# Amp limit of a Battery

1. Aug 8, 2014

So I am in the process of building various solenoids for some projects I have. But the issue I have is that the battery I use is over heating. From my understanding, when I connect a circuit to a battery, the energy gets dumped very quickly and overheats the cell. Is there a way to calculate how much amps is being delivered through the circuit? and also a way to calculate if a certain battery will tolerate that current without it getting extremely hot? I am not sure if what I said made sense, but any help will be appreciated.

2. Aug 8, 2014

### Okefenokee

You can easily calculate how much current the battery will deliver.

Current = Voltage / Resistance.

Measure the resistance of your solenoid. Let's say that it's 0.1 Ohms. Measure the voltage of your battery. Let's say that it's 12 Volts. Your current is going to be 12/0.1 = 120 Amps.

Your second question is a bit more complicated. The Ampacity of a battery depends on many different things. The manufacturer will usually have a rating for the battery that tells you how much it can continuously deliver. Don't get confused though. There are many figures that just tell you how much the battery can deliver in a short burst. Also, there's no one standard way to describe how much current a battery can deliver. Every manufacturer has their own quirks.

You may see some ratings listed as something like 0.8*C. That just means that batteries of that type can continuously discharge 80% of their charge capacity. For example, if you have a lead acid battery rated for 35 Amp/Hour and for 0.8*C then you can get a steady 28 Amps from it.

0.8*C is fairly average for lead acid batteries by the way.

If you have a specific question about a specific battery then provide a link to it and I'm sure someone can help you.

Let's talk about your solenoids. The coil in the solenoid doesn't have much resistance. It takes an extremely long wire to offer much resistance. This means that you are practically shorting your battery. You need a current limiting resistor in series with your solenoid. The resistor should allow only enough current to flow so that the solenoid operates reliably. We would need to know much more about your project to suggest a resistor value.

Last edited by a moderator: Aug 8, 2014
3. Aug 8, 2014

### sophiecentaur

Using high values of current is a slightly unusual problem and conventional circuit ideas do not always help. It is important to match the supply to the coil resistance and current that you want. You could be cooking your battery unnecessarily.

It strikes me that you may do better with a different battery. What is the voltage of the one you are using? There are some very high current capacity rechargeable D cells, which could be connected in series to give you 2, 4 or 6V. and which have a 400A short circuit current. (Cyclon)

Another way to deal with this problem could be to use a short duty cycle for your solenoid - if that is possible in the context of your experiment.