Craig Truglia is an ASE A6, A8, and L1 certified technician who presently works as a service writer for Patterson Auto Body, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field. Technicians Truglia and Fred Byron took part in diagnosing the different vehicles in this article.
Key On Engine Off (KOEO) battery draws have always been a relatively difficult concept for customers to wrap their heads around. They figure when the vehicle is turned off, nothing is on any more. However, these problems are hardly new, as vehicles have had shorted wiring, switches, and other simple problems for years.
For the last few years now, many parasitic draw issues have become increasingly complicated, thanks to the computer networks on vehicles. This means that tried-and-true diagnostic techniques may actually complicate a KOEO battery draw diagnostic.
Thankfully, even though there are a few new techniques and increasingly complicated systems to be dealt with, the process is still essentially the same in principle..
Pulling fuses. Many technicians believe they do not need a review in the “pulling fuses” technique, but forgetting the basics can cost valuable time. The first step is to test the batteries after having them fully charged. A technician can waste hours diagnosing a vehicle only to find that a shorted battery cell is literally draining power, even after it is charged or the vehicle is run for some time. Make sure the vehicle is working with a known good battery.
In fact, recently a Ford diesel (equipped with dual batteries) came into the shop and the customer demanded that the batteries not be changed, because he wanted to change them himself after the problem was diagnosed. Lo and behold, one battery was measurably warmer than the other because it had a shorted battery cell. His battery was the parasitic draw all along.
After putting on an amp clamp (or meter on series), it is good to know that a battery generally should not be pulling more than 100 mA or so. A nice rule of thumb is 25mA to 75mA.
A fairly simple way to know if a parasitic draw is in acceptable range (because manufacturers do not generally publish specifications) is to divide the reserve capacity by four. So, if the reserve capacity is 120 minutes, if you divide it by four, you get 30.