Portable speaker system

Hello everyone I'm starting a project for work we recently have been aloud to bring in a small radio or battery powered speakers i am using a tablet computer powered from a large battery used in sit down hilos and such but I'm constantly on the move and i don't have the luxury of a wall socket so battery's are going to be needed I plan on building a small wooden box putting a 6 inch speaker in it and stuff some kind of battery power in it

I have decided on using a sonic t amp which is powered by 8 AA battery's I ditched that idea and decided to wire in 8 D battery's so it would last longer but I'm still doubting that it will be much more cost effective so i was wondering if i could use a 12 volt rechargeable tool battery

The amp comes with a 12 volt 1200 mah ac adapter but I'm thinking the rechargeable battery will have a higher amp rating then necessary and it only needs up to 1.5 amps i did find a couple around 1.5 but would measure before i decided to go through with it.

Would it be possible to do this I know with a lower amp rating the voltage could fluctuate on the battery but being that alkaline battery's normally measure more then 1.5 volts new the amp should be able to accommodate for it. I hope :-/ is this a lost cause or should i just stick with alkaline i really don't want to keep dumping money into batteries if i don't have to.

Also if a higher amperage is the case is there a way to lower it even though there may be a slight voltage drop the amp should still be able to operate on it
Thank you in advance mike

An amplifier draws only as much current as it needs. an amplifier does not operate full blast all the time so its average power and current are low. Use a battery that has a very high Ah rating so it will last longer.

Why not use AA Ni-MH rechargeable cells? They are 2500mAh about the same as throw away alkaline cells except their voltage stays higher for longer.

The Sonic-T amplifier that uses the Tripath chips has an output of only 6W into 8 ohms. Article on the web say Tripath went bankrupt in February, 2007.

I would recommend using a small sealed lead acid battery.

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WARNING:
If you go with this idea then you must use a fuse or PTC resistor (polyswitch/fuse) to provide over current protection; a shorted lead acid batter can explode and cover you with battery acid and molten lead!

audioguru. NiMh's start high, but their nominal voltage even slightly discharged is only 1.2 volts per cell so he'd effectively be running on 9.6V DC with a much higher start voltage and the added series resistance of each cell.
Much the same with a 12 volt lead acid battery, in moderately discharged states especially under high current load the voltage produced is relatively low.

Although the charging circuitry is typically not as lenient and overall system design wouldn't be as simple as with lead acid or NIMH packs Lithium packs with good charge control circuitry are well purposed for this kind of application. Nearly all modern battery powered devices should be using Lithium batteries, economy of scale has kicked in and what it lacks for in the total cost deparement may seem to be lacking compared to simpler NIMH recharageables the size/weight benefits of a Lithium pack outweigh it's disadvantages.

Sceadwian said:

audioguru. NiMh's start high, but their nominal voltage even slightly discharged is only 1.2 volts per cell.

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The alkaline battery voltage keeps dropping until soon it is less than the fairly flat voltage from Ni-MH battery. Then the voltage from the Ni-MH battery is higher than the alkaline battery for most of the discharge.
It is more obvious when the current is higher.

Those are AAAs What about AAs or Ds under the same load?
What about Lithiums as I was suggesting?

Sceadwian said:

Those are AAAs What about AAs or Ds under the same load?
What about Lithiums as I was suggesting?

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AA alkaline cells have 2.5 times the capacity of AAA alkaline cells.
AA Ni-MH cells have almost 3 times the capacity of AAA Ni-MH cells because they weigh almost 3 times more.

Energizer sells c and D size Ni-MH battery cells that have a little 2500mAh AA cell inside.

I have not seen a datasheet for a Lithium battery but the Lithium Ion battery in my RC electric airplane begins fully charged at 8.4V and the voltage slowly drops to 6.0V, similar to the drop from an alkaline battery. The 7.4V Lithium battery is lighter, has a capacity about 3 times and is much more powerful (lower internal resistance) than the Ni-MH 7 AAA cells battery (8.4V) it replaces.

Lithium batteries are expensive. I use used cells from a laptop computer battery, a cable modem and a DVD player.

A laptop and portable DVD player I can understand having a lithium secondary pack, but not cable modem, must have been a primary lithium backup battery. VERY different beasts, not even comparable.
There is a reason why almost all modern portable devices that require energy storage are using Lithium Ion secondary cells. The econmy of scale is making them less 'expensive' than you think. It has in fact made them so much less so that all you need to do is find the right supplier, they're superior to all other commonly used battery technologies.

All my Lithium-Ion cells are the same size: 65mm long by 18mm in diameter. they have a metal case.
The two from the cable modem are the strongest maybe because they were never used.
The four from the DVD player are fairly strong and the six from the laptop computer were used a lot and are not strong.

sorry it a dayton audio t amp exact same just rebadged I appreciate all the help guys I guess what i'm really looking for is can i use a 12 volt NiCd drill battery since i have an old one with a charger. the drills gear box broke after about two weeks from a ten foot drop so its practicaly new and i just kept it around I have read of ppl putting 14.8 volts for short periods of time and surviving. If i could get out of this just buying the amp and a grill for the speaker that would be great if it won't work i'll do other wise mentioned.


also thinking of adding one of these so i know when to charge it is that all i would need to add it is the meter itself

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A 12V Ni-Cad battery is 14v to 15V when it is recently fully charged. its voltage drops quickly to 12.5V or 12V then slowly drops to 10V when it should have the load disconnected and recharged soon. The meter will be fine.

it sounds like it should work just fine just trying to make sure. I appreciate your help as this is first time doing it this way and was'nt sure if it was really safe for the amp i did a similar thing for a radio in my canoe but that's not getting used everyday so batteries for that is ok. thanks again

Lithium batteries are more dangerous than NiMH batteries.

Nickel is more expensive than lithium.

I have some real C and D NiMH cells, they are 4500mAh and 9000mAh respectively.

Hero, why do you keep posting blanket statements which don't have much to do with reality?

Lithium batteries are not more dangerous than NiMH's in general use. Modern Lithium cells have built in thermal overload protection that prevents overcharging except under extremely high voltage, and the only way to get them to go up in flames after that is to intentionally pierce them while they're hot and fully charged. All the stories you see about lithium batteries going up in flame are from defective or improperly made chargers and batteries which were thermally insulated or involved physical damage to set off the pack.

Nickle vs Lithium metal prices I'm not so sure about because I'm having trouble even looking up the price of Lithium, not to mention that the amount of metal in the batteries isn't so incredibly high as to make the cost of the raw materials an overwhelming concern, the processing and manufacturing of the materials is the bulk of the cost of a battery.

If a Ni-MH battery is overcharged then it hisses like a snake and releases some gasses. No fire.
But if a Lithium battery is overcharged or over-discharged then it catches on fire.
Lithium batteries are classed as hazzardous on airplanes, not Ni-MH batteries.
Lithium batteries need a protection circuit, not Ni-MH batteries.

Lithium-Ion cells in notebook pcs, DVD players and other products have the protection circuit on a pcb in the pack, not built into the cells.
Li-Po cells also have the protection circuit separate.
My cell phone Li-Ion battery pack has two cells in parallel and has a protection circuit attached on a small pcb. Its cells do not have a protection circuit built-in.

I saw a pretty big RC electric model airplane catch on fire in flight when the pilot gave it full power. The protection circuit was separate from the battery and did not match the lower max allowed output current of the battery. The battery burned buried in sand for over half an hour.

My RC electric model airplane does not have any battery protection for its Li-Ion battery. The battery is pretty hot after a long flight but the motor is hotter. I have flown it many times.

In general use there is no more risk from using a lithium pack than a NiMH pack. RC models are definitely not general use, those types of packs are often used well beyond recommended discharge limits and generally don't have thermal protection systems because the discharge currents are so high they're always running the pack near it's failure point anyways. You might be wrong about your cell phone's protection circuit, there's often a thin strip that runs up the side of the battery to bring the electrodes out to a common point that are made of thermally sensitive material, if the battery ever over heats this strip goes high resistance and prevents overcharging. Not all batteries have this but I've seen it on several lithium packs I've gotten out of devices. It doesn't look much different than a thin metal strip and can be easily overlooked.

I've seen several Youtube clips of people intentionally flaming up lithium packs, you have to put really high currents relative to their normal operating currents and possible overcharge current to get them to go up. The articles you see in the papers about this or that laptop or cell phone having gone up in smoke on someone is from an improperly engineered cell or cheap Chinese battery nockoffs or devices that have zero protection ability. I've heard of a couple accounts of fully charged lithium packs catching on fire if crushed or punctured with high energy as the sudden kinetic impact can cause a run away thermal condition in the battery, but under normal circumstances this can't happen.