Category Archives: Batteries

Quick Backup Current Limit Calculation

I was in a meeting this morning when I was told a battery pack had insufficient capacity to meet a 24 hour backup time requirement. While in the meeting, I grabbed a specification for the battery the test engineer was using, did a quick calculation of the backup time results that the test engineer should have expected, and what needed to change in order to pass the backup time requirement. While I have discussed different kinds of battery calculations in numerous blog posts, I thought this one was more typical than most and reflects the kind of work done daily by battery engineers. Continue reading

 
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Relationship Between Battery Cold Cranking Amps and Capacity

Many battery manufacturers do not specify the Ampere-Hour (AH) ratings for their automotive products because Cold Cranking Amperes (CCA) more important in automotive applications than AH ratings. Car applications tend to focus on the ability of the battery to crank the engine when both the battery and car are cold. While reading a post on an automotive forum about batteries, I saw the following statement made about the relationship between a battery's CCA and AH ratings. Continue reading

 
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Remote Car Starter Can Drain Car Battery Within a Week

My Montana-based son mentioned that his wife's 2016 Honda CR-V (Figure 1) will not turn-over after sitting in the garage for seven days. No starting problems had occurred prior to early November. Unfortunately, I have had my share of car electrical problems and some of these problems have been hard to find. However, this is a new car and under warranty, so I recommended that he just take it into the dealer. He took the car into the Honda dealer, who told him that this is the result of the current drain imposed on his car battery by his aftermarket remote start system, which was verified to be within specification –the car was operating normally. Continue reading

 
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Battery Room Ventilation Math

In this post, I will be looking at how these ventilation calculations are performed. My focus here is on standard wet-cell batteries, which simply release any gases generated during charging. Other battery types, like Absorbed Glass Mat (AGM), will attempt to recombine the H2 and O2 released during charging. The AGM battery's internal gas recombination will reduce the amount of H2 released by these batteries – example shown here. I will compare the output of my model to those generated by a number of web-based and textbook sources. The results are in good agreement. Continue reading

 
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Using SUMPRODUCT to Evaluate Two-Variable Polynomial

One of the most common computation tasks that my customers face is estimating battery capacity based on the battery's temperature and discharge current. Figure 1 shows a example of the capacity curves for a typical lead-acid battery. Ten years ago, I chose to implement this function with an Excel spreadsheet that used a polynomial approximation for this function. An engineer today asked me to explain how my Excel implementation works, and I felt this would be a good topic for a post. This approach is implemented using SUMPRODUCT – no helper cells were required. Continue reading

 
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Molarity, Molality, Mass Fraction Conversion Formula

I am doing some work with lead-acid batteries and their state of charge versus their H2SO4 electrolyte concentrations. While we normally use specific gravity to assess a battery's state of charge, some folks use molality (m), molarity (M), or mass fraction (w). These concentration metrics are all related by relatively simple formulas (Figure 1). Continue reading

 
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Backup Power For My Cabin

My wife and I are in the process of designing our northern Minnesota retirement cabin – the current structure is too primitive for any extended stay. Because power is unreliable in the north woods, I am researching whole-home battery backup options. One possible option is the Tesla Powerwall, which provides 6.5 kW-hr of energy per battery pack. You can increase capacity by adding battery packs as you need. Continue reading

 
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Temperature-Compensated SLA Battery Charging Voltage

Quote of the Day If the tiger ever stands still the elephant will crush him with his mighty tusks. But the tiger does not stand still. He lurks in the jungle by day and emerges by night. He will leap … Continue reading

 
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Worked Calculation Example of Lithium Battery Capacity Versus Load

I frequently am asked to comment on data that other engineers send me. This morning I received some test data obtained from an engineer measuring the backup time of an Uninterruptible Power Source (UPS) containing multiple lithium-ion (Li-Ion) batteries. The engineer was disappointed with the backup time provided by this UPS and wanted to know if his test results were reasonable considering the battery capacity of the UPS. While there were numerous circuit parameters measured during this testing, the critical information was the battery voltage versus time. Continue reading

 
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Interesting Discussion on Lithium Battery Fire Hazard

We have all heard stories of the  fire hazards that lithium-ion batteries present – just to remind you, here is a partial list: Boeing Dreamliner Electric Cars Samsung Phones There have been many other documented "thermal events". I personally have … Continue reading

 
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