Don't get stuck at the light: start-stop technology and battery management

August 05, 2015 // By Paul Pickering
If the futurists are correct, in 50 years our grandchildren will be driving autonomous electric vehicles that they summon up from the cloud on demand. Gasoline will be a historical curiosity, and Middle East will be known primarily for its solar power.

These days, though, gasoline is still King of The Hill, and the DOT [USA’s Department of Transport] has been slowly turning the screws on vehicle fuel economy ever since the first CAFE regulations passed Congress in 1975 following the Arab Oil Embargo.

There are a couple of well-recognised ways to reduce fuel consumption – make smaller or less powerful vehicles. But [American] buyers love their trucks, SUVs and V8 power.

The quest for reduced fuel consumption, while still catering to consumer preferences, has led to the introduction of several new technologies. One of them is the “start-stop” (or stop-start – we'll leave that discussion to the “chicken or the egg” folks) system; it automatically shuts down and restarts the engine to reduce the amount of time the engine spends idling, thereby reducing fuel consumption and emissions.

This is most advantageous if you a spend significant amount of time waiting at traffic lights or frequently get stuck in traffic jams. This feature is present in hybrid electric vehicles, but has also appeared in otherwise conventionally-powered vehicles, so-called “micro” hybrids. For such vehicles, fuel economy gains from this technology are typically in the range of 5 to 10%.

Turning off the engine at a stop light. What could possibly go wrong?

Predicting battery failure

Battery failure is the single biggest cause of vehicle breakdown, accounting for 52% of failures in one study of 1.95 million vehicles less than 6 years old; in another study, failure rates ranged from 1000ppm to over 10000ppm for batteries older than 3 years. Battery failure is exacerbated by operating conditions that don't allow the battery to fully charge, such as driving short distances using heavy accessory power. Excessive heat is another contributing factor.

Battery death can occur without warning especially if you inhabit the USA’s Desert Southwest – both [EDN editor] Steve Taranovich and I live in the Phoenix, AZ, area. I once drove 40 miles, turned off the engine for a moment outside a restaurant, and had to be pushed to the nearest parking space. Lucky I wasn't on te intersate highway during rush hour.

To forestall this possibility, it's critical to have precise knowledge of the current state of the battery to disengage the start-stop system if there's a possibility that the vehicle may not start following a deliberate shutdown, as well as alert the driver of impending battery failure.

It might seem reasonable that a measurement of minimum battery voltage during crank would give a good indication of impending doom, but this is not the case: Figure 1 shows a comparison of cranking voltages of a battery that was subject to accelerated ageing. In week 13, a week before eventual failure, both the worst-case droop voltages are essentially the same as a cranking sample from week 9, so a more sophisticated approach is needed.

Figure 1: Comparison of cranking voltages of battery with accelerated ageing (source: Ryan J. Grube)

In commercial battery monitoring systems, there are two primary battery metrics used to forecast incipient failure: State of Charge (SoC) and State of Health (SoH). Related to these, battery State of Function (SoF) provides a yes/no rating as to whether the battery can perform its required function of cranking the vehicle.