I know there might be an obvious answer to this, but I am asking it anyway.
All of the cars I’ve owned have had a 12v battery.
None of the batteries has ever been the same size.
Why is there so much difference between the physical size of car batteries, to the point where I cannot just walk into a store and buy any 12v? I would think that this is one thing that should have become standardized over the years, so any car operating with a 12v system would have the ability to use the exact same battery.
If I were designing a vehicle, it would make sense that the only requirement for a battery would be that it would meet the power needs for that vehicle. As for size, the smallest possible battery would seem to be the desire of every engineer…
Clearly there are other factors in play. What are they?
A battery can only put out so much current. If you need more current, you need a bigger battery. The battery from a tiny 3 cylinder Yugo isn’t going to be able to start a huge V8 from an F150 pickup truck in the dead of winter.
On the other hand, a bigger battery adds a lot of cost and weight. The battery for an F150 costs almost twice as much as the battery for a Yugo, and also weighs a heck of a lot more.
So every vehicle has a battery that is big enough to do the job but not too big so that it doesn’t cost too much or weigh too much. You were on the right track when you said every engineer’s desire is the smallest possible battery size. Because engines come in different sizes, the smallest possible battery varies from vehicle to vehicle.
In the quest to make car engine compartments smaller and smaller (so you have more room for passengers or trunk space or whatnot), batteries now come in even more variety. You can have one that’s tall and thin or short and fat, depending on which size the engineers felt would fit better into their cramped engine compartment.
There’s also two different types of terminals, so that adds even more variety.
And, if like me, you own an old Beetle, those have a particular problem. The battery is located under the back seat, and as Beetles age, the seats start to sag, and sooner or later the metal springs start touching the battery and shorting out. So Beetle batteries have a plastic cap over the hot terminal to stop you from ending up with the infamous Beetle “hot seat”. So there is yet another type of battery.
Mostly though, it’s size, cost, and cranking amps.
Assuming NiMH AA batteries (they need to be rechargeable), you’d need 10 of them in series to get 12 volts, and that gives you about 5 amps, tops. A typical car battery is about 800 amps. If I did the math right, that works out to 160 strands of 10 batteries each, or 1600 batteries.
This becomes more interesting in the case of electric cars. It’s one thing for an auto parts store or Walmart to stock 30 or whatever different sized car batteries, but it would get extremely expensive for a company to stock a large number of the very expensive batteries used in electric vehicles. So you break down in the middle of nowhere because of batteries problems. It looks like days to get a replacement from a dealer, and they are only open on normal weekday hours…
There was a car like that, made by Joe Newman, a crazy-ass inventor who died in 2015. According to critic and physicist Robert Park, Ol’ Joe tried to raise funds to produce his miracle car, and after driving it – very slowly --once around a demonstration track, would hold up a single AA cell, saying, “It runs on this!”
What he avoided doing was opening the hood, which would have revealed not a single AA, but thousands and thousands of them, in series. A high-voltage DC motor isn’t the most efficient of machines, and Newman’s invention never saw commercial production.
Neither did his other perpetual motion inventions.
This isn’t even comparable, though, because it isn’t just the steady state draw the batteries can provide but the ability to deliver the required current during the high crank load of ignition, which NiMH batteries probably could not do, nor would NiMH batteries withstand the constant charge/discharge cycle that car ignition requires, hence why lead acid batteries are still used despite being less than desirable from an environmental point of view.
Practically speaking, there is no reason that four or five size/configurations of lead acid batteries wouldn’t suffice for all passenger vehicle ignition systems around which vehicle designers could package, but a minimum set of configurations has not been developed by the National Electrical Manufacturers Association and the International Electrotechnical Commission or endorsed by automobile manufacturers, so we get the large number of different configurations for no particularly good reason except inertia.
Electric vehicle battery packs are a unique situation because in order to provide sufficient range they end up being a significant amount of the vehicle mass and often an integral structure. Nonetheless, modern electric cars use what are essentially consumer grade lithium polymer (Li-Po), nickel metal hydride (NiMH), or molten chloroaluminate sodium (sodium salt or “ZEBRA”) cells joined in series in to form banks which are attached in parallel to provide the necessary current and voltage requirements. Since these don’t need to provide amperage against an aggressive cranking torque but are required to provide a steady load through deep discharge they are in a very different application from 12v automotive ignition batteries. The banks and packaging itself is custom designed for each application and making them interchangeable is not really feasible any more than you could build a single internal combustion engine suitable for all vehicles from a kei car to a heavy contractor van.
The need for different diameter/sidewall height/width/tread designs and compounds in tires is actually explicable. Although most people think that tires are just black blobs that hold the chassis off the road, they are actually one of the most complex unified mechanism machines ever designed. Tires are designed in specified for a vast number of different needs and purposes and are selected based only only on the size, mass, and speed of the vehicle but the desired ride effects, behavior of the suspension, type of environment they may operate in, desired lifetime cost, damage resistance, and other factors. I can’t say that every single tire configuration is absolutely necessary but this is a case where the market generally dictates which configurations are best suited for certain applications and those tend to dominate.
I got nothing for you, here. You should need maybe a dozen design configurations depending on oil weight, flow, and operating pressure of the engine. Ditto for air filters, and if Toyota or Subaru can build an entire car using three or four sizes of bolt/nut I’m at a loss why a General Motors compact car requires the entire range of SAE wrench sizes just to change the serpentine belt.
They are based on “group size”, and these offer different CCA and reserve requirements in a given size, based on engine requirements and accessories, actual battery location, styling, etc. People who live in very cold climates will often by the biggest, heaviest battery that will physically fit. Sometimes the OEM will provide a battery that is a little light in that department.
I knew there would be no way to have one standard, one-size-fits-all battery. I get, for the most part, what you are pointing out with the size of the vehicle/cranking amps theme. I understand that a large V8 F150 will have different needs than a Yugo.
But what about similar vehicles from the same manufacturer? (This is the origin of the OP).
Over the weekend I was visiting a friend who has two older BMW’s. A 2002 3-series, and a 2002 5-series. Both cars have inline 6 cylinder engines, and although the 5-series has a longer wheel base, for the most part the cars appear to have the same major options and relative power requirements. I thought the batteries would be the same.
They are not.
Now perhaps someone screwed up along the way and/or put the wrong battery in one of the cars. But the 5-series battery is larger than the 3-series battery.
Both batteries are in the same location in the vehicle (trunk) and although I couldn’t tell for sure, the eyeball test indicated that the battery bay was about the same size in each vehicle. They certainly looked like they should have been able to accomodate the same size battery. The 5-series trunk appeared to have more space, but the battery compartments looked close (if not identical).
I did not look up the specs on the OEM battery for each car, but I cannot imagine there is a huge difference in power requirements between a 3 and 5 series BMW from the same model year.
From a manufacturer’s POV, I can’t think of a reason why they would not not want to standardize this where possible. Have they fine-tuned the needs on these cars to a point where the savings BMW sees on a slightly smaller battery for their 3-series offsets the added manufacturing costs of a larger battery for their 5-series? (Again, not sure if the correct batteries were installed in those cars).
