is battery drain linear with resect to Mah?

[daz]

What i'm trying to determine is this....if you have two 9v batteries and one is 500 Mah and the other is 250, will the 250 drain down to a given voltage twice as fast as the 500?

Here's another question to irritate you all with. If you run 2 or 3 low current effects like tube screamers and such off one battery, what other negative considerations might there be besides faster drain? I'm running 2 off one battery now, a RAT and a tube screamer and it sounds fine with no ill effects what so ever. I'm thinking about running the 3rd off the same battery too, a TC electronics spark boost.

[eschertron]

2x 9v batteries in parallel you have effectively 750Mah; the voltage is common so the drain is proportional to the reserve.
2x 9v batteries in series you have a problem; the smaller-capacity battery is a weak link and goes dead long before the other battery is taxed.

[daz]

So u r saying voltage will do as i suggested.....drain to (as an example) 8v twice as fast with a 250mah battery as a 500 mah battery?

As to running two 9v in series or parallel, i'm not doing either. I am looking at rechargables and the one i want to get (which is the only one i know of that charges to an actual 9.6v) has 1/2 the Mah as a regular alkaline so i wanted to know if it will then last 1/2 the time too. IE: hit 8.5v in 1/2 the time as the alkaline. (8.5v is where one of my pedals seems to starts losing gain)

The other question was ***aside*** from faster drain, if i ran 3 low drain effects off a single 9v would there be any tonal or other issues.

[nickb]

You mustn't connect batteries in parallel. The stronger battery will try to charge the weaker one. It might be okay if the batteries were exactly the same type and condition but that is hard to do.

If you really want to increase the effective capacity you will need to add a diode in series with each battery before joining them together. The diodes will drop some voltage ( and hence power) so best to use a low forward voltage schottky types.

[daz]

Man, what r u guys smokin?! I never even suggested i'm gonna connect batteries in series or parallel ! Put your glasses on !

[Mick Bailey]

Different batteries have different drain curves down to a given voltage and it doesn't necessarily follow that a battery of twice the capacity of another will take twice as long to get to the same voltage. It could do by coincidence, though. One factor is how the increasing internal resistance as the battery discharges not only reduces the voltage and current availability, but also acts to reduce the discharge. Different battery chemistry has a dramatic effect on discharge curves.

Running multiple effects of a single battery isn't a problem if the effects have effective AC bypassing internally, which pretty much everything has apart from some really old designs. I have a customer who powers a handful of pedals off a rechargeable 9v - Just long enough to do a show or two. He's pretty diligent about carrying a spare and recharging.

[nickb]

Man, what r u guys smokin?! I never even suggested i'm gonna connect batteries in series or parallel ! Put your glasses on !

No you didn't but other's did.

The simple answer to your question is no, a 250mAH battery will not drain twice as fast a 500mAH one, but it wont be too far off. You need to look at the discharge curves on the manufacturer's data sheet for the batteries in question to get an accurate answer.

An issue with using a common battery connection for several pedals is that a ground loop is formed which may give you troublesome hum issues.

[daz]

Different batteries have different drain curves down to a given voltage and it doesn't necessarily follow that a battery of twice the capacity of another will take twice as long to get to the same voltage. It could do by coincidence, though. One factor is how the increasing internal resistance as the battery discharges not only reduces the voltage and current availability, but also acts to reduce the discharge. Different battery chemistry has a dramatic effect on discharge curves.

Running multiple effects of a single battery isn't a problem if the effects have effective AC bypassing internally, which pretty much everything has apart from some really old designs. I have a customer who powers a handful of pedals off a rechargeable 9v - Just long enough to do a show or two. He's pretty diligent about carrying a spare and recharging.

Now THATS the kind of answer i was hoping for. So no way to know i guess. But the rechargable i'm looking at is a lithium type whatever thats called....Lion or something like that. I was wondering just how that would hold up but if u say you know someone who powers a hand full of pedals on a single rechargable i guess my 3 should be even better being that one is only 4 Ma drain, one is 7, and one is 16. I may just leave it as is now with one battery running the 4 and 7 Ma pedals and the other one on the 16 Ma one. I was waiting for a reply to buy them and i suppose i may as well go order them. Waiting because of the price....Over $50 for 2 batteries and charger. But this is the only manufacturer i know of with rechargable 9v batteries that charge up to an actual 9.6v. Thats for that informative reply.

[daz]

No you didn't but other's did.

The simple answer to your question is no, a 250mAH battery will not drain twice as fast a 500mAH one, but it wont be too far off. You need to look at the discharge curves manufacturer's data sheet for the batteries in question to get an accurate answer.

An issue with using a common battery connection for several pedals is that a ground loop is formed which may give you troublesome hum issues.

I see. As far as ground look hum, none at all running 2 of them on one battery. But we'll see what happens when i try hooking the 3rd up to the same battery. Just very low drain drive pedals and no digital stuff. It there were any of that i wouldn't have even considered doing this.

[nickb]

I see. As far as ground look hum, none at all running 2 of them on one battery. But we'll see what happens when i try hooking the 3rd up to the same battery. Just very low drain drive pedals and no digital stuff. It there were any of that i wouldn't have even considered doing this.

There is a more serious case of concern I just thought of. Most pedals have negative to ground, but some, especially some older ones, use positive to ground. If you mix these then you'll short the battery out.

[daz]

There is a more serious case of concern I just thought of. Most pedals have negative to ground, but some, especially some older ones, use positive to ground. If you mix these then you'll short the battery out.

Yes, i'm aware but none of mine are + to ground.

[eschertron]

Man, what r u guys smokin?! I never even suggested i'm gonna connect batteries in series or parallel ! Put your glasses on !

No you didn't but other's did.

...and I ain't sharing

[daz]

I hooked the 3rd pedal up so i am running 3 drive/boosts from one 9V and it works the same as using three 9v's. Now i just have to see how fast it drains a 9v but i have ordered 2 of the powerex lithium rechargables and charger. The charge to a full 9.6v so hopefully one will last thru a full nite and then some, but i will have a second one if not. $50+ for 2 batteries and charger but if they last thru a few hundred charges it'll be a lot less $ than alkalines.

[J M Fahey]

What i'm trying to determine is this....if you have two 9v batteries and one is 500 Mah and the other is 250, will the 250 drain down to a given voltage twice as fast as the 500?

Yes.
Notice capacity is given in milliamp-hours , so same milliamps, half the capacity=half the hours.

yes, one battery can feed a couple pedals, as long as total drain is not too high.

Battery capacity is indicated by a capital "C" and in general plain vanilla batteries are guaranteed full capacity if drained C/20 , so, say, 500/20=25mA (which is ample for the pedals you mention) and not recommended going above C/10=50mA

SOME loads go above that, say motor driven toys, flashlights, photo flash, etc. but they are not used continuously.

I am looking at rechargeables and the one i want to get (which is the only one i know of that charges to an actual 9.6v) has 1/2 the Mah as a regular alkaline so i wanted to know if it will then last 1/2 the time too. IE: hit 8.5v in 1/2 the time as the alkaline. (8.5v is where one of my pedals seems to starts losing gain)

You must get the proper datasheet with full curves to compare, or measure yourself; they generally give discharge time to a quite lower voltage (meager 6V or so?); for many 8.5V is almost "full charged" and they donīt bother stating that ... unless itīs a full detailed curve as I mention above.
many "9V" rechargeables are actually 8.4V **fully charged**, go figure.

Rechargeable and not (Carbon or alkaline) curves are different and canīt be directly compared.

You mustn't connect batteries in parallel. The stronger battery will try to charge the weaker one.

Not so sure about that, not if same chemistry.
There is a built-in "voltage" value for every chemical compound and that does not change with "discharge" condition.

WE measure lower and lower voltage because battery gets worn/tired, internal resistance increases so we get less voltage at terminals, but "chemical" voltage does not change, thatīs why a worn battery wonīt pull current from a fresh one.
Again, IF same chemistry.

In zinc carbon battery cell, zinc is the anode material and manganese dioxide is the cathode material. Electrode potential of zinc is – 0.7 volt whereas electrode potential of manganese dioxide is 1.28.
Therefore, theoretical voltage of each cell should be – (- 0.76) + 1.23 = 1.99 V but considering many practical conditions, the actual voltage output of a standard zinc carbon battery is not more than 1.5 V.

[daz]

This is crazy....just got the powerex batteries and charger today. The batteries come pre charged so i measured on....10.5 volts ! And they don't drop much under load with a couple pedals on, 10.3v. Not sure how this will relate to how long they last before needing recharge, but while i don't have EJ ears i can hear a obvious difference and i don't like it. Too much low end. I assume this will go away once it drains to a normal voltage but i hate to have to compensate with tone controls then set it back once they get down to a normal voltage every time i recharge them. Maybe i shoulda got the 8.4v ones but the manufacturer says they charge to 8.4 which is not far from where i'd recharge. But now i wonder if they would charge to 9.4. Oh well, nothing is ever as it seems or as was said is it?