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1.) Analysis of Battery Capacity - The following (was also added to our website - Battery tab) is a response to a user's question regarding Battery Ratings and Capacity.

Below are data tables and their resultant graphical representations of % Battery Capacity versus Measured Battery Voltage.The major difference between the data sets is that in most scenarios, lab testing data will exceed field results. When analyzing the data, one will notice that in field use, batteries yield approximately 80-85% (80% shown below) of their rated (Laboratory Derived) amp-hours. This appears to be a common efficiency factor used throughout the electrical industry. This is not a reflection on any manufacturer, but the manner in which testing results are derived and used by most when it comes to marketing their products. We have provided typical differences below for your evaluation and use in determining your requirements. When calculating a house bank size requirement to fill one's needs, multiply your house bank amp-hour requirement by 125% (actual AH is apx. 80% of rated AH) in order to arrive at the correctly sized battery bank when using published battery rated amp-hours.

The graph on the left displays specifications from an owner's manual based on lab testing results under perfect tightly controlled conditions. The graph on the right is from actual observed field measurements performed by ZRD. Manufacturer lab testing reaches its 50% capacity level at 12.20 vdc. ZRD reached it's 50% capacity at 12.05vdc. The disparity is due to the fact that the laboratory tested battery bank is starting at a higher state of laboratory initial charge that is not available or possible to acheive in a field setting.


Capacity 0 % 20 % 40 % 50 % 60 % 80 % 100 %
Manufacturer Specified voltage 10.50 11.55 12.05 12.20 12.35 12.60 12.80
ZRD Observed voltage 10.50 11.25 11.80 12.05 12.25 12.55 12.80

4D AH available per Specifications 0 42 84 105 126 168 210
ZRD Observed AH available 0 34 67 84 101 134 168

8D AH available per Specifications 0 51 102 128 153 204 255
ZRD Observed AH available 0 41 82 102 122 163 204


Lifeline Owner's Manual Battery Capacity Specifications

Observed Battery Capacity Data

Click on the above graph to view it in full-size detail.



The ZRD derived data (graph on right) was based on a typical daily consumption of 15 AH per battery per day (significantly less than the 20 hour rate). At the end of 6 days a recharge cycle was started to return the battery bank to it's 100% recharge state - correctly (3-STAGE), completely (100%), shore power (Stable) recharged. One is still using 50 - 100% (discharge - recharge) states, but the numbers are in proportion to the manufacturer's specifications. If your rate of consumption (AH per day) is significantly greater than the manufacturer's or ZRD test data, your will reach the various levels of discharge at a faster rate with a corresponding reduction in available amp hours. Remember, deep cycle battery capacity is usually stated as a 20 hour discharge rate.. This means that if you divide the battery's amp hour rating by 20 and consume this number of amps per hour, you will have a 100% discharged battery at the end of the 20 hour period. As an example, a 100 AH battery will provide 5 amps for 20 hours.

When using power at discharge rates above it's 20 hour rate, a battery will not supply it's rated AH. It will reach 100% discharge in a shorter time perior. The greater this increased rate is, the more significant the decrease in time to reach the 100% discharge state. Lastly, consider the following table.

CAPACITY AT VARIOUS DISCHARGE RATES

Hours to Reach 100% Discharge of Capacity 20 10 5 3 1
Percent of Rating 100 % 89 % 78 % 66 % 45 %

If less than 20 hours passes before battery voltage falls to 10.5V you can still determine your capacity with some simple arithmetic. Using a 12 vdc 100 AH rated battery with a constant 5 amp load applied, takes only 10 hours to reach 10.50 vdc, you will have consumed 50 AH. This is the 10 hour capacity. Dividing the 50 AH consumed by 89% (10 hour rate from the table above), you determine that the actual 20 hour capacity for this battery is 56 AH. This is either a defective battery, one that has not been maintained, or one that is very close to the end of it's life. If that same battery had consumed 85 AH (using a higher current draw of 8.5 amps over the same time period), the capacity for this battery would be 95.5 AH (85 AH consumed divided by 89%). This battery would be significantly in better condition than the battery in the previous example.

2.) Analysis of Battery Capacity - Similar Conclusion reached by another user

I've been looking at buying a pair of Lifeline GPL-4DA AGM batteries and have come across something curious. The rated capacity of these batteries is 210 Ah at 20 hr discharge rate. This means: apply a 10.5 A load and after 20 hours the voltage will have dropped to 10.5 V, meaning the battery is effectively discharged. So far so good. Then I looked at the "minutes of discharge" at 25 A, 15A and 8 A. They are 390 mins (6.5 hrs), 680 mins (11.33 hrs) and 1375 mins (22.92 hrs) respectively. This means that the capacities are as follows:

1. At 25 A discharge rate - 6.5 hours equals 162.5 Ah (6.5 x 25 = 162.5)
2. At 15 A discharge rate - 11.33 hours equals 170 Ah (15 x 11.33 = 170)
3. At 8 A discharge rate - 22.92 hours equals 183.33 Ah (8 x 22.92 = 183.33).

From the above figures it is immediately obvious that the capacity at the 20-hour discharge rate should fall between 170 Ah and 183.33 Ah. Try to calculate the Peukert's Exponent (PE) based on these figures.

PE Points 1 & 2 = 1.09
PE Points 1 & 3 = 1.11
PE Points 2 & 3 = 1.12

All these line up very nicely and I'd use 1.11 to set up a Link 10 or Link 20 monitor. Competing comparative batteries have a similar PE, which is btw very good. But try to calculate the PE using the rated capacity of 10.5 A and 20 hrs in combination with say point 1 and you'll get 1.3, which makes no sense for a good quality AGM battery and is completely out of whack with the rest of the figures...