what capacity battery for mini bluetooth amp

[Sub_2_XanimalXG]

im building a custom bluetooth speaker using this amp:     https://www.ebay.co.uk/itm/Car-Bluetooth-2-1-Hi-Fi-Bass-Power-AMP-Car-Mini-Digital-Amplifier-Audio-USB-ald/254160724883   

i want to know what capacity battery would i need if it was running for 12 hours, thanks in advance -Xan

[HanZie82]

Well thats a calculation of the voltage and wattage it uses. I checked the description its not really clear on power usage. Just that its 50Watt and can use AC 110-220 V, DC 12-24V.

 

I would think if this is really what you want, order it then use it on a lab power suply to find out the Amps drawn at certain voltages/volumes So then you can calculate the battery you need.

 

 

edit: please correct my math if its wrong, im sleepy.

But 50 Watts at 12Volts is about 4Amps. So 12(for the hours) times 4 amps is the size of the 12V battery you would need. (edit; which means 48AmpHours)

edit2: If you go for a 24V battery the Amps need can be halved.

[johnnyappleseed]

To run at max power, 50 watts for 12 hrs is 50 * 12 = 600 watt hours.  With a 12V battery, 600 wh / 12V = 50 amp hours. With a 24V battery, thats 600 wh / 24V = 25 amp hours.  Those are some pretty large numbers, would get expensive fast.  Random google search found this store https://www.batteryspace.com/12-8v-lifepo4-battery-packs-from-19.8ah-to-200ah.aspx , not sure if these are the best price you could get.  Probably better to assume you wouldn't be using it a max power all the time if you want it to be reasonably priced.  Maybe assume 1/2 power for three to six hours? Could start to look into constructing your own battery pack from 18650 cells. I would consider purchasing a nicer amp, since you will probably end up spending a fair amount on batteries.

 

I believe the number @HanZie82 came to was the rated amps your pack should be capable of.  50 W / 12 V = 4 and a bit amps. 50 W / 24 V = 2 and a bit amps.  There's a difference between amps and amp hours.  Amps is the amount of current flowing thru something, think flow rate of water.  Amp hours is the capacity, so think how much a tank of water would be filled up in a period of time with a certain current.  https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/

[HanZie82]

33 minutes ago, johnnyappleseed said:

I believe the number @HanZie82 came to was the rated amps your pack should be capable of.  50 W / 12 V = 4 and a bit amps. 50 W / 24 V = 2 and a bit amps.  There's a difference between amps and amp hours.  Amps is the amount of current flowing thru something, think flow rate of water.  Amp hours is the capacity, so think how much a tank of water would be filled up in a period of time with a certain current.  https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/

Totaly correct i was not mentioning Ah enough. Im sorry, im sleepy and have indeed dropped the bal on that one.

 

 

Also keep in mind 50Watts is the max. It would not use that continuously, even on full volume. So you might be able to get away with smaller pack than rated for 12 hours on max power usage.

[mariushm]

Can you take some CLEAR pictures of the boards inside that product and post them here?  Maybe write down what's written on the chips?

If so I and others can give more details. See below vvv

 

It's a bit more complicated.

 

The numbers in the listing are most likely the PEAK values, the maximum the audio amplifier chips can do with significant distortion and bad quality.

In order to accurately determine how much this thing would consume, you actually need to figure out what amplifier chip or chips are used.

 

The maximum output of an audio amplifier chip is directly related to the input voltage the amplifier chip gets.

For example, an audio amplifier chip may work with anything between 10v and 36v, but with 12v it may only output maximum 5 watts, at 24v it may do 15 watts and at 34v it may do 25 watts per channel with minimum distortions, like 0.1% THD. With gain set high and assuming 10% THD, the amplifier chip could advertise 10w at 12v , 25w at 24v and 40-50w with 32v input.  But with 10% THD, the music quality would suck, you'd get distortions so you don't want that.

The thing accepts either DC input 12v .. 24v , or can be powered from mains, which means the device has a small switching power supply that probably outputs 24v from AC. 

This means the device has an amplifier chips which works with less than 12v and at least 24v. It probably works with higher voltage but I think they use some cheap voltage regulators which convert 12v...24v to 3.3v or 5v, whatever is used by the bluetooth microcontroller and mp3 decoder and card reader to work

Without checking the regulator chips and the capacitors used, it would not be a good idea to use a voltage higher than 24v.

 

Anyway, so you can use voltage as low as 12v and as high as 24v... higher voltage means amplifier has the possibility of outputting higher volume, more power.

 

Let's be a bit more realistic and say that with around 12v input, the amplifier will produce up to 20w of sound, with around 90% efficiency, so overall it will take 24-25w from the DC input.... or around 2A.

Let's look at a lead-acid battery datasheet.

 

Here's a 7.2Ah one, that you'll typically find in a cheap UPS : https://www.digikey.com/product-detail/en/panasonic-bsg/LC-R127R2P1/P218-ND/284836

Here's datasheet : https://na.industrial.panasonic.com/sites/default/pidsa/files/panasonic_vrla_lc-r127r2p.pdf

So you can look at that picture on the right, and you can see that for 1.44 A constant discharge, the battery will go from 13v to 10v in around 4 hours: 

 

If it discharges in around 4 hours at 1.44A, then at 2A it will discharge in less time. This is assuming you'll output a constant 20w of audio power, if you keep the volume the same.

 

But, you can go with a bigger battery, like one used for cars or trucks, but the UPS variety - the lead acid batteries used for motorbikes and cars and trucks are designed for short high current bursts (to turn on engine), not for continuous discharge.

Here's for example a 28Ah battery, which costs a lot more : https://www.digikey.com/product-detail/en/panasonic-bsg/LC-X1228P/P049-ND/265784

Here's the datasheet: https://na.industrial.panasonic.com/sites/default/pidsa/files/panasonic_vrla_lc-x1228p_lc-x1228ap.pdf

 

You can see the graph there, which shows how at 1.4A of constant current discharge (~15-20w), the battery would last around 20 hours :

 

If you don't want to pay so much for lead acid batteries (and be so heavy), you have options of using lithium batteries ( power banks) or NiMH batteries.

 

NiMH batteries ... there are C or D size NiMH batteries that have high capacity. C size is typically 3600 to 5000mAh , D size is up to 9000mAh but it's common to have around 5000mAh.

Here's a 5Ah C size example, 10$ for 2... in reality it's probably more like 4..4.5 Ah : https://www.amazon.com/EBL-5000mAh-Ni-MH-Rechargeable-Batteries/dp/B00ZA7PYE0/

So with 10-12 of these in series, you would have a battery that's around 10-14v DC 5Ah, almost as much energy as that 12v 7Ah lead acid battery. but lighter.

 

With lithium batteries/power banks, you have to keep in mind that their voltage is much lower at 3.7v..4.2v and there's a circuit inside which raises this voltage to 5v and that boost process is not 100% efficient, it's only around 92-95% efficient. If you change the circuit to raise the voltage to around 12v, it would be even more inefficient. 

So if you need 28Ah from a lead acid battery to have an amplifier consuming 25w for around 20 hours, you'd probably need 3..3.5 times as much from a lithium battery pack (because the average voltage from the lithium battery is 4v, so the step-up regulator takes 3 units of "4v energy from lithium battery" to create 1 unit of "12v energy" and wastes around 10-15% due to inefficiency of conversion... so 3.2-3.5x more capacity is required to match the lead acid battery.

So my guess is for 20 hours or so of continuous running at 25w, the amplifier would need a power bank with at least 50000 mAh

 

There are battery packs made out of multiple lithium batteries in series and in parallel, and there are chargers which can handle such batteries.

For example HobbyKing has such batteries... here's just an example (can't vouch for it, haven't use them) :

 

https://hobbyking.com/en_us/turnigy-high-capacity-battery-20000mah-4s-12c-drone-lipo-pack-xt90.html

It's a 20000mAh pack with 4 batteries in series, giving you a voltage between 4 x 3.7v..4.2v

Here's a charger that's designed to charge battery packs with up to 6 batteries in series (up to 6x3.7..4.2v = ~ 24v): https://hobbyking.com/en_us/imax-b6-50w-5a-charger-discharger-1-6-cells-genuine.html?___store=en_us