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Battery Efficiency - AGM Batteries

Battery Types and Battery Efficiency - VRLA vs GEL vs AGM
Peukert's Law

Click here for an explanation of the difference in VRLA vs GEL vs AGM battery Construction

Now that we understand the difference in the construction, let's look at the individual batteries efficiencies.

Peukert's Law, presented by the German scientist W. Peukert in 1897, expresses the capacity of a battery in terms of the rate at which it is discharged. Mr Peukert devised this formula to illustrate numerically how discharging a battery at higher rates (amperages) actually removes more power from a battery bank than one would expect (aka shows numerically the battery efficiency). This law explains why a battery discharged at 20 amps does not remove twice as much power as one discharged at 10 amps, the battery actually realizes a slightly larger discharge for the 20 amps due to effeciencies.

As the rate of current (amperage) increases, the battery's capacity (ampere hours at current x) decreases. This does not affect the long term capacity of the battery as it remains fairly constant if recharged properly and not overly discharged.

The equation below is Peukert's equation. This is the mathematical illustration of battery consumption at various discharge and charge rates. When we compare these outputs at different rates, we can compute the efficiency as a percentage.

Peukert's law is as follows:
Peukerts Law of Battery Capacity
C_p = is the capacity according to Peukert, at a one-ampere discharge rate, expressed in Ah.
I = the discharge current, expressed in Amperes a.
k = the Peukert constant,
t = the time of discharge, expressed in hours h. The Peukert constant increases with age for any of the battery types above, but generally ranges from 1.05 - 1.15 for VRSLAB AGM batteries, 1.1-1.25 for Gel, and 1.2-1.6 for Flooded Batteries.

To get to a useful derivative of Peukert's law to do your own calculations is a bit tricky. However, the list of equations below should take you through the process painlessly. Start with equation 1 and move forward through the process to get your own numbers. Remember that your system has inefficiencies in the wire, inverters, chargers, and other connections. Each contributes to the overall efficiency, and will affect your calculations.

Equation 1: Peukert's Capacity of your Battery

Hardest part mentally (it looks wrong).... hour rating x (capacity/hour rating) ^ peukerts coefficient = peukerts capacity
The equation for a 100 amp hour agm battery rated at 24 hours would look like this
24h*((100ah/24h)^1.1) = 115.339 ah
(It looks really wrong at this point, but think of your tax forms, you have to get to the end)
With this corrected capacity, we have half the equation to see real power available.

Equation 2: Modified Amperage for Peukert's Equation

I(amps)^k(peukert constant) = corrected amperage usage
For the same 100 amp hour battery we are going to discharge at 20 amps, and again at 5 amps.
20a^1.1 = 26.98 amps
5a^1.1 = 5.873 amps
With this corrected amperage usage, and corrected capacity we can calculate the time it will run.*
(Depth of discharging batteries has a direct correlation to battery cycle life - deeper discharge = fewer cycles)

Equation 3: Modified Capacity expressed as Minutes of Discharge

The whole point of this operation is to know how long your battery bank will work. This is the equation to determine that number in minutes. Divide Peukert's Capacity above (equation 1) by Modified Amperage (equation 2) to view Capacity in AH
C_p / I(amps)^k(peukert constant) = hours of discharge to 100% discharge......
115.339ah/26.98 amps = 4.27 hrs or 4 hours 16 minutes or 256 minutes @ 20 amps*
115.339ah/5.873 amps = 19.6385 hrs or 19 hours 38 minutes or 1178 minutes @ 5 amps*
Discharge batteries to 50% or face reduced cycle life - NO CHEMICALS LEFT = NO MORE REACTION.
Many manufacturers engineer systems to prevent complete and total discharge for your protection.

Equation 4: Modified Capacity expressed as Ampere Hours for amperage discharged

This equation will express the ampere hours of any battery at your amperage. This is helpful for comparing apples to apples when shopping for batteries as well.
Equation3 hr x amps discharged = amp hours of battery
4.27 hrs x 20 amps = 85.4 Amp Hours
19.64 hrs x 5 amps = 98.2 Amp Hours

Equation 5: Battery Efficiency

Result of equation 4 / Rated Capacity of Battery
High amperage example, the 100 ah battery at 20 amps has an efficiency of 85.4%
85.4ah real /100ah rated = 85.4% efficient at high amperages.

Low amperage example, the 100 ah battery at 5 amps has an efficiency of 98.2%
98.2ah real /100ah rated = 98.2% efficient at high amperages.

Temperature Losses

Generally speaking, as a molecule is reduced in temperature, the less active atomically it becomes. This affects many exothermic (heat inducing) reactions including the power generated by lead acid batteries. In cold Colorado winters, be they AGM, GEL or flooded in composition, all batteries efficiency are negatively affected. The graph shows the negative correlation between temperature and battery capacity in an AGM battery. Notice how a 100 amp/hour battery at 90° F has only 55% of that capacity at 4° F and 43% capacity at -22.

Indoor Installations

This raises issues when it comes to building many systems, as you would like to avoid these losses due to the cold. Indoor installations, however, bring their own challenges due to the hazardous materials acid, hydrogen, and lead. Many would rather not introduce these into the living areas of their home, rv, or yacht. In order to properly install batteries into a living quarters, one must take into consideration these 3 things......

Hydrogen Venting, discussed here.
Safety and Testing - all our batteries are UL Listed, MILITARY SPEC
Mess, Acids, and Battery Choice

The proprietary technology in Lifeline & Sun Xtender Batteries solves these and many other isses by physically sealing in the acids, preventing all corrosion on terminals / wires / battery box, and facilitating indoor installation at any angle. This in turn eliminates cold losses in Colorado's winters. This can contribute more than 50% more power from your system and eliminate costs that would otherwise be associated.

Flooded batteries are very messy (gassing & acid), need maintenance, and are not the most efficient.

GEL batteries are less messy (gassing & acid), maintenance free, but are not tolerant of high amp loads.

AGM Batteries from BD Batteries are BOTH maintenance free and amperage tolerant.


Manufacturing Considerations

Each manufacturer builds their batteries to fail differently. Some are designed to dissolve all the electrolyte (electrolysis), some are designed to sulphate (crystalize) the electrolyte, some short out the plates, yet others completely decay the anode or cathode in the battery on their way to eventual failure. Each method has pros and cons, some are temporary and others are permanent. Each dictates the cut out points for high and low amperage on the peukert curves toward 100% DOD. Some manufacturers even have methods for preventing complete and total discharge of the battery, allowing the customer to "RECOVER" the battery and continue to use the product.

In order for the manufacturers to guard their competitive secrets, we are not allowed to divulge how they do it, and as there are a myriad of technological advancements designed to terminate the current in a battery prior to complete discharge we can talk about it. This is a step toward the protection of "YOUR" investment. Please do not mistake these protections as a loss in power or reduced AH capacity, reduced battery efficiency, or other way the company is cheating you. Rather these are an insurance policy against your accidental complete discharge and investment loss.

AGM Batteries Available

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Lifeline Deep Cycle AGM Battery Specifications
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Lifeline OEM - RV & Marine List
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AGM Battery Users Manual Part 1 (1 of 2)
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Sun Xtender Technical Documentation
AC - DC Sun Xtender AGM Battery Bank Sizing Calculator
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Chairman Traction, Standby, and Wheelchair Batteries.
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Battery Chemistry Comparison - VRLA vs GEL vs AGM
Comparison Between Gel Batteries and AGM Batteries

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Total Power AGM Racing Battery Specifications
Total Power Racing Lightweight Batteries by Size
Battery Chemistry Comparison - VRLA vs GEL vs AGM
Comparison Between Gel Batteries and AGM Batteries

Rock Racing Battery Dealer
Rock Racing Battery Features
Rock AGM Racing Battery Specifications
Total Power Racing Lightweight Batteries by Size
Battery Chemistry Comparison - VRLA vs GEL vs AGM
Comparison Between Gel Batteries and AGM Batteries

Shipping, Safety, and Legal

Lifeline, SunXtender, Total Power, and Rock Racing Battery Material Safety Data Sheet - MSDS (pdf)
Lifeline, SunXtender, Total Power, and Rock Racing Battery Safety Bulletin
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AGM Battery Applications and Benefits

AGM Battery Efficiency and Peukert Coefficients
Custom Uninterruptable Power Supply - Custom UPS
Uninterruptable Power Supply Kits - UPS Kits
Solar Powered Off Grid Housing

AGM Battery Installation Considerations

Hydrogen Venting Systems

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The information contain on this deep cycle AGM batteries website is deemed reliable, but not guaranteed. Please verify specific information on marine AGM batteries, rv AGM batteries or auto AGM batteries listed here by calling us direct. We are proud to offer the highest quality AGM batteries and hope you will consider using our deep cycle AGM batteries in the near future!