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I have a motorcycle battery rated as 8.4Ah (20HR). It also says 8Ah (10HR) and I am not sure what´s the difference.

Anyway, I have a GPS tracker fitted with a rated consumption of 5mA. According to this formula I have to divide battery capacity by consumption:

8.4 from battery / 0.005 consumption = 1680 hours (70 days)

Is this correct? The reason why I am posting this is because the battery says 8.4Ah, which is only double as many mobile phone batteries:

Why this motorbike battery is so heavy then if it's only double the capacity? Is it because it's a 12V? My knowledge in electronics is zero as you can see.

Thanks and happy NY!

I have a motorcycle battery rated as 8.4Ah (20HR). It also says 8Ah (10HR) and I am not sure what´s the difference.

The capacity decreases at higher discharge rates. The figures are telling you that discharge current of $ frac {8.4 text {Ah}}{20 text h} = 0.42 text A $ will last 20 h and $ frac {8.0 text {Ah}}{10 text h} = 0.8 text A $ will last 10 h. Given that doubling of discharge rate the total energy out is actually surprisingly close.

8.4 from battery / 0.005 consumption = 1680 hours (70 days).

Correct method. (I didn't check your numbers but they look right.)

Why this motorbike battery is so heavy then if it's only double the capacity? Is it because it's a 12 V?

Two reasons:

• As you suspect, the 12 V is related. The energy stored in the battery is given by $ V times I times t $ so a 12 V battery will have three times the energy storage of a 4 V battery. Your 12 V battery has a capacity of $ 12 times 0.42 times 20 = 100 text {VAh} = 100 text {Wh}$.


• Battery chemistry. Your mobile phone battery is a lithium based battery. According to Green Transportation :
  
  
  
  
  • Lead acid energy density is 33 - 42 Wh/kg so we would expect your battery to weigh about 3 kg.
  
  
  • Lithium ion energy density is 100 to 265 Wh/kg so we could get the same energy storage in a 0.4 kg battery.
  
  
  
  

Figure 1. Energy densities for various technologies. Source: EPEC.

From Here:

If a battery has a rating of 100AH @ 20Hr rate, then that battery
was discharged over 20 hours with a 5 amp load. Starting batteries, on
the other hand, are typically rated at 10Hr rate, because they are
used faster, so the 20Hr rate is not as important.

A battery has an internal serial resistance. Faster you discharge, higher are the losses, that is why they provide different capacity for different discharge rate.

In your case since you discharge very slowly you can take the first value.

Your formula is correct. However batteries also have a self discharge rate that you might want to consider if you need long period.

Why this motorbike battery is so heavy then if it's only double the
capacity? Is it because it's a 12V?

This part isn't clear, you can edit your question and I will edit the reply.

If it's only the double capacity of "what" ?

You need to multiply the capacity by the voltage to know the total energy stored, so 8.4 * 12 = 100.8Wh.

A 8.4Ah 5V battery will have less than half capacity.

Also Lead-Acid batteries (probably what you have) are heavy and low power density. Li-ion batteries (like on the phones/laptop) have much higher power density so it will be much lighter for the same amount of stored energy.