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By Scott Shotton

Batteries that go dead overnight are nothing new. Parasitic draws have been around longer than we have had glove box lamps. I remember, before I really knew much about cars, an old gentleman showing me how to disconnect a battery terminal and put it back in place while looking for a spark! I am positive this has not been a good technique for many decades and many reasons. But I digress.

Today’s case involves a 2017 Chrysler Pacifica (non-hybrid) equipped with a stop-start system.

The customer’s complaint is that the battery goes dead overnight but can be jump started and remain functional for the remainder of the day. Once parked overnight the symptoms repeat themselves again. Because I am a mobile technician, and the shop called me on this one, I requested that they charge and test the battery to have the vehicle ready when I arrived. The battery was charged when I arrived and I was told that the battery tested good.

Our diagnostic approach to this vehicle is going to be testing for a parasitic draw. One of the tests that we could use is to disconnect the battery and install an ammeter in line. Just as the “spark” test mentioned earlier, I no longer like this test. Actually, I prefer not to disconnect any vehicle’s battery unless I absolutely have to. Who knows what memories can be lost, radios locked or adaptive strategies erased?

I prefer a scope and a current probe. It is easy to grab a battery cable and clamp a current probe around it to see the resting draw.

In this case the vehicle has two batteries: a main battery and an auxiliary battery that is much smaller. This is going to affect our diagnostic path. First, we will need to access a wiring diagram. Figure 1 was obtained from OnDemand5 and is quite busy. I did have to trace some wires around to get the lay of the land. For simplicity, Figure 2 is a re-drawn version of the system.

Fig.1: The wiring diagram for the Pacifica in the bay. (below)

Fig. 2: A re-drawn version of the Pacifica diagram for simplicity. (below)

Note that in Figure 2 above, I made some annotations (in Blue) of where I will be making connections. These annotations will be referred to throughout this article.

Using my PicoScope 4823 scope and a Pico TA189 30-amp current probe (Fig. 3 below) I started doing my testing without disconnecting anything.

Fig. 3: Pico 8-channel scope connected and ready to measure (Enlarge and open in new window)

My first connection was at Point A of figure 2 and actually where the current probe is shown in figure 3. This is the main ground for the vehicle and I got a reading of approximately 800 milliamps. FCA service information says that around 35 milliamps is acceptable on this vehicle. My next step was to move the probe to Point B and I measured approximately 800 mA. My next step was to move to Point C and I obtained a 400 mA reading.

At this point, since a 400 mA draw was going through the Power Distribution Center (PDC) I chose to do a voltage drop test across each fuse to see which circuit was the culprit. This test, by the way, is much better than pulling fuses. Removing a fuse can reset a module and completely mess up our path. During my testing I found that around half of the fuses had a voltage drop which indicates that current is flowing in those circuits. I guess now it is time to plug in a scan tool and see if we have any codes. I have been trying to avoid this task because I did not want to wake up any modules simply by opening the door.

I connected the wiTech2 and retrieved low battery voltage codes from almost every module. This was no surprise given how many times the battery went dead and had been jump started. My train of thought at this point was: do we have multiple modules staying awake and causing half of our parasitic draw?

To test my theory, I left the wiTech2 connected, powered down the vehicle and observed the topology screen to see if modules went to sleep. I waited over 30 minutes and Fig. 4 below is what I observed. The modules that are red went to sleep. The modules that are blue or yellow are still awake. If we do a count, 21 modules are still awake and 9 modules went to sleep. The 21 modules that are still awake could account for our parasitic draw couldn’t they?

Fig. 4: Chrysler Topology. (Enlarge and open in new window)

My point is, with a dual battery system, our approach may need to be different. If you look closely at the wiring diagram in figure Fig. 2 (below) you will see, in the rest position, both batteries are wired in parallel.

In this case I chose to leave my current probe connected and disconnect the power feed at Point D to eliminate almost the entire vehicle. I still observed a 400 mA reading. For reference, the starter and generator circuit were also eliminated from the equation. If we look closely at the diagram given what I have disconnected, is there anything left besides the two batteries wired in parallel? Also, should there be a 400 mA draw between the two?

The shop told me that they tested the battery, but if there was a second battery wired in could that effect our results? Time to go back to basics and do some addition research.

First, on a multiple battery system, batteries need to be disconnected to test them. In this case the shop neglected isolate the batteries and never even tested the second battery. The Aux. Battery on this vehicle was bad. Additional research explained how the system works. During cranking the power control relay (Fig. 2) opens to allow the starter motor to use the main battery while the Aux. Battery supplies power to everything else on the vehicle. In this case, OCV of the Aux. Battery way 6.2 volts when isolated. Do modules like that voltage?

Second, if two batteries are wired in parallel and one of the batteries has failed, will the good battery try to charge the bad battery even in a state of rest? Yes!

What was happening on this vehicle? The bad battery was draining the good battery hence the measured 400 mA draw at Point B of Fig. 2 with disconnection at Point D. That accounted for half of our draw. The second half of the 800 mA draw was due to modules staying awake as shown in Fig. 4. Why were they staying awake?

Research into system operation revealed that during cranking the power control relay opens. At this point, the main battery powers the starter motor to crank the engine over. The Aux. Battery is isolated and is responsible for powering all of the modules on the vehicle. If the Aux. Battery has too low of a voltage modules get confused and, in this case, stayed awake causing the second 400 mA draw measured at Point C of Fig. 2.

The total is the 800 mA draw that is draining the battery. The failed Aux. Battery was the cause of both draws. Replacement of the battery, the shop opted to replace both, resolved the issue. My point is: be aware of how multiple batteries behave when wired together, isolate them for testing purposes and do not be surprised when low battery voltage causes unusual electronic module behavior.