work, but even if you found a second use for the 7v batteries, they will still find themselves in a landfill at the end of their lives.

I usually try to take the easy way out, but when it comes to batteries, I try to either just say no or use rechargeables.

</soapbox>

[jacobyjd]

I'd agree that you may be looking at this from the wrong direction--the used batteries are all but at the end of their life. It seems like far more of a hassle to deal with some sort of pedalboard power supply that would be short-lived at best.

Have you considered, rather than using regular 9v-sized rechargables, using some sort of external battery pack? It might be a little heavier, but you could build a killer regulated 9v supply using a 12v battery of some kind--Lithium-Ion? Laptop battery? Something like that. Fairly small, light, holds a charge a LONG time, and an easy recharge solution.