As technology progresses, there will soon be millions of clean electric cars on the road. Every major automotive giant has plans, or have already built the electric vehicles. Many say that they are doing so to cut greenhouse gas emissions to leave less pollution in the world. And all in all, there is absolutely nothing wrong with the direction that this new trend of electric cars will take.

However, underneath the hood of each electric car sits a lithium-ion battery. Research has shown that these batteries are manufactured in some of the most polluted manufacturing grids in the world. In fact, by 2021, there will be enough capacity to build batteries for over 10 million vehicles running on 60KW/h battery packs. Most of these batteries will be supplied by places like China, Thailand, Germany and Poland that rely on non-renewable sources like coal for electricity.

What this means is that while the cars themselves do not produce harmful emissions like the traditional engines, the manufacturing process itself for the batteries does. According to Munich-based automotive consultancy Berylls Strategy Advisors, the resources used to build just one car battery would emit up to 74%  more carbon dioxide than producing an efficient conventional car if it’s made in a factory powered by fossil fuels in a place like Germany.

“We’re facing a bow wave of additional CO2 emissions,” said Andreas Radics, a managing partner at Berylls Strategy Advisors, which argues that for now, drivers in Germany or Poland may still be better off with an efficient diesel engine.

Despite this, regulators have not come up with a set of clear guidelines on the acceptable carbon emissions over the lifecycle of electric cars. This may be due to the fact that electric cars are still new in the market and have not exhibited clear carbon emissions for regulators to properly come up with guidelines. Still, this is happening just as the likes of China, France and the U.K. move toward outright bans of combustion engines within the coming years.

“It will come down to where is the battery made, how is it made, and even where do we get our electric power from,” said Henrik Fisker, chief executive officer and chairman of Fisker Inc., a California-based developer of electric vehicles.

To put it into perspective, in a comparison between an average German gas-guzzler and the electric Nissan Leaf, it’s been shown that the German car can run up to 3 and a half years, or more than 50,000 kilometers, before a Nissan Leaf with a 30 kWh battery would beat it on carbon-dioxide emissions in a coal-heavy country.

This is not including the fact that the Nissan Leaf has one of the smallest batteries in the market now. BMW’s i3 has a 42 kWh battery, Mercedes’s upcoming EQC crossover will have a 80 kWh battery, and Audi’s e-tron will come in at 95 kWh. With such heavy batteries, an electric car’s carbon footprint can grow quite large, depending on how it’s charged. Driving in France, which relies heavily on nuclear power, will spit out a lot less CO2 than Germany, where 40 percent of the grid burns on coal.

“It’s not a great change to move from diesel to German coal power,” said NorthVolt AB CEO Peter Carlsson, a former Tesla manager who is trying to build a 4-billion-euro ($4.6 billion) battery plant in Sweden that would run on hydropower. “Electric cars will be better in every way, but of course, when batteries are made in a coal-based electricity system it will take longer” to surpass diesel engines, he said.

However, the benefit of driving battery cars in cities will be immediate: their quiet motors will reduce noise pollution and curb toxins like nitrogen oxide, NOX, a chemical compound spewed from diesel engines that’s hazardous to air quality and human health.

“In downtown Oslo, Stockholm, Beijing or Paris, the most immediate consideration is to improve air quality and the quality of life for the people who live there,” said Christoph Stuermer, the global lead analyst for PricewaterhouseCoopers Autofacts.

But electric cars aren’t as clean as they could be. There is a need to switch to using renewable energy to manufacture the car in order for it to be clean, and doing so will allow for the production of electric cars to slash emissions by over 65%. As it is now, manufacturing an electric car pumps out “significantly” more climate-warming gases than a conventional car, which releases only 20%of its lifetime carbon dioxide at this stage, according to estimates of Mercedes-Benz’s electric-drive system integration department.

Due to this fact, some manufacturers are exploring ways to produce batteries in a more sustainable way. Tesla uses solar power at its Gigafactory for batteries in Nevada and has plans for similar plants in Europe and Shanghai. Chinese firm Contemporary Amperex Technology Co. is also looking to power its future German plant with renewable energy like hydropower. In Norway, the electric grid is powered entirely by hydroelectric energy, and studies have shown that the electric cars there generate nearly 60% less carbon dioxide over their lifetime, compared with even the most efficient fuel-powered vehicles.

Until this issue is resolved, electric vehicles will have to wait to be crowned as one of the world’s best chances at environmental friendly rides.