batteries - How does voltage progress during discharge of a battery?

The build:

I'm building a portable bluetooth boombox. It's a very simple build. The components I'm using are:

• Bluetooth amplifier board (TDA7492P)


• 2x50W speakers


• Rechargeable battery

The amplifier needs to receive 12V. So the battery I will get will be able to deliver this voltage.

My questions:

Now before I buy the battery I have 2 questions that come to my mind.

Will the voltage at the terminals of the Amp be 12V for the entire discharge of the battery or do I need some kind of a voltage regulator?

My other concern is the autonomy of the boombox. I want to be able to combine 2 batteries to double the battery life of the device.

Is there a way I can put together 2x12V batteries and obtain double the autonomy? But keep the voltage to 12V. Do I need to connect them in series or in parallel?

As you can guess i'm new to electronics, and any help would be very much appreciated.

Answer

Battery discharge curves and you

Batteries indeed vary in voltage as they are discharged. This is a function of the chemistry of the battery, and specified by the battery maker as a discharge curve, characteristic of the chemistry of the battery but also varying with the discharge rate and a few other parameters (such as temperature).

For instance, a 12V sealed-lead-acid battery's discharge curve is shown below (this is one of the most common types of small-ish 12V batteries):

As you can see, the battery starts off at around 13V and discharges to 9V before not having much life left in it. The good news, though, is that power amplifier ICs like yours don't care much as they are designed to work over a range of voltages; for instance, your TDA7492P can run on anywhere from 8V (a single, rather flat, 12V lead-acid battery) to 26V (almost enough to run on 2 12V lead-acids in series). The volume may go down as the battery drains, but that's OK.

I want more runtime!

You can connect batteries in series to boost the voltage (a 9V alkaline battery is really 6 1.5V alkalines in series, in a neat package), or in parallel to boost the available current capacity without changing the voltage. Since the TDA7492P can't quite take 2 12V lead-acids in series (especially during charging), you'll have to connect your 12V batteries in parallel. (This does introduce complications when you go to charge them, but lead-acid batteries are tolerant of this. Lithium-ions, on the other hand...)