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Originally Posted by Dougigs
The benefits for headphone amps are real. An 11.1v li-poly cell occupies slightly less space than 2 9V batteries; it offers 2400 mah of current whereas the 2x 9v, if series-connected, give you just over 200 mah.
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Who said you had to use only these or 2 x 9V? We'd have to count the extra space to end up with 18V of Li-Poly cells if you're going to compare these two alternatives, else a declaration about what kind of cans someone will need to have per a particular gain scenario, as well as which amp.
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They also weigh less than one of the 9v cells. If the 9v into a 40ma headphone amp gives you 5 hours of play time, the li-poly will give you 50 hours. |
The thread opened with mention of 900mAH cells. I may have overlooked these 2400mAH cells? I don't think weight is an issue though, we're talking about a few ounces as a percentage of total amp weight.
On the one hand you're trying to argue Li-Poly is smaller, then here you're arguing about better runtime when the 9V is smaller. Do we really need 50 hours of runtime? AT what cost and time to implement?
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That 11.1 cell will cost you US$50. I've used that much worth of copper-top 9V in the last month, even though I recharge my copper-tops. |
We both know $50 is significantly more expensive than one or two 9V NiMH, put in the context of price comparisons. IF you find a particular application where Li-Poly works well, you might justify it, but that's a completely different matter than trying to argue outside of that context.
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And the battery-life figures for these things are much better than for li-ion cells or many other rechargables: After 500 full discharges on a typical li-poly cell, full capacity drops only to 80 per cent. |
Considering their cost, AND the additional cost to implement them, that's pretty much irrelevant. It makes them seem marginally better but is one of many factors.
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The charging and discharge-cutoff circuitry is cheap, easy to design, and extremely small. |
Compared to a car, yes all of the above is true. Compared to already employed means of powering headamps, no, it'll be a little smaller and that will only be of benefit in particular cases, with particular amps where you'd run out of room otherwise.
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I'm working on a prototype the size of half a MINT circuit board that contains a) a 14-pin SMD li-poly smart-charging chip that cuts off at full charge etc; b) an 8-pin SMD discharge-protection chip that cuts the battery out at 2.8v (or slightly higher, for a safety margin) and provides a power-switching circuit; c) a single small MOSFET; d) about 5 resistors and 3 other passive components, all of which can be SMD. |
I don't doubt that it's nice and that you should be complimented on your work when it's all done. However, you seem more interested in swaying others towarsd this goal you haven't even realized yet. After countless hours you will have educated yourself well on these issues but unless you hide away your design, it's pretty inherant that others will be less educated about it, else they too will spend quite a bit of time reinventing the wheel so to speak, at great cost, again only benefitting with particular case, amp, and useage or recharge pattern (intervals). What you end up with might be a godsend to someone out backpacking for the weekend but I don't see where 50 hours between charges is worth the time or $ to "most" typical uses.
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Using such a unit would be pure simplicity: It has a 2.5mm socket in the back. When the battery discharges and the audio cuts off, you plug your laptop-computer power pack into the back for 2 hours, until the charge-done LED lights, and then you've got 4 more days of dawn-to-dusk listening on a PIMETA or a loaded MINT. |
So you're going to be lugging around a laptop battery to recharge a 2nd, lower capacity but just as expensive (when all is said and done) battery? I would tend to think there's plenty of time to recharge batteries while the owner sleeps unless you have some kind of special mission you're not telling us about.
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Total cost for all these parts is about US$20. Or you could get a pre-made charger board, about 2 inches square and fully SMD, for US$16 from various radio-controlled hobby shops, and build a discharge-cutoff board -- same cost, maybe a bit easier. |
So it's about $70 minimum to lose a few ounces and "maybe" a few cubic CM of space... or maybe not, if this control board doesn't integrate into any particular case well it may end up being larger. 2 sq. inches is a lot of space for batteries.
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It won't quite fit into an Altoids tin but it will fit into one of the smallest Hammond cases. Using the flattest, smallest mobile-phone cells could get the whole thing into a mint tin, but the control PCB and extra LEDs etc would make it a very tight squeeze indeed. I'll leave that experiment to some non-drinker with tiny fingers. |
What about impact protection? It's not going to be good to squeeze any such cells right up against a thin-walled metal (container). Are you considering a Mint or Pimeta? $70 for powering a Mint seems like a very luxurious endevour to save a few cu. cm of space considering there's still the source and laptop - unless the laptop IS the source, but then you completely avoid the whole issue and could've just ran the amp from the laptop battery and completely avoided the losses from recharging the Li-Poly cells. You must have some specific goal a bit more involved than mentioned thus far.
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You're right that there are easier battery approaches. The nimh fast-charge board for the PPA is a great design that can easily give you the same amount of charge capacity with a dozen nimh AA cells. I'm interested in experimenting with these because they provide the same capacity and rechargeability in a truly pocket-sized enclosure. |
Contrary to what my devil's-advocate post would seem to imply, I'm not dead-set against using Li-Poly cells given the need... but it seems like like a "I can therefore I will" rather than an expressed need, at a substantially higher cost, complexity, and potential danager. Accidents tend to happen because of the things one
doesn't forsee.
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NOTE AGAIN: YOU MUST NOT USE THESE WITHOUT A RELIABLE CIRCUITS THAT CUT THE BATTERIES OUT WHEN THEY'VE CHARGED UP TO PEAK POWER OR DISCHARGED BELOW 2.8V PER CELL. ANY OTHER CONFIGURATION IS POTENTIALLY VERY DANGEROUS. |
I think it's good and useful that you're being a pioneer here, but along with the benefits also come the drawbacks. It would only be fair to point those out to keep the discussion on an even keel.