Current Issue

Back to ARA Magazine

Battery Operated

A lesson in high voltage batteries.

By Craig Van Batenburg
Craig Van Batenburg, CEO of ACDC has 40 years of automotive experience. Driving a Chevy Volt these days, he also owns and drives HEVs from Ford, Honda, and Toyota. Craig helps independent shops, technical schools, salvage yards, and more worldwide. He can be reached at (800) 939-7909, or

Hybrid vehicles are here to stay. If you haven’t already, it’s time to learn what you need to know about these unique car parts. Here is an overall guide to handling hybrid batteries. For more information and training, go to You can also purchase the Hybrid Vehicle Dismantling Guide, developed by ARA’s Technical Advisory Committee, which contains information on how to remove and handle hybrid car batteries. The order form is found at

Why Do We Have Two Batteries?

When your 12-volt brain thinks about a battery that starts a car, we picture a black heavy object with a price tag of under $100 for most cars. Back to school. A lead acid cell is about 2.1-volts when charged up. Add six together and we have a 12-volt battery. When a 12-volt battery fails we install a new one. Fast. Cheap. No big deal.

Hybrids (HEVs) have at least two batteries. A 12-volt is there along with a high voltage battery. Why still have a 12-volt battery when we have hundreds of volts in the big battery? The 12-volt battery is there to:
• Keep memory alive for months.
• Allow communication between computers.
• Allow the computers to test high voltage circuits for safety.
• Close contactors that allow the high voltage (HV) battery to connect to the car.
• Supply Fuel/Spark for one second or more on some HEVs.
• Run emergency flashers.
• Power for accessory mode.
• Power for key on/READY-off (the old key-on/ engine-off) mode.

As you can see the 12-volt battery is critical.

High Voltage Battery Terms

To understand the construction of high voltage battery packs lets define some terms. The “cell” is the smallest part of the battery pack and in the case of older Honda HEVs they are easy to identify as they look like a D cell you would put in your flashlight. In the case of the Ford Fusion HEV (2010-2012), that also uses D cells, they are stacked in series with four cells, wrapped in plastic, and we call them a stick. Ford uses nickel metal hydride (NiMH) cells at 1.25-volts per cell so each Ford Fusion stick is about 5.0 volts when charged up. Put 56 sticks in a series and you have 280-volts. Ford rates their battery at 275-volts. Round cells seemed simple. When the Prius came out in model year (M/Y) 1997 they had round cells too. In M/Y 2001 the Prius introduced the rectangular cell (it packages better) and put 6 cells in rectangular plastic housing. This part now had a name, a “prismatic module” or module for short. It was also NiMH, so 6 cells at 1.25-volts made each module about 7.5-volts. Put 38 of these modules together in a series (a 2001 Prius) and you had 285-volts, but it was reported as a 275-volt battery, like the Ford Fusion. Add one more twist. A “V block” is how Toyota reports the voltage of two modules on their scan tool. The entire collection of cells makes up a battery “pack.” So we have cells, sticks, modules, V block, and packs.

General Note About All HV Battery Packs

NiMH, was not commercially used until 1990. It was invented in the U.S., and was so new that a lot of durability issues were unknown when they went into the hybrid cars of today. We know now they will last longer than we first thought.

NiMH batteries have significantly higher power and energy density than lead acid. They are much smaller, freeing up valuable space. They are safe and are easily disposed of, without damaging the environment. Purchase a $2,000 to $3,000 battery charger and store your batteries in a compartment that is protected from the elements, monitor their temperature, and cool them when they get hot. Never let them overcharge or undercharge, and stop using them if you approach a limit that may be harmful.

A comparison battery is lead acid or the NiMH batteries used in old-fashioned cell phones, with its 40 buck charger. The other is nickel cadmium – yet it has an issue with memory. NiMH does not have a memory issue unless it is charged in the mid 90% range.

NiMH HV Battery Concerns

The big fear is that the expensive NiMH battery pack will fail after the warranty is up and all potential fuel savings will not be realized. Because this fear is widely reported, it keeps people away. Here is what I know; the HV battery pack in my 13-year-old 120,000 mile Insight is as good as new. Some NiMH cells have failed, but not many. However, with millions of hybrids on the road worldwide, something is bound to go wrong.

Most NiMH battery packs that develop codes were swapped out under warranty. The Insight battery pack is a problem for some owners after 100,000 miles mostly due to a poorly designed regenerative braking system. Toyota has had warranty battery pack replacements that were not cell related, but corroded terminals within the pack. Honda had some Insights that sat too long and the cells degraded. They were under warranty and now a warning label is under the hood. HV Battery Packs will need service and eventual replacement, but the lifespan is more than most expected a decade ago when hybrid’s came to the U.S.

Storage of Hybrids

If you are going to store a hybrid, here is a protocol to follow. Get the State of Charge (SOC) as high as possible, then you can store the car for up to 3 months without a problem. Heat is a factor, cold weather extends the time a hybrid can sit. The 12-volt battery may need a battery tender on it while in storage (do not use on an AGM 12-volt). Over a few months the NiMH battery pack could start to lose its charge. If the NiMH drops below 20% SOC, it could cause some long term battery life issues. SOC is measured with a scan tool. Honda hybrid cars will need to be started and run for 30 minutes every 3 months, although Honda would disagree with me.


If a NiMH battery is left at 100% SOC it will discharge at a faster rate than at 60%, the normal SOC on a hybrid car. The lower the state of charge, the slower the chemical reactions. The slower the chemical reactions, the longer the cell will last. Reduced temperature slows both the chemical reactions and the self-discharge rate, so cooler temperatures is better for storage than warmer climates. Also, the OEMs never go to 100% SOC on any Li-ion or NiMH battery to help the longevity of their battery packs.

Lessons from 13 years of HEV Research

Nickel Metal Hydride is the best battery we have for hybrids at this time. The upside is good service life, relative ease to recycle, and a reasonable cost, if they last 200,000 miles plus. The downside is they are heavy, bulky and have low energy density. A fully charged Gen II Prius (about 80% SOC is fully charged) can move the car one mile. But with a gas engine and the constant charge/discharge/recharge cycle, it works well. The battery ECU watches temperature, SOC, energy output and input, voltage, and more.

With a close eye on the HV battery the computer in charge makes sure the NiMH cells never overheat, over charge, over work, get too low on SOC or work hard when extremely cold or hot. Operating ranges are usually between 40% to 70% SOC (almost never below 25% or over 80%), temperature -40°F to 140°F (-40°C to 60°C) but is usually kept between 40°F and 100°F (5°C to 38°C). The A/C system helps keep it cool and if the NiMH is very cold it is used sparingly until it achieves 35°F to 40°F (3°C to 5°C).

NiMH batteries have failsafe software to protect them. Each cell produces about 1.25-volts when fully charged. If you see an OEM spec for the HV battery voltage when you scan the car you may see a variation of plus or minus 25%. NiMH batteries can also charge and discharge rapidly. It can take less than 5 minutes to cycle through a low to high SOC. At the present time no individual cells or modules are available new so a new battery pack is always recommended at the dealership. This is an area where the aftermarket has a big advantage.

Our research is based on the fact ACDC owns one of the oldest hybrid fleets in North America, something we are very proud of. Our test fleet and many other hybrids we take care of allow ACDC to have empirical data on which we can base estimates of length of service life. In simple terms, the NiMH battery experiment in hybrids cars is now 16 years old worldwide and over 13 years old in the U.S. It is an overwhelming success with most battery packs still in the original cars doing just fine. But now, the battery issues are starting to show.

It is always good to know more about the parts you remove and sell. Electric Advanced Vehicles (EAVs) are here. They are our future and we all need to step up to the plate.

Safety Issue

Be careful when handling a leaking NiMH cell (this is rare).
Use a pair of gloves as the electrolyte may cause some irritation.
If you get the NiMH electrolyte on your skin, flush with vinegar and water. I have rarely seen a leaking cell, as there is no liquid inside only a paste type electrolyte. This is more of an issue for EMT and Fire departments. Lead acid is worse. To dispose of, call the toll-free number on the battery case for help. For the first time NiMH batteries are being carefully monitored and protected.

What to do Before Removing or Touching HV Parts (Orange in color)
• Remove ignition key and make sure car is off, not just the ICE
(if it is a Toyota or any keyless type, make sure the dash is dark, off).
• Disconnect the 12-volt battery.
• Remove or switch “off” the main HV switch or plug.
ACDC sells charts for this purpose.
• Test all HV parts with a CAT III DVOM rated to 1000-volts while wearing the HV gloves.
• Don’t take chances. Work Safely.