Quote of the Day
If a free society cannot help the many who are poor, it cannot save the few who are rich.
— John F. Kennedy
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 five 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 discharge imposed on his car battery by his aftermarket remote start system (with cell phone interface, sometimes called a drone), which was verified to be within specification –the car was operating normally.
In this post, I will discuss why this behavior is normal considering how the remote starter works and recent weather changes in Montana. I should mention that all modern cars will discharge their batteries over time because their computer systems are always on – their nominal current drain is about 30 mA. Even without a remote starter, the cars will discharge their batteries after about 3 weeks (Appendix A).
- Cold Cranking Amps (CCA)
- Cold cranking amperes (CCA) is the amount of current a battery can provide at 0 °F (−18 °C). (Source)
- Parasitic Current Drain (iP)
- Parasitic current drain can be defined as any electrical device that draws electric current when the ignition key is turned off. In the case of a remote starter, the radio receiver must be powered so that it can receive the starter signal. This receiver current draw is parasitic current drain. (Source)
- Ampere-Hour (Ah) Capacity
- An ampere hour or amp hour is a unit of electric charge, having dimensions of electric current times time, equal to the charge transferred by a steady current of one ampere flowing for one hour, or 3600 coulombs. Battery charge capacity is usually rated in Ah. (Source)
Battery Capacity Versus Temperature
Figure 2 shows the impact of temperature on the capacity of a lead-acid traction battery.
CCA Versus Battery Capacity
Figure 3 shows that a battery's available Cold Cranking Amperage (CCA) decreases with reduced battery capacity. For the analysis of this post, I will assume that a battery's available CCA reduces proportionately with its capacity. Figure 3 originally came from a Yuasa technical specification.
Cranking Current Required By a Car Versus Temperature
Figure 4 shows that the battery's available CCA decreases with lower temperature as the car's need for cranking amperage increases. I refer to this situation as "bad squared".
Here is what I was able to find out about my son's situation:
- His remote starter is rated to have a maximum parasitic current draw of 75 mA.
- The measured remote starter parasitic current drain is 70 mA – it is within specification for remote starter units with a cell-phone interface. Other folks report similar parasitic loads.
- Separate from the cell phone-based car starter parasitic current drain, the car itself has a parasitic current drain of 30 mA to support its onboard computer. This means that the total expected parasitic drain is ~100 mA = 70 mA + 30 mA, which is in rough agreement with reports on other web sites.
- The car battery experiences the parasitic current drain for 5 days.
- His wife just had a baby and is on maternity leave. The car had been driven every day prior to October 30th. After October 30th, the car has been driven sporadically.
- The car is only driven short distances. I am guessing that the battery is never fully charge because of how it is started, driven a short distance and stopped. I will assume the battery routinely sits at 75% charge during normal use. In fact, cars brought in for service typically have batteries with 70% of a full charge.
- The weather recently turned cold. The temperature had been in the 70 °F (~25 °C) range and now is about 18 °F (-8 °C) in the morning when the car is being started. This temperature drop reduces the battery capacity to 60% of its charge at the specification temperature (77 °F).
- I will assume that the effective CCA rating is proportional to the Ah rating, which is illustrated in Figure 3.
- The car battery is rated for 500 CCA. I do not have a specification for its ampere-hour capacity, which is important when determining capacity. Batteries of similar size and CCA ratings have a capacity rating of 50 Ah.
This is enough information for me to figure out what is going on.
Figure 5 shows my analysis of the battery capacity. What I calculated is that the battery's capacity has reduced by ~75%. This results in a large decrease in the cranking amps available (justification in a later post) and results in slow or no cranking when attempting to start the car.
I am afraid that an additional parasitic battery load of 70 mA is enough to drain the battery sufficiently after five days to make starting difficult or impossible. This reminds me of a battery problem I had in my youth where a glove compartment light did not turn off and would slowly drain my battery. I could fix the light problem, but the remote starter must draw current to detect the radio start signal.
Appendix A: Computer Parasitic Leakage Example
Figure 6 shows a manual excerpt that illustrates how modern cars have computers that constantly drain charge from their batteries.