Q About Lithium Batteries in Series

[Paul Marossy]

If I were to use three 3V CR2325 lithium batteries in series to get a "compact 9V battery", how long could I expect them to last with something like that Tillman FET guitar preamp? I want to put this preamp in one of my guitars, but there just isn't the space for a conventional 9V battery in the control cavity...  :?

[R.G.]

Find the ma-hr rating of the battery. Measure the current drain of the circuit. Divide the ma-hr rating by the current drain of the circuit to get an estimate of the hours.

[gez]

I want to put this preamp in one of my guitars, but there just isn't the space for a conventional 9V battery in the control cavity...  :?

If you just need a buffer/line driver, then the ICL76X1 series of micropower amps can be run from a 3V supply.  They're CMOS output so you can get away with a 3V signal at unity gain.

[Paul Marossy]

Find the ma-hr rating of the battery. Measure the current drain of the circuit. Divide the ma-hr rating by the current drain of the circuit to get an estimate of the hours.

I figured it was something along those lines. The part I am not sure on is if one battery is rated for 190mAh, do I triple the mAh rating or does it stay at 190mAh? I would intuitively think that it would give me 570mAh or so.

If that is the case, I would think that three of those batteries in series should last about half as long as a 9V battery. And a 9V battery lasts me quite a while before I have to change it. It might be worthwhile for me to give it a try.

If you just need a buffer/line driver, then the ICL76X1 series of micropower amps can be run from a 3V supply. They're CMOS output so you can get away with a 3V signal at unity gain.

Is that an SMT device?

[Mark Hammer]

From what I understand, lithium batteries are intended for circuits where current drain is minimal;  think of it like someone who can walk slowly for miles on end, but can't run.

The surfaces of these batteries are nominally and visible flat, but I doubt that contact is made along the entire surface.  Is there any utility in attempting to improve the conduction between contacts with any of the conductive pastes out there?   Or is current delivery overridingly constrained by what happens *inside* the battery?

[Paul Marossy]

From what I understand, lithium batteries are intended for circuits where current drain is minimal; think of it like someone who can walk slowly for miles on end, but can't run.

Hmm... that colors my thinking a little bit. I suppose I could phantom power it using a stereo cord, but that presents a whole new set of challenges.  I really would like to be able to just plug the guitar in and not have to do anything special. :?

[Mark Hammer]

That doesn't necessarily imply that litium batteries would NOT be appropriate for on-board stuff, merely that one would have to be selective about what types of circuits are being installed.  If it was a simple FET-based, unity-gain buffer, the chances are pretty damn good that a lithium battery would be just fine.  If the goal was to power an on-board delay-line, well.......

[Paul Marossy]

I hear what you're saying Mark. It seems a little borderline, though, in terms of longevity of the batteries vs. cost. But, that Tillman FET preamp has a pretty low current draw. In one of my guitars, the last 9V battery probably lasted a good ten to twelve months playing the guitar about 4 hours a week. In my other guitar, probably six months playing it about 8 hours a week. I guess I ought to just try it and see how it works out. Now, onto how to make a battery holder...  :wink:

[R.G.]

The part I am not sure on is if one battery is rated for 190mAh, do I triple the mAh rating or does it stay at 190mAh? I would intuitively think that it would give me 570mAh or so.

Stacking batteries in series gives the same current but higher voltage. Putting them in parallel (although not a good thing to do for other reasons) would give more ma-hr. So three batteries in series gives the same ma-hr rating as one battery, but three times the voltage (and therefore three times the power/energy).

[R.G.]

Stepping sideways...

Have you considered using NIMH batteries  semi-permanently inside the guitar and a charger jack?

[Paul Marossy]

Thanks RG. That being the case, it's not real practical to do what I want to do.  

At least I learned something new about batteries that I didn't know before.  

[Paul Marossy]

Stepping sideways...

Have you considered using NIMH batteries  semi-permanently inside the guitar and a charger jack?

No, but that's an idea.  :idea:

[gez]

Is that an SMT device?

They can be if you want, but the ones I use are bog-standard 8 pin DIL.

The ICL7611 is probably the best choice for lithium batteries as you can set the quiescent current via one of the pins (think it's pin 8 - been a while)!  Other than that, wire it up as normal.  

Even if you go for the dual version (7621) it has really low current consumption.  Mouser sell them.

[Paul Marossy]

Thanks gez, that's another idea to think about.