Product Review: Current Clamp
In Product Review: Bluetooth Multimeter, I wrote about troubleshooting a difficult to diagnose battery drain on my ’02 Mustang GT. Extensive diagnostics lead me to think I have a six-month-old battery that is permanently weak and will probably need to be replaced within a year or two. I’m not sure what caused this.
Following is a quick recap of the testing I’ve already performed. When I began troubleshooting, the battery would only hold a 60% charge. So, I had the car alarm removed to rule it out as a parasitic drain. A couple months later after a lot of troubleshooting, but no other changes, the battery would consistently hold an 80% charge. A couple months after that, the battery is now holding a 90% charge overnight. But a battery only 7 months old ought to be close to 100% overnight with the tiny constant 10mA battery draw I previously confirmed with a parasitic drain test.
I have two theories that are interconnected:
1) The car alarm was a problem
2) This caused some sulfation of the battery plates, but with a lot of interstate driving over several months, the sulfation was slowly reduced resulting in improved battery health
Maybe the alarm was never a problem, but at this stage I’m not going to bother putting it back in to confirm.
My two-part theory could be wrong, or there could have been multiple problems that caused the battery problem I’ve been trying to diagnose. So, my next steps are to confirm that starter cranking current is in an acceptable range and to perform a more thorough analysis of alternator output current under different conditions. For that I ordered the BTMETER BT-605A current clamp to plug into my multimeter.
The BT-605A measures AC or DC current and can be connected to any standard multimeter. It outputs 1mV per amp. You set your multimeter to read mV and connect the BT-605A leads to the meter’s voltage inputs. Then you place the BT605-A clamp around the wire you want to measure.
I wanted to measure the starter motor’s maximum current draw from the car battery, so I clamped the BT-605A to the negative battery terminal, set my multimeter to record max mVDC, and started the car. I got a reading of 110A which seemed low. I repeated the experiment six times on cold and warm starts and got various readings between 110A and 260A. (260A is probably close to the correct number.) Why am I getting such a wide range of results?
I contacted BTMETER support and explained what I was doing. They introduced me to inrush current theory (the maximal instantaneous input current drawn by an electrical device when first turned on) and unfortunately, the BT-605A cannot measure it. They thanked me for letting them know and they would add that feature to the next revision of the BT-605A circuit board. They recommended a different product on Amazon that supports DC inrush current, but I haven’t purchased it yet to retry my experiment.
But, the BT-605A clamped around the output wire of the alternator could accurately measure how many amps the alternator was supplying under various vehicle electrical loads. At idle with all accessories off it measured 11A. At idle with a large electrical load: brights on, blower fan on high, and the electric radiator fan on, the alternator supplied 50A. With brights on, AC on, blower fan on high, and the electric radiator fan on, the alternator consistently supplied 70A at idle. Repeating those three tests with the engine revved to 2000 rpm, the alternator supplied the same values. Those measurements indicate the alternator is performing as expected. It is properly sensing the amount of current required under various loads and quickly adjusting to supply the amount required. In addition, I used a second multimeter to measure battery voltage during all six tests, and it consistently remained well within the Mustang shop manual specification of between 13.0 and 15.0V.
I give the BT-605A current clamp 5 stars. It is compatible with any multimeter that can measure mV from standard 4mm banana plug probe inputs. It is easy to use, seems durable, and at $22 from Amazon it’s very affordable. However, opening the clamp jaws seems to take an excessive amount of hand strength and a steady grip. I would think this could be improved with a minor design change. I measured various DC amp values from 1A to 70A on different wires and the results seem to be accurate, but I did not have a way to verify the accuracy. However, if you need to measure inrush current, the BT-605A cannot do that.
I still want to accurately measure the starter motor’s maximum current draw from the car battery, so stay tuned for a future product review on a different clamp meter that reads DC inrush current.
