Batteries for chip amps

[Ben79]

Hi all,

I'm gonna be doing some busking and I want to be able to power my little rig from a battery.

I'm thinking about something like this since the RC industry take their batteries seriously and their customers can't be easily fooled by false ratings...

http://www.hobbyking.co.uk/hobbyking/store/__9463__Turnigy_4000mAh_4S_20C_Lipo_Pack.html

I'll need 12v for my amps (Ruby and Tiny Giant) and obviously 9v for pedals (all fuzz) and I'd like be able to run for 2-3 hours.

Any advice on how I can accomplish this?

Thanks for any help.

[J M Fahey]

4000mAH (4AH)  is fine.

What does R$ mean and how does it relate to U$ or Euro?

[Roly]

Hi Ben, welcome.


Google says R$ = 1 Brazilian Real = 0.32US$.


The Ruby is a one watt amplifier, the Tiny Giant uses a TDA7240 which appears to be a 20 watt amplifier.

The important rating for a battery is its amp-hour rating, how much current you can draw out of it for how long before it's flat and needs recharging.

At 12 volts, and cranked flat out at one watt, the Ruby will draw;

P = E*I
I = P/E
1W / 12V = 0.08333333A or 83mA.

With a 12 volt battery rated at 4AHr (4000mA/Hr) the operating time is;

4000mAHr / 83mA = 48.19277108 hours, or a couple of days continuous.

For the Tiny Giant on 12 volts flat out at 20 watts;

20W / 12V = 1.66666667Amp

4AHr / 1.66A = 2.41hours

The saving grace here is that the power consumption of the amp is directly related to how hard you drive it, so you might get two or three hours or continuous thrash metal playing, but it could easily go twice as long in a more moderate show.


Caution: you can be pretty casual with most battery chemistries and get away with it, but NOT lithium technologies - they are unforgiving of mistakes and can bite back hard.

- these batteries do not like to be over-discharged, so you really need a low voltage cutoff;

- they are quite picky about their charging regime so a specific charger for the cells/pack used would be a good idea (one aeromodeller burned his kitchen down while recharging and now recharges his Lithium batteries inside some concrete breeze blocks).

- they really do not like physical abuse, being spiked or crushed, or being short-circuited.

https://youtu.be/HCGtRgBUHX8

https://youtu.be/LExMC5buoFg

All battery technologies store very considerable amounts of energy, and the better they are, the more forcefully they can unload that energy back to you.  Particular dangers of lithium batteries are suddenly venting hot material, long jets of flame, taking off like a rocket, and literally exploding - and it doesn't take a lot of mistreatment to trigger this off.


I'm thinking that there are battery powered tool sets with spare batteries and proper charger in the 9-18V range with lots of amp/hours, so I'm thinking something like take the torch/flashlight tool and turn it into a power socket for your amp rig.  As for your 9V stomps I'd just get some 9V rechargables and a charger for those.

HTH

[Ben79]

Thanks guys and thanks Roly for such a helpful and insightful reply - much appreciated.

I hadn't given the safety aspect of the battery any consideration so that's good to know.  If I go the RC battery route I'll make sure I'm clued up on how not to make it explode....unless I want it to at the end of a set.

There are LiPo batteries that come with chargers and circuitry to prevent all that inconvenient fire and exploding for charging laptops and phones and I've seen them rated (probably bogusly) at 18Ahr for not too much money.  Perhaps that's the way to go.  I can play through my Ruby for 9 solid days on one charge.

What effect might a lithiium battery have on the behaviour of a fuzz circuit?  I know internal impedance can be a factor but I don't understand it.

[Roly]

There are LiPo batteries that come with chargers

That sounds like a good option, much more capacity too.   :dbtu:  Most of the laptop and power tool batteries I've investigated seem to have built-in battery management, so this type of battery would be a good choice since it is likely to have all the required protection built-in.  (that's what that other connector with several contacts is about, it talks to its charger).

What effect might a lithium battery have on the behaviour of a fuzz circuit?  I know internal impedance can be a factor but I don't understand it.

Well that's a question I don't think has been asked before.   8|

Happily for simplification the internal resistance (DC) of a battery and its internal impedance (AC resistance) are pretty much the same thing for our needs.  To a first (and even second) approximation a battery behaves pretty much like a pure resistance, not much in the way of reactive (AC) elements such as shunt capacitance or series inductance to bother us even at audio frequencies.



The internal resistance, R_int or r above, isn't a fixed value but varies over quite a wide range, mainly depending on the State of Charge of the battery.  These two are so closely linked that the State of Charge can be implied from the internal resistance - high internal resistance means the "perfect" internal battery can't deliver much current to the outside world, your torch bulb only glows dimly, and thus you reason that the battery is flat.

A battery in a low State of Charge, flat or dying, has a high internal resistance, while a freshly made or charged battery has a very low internal resistance.  Shorting a fresh D-Cell for example can result in a current of several amps, and applying Ohms Law to 1.5V tells us the R_int must be well under one ohm.

But we don't go around shorting batteries if we can help it.  Internal resistance is determined by accurately measuring the battery terminal voltage for various safe currents between zero and rated maximum, then extrapolating the curve.  (The internal resistance is the change of voltage delta-V (= change in V, divided by current).  It simply isn't safe to directly measure if a car battery will indeed deliver 350 "cold cranking amps" at 12 volts.


(delta-V is 0.9 volts)


What is a bit scary is that battery manufacturers have been beavering away for decades to make "better" batteries, and an important way a battery can be better is to have low internal resistance.  The result is the modern generation of lithium batteries that have internal resistances so low that they can deliver so much current they will catch fire or explode if shorted (or overcharged).  As JMF implied above, if the battery in my drummer friends pocket had been a lithium rather than an common dry battery he would have needed a trip to the hospital burns unit (and new jeans).


{aside: as a theater tech I have been exposed to the fear of on-stage fire that is theater traditional but not shared by rock and roll bands at venues, and the fear is real because there have been some really terrible disasters, cinema, theater, and club fires started by on-stage pyrotechnics.  Even as a sometime accident investigator I could only watch the harrowing start-to-finish YouTube video of the 2003 The Station nightclub fire, Rhode Island, once.  It is just too distressing, but it is also a very salutary lesson in just how fast an on-stage fire can get out of hand.

Onebaldbob has a story about how the gas jets for the Count Down show were fitted backwards (by a very professional crew) when the show moved to Sydney, and instead of framing the drummer (during rehearsals) they ended up flaming the drummer, roasting his arse and causing him to jump over the kit to escape.  Add some of my own near-miss or close-call experiences and I have pretty negative feelings about any potential source of ignition anywhere in a venue.  I've stage managed fire-eating and pyrotechnic acts and I.don't.like.it.  I'm the guy lurking over there with an industrial fire extinguisher.  }


Now in AC Theory all DC voltage supplies are a short circuit to AC, so with a fresh battery the voltage supply rail in your fuzzbox might be at 9VDC but it is also ground for AC.  And if the fresh battery has an internal resistance of say 0.1 ohm, then it appears to AC signals as if ground and the supply are connected by a 0.1 ohm resistor - "ground" close enough.

But when the battery goes flat the internal resistance may now be as high as 100 ohms and this may upset the AC operation of the circuit. (note that portable radios often show low frequency instability with a flattening battery, pumping out pulses of programme with silent gaps.  Generically called "motorboating".)

The normal way that equipment manufacturers deal with this is placing something like a 100uF electrolytic cap across the supply.  When the battery is fresh this cap is effectively shorted by a 0.1 ohm resistor and so doesn't have anything to do, but when the battery goes flat and its internal resistance climbs this cap now provides an alternative low impedance path around the battery to ground for AC signals, hence why it is sometimes called a "bypass" cap.

I'll give you 10:1 odds that if you trace back from the battery snap in your fuzzbox (via the external power connector and on/off switching) you will find the first thing it comes to on the board is a 100uF cap - right across the supply.  And if it's not the first it will be there somewhere.  It's almost universal (except perhaps with home-brew pedals).


So the short answers are;

- lithium batteries typically have much lower internal resistances than the normal dry batteries used in stomps;

- your stomps almost certainly already have taken rising battery internal impedance with falling State of Charge into account;

- and if they haven't it's a 50-cent fix.   :dbtu:


HTH

[Ben79]

Thanks again Roly.

I build all my pedals myself and put a big electro between the rails so that's not a problem.

I've bought a 5Ah Lipo battery on Ebay.  It's a 14.8v model battery so I'll need a charger, something to regulate the voltage to 12v and some kind of housing for it all so it's safe and easy to transport.

Any advice on the 12v regulator would be appreciated.

I'll report back once it's all up and running. 

[J M Fahey]

It's a 14.8v model battery so I'll need a charger, something to regulate the voltage to 12v

What for?

"12V" stuff designed for use in a car is actually designed for 14.4V which is what you actually have in a car with the engine running and the battery well charged.
14.8V is practically the same, within a couple % points, ver close.

In fact a lot of equipment, including amplifiers and Communications stuff have their  "official" output power measured at 14.4V , for 2 very good reasons:

1) power is higher than with mere 12.6V which is what a well charged battery puts out some time after charging has stopped.

2) "they are not cheating" , since 14.4V is actually present in a car with a running engine, which means most of the time.

Some "split the difference" and quote power @ 13.8V , which is a "safe" battery charging value if you want it lo last a lot.

The point is that all those values are "nominal 12V" .

[Ben79]

I want to run a few things at 12v - a Ruby amp, a Tiny Giany and a Valvecaster.  I already destroyed one lm386 in the Ruby by accidentally feeding it 14.3v from a faulty transformer so I'm pretty sure it needs 12v max.

[g1]

  There is a version of the LM386 (N-4) that is good to 18V.

[Ben79]

It's 32 ohm though so I went for the N-3....perhaps misguidedly.