Rechargeable Battery solution for Homemade Blooth Speaker

How many amps?

I have done something similar but for ease of use made sure everything would run from 5v and use a powerbank as the power supply.

Mine uses a raspberry pi rather than a Bluetooth board but the concept is the same.

What old radio are you using?

There is a range of standard Lithium Polymer batteries in cylindrical format, and various modules for working with them, such as the one below:

Ali Express

You may be able to find similar on eBay, perhaps from UK Sellers.

These are the cells. The standard voltage is 3.7v, so if you want to get to 25v, you'll need several, or some kind of voltage converter.

GENUINE-LG-HG2-18650-3000-MAH20-30A-HIGH-DRAIN-IMR-RECHARGEABLE-Li-BATTERIES

Using this technology, you'll need to ensure that either the cell or the charging circuit has built in protection for over/under-charge.

andybirch wrote: »

Not sure, if I'm honest. Will be running a bluetooth amplifier board (2 x 50w), 2 x 4 ohmn 40w speakers, so not a great deal I don't think.

honestly I would rather spend £50 on something like this anker bluetooth speaker as it probably would be cheaper, and perhaps up to the job? https://www.amazon.co.uk/Anker-Wireless-Bluetooth-Radiators-Subwoofers-Black/dp/B0109TWZ54/ref=sr_1_7/258-3838293-9989061?ie=UTF8&qid=1537864802&sr=8-7&keywords=anker+bluetooth+speaker


between 8 & 25v. You do realise that that 50w quoted is at full 25v power which calculates to a 12.4ohm speaker, but it gives gives of room to play with voltage. Also if you double the voltage on the same resistance the power goes up four fold.

for 50w @ 4ohm generally speaking you will need a 14.142135623730951v power source (car battery).
if the battery drops to 12v@4ohms you get 36w.

if you go mains, using a linear power module you will loose a shed load of power just to keep the voltage stable, and switched mode is the preferred way to go - if possible, and if it can deliver the peak amps.

The LG HG2 18650 3000mAh20/30A HIGH DRAIN IMR RECHARGEABLE Li BATTERIES, you are looking at 4 batteries, which when they are brand new fully charged, connected in series gives 14v@3Ah into 4ohms, and to redo ohms law is 49w power which needs 3.5A per channel

so if you are pumping 2x50w=100w. The four cells can supply 3Ah, and with 4 connected in series gives 14v, but lets being generous call it 12v, as that peek voltage will drop below this in an hour, so 12x3ah=36w, but you need 100w. You need three times as many batteries for one hour = 12 batteries per hour, and how may hours do you want to run this?

Not taken into account is the discharge curve and discharge voltage, nor losses of circuit, or battery or heat. How long will it take to re-charge all these batteries? You will probably not be running this at 100w all the time, or any time.

If you are using a 40w speaker as a 3w speaker, then that 40w speaker will waste more power compared to a 4w speaker, as 40w is more robust and needs much more energy to make it move