How do I lower the amps without tampering with the voltage?

Question

I have old speakers and decided to make them useful again by making then battery powered but I am new to this so I'm not 100% sure on what to do. In the speakers there is a small transformer which converts the 230 volts to 12 volts and 1.2 amps but I have two 12 volt 4.5 amp batteries which I am going to use in parrellel so that I get a longer battery life. This means I will have 9 amps coming from the batteries but I don't know how to reduce the amps without interfering with the voltage. Thank you in advance.

Answer 1

Electrical devices fed from a constant voltage source such as the mains or, in your case, a battery draw the current required to operate. This is why plugging your desk lamp into the national grid which can supply gigawatts of power does not vapourise your desk.

If your amplifier requires an amp or so to run that's all it will draw from a 12 V supply no matter what the supply is capable of (providing it can supply at least an amp).

To help further, you can add up the amp-hour (Ah) ratings of your batteries and calculate the run time.

$$ h = \frac {Ah}{A} $$

so if you had two 8 Ah, 12 V batteries in parallel and your amplifier draws 0.9 A at reasonable volume your run time is

$$ \frac {8 + 8}{0.9} = 17 \; hours $$ approximately.

Put a 2 A fuse into your circuit to protect the batteries and wiring in the event of a short circuit.

Answer 2

A battery rating of 4.5A means that the maximum current it can supply shouldn't exceed 4.5A, or you risk damaging it (this can happen when an insufficient load is connected, e.g. when a battery is shorted).

As such, in this scenario two batteries working at 1.2A (connected in parallel) will operate at roughly 1.2/9 of their total available power without any negative side effects.

That being said, before connecting anything, make sure you read and interpreted the ratings correctly. Batteries are often labeled with their capacity in Ah, or ampere-hours. This is merely an indication of the energy stored in such a cell, and not the available power. If the battery was actually rated 4.5Ah, it would mean it can operate for 4.5 hours providing a current of 1A. Such a discharge rate is rather low for most common batteries I personally know of.

Answer 3

The 4.5 amp rating on the battery tells you the maximum you can safely draw from the battery.

The actual current will follow Ohm's law: I = E / R. So, at 12 volts, you'd only draw 9 amps if the battery saw an impedance of 12/4.5 = 1.3 Ohms.

Since your normal power supply is limited to 1.2 amps, we can guess that the circuit actually has an impedance of around 12/1.2 = 10 ohms, at minimum. In reality, it probably has somewhat higher impedance than that, so the current is a little lower, giving the power supply a little bit of head room--typical engineering rule of thumb would be to leave at least a 10% margin, so it's probably intended to draw no more than around 1.2 - 10% = 1.08 amps.

In any case, the current will be limited by the impedance of the circuit, not by the rating on the batteries. A problem would arise primarily if the batteries had a lower rating than the circuit might draw. That's generally a dangerous situation--connecting a battery to that low of an impedance may destroy the battery (e.g., cause overheating). In mild cases, this might just degrade battery life. In more serious cases, it can cause such serious overheating that it catastrophically destroys the battery, starts a fire, etc.