Watts Hours 

Watt Hours (Wh) Please read my article "Amp Hours a Beginners Guide" before you read this one. Watt hours are slowly becoming a more universal term than Amp hours to describe battery capacity.  Watt hours is a more accurate way of describing the capacity because it takes into account the voltage of the battery. It is a truer measurement of the actual amount of energy in a battery compared to Amp hours Different batteries have different chemistries.  They are made differently and perform differently.  Companies use the term Nominal Voltage to describe the basic voltage of a battery.  The voltage of all batteries, no matter what type, changes as it is discharged. For example a Flooded Lead Acid (FLA) battery may go from 12.65V when it is fully charged, down to 11.58V when it is fully discharged. These batteries have a Nominal Voltage of 12V.  It is important to note that this number is not the average voltage. This is a realistic number chosen by the manufacturers of Flooded Lead Acid (FLA) batteries to market the battery based on the chemistry on this type of battery.  An Absorbent Glass Mat (AGM) battery may have a voltage of 12.89 at a full State of Charge (SOC). and a voltage of 11.64V when fully discharged. This battery also has a nominal voltage of 12V. Lithium Iron Phosphate (LiFePO4), May have a full SOC voltage of 13.8V and drop down to 11.2V when fully discharged. These batteries have a nominal voltage of either 12.6V or 12.8V depending on the manufacturer. 

 

So, how does nominal voltage apply to measuring a battery's capacity using Watt Hours? Let's first understand what a Watt is. A Watt is a joule per second. It is a measurement of Energy per Time. At the moment, that does not help us though. Instead we will consider a Watt the current of 1 Amp and a potential voltage of 1 Volt.  More simply explained, as a Watt = 1 Amp X 1 Volt.  Even more simply, Amps times Volts = Watts.  Being we are comparing the concept of Amp-hours to Watt-hours we can jump right to the conversion.  We just said that Watts are A x V.  We can simply convert Ah to Wh by multiplying by the voltage! 

 

Let’s look at a few examples of this and then see why Watt hours are better than Amp hours when describing battery capacity.  Example 1. A 100Ah Flooded Lead Acid (FLA) with a nominal voltage of 12V has a theoretical Wh of 12V X 100Ah = 1200Wh.  (sadly, this lead acid battery can only be discharged to ½ its rated capacity without damaging it) So, 1200Wh is really only 600Wh. Now let’s look at a 100Ah Absorbent Glass Mat (AGM) with a nominal voltage of 12V. Again, 12V X 100Ah = 1200Wh. This battery type can be discharged to about 60% its rated capacity.  1200Wh X 60% = 720Wh.  Next we will look at a 100Ah Lithium Iron Phosphate (LiFePO4). This type of battery has 6 major advantages and 1 disadvantage.  1) It can be discharged to 100% of its rated capacity, 2) It can be charged and discharged at higher currents than the other types, 3) It can last up to 10 years, 4) It has a greater energy density, meaning that it is drastically lighter, 5) It has a higher nominal voltage. 6) It can have a nominal voltage of 12.6 to 12.8. The significant disadvantage is its cost.  Okay, let's look at how that higher nominal voltage applies to Wh. A 100Ah LiFePO4 battery may have a nominal voltage of 12.6V  It's Wh measurement is 12.6V X 100Ah = 1200Wh. Because this does not have the discharge limitations like the other two types we discussed, it can be discharged to 100% of its capacity (best to not go below 90%). 

 

So, why are Watt Hours better than Amp Hours when measuring a battery's true energy?  Here is an example.  I have 2-100Ah batteries.  Although the Ah are the same the voltages are different.  One is a 12V battery, the other is a 24V battery.  Although they have the same Ah rating the 24V battery has twice the Wh rating. Another example is 2-100Ah batteries, one with a nominal voltage of 12V and the other with 12.8V.  The 12.8V battery has 6.7% more Wh.  See the graph below for an approximation of the Depth of Discharge of a Lithium Iron Phosphate (LiFePO4) and a Flooded Lead Acid Battery. 

​

The following is also in my "Amp Hours a Beginners Guide" article but I felt it is worth putting in this article too.

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 charges.  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. 

​

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

​

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

Furrion High Precision Battery Monitor

Here is a link to the user manual User Manual 

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

​

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

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

​

Of all of the battery monitors I have used, the Victron Energy Bluetooth Smart Shunt 500 amp Battery Monitor is by far the easiest to use and to install and it gives you the most data. The price was $130 for a long time but I am seeing now at $117. 

​

Other companies are slowly coming out with battery monitors that work well. For example, The LiTime Battery Monitor with 500A Shunt   and Renogy Battery Monitor with 500A Shunt   are both decent monitors.  However these are not bluetooth.  With both of these you need to snake wires from the shunt to where you want the screen.  Both are around $85.  I would spend the extra $30+ to get the Victron Energy Bluetooth Smart Shunt.

​