# Homework Help: Battery specifications question

1. Mar 4, 2015

### xzibition8612

A battery is rated as follows:

2600 mah
12 V
5C

My question is what does each of these values mean?

2600 mah is the "capacity" of the battery. To my understanding this means you can draw 2600 ma current from this battery for exactly one hour; after one hour the battery is dead. Or you can draw 1300 ma for two hours...etc. Whatever your design desires.

12 V is the voltage of the battery. My understanding is if you want to connect this battery to a motor or something similar, the voltage needs to be the same or the battery/motor would be damaged? But I've also read that as the battery is drained its voltage changes, so how does this not damage the device its connected to?

5C is the "capacity" multiplied by 5. This means this battery can produce 5*2600 = 13000 mah. This means the battery has a max capacity of 13000 ma for one hour. Then doesn't this make the first 2600 mah value meaningless and redundant? This is the one that is confusing me greatly.

Also suppose I know this battery has energy density of 173 wh/kg. Also assume the battery weighs 10 kg. Then it has 1730 wh. 1730 watts for one hour. How is this related to 2600 mah value? They must have some relation because both are basically describing power for one hour.

Thank you very much for answering my questions.

2. Mar 4, 2015

### Staff: Mentor

The C is a standard abbreviation*, here it's 2600mA. It's likely that 5C is the maximum discharge current, though it's possibly the recommended charging current. I'd need further information before being able to conclude which.

* actually 'capacity'

3. Mar 4, 2015

### xzibition8612

discharge. this isn't an actual battery, just roughly of the things I'm seeing. I need to know what they mean. Its discharge.

4. Mar 4, 2015

### Staff: Mentor

Then 5C would be the maximum safe current drawn.

5. Mar 4, 2015

### xzibition8612

so I could draw 13000 ma for one hour? But it said originally 2600 ma for one hour. I'm confused about this. thx

6. Mar 4, 2015

### Bystander

For "0.2 hr.," probably not more than a tenth. That's maximum safe current until the battery runs out of stored energy.

7. Mar 5, 2015

### xzibition8612

i see. So the original 2600 mah is the actual capacity, and the "C" is the maximum amount of charge you can get from the battery, provided it still is limited by that original 2600 mah limit.

8. Mar 5, 2015

### Staff: Mentor

Actually, C is the 2600mAH capacity. But C is thought of as 2600mA when discussing charging or discharging current. In this case, the maximum safe current is specified as 5 x 2600mA, i.e., about 13A

9. Mar 7, 2015

### CWatters

The simple answer is... When a battery is discharged the voltage falls. Motors, light bulbs etc are usually damaged by over/excess voltage but not under voltage. So they just slow down or get dimmer.

It's not always that simple... Some types of load (for example some pressure washers) draw constant power from their supply. Power = voltage * current so if the voltage falls the current might increase to keep the power constant. If you use one of these on a long extension lead there can be voltage loss in the extension lead due to it's resistance. That can cause the pressure washer to draw more current and that increased current can damage the pressure washer. If you don't understand this perhaps stick with the simple answer for now.

No they aren't quite describing power for one hour...

1) The "wh" in 173 wh/kg stands for Watt Hours per Kg. That's equivalent to Power * Time/Mass
Now
Power * Time = Energy
so
WH/Kg is actually Energy per unit mass or "Energy density"

2) The "mah" in 2600 mah stands for milli Amp Hour. That is Current * Time

Remember that Power = Current * Voltage so to convert AH to WH/kg you need to multiply the mAH by the voltage and then divide by the weight. For example lets say you had a 2600 mAH 12V battery that weighs 0.5Kg.

The energy in the battery is 2.6AH * 12V = 31WH (or 31 * 60 * 60 = 111600 Joules)
The energy density is 31/0.5 = 62 WH/kg

Last edited: Mar 7, 2015