Product Review: Automotive Battery Load Tester
In the past seven years, I’ve needed to replace the battery six times in my ’02 Mustang GT. On three occasions I’ve asked local repair shops to try and diagnose the root cause but it has remained elusive.
Early this year, within two months of replacing the battery again, I experienced slow startup cranking in 28°F weather, so I decided to start digging into the problem myself.
I started by looking on Amazon for a battery load testing tool. I found the Foxwell BT705 on sale for $99. It’s a rugged handheld electronic tool with a long 7.8 foot cable that clamps onto the posts of the battery like jumper cables. The cable was long enough for me to set the tool on the windshield, sit in the driver’s seat, and with the window open reach around to press the buttons.
The first test is an initial voltage check with the vehicle off. It reported 11.9V. I consulted a battery voltage chart that I found online. 11.9V is about 30% charged — quite bad. But the temperature was about 55°F and the car started OK so I drove for about 45 minutes on the interstate to recharge the battery. To get an accurate “resting voltage” of a battery, it should first be left to rest, neither charging or discharging for 8–24 hours (see the section Fully charged voltage in the article here).
The next day I start the testing procedure over and the first test is 12.24V. That’s around 65% charge. Not good, but better. So, I proceed to the next step which is start the car. That test passes which means the battery is OK while the starter is cranking. The third test checks the alternator output, parked, at idle. That test passes. The final test is with a high electrical load. You turn on the high beams, put the blower fan on high, and rev the engine for the duration of the test. This test passes.
Alright, first full round of testing complete, and nothing significant found. But that initial battery test should have been at least 12.53V (90%), not 12.24V (around 65%). I drive it again for about 45 minutes on the interstate to charge the battery and then call it a day.
Next day, after sitting overnight I perform the initial battery test again, and it’s the same as last time 12.24V. There are two likely explanations at this point. Explanation one: it’s a bad battery that isn’t going to hold a charge over 65%. Explanation two: something in the car’s electrical system is malfunctioning and causing a significant battery drain when the car is off.
I’ve been reading on the internet that some car alarm systems can cause an unacceptable battery drain overnight. The original Ford dealer installed an aftermarket security/alarm system before I purchased it brand new. My first experiment is to put the alarm to “valet mode”. I then drive it again for about 45 minutes and then call it a day.
Next day, after sitting overnight I perform the initial battery test again, and it’s the same as last time 12.24V. Putting the alarm in “valet mode” makes no difference, but the alarm system could be the problem. So later that day I visit a local mobile sound shop that has helped me in the past. I explain to them what’s going on and ask them to completely disconnect the alarm system so I can rule it out. Within 15 minutes they were able to not only disconnect it, but physically remove the main module (quite small) and hand it to me. Once again, I drive for about 45 minutes and then call it a day.
Next day, after sitting overnight I perform the initial battery test again, and it’s now 12.35V (75%). An improvement, but still nowhere near the target of 12.53V (90%).
My next step is to drop the car off at Quick Lane (owned by Ford Motor Company) with a short, written summary of everything above and ask the Ford Technicians there try and find a parasitic battery drain. They perform a parasitic drain test multiple times, over multiple days, which consistently reports a 10mA drain, which is well under the factory specified limit of 50mA. I’m disappointed they couldn’t find anything, but at least this is additional information.
I think about it some more. Perhaps there is a larger intermittent drain overnight which wouldn’t have shown up on the tests Quick Lane performed. With the help of the internet, I found the official Ford shop procedure for the parasitic drain test. And with my old Radio Shack digital multimeter, I set up the parasitic drain test myself and let it run for two hours with the meter set to record the maximum value. The result was a max of 10mA.
It’s still possible that a large intermittent drain just didn’t occur within my two-hour test. What I need next is a multimeter than can continuously take readings over a period of 24 hours and provide a data file export that can be loaded into Excel to look for intermittent spikes. My old Radio Shack digital multimeter doesn’t have that feature, so back to Amazon to find one that does.
The Foxwell BT705 battery load tester provided a helpful general assessment of battery and alternator health. I give it an overall rating of 4 stars. A key feature of this tool is only requiring one technician to perform the testing. This is only possible if you can press the buttons on the control pad while sitting in the driver’s seat. The cable on the tool should be at least three feet longer to ensure that’s possible with most vehicles. Fortunately, in my case I was able to set the tool on the windshield, sit in the driver’s seat, and with the window open reach around to press the buttons.
Stay tuned for a future article where I’ll cover purchasing and using a multimeter to perform continuous data capture and then export the results to a data file.
