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A Yamaha digital piano is powered by a mains power-supply. The power-supply states the following input/output values:
INPUT 100-240V~50/60Hz 0.5A
OUTPUT 12V === 1.5A
(Where I have written === there is actually a solid line over a dotted line).
He is keen to go busking with this piano. I am therefore looking to substitute the mains adapter with a 12V battery which I have to hand. I don't want to risk damaging his piano. Should I place some sort of circuit (voltage regulator?) around the battery to protect the piano. Can anyone assist in telling me exactly what the circuit diagram should be?

pic1 pic2

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    \$\begingroup\$ @newbie 7 Ah! It’s an entirely different physical quantity. \$\endgroup\$
    – winny
    Jul 28 '19 at 11:08
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    \$\begingroup\$ @newbie That’s besides the point. You are comparing kilometers and kilometers per hour as they where the same thing. “How far is it?” “120 km left now.” “How fast are we going?” “90 km/h” “Do we need to walk the last 30 km?” \$\endgroup\$
    – winny
    Jul 28 '19 at 11:14
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    \$\begingroup\$ @journeyman Please measure the unloaded output voltage from the adapter with a multimeter and post the result to give a definitive answer for you. \$\endgroup\$
    – winny
    Jul 28 '19 at 11:15
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    \$\begingroup\$ @winny oh lol i didn't get your point the first time! yes it's 7A per hour not 7A thanks for correcting me. \$\endgroup\$ Jul 28 '19 at 11:16
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    \$\begingroup\$ @newbie Gah! You are making it worse. IT’S not per hour! Ampere MULTIPLIED by hour = Ah. \$\endgroup\$
    – winny
    Jul 28 '19 at 11:24
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The power supply looks like a regulated 12V switchmode power supply, therefore it should output 12V under varying load conditions. The battery is not regulated, a charged battery can have more than 13V and the voltage drops when discharging the battery. If the piano needs regulated 12V then connecting to a battery is a bad idea. It is also possible that the power supply is unregulated so the piano might work with the battery. If you have the service manual of the piano maybe a safe voltage range can be determined. Perhaps a range reads on piano 12VDC input connector. It is not even clear how much current the piano needs, it may read on the piano or manual. If you do try this, at least put a fuse (2 Amps?) right at the battery terminal to protect people from burning wires and exploding batteries.

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  • \$\begingroup\$ The 12V SLA battery shown will work just fine, until its voltage drops below some threshold. Then you will start to lose volume, and if you persist, you will lose functionality. But a well designed piano will not be harmed. Be careful to observe the correct polarity when connecting the battery. And there is some merit to the idea of a 2A fuse (it certainly can't hurt). \$\endgroup\$ Jan 20 at 2:02
  • \$\begingroup\$ @PhilFreedenberg How do you know it will work just fine? Have you seen the schematics to verify it, or tried yourself, and there is no long-term effects? Otherwise, you are just guessing. \$\endgroup\$
    – Justme
    Jan 20 at 5:16
  • \$\begingroup\$ Because Yamaha doesn't put their name on junk \$\endgroup\$ Jan 20 at 18:27
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A lot of people do exactly this with these electronic pianos (not only Yamaha) with no ill effect.

There is no such thing as "exactly 12 volts". Power adapters have voltage tolerances (because of varying load, varying mains voltage, internal elements aging and just because they are not created equal).

That's why electrical appliances (both using adapters or directly powered from mains) do tolerate some range of power voltages.

If there is no clear tolerance stated on the label, one can safely assume +/- 20% is at least safe and +/- 10% should not affect the appliance performance in any way.

A "12V" lead-acid battery in fact goes between 10V (fully depleted) and 15V(end-of-charge state for a rather bad charger). If the charger is disconnected, the battery voltage can be assumed below 13.5V . The useful voltage range more or less fits between these 10% limits.

What can go wrong, then?

  1. Fire safety. The original power adapter has a reasonable limit of what amount of power it may put out. This practical limit is usually supplemented by various protection features (short protection, overload protection, thermal protection) that together make a dangerous failure modes pretty much unlikely.

A battery like this has none of these features. When not fully depleted, it can happily give out as much as 70-100 amperes, melting wire insulation, heating wires to red glow and, given chance, starting a fire.

It is up to you to think about at least some basic protection (e.g. fuse) and operate the whole thing in a safe manner.

  1. A battery quickly deteriorating because of over-discharge Out of thin air, I think this piano will play just fine when powered as low as 7 or 8 volts. Maybe the max volume will be somewhat less, but one rarely plays always at 100% so at some point the piano will just stop (or the sound will become distorted) and at this point the battery is probably over-discharged.

A small panel voltmeter with low self-consumption may prove useful. Just take care not to discharge the battery below ~10.5V .

  1. A battery quickly deteriorating because of inadequate charging.
  • Lead-acids don't like being stored in even partially-discharged state. Charge them as soon as practical and don't expect anything good if the battery is left discharged for a month or two.
  • Not all chargers are made equal. There are good and bad ones. A 3-stage, microprocessor-controlled charger may make the battery good for as much as 1000 cycles (but may as well cost more than the battery). A cheap constant-voltage or (almost) constant current charger may fry the battery for 10 cycles or less. YMMV.
  1. The battery dying exactly when important and demanding listeners expect the best performance.

Some experimenting, bigger battery and/or a spare battery may work around the problem.

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  • \$\begingroup\$ Can you provide any authoritative basis for the statement "one can safely assume +/- 20% is at least safe"? Is this some sort of industry standard or is it really just your opinion, possibly based on your experience? Given that modern power supplies can easily achieve a 5% tolerance (suggesting that equipment could be designed to expect such accuracy) this seems like a bold and dangerous claim. \$\endgroup\$ May 21 at 22:47

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