What is an Amp Hour?

The term Amp Hour- (Ah) comes up a lot while talking about RV batteries.  I have often heard people express that they do not understand what this means. My hope here is to explain this concept and to give some examples as to where it applies.  

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Amp Hours is basically a measurement of capacity. It tells us the capacity of a battery.  Essentially it tells us how much energy it can supply.  The more Ah a battery has, the more energy it can supply.  A 200Ah battery has twice the capacity that a 100Ah battery has. Think of it as a bucket of water, The larger the bucket, the more Ah or, in the bucket’s case, the more water it can hold.  So what exactly is an Ah and what does it really mean?  One Ah, is the ability to supply 1 amp of current for exactly 1 hour.  For example, a 100Ah battery can deliver 1 amp for 100 hours. If it was delivering 100 amps it would only last 1 hour.  It can also supply 50 amps for 2 hours or 25 amps for 4 hours etc…  Notice that no matter the amps and times referred to in those above examples, all equal 100Ah.  1A X 100h =100Ah, 100A X 1h=100Ah,  50A X 2h = 100Ah, and 25A X 4h = 100Ah. 

 

How does this apply to its actual use?  Okay, let’s say you are using 10 amps from your 100 Ah battery.  How long can you do this before it is dead. First, an Amp Hour is really amps multiplied by hours. Amps X hours = Ah, as shown in the above examples.  To standardize the measurements of batteries, we always state the number of amps it can deliver in ONE hour.  A 100 Ah battery can deliver 100 amps for 1 hour.  A 200 Ah battery can deliver 200 amps for 1 hour.  Notice that in describing the battery capacity, we always use the amps delivered in ONE hour.  However that is not really how we use batteries.  We use them for more than 1-hour, I hope.

 

Okay, back to that 100Ah battery using 10 amps.  How long will it last?  100Ah / 10A = 10 hours. Wait, where did that equation come from? I used algebra: If A X h = Ah then Ah/A=h. Now when I know the capacity of a battery and I know the current (in amps), I can find the time (in hours) it will last. 

Let's use some more complicated numbers this time. Now let’s assume the 100Ah battery is delivering 7.5 amps.  How long will this last? 100Ah / 7.5h = 13.33 hour or 13h and 20 min. Let’s use that 7.5 amps now with a 200Ah battery.  We know the 200Ah battery is twice the capacity so we can guess it will last twice as long. Let’s check the math anyway.  200ah / 7.5 A = 26.66 or 26h and 40 min. Yup, that is twice the time. We got it, battery capacity divided by the current = the run time.

 

So here is a real world example for your RV. You have a 50 Ah battery, the weather is hot. You need to cool off the RV.  Your overhead vent fan uses 3.5 amps at full power.  How long will it last?  50Ah / 3.5A = 14.29h or 14h 17.4min.  Now is this realistic?  Not exactly. The reason is because you have other things running and may or may not also have some solar recharging your battery.  However this does give a good base ground to start at understanding how the Capacity measurement of a battery applies to its use in your RV. 

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Using a Battery Meter

One thing I need to point out here is that a battery's voltage is dynamic. Yes, it changes as its state of charge changes.  But it also changes as you use it.  Your battery may have a resting voltage (not being used) of 12.6 volts.  However as soon as you begin to drain the battery, the voltage immediately drops.  Also, the harder you work it, the lower the voltage will drop. But, when the battery stops being used the voltage will slowly recover. For example, the 12.6V battery may show 12.1V when being used.  Then, it will slowly recover to 12.5V when the load is removed. It is important to understand this when using Voltage to measure your batteries remaining capacity.  A voltage meter will work but it is not a very accurate measure of the battery’s true SOC.  You can find tables published online to relate a battery’s voltage to its SOC. See the graph below for an approximation of the Depth of Discharge of a Lithium Iron Phosphate (LiFePO4) and a Flooded Lead Acid Battery. 

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Here is link to some well written articles by Alex Beale found on his website, Footprinthero.  He has some tables he created showing different batteries Voltage Vs. SOC

https://footprinthero.com/lead-acid-battery-voltage-charts

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The following Battery monitor comes preinstalled in the stock Grand Design 226RK

Furrion High Precision Battery Monitor User Manual 

https://support.lci1.com/documents/furrion-high-precision-battery-monitor-user-manual-im-fen00032-v3.0

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Here is a link to the many battery monitors from Victron Energy

https://www.victronenergy.com/battery-monitors

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