Battery capacity?

knightstyle

Is a 40v 2Ah  battery the same as a 40v 80Wh battery?  Looking for one for a Swift mower and confused by the specs.

[Deleted User]

Yes it is the same.

Watts = Volts x Amps 
or
Amps = Watts / Volts

You can use the same formula when converting Amp-Hour and Watt-Hour.

nottsphil

Watt-Hours and kWh I find quite easy to understand, perhaps because I've been monitoring my solar PV for 8 years. But I'm struggling with the concept of amp hours, so was wondering if anybody could provide a suitable analogy. 

pbartlett

amp is current flow so it is like having a 20 gallon-hr garden water barrel. it can provide one gallon an hour for 20 hours or 2 gallons an hour for 10 hours.

forgotmyname

Ah ...   100Ah = 100amps for 1 hour or 1amp for 100 hours or 25amps for 4 hours.

Not quite that simple due to the battery chemistry, that can alter those figures and drawing a smaller current

may last quite a bit longer than a high short term load. a 100Ah battery may only put out 100Ah if its a 1amp

load for 100 hours a 100amp load may only last 30 minutes.

Lead acid wont store as much energy as a LiPo battery.  I repaced 3 x 12V 4Ah batteries with a pair of

18v 5Ah batteries which weighed a mere 1kg in total yet lasted a lot longer and put out more power.

(Electric bike)

[Deleted User]

If you understand kWh then you'll know that kWh is kW x hours. An appliance that is rated at 2kW of power will use 2kWh if it is operated for 1 hour (2kW x 1hr = 2kWh). If it is used for half an hour, then it will consume 2kW x 0.5hr = 1kWh. 

Amp-hours work exactly like this but instead of being a measure of power, it is a measure of the quantity of electricity. The power (watts) from electricity comes from a combination of voltage (pressure) and amps (quantity) hence the formula Watts = Volts x Amps - more on that in a bit.

Think of a battery like a tank of water containing 100 litres of water. If you drain the tank at a steady rate over 1 hour, then 100 litres of water come out giving 100 litre-hour capacity. Substitute the word 'litres' for 'amps' and the analogy holds up. So a 100Ah battery will provide 100 amps of current for one hour.

But just like a battery, you don't have to drain the tank in one hour, you could drain it over 10 hours. So you could drain 10 litres or amps for 10 hours - either way the total (amps x hours) or (litres x hours) will still equal 100.

Coming back to what amps mean in relation to watts. We measure electricity consumption in watts because it is a measure of the useful work done by the electricity so that's why the electric companies charge us by the watt-hour and not amps or volts. 

Think of water running through a pipe, the pressure of that water is the volts, the quantity of water passing through per second is the amps. Turn on an ordinary hose pipe and it will pass say 1000 litres of water per hour, that's the amp-hours in the analogy. Water pressure is about 4 bar (volts) so the power could be said to be 1000 x 4 = 4000 power (made up units for the example but like "watts"). 

We know that garden hose pipes aren't that powerful compared to say a jet washer - we know a jet washer output has more power. Now a jet washer will actually use less water per hour - say 500 litres per hour, but the pressure is increased massively to 100 bar so the power output from the jetwasher could be expressed as 500 x 100 = 50,000 power (made up units again but like "watts").

Sorry that was long winded but the point is, the jetwasher is using only half the litres per hour [amps] of the garden hose but we know that it is far more powerful because the pressure [volts] is 25 times higher than the hose, the jet out of the end is much more powerful and therefore far more useful in certain jobs because the water does more work (power).

Overhead electricity powerlines use this pressure washer effect to great advantage - notice the jet washer pipe is much thinner than your hosepipe yet delivers far more power from the water. Electricity grids crank the volts up to 400,000 volts (pressure) and so don't need to send as much amps (litres per second) and therefore can use thinner wires (or pipes in the hose example) to transmit a massive amount of power.

Going back to batteries. I have 2 batteries that can both run 2 different 1kW heaters for one hour (one heater is 250v the other is 12v). They will both give off exactly the same quantity heat for my room but lets say:

Battery 1 is a 250v battery so to run the heater it is 250v x 4 amps = 1000 watts
Battery 2 is a 12v car battery so to run the heater it is 12v x 83 amps = 1000 watts

So battery 1 is a 4Ah battery and battery 2 is an 83Ah battery. Sounds like battery 2 is a higher capacity which is why measuring battery capacity in watt-hour is far more useful than amp-hour because they are both 1000 watt-hours batteries (1kWh) when measured in watts.