Battery talk- types and applications

[Mark Hammer]

I think it is important to remember that in any discussion of the role of battery types and lifespan and such, in the context of effects, we have to keep remembering that 95% of the interest in this subject applies to circuits that apply large amounts of gain.  Indeed, some circuits, like all those MN3207-based chorus and flanger pedals, and FET-based phasers with zeners in them somewhere, go out of their way to assure maximum stability of the voltage supplied to the critical components and bias points.

So, the key question we're really addressing here is how a battery CAN and DOES respond to a circuit that applies large amounts of gain to a signal that fluctuates and demonstrates urgent instantaneous current draw alternating with  periods of "relaxation".

[demonstar]

We were talking about why the battery indicator seems to go down quicker near the end of the batteries charge. I proposed that the battery indicator reading was directly proportional to the voltage of the battery. So this made me think that the phone battery is probably running into a regulator to obtain a constant voltage for the circuit. This regulator will probably put out a constant current (the current required by the phone circuit) and by nature it's putting out a constant voltage. Lets assume once the battery voltage drops below the regulators output voltage the phone dies.   As the voltage falls closer to that point in order to maintain the power input required to allow the voltage and current output to stay stationary a larger current must flow in to the regulator. So to summarize that bit... the battery voltage is falling so to keep the power input into the regulator the same (as the power output is still fixed by the phones requirements) the current flowing from the battery into the regulator increases.

So this is where my thoughts (question) lies. If the battery starts drawing a larger current as it's voltage is falling does the voltage on the battery then fall quicker? If so this would mean that, that would explain why a phone battery indicator would fall quicker as the battery gets closer to needing to be charged.

Wow... there are lots of assumptions in there but anyway that's what I was thinking. Any thoughts on this? Especially as to if the battery drawing a larger current as it's voltage is falling does the voltage on the battery then fall quicker? I hope it makes some sense!

You're correct on all counts. All of the simple/cheap battery indicators are voltage readers. Battery voltage does fall off much more quickly when they near exhaustion. And a constant power load, like a step-up regulator, does pull more current as its input voltage goes down.

A battery is a little container of chemicals. The chemicals react to force the electricity out. As more electricity is used, more of the chemicals get used up. They get used up first nearest the  + and - electrodes, and this gets further away as it's depleted. So the internal resistance from the active chemical region to the terminals gets further away as it's used up. Heating makes them more chemically active, giving a boost. Mechanical agitation shakes them up and moves fresh chemical slightly nearer the terminals.

Thanks R.G.!

[joegagan]

this is a great thread, thanks for all the input so far.

what i can add to this is that in my experience the rechargables have finally arrived.

the Ni MH 9v rechargables are way under 5$ when on sale at harbor freight. i charge them constantly when not in the pedal.

my main battery sucker is a digitech digidelay. it will use up a duracell alkalilne in under two hours. the digidelay seems to have something in the circuitry that will not let the pedal turn on when the voltage of the batt falls below some threshold.

but for jamming an hour or 90 minutes here at the house the rechargeable provides plenty of power over and over.

i haven't measured ma but the batt still has v8.95+ after an hour of use in the digidelay.

now , of course i could use a power supply, but i like the convenience of less cords and all that.

i bet a NiMh rechargeable would last a very long time in a low current draw stompbox. i don't have any data on the tone diff in that applcation tho

[sfx1999]

The problem with NiMH batteries is that they lose charge even when they are not in use.

[joegagan]

The problem with NiMH batteries is that they lose charge even when they are not in use.

true dat.

that is why i keep one charging all the time.

i have about 16 ni mh aa cells, mostly for my camera. they last as long or longer than alkalines when charged fully.

the 4 pack of aa ni mh  is sometimes $5 at harbor freight!!!

i am pretty sure i have saved $200+ in the 10 months i have had the camera

just takes a little discipline to stay on top of all the charging.