Understanding Amp Hours
By Bill Peterson
If you have never dealt with deep cycle batteries, you may have run across the term “Amp Hours” abbreviated Ah. This term is used to express the storage capacity of deep cycle batteries. This is the maximum sustained amperage drawn from a fully charged battery over a certain time period to a point where the battery is at 100% DOD (depth of discharge), for all purposes dead! This amperage is then multiplied by the discharge period to give the battery its Ah rating. The period of time used to drain and measure the battery is usually 20 hours, although longer or shorter time periods may be used depending on the application.
As an example, a 200Ah (20 hr rate) battery can sustain a 10-Amp draw for 20 hours before reaching 100%DOD. 10 Amps X 20 hours = 200Ah. You may say that this same 200Ah battery should last 4 hours with a 50Amp draw, but this is not the case at all. The reason for this non-linearity is that the capacity of a battery actually decreases when the load increases. Take a look at fig. 1 to understand how the Ah ratings vary depending on the discharge period.
Let’s use the example above of a 200Ah battery and see what its Ah capacity would really be for a 4-hour discharge period. Using the chart, find the 4-hour mark and follow it up vertically until you cross the flooded battery curve. Then follow the horizontal line across until you reach a percentage. In our example the 4-hour line intersects the flooded battery curve at about 70%. This means the battery has 70% of the capacity it had when discharged at the 20 hour rate. This comes to 140Ah or a 35 Amp draw, quite a bit lower than the 50 Amp draw expected.
200Ah X 70% = 140Ah @ (4-hr rate)
140Ah / (4-hr rate) = 35 Amps
As you may have noticed, if we chose an AGM or Gel type battery, we would have had 84% of our original 200Ah, or 168Ah (4-hr rate). This means we could draw 42 Amps from the battery for 4 hours. Does this mean that AGM or Gel cell batteries are always better? In short the answer is no, not in all applications. All batteries have their strengths and weaknesses and it is beyond the scope of this article to explain them.
When shopping for a battery, do not try to compare batteries with Ah ratings based on the 20hr rate with batteries based on a 100hr rate. Flooded batteries may have between 25% to 40% larger Ah ratings when using the 100hr rate vs. their 20hr rate. AGM and Gel cells may have 20% to 25% higher Ah ratings if they were based on the 100hr rate vs. their 20-hour rate. To keep yourself from being confused, always use the 20hr rate when designing home power battery storage.
So how long can we run a 50 Amp load on a 200 Ah flooded battery? How does 2.5 hours sound? 2.5Ah rate = 63% capacity X 200Ah = 126Ah (2.5hr rate). 126Ah / 2.5hrs = 50.4 Amps! Hey we have a little room to spare, or do we?
How about throwing a few more wrenches into the works by talking about some other factors that effect battery capacity. The largest factor is battery life. The deeper you discharge a battery during normal use, the shorter its overall life span will be. In the examples above, we discharged the battery to 100% DOD. Repeatedly discharging a battery to this depth will severely shorten its lifespan. Although you can periodically cycle a battery down to 80% DOD with virtually no damage to the battery, it will still shorten its life if done on a regular basis.
Most deep cycle battery manufacturers recommend a Depth Of Discharge of 50%. This is a good compromise between battery life, storage capacity, and economics. In our example above this immediately cuts our 200 Ah battery down to 100 Ah if we want it to have a good life span.
Let’s talk about another factor and that is battery temperature. In general, a warmer battery has a higher capacity. The opposite is also true; a colder battery has a lower capacity. If taken to extremes, the battery may be damaged by excessive heat or cold. Take a look at fig. 2 to see how capacity is affected by temperature.
If we were to use our 200 Ah battery outside in winter and the battery’s temperature was just above freezing, say 34 deg F. We could use the chart to see what kind of capacity we could expect from it. Find 34 degrees and follow it up to the capacity curve and we see that the capacity of the battery is only 80% of normal.
To go a step further lets say we wanted to only discharge our battery to 50%. So our 200Ah battery now becomes an 80 Ah battery when used in the cold near freezing. One nice advantage about cold batteries is that their useful life is extended. Conversely, if a battery is always hot, its life will be shortened. Most installations see both hot and cold, so the life expectancy pretty much evens out. Bottom line is, give your batteries a sheltered place to live that will limit the temperature extremes.
Two other factors that greatly influence battery capacity are charging and maintenance. Over and under charging of deep cycle batteries is the most common cause of premature loss of capacity. Overcharging can “boil off” the electrolyte leaving the plates high and dry in a flooded battery. In the case of gel cell batteries, permanent damage can occur if the maximum charge voltage is exceeded and pockets of gas deform the gel around the plates. Undercharging can cause a buildup of lead sulfate on the battery plates. When you bring a battery back to full charge each cycle, the lead sulfate is recombined into the plates and electrolyte. Never leave or store a battery in a semi-charged state, keep the batteries charged if not in use. Stored batteries can be left on a trickle charge or at least topped off every couple of months. Always follow the battery manufacturers recommendations for charging and maintenance to ensure maximum capacity and life.
Hopefully this information will make more sense out of the battery sizing worksheets and why it is important to select a battery bank size that is correct for your installation.