Can I use a bigger battery

Published March 4, 2018 by Sean

This is a common question which people starting out in Electronics struggle with. Batteries have ratings in volts, and mAh. Sometimes they’re rated in C or A too. Trying to understand what you can safely change, and what won’t work is difficult.

Batteries come in multiples of 1.2, 1.5, 2, 3 and 3.7 volts (actually, there are a lot more, but these are the most common ones). The rectangular 9 V batteries are actually 6 of the 1.5V type packed together in a small case. A car battery gets to 12V by taking 6 of the 2V type. In a laptop, you’re likely to have 3 of the 3.7V, giving 11.1 volts. Given this, it might be easy to see that sometimes you will want to swap out one battery for another in a given circuit.

In the other side, you’ll have a circuit so some sort, rated to take a specific current (or power) at a defined voltage. A small computer board (like a Raspberry Pi) will take about 200mA at 5 volts. Actually, this varies quite a bit, and when it’s working really hard, it might take 500mA. Circuits usually specify a maximum current, and sometimes will also specify a typical current. What matters most with a circuit like this is the voltage range which the circuit is designed to operate on.

In the case of the raspberry pi, this specification is 4.75-5.25V. What we can assume from this is that providing less that 4.75V might cause the circuit to stop working (temporarily, possibly causing problems depending what you’ve using it for). Between 4.75 and 5.25, you’re guaranteed to be safe and reliable. Above 5.25V, there is a risk of permanant damage. Not always immediately, and not always for every part. We can’t make any guesses about how safe 5.3V would be, even if we have tried it already on one part and not seen any problems. Based on this, we can’t safely use a combination of 1.5 V (alkaline cells) or 2V (lead-acid), but we could try 1.2V (NiCd) because four of them would give 4.8V. Unfortunately, as soon as we start to use a battery, it’s voltage drops a little (like a small engine under load), and we’d see that the computer might start to work, but fail after a few minutes or when it takes a bit more power than the average.

In the case of powering a small computer from batteries, we probably need to use a circuit called a voltage regulator. This either absorbs some of the extra voltage (a linear regulator), or actually translates the voltage directly to what we need (a switching regulator, more efficient, but more complex).

Lots of circuits already have built in regulation (so they are more tolerant of input voltage). Maybe a circuit can accept anywhere between 3V and 7V. In this case you can safely use any combination of batteries which give this voltage.

The current or charge capacity of a battery is usually only critical in determining how long a circuit will operate for. The voltage is what drives the operation, and the circuit only takes as much current as it needs. This means that there is no harm in providing a battery of the same voltage, but with a bigger capacity. On the other side, it’s not quite as simple. If your circuit is a motor which takes 5 amps (in a robot, for example) then a small battery won’t be able to provide it’s rated voltage when you draw such a large current. Different battery types vary, but if the battery would be completely discharged in less than an hour, you need to check if the battery is suitable for the application. High current batteries can be designed to discharge completely in 5 minutes or less, but these are very specialised applications and risk both overheating when there is any small fault, or damaging the battery (if any part of the battery overheats internally).

In summary:

  • Battery voltage must be close (depending on the specification of the equipment.
  • Higher capacity batteries are OK as a replacement.
  • Lower capacity batteries might not work.

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