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SLI battery technologies

To say that batteries are like other technologies might not be 100 percent accurate. Except that they are changing just as fast.
Wednesday, August 28, 2013 - 07:43
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Like everything else in the automotive world, batteries and charging systems have gotten a bit complicated. Even the old standby lead acid battery now has many flavors. When you toss in hybrids and electric vehicles (EVs) from different manufacturers, you end up with a multitude of different charging and starting systems. This is due to a combination of advancing battery technologies and the many different drive systems available today.

While the EVs and hybrids use exotic batteries running at hundreds of volts for actual propulsion, the good old 12-volt system remains in almost all vehicles. The 12-volt system remains for the purpose of Starting, Lighting and Ignition (SLI) like it has always been used.

All vehicles currently require an SLI capability, however there are different levels of SLI. The “normal” vehicle still has a need for a lot of starting current, and still is charged by an alternator being spun by the engines belt system. Most EVs and hybrids that use a 12-volt starter use much less current for starting due to the smaller engine and starter motor. EVs and hybrids that use the high voltage (HV) system to start the engine only need the 12 volts to run the controllers and systems necessary to get the car started. Most of the electrics charge the SLI battery using an auxiliary output from the high voltage system rather than a standalone alternator. This auxiliary output comes from the DC to DC converter, or the Auxiliary Power Module (APM)

The many different types of SLIs and drive systems call for many different battery specs. Other factors in choosing the right battery for a given design include the cost of the vehicle and the replacement battery, vehicle weight and associated fuel mileage, instant (starting) and constant current demands, charge cycle length, battery life and normal parasitic draw.

Most SLIs continue to use a Lead Acid battery of some type. Plates of lead and lead oxide get converted to lead sulfate by sulfuric acid making electricity. The lead sulfate is converted back to lead oxide when electricity is inserted during charging. During this charge/discharge cycle the lead sulfate begins to build up on the surface of the lead plates increasing the batteries internal resistance and eventually killing the battery.

The list of variations in the lead acid battery has grown quite a bit. Not all that long ago all that was available was the standard flooded cell battery. The plates and a liquid sulfuric acid solution in cells with a hole and a cap at the top. As outgasing of hydrogen and other chemical effects occurred the sulfuric acid levels would get low and needed to be refilled with water.

Then came the Maintenance Free (Sealed) battery, and its cousin the Valve Regulated Lead Acid (VRLA) taking away the need to monitor electrolyte levels at the cost of battery life.

Then the Absorbed Glass Mat (AGM) extended the battery life back to normal levels. A porous mat of glass material kept fluid levels in control.  

Then the Gel Cell and the Gell Cell AGM hybrid. This gave better specs by putting the electrolyte into a gel form, but needed a more cautious charging system to prevent the gel from hardening.

Now take all of the variations of the lead acid battery, make the plates thicker and you get all of them in a Deep Cycle variation. In a deep cycle lead acid battery the thicker plates cause less surface area to come in contact with the electrolyte. This means less instant current, but longer charge cycles and battery life.

NiCad (Nickel Cadmium), NiMH (Nickel Metal Hydride), and Li-Ion (Lithium Ion) also might be found powering SLI systems in electric vehicles that have low instant current requirements. These batteries have significantly higher energy density (watt-hours per pound). They have longer charge cycles as well.

With all of these variations, it becomes extremely important that battery replacement and eternal battery charging follow OEM specs to the letter. A given battery in the AGM category might take a constant charging voltage of 13.5VDC, while an identical AGM/GEL battery requires 14.75VDC. In the absence of vehicle OEM specs or procedures, battery specs or procedures should be found. The damage to the battery that might occur may not be apparent right away. It might just cut battery life by a substantial amount. It might change the batteries typical voltage readings by a small amount. In today’s computer controlled SLI charging systems, these changes can be significant.

The same issue applies to testing. We all know the old rule of thumb. Open circuit battery should be 12.5 to 13.5 and the running (charging) voltage should be 13.5 to 14.5. This was intended only for flooded cell and maintenance free batteries in an alternator based charging system. Everything else requires specs from the vehicle or battery OEM.

Most vehicles that do not use an alternator for SLI charging (and some that do) are now equipped with On Board Diagnostics (OBD) for the SLI system. This is always the preferred method for testing these systems. Both error codes and PID readings should be taken from an appropriate scan tool when SLI OBD is available. PIDs should always be compared to OEM specs. In hybrids and EVs, always make sure the PID and/or error code is for the SLI system and not the drive battery. The SLI system is often referred to as the “auxiliary battery” and the drive battery is often referred to as the “traction battery.” Modern electronic battery monitoring is no longer limited to simple voltage readings. Battery State of Charge (SOC) and internal resistance is calculated by the computer using a variety of data from voltage and current readings to temperature measurements.

Another issue that requires strict adherence to OEM procedures is jump starting. Again, most alternator-based vehicles still might use industry standard procedures. Some systems that use DC-to-DC converters might be damaged by improper jump-start procedures. In all cases, if there is a special jack, connector, or terminal for jump starting it should always be used. Some of these systems will be using the jump start current for spinning the starter motor directly while providing voltage for control systems. Some will only get control systems running so that the HV battery can be used. Some will wait until a specific voltage has been reached for a certain amount of time before it will attempt a start. Again, consult vehicle specific OEM procedures normally found in the owner’s manual.

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