Car batteries are going to be a real challenge in finding them in Escape from Tarkov. You sometimes get lucky to see one, but then it is too big or too heavy to be lifted by you. Having said that, you are not going to leave any behind until your hideout is complete or other tasks that need car batteries are complete. There are a few spots of luck on some maps where Car Batteries have a higher chance of spawning compared to others. The best way to easily and quickly gather Car Batteries in Escape from Tarkov is to hit those areas, and here's how to accomplish it.
You can find car batteries in Technical Supply Crates, and other forms of random loot containers that tend to contain technical items as they are treated as a "technical" spawn. Fortunately, you're not going to find these Car Batteries in all technical crates and boxes. To put it into perspective, while Toolboxes can spawn technological treasure, they cannot spawn Car Batteries. That makes sense because of how large car batteries are.
These regulations are going to make vehicle batteries pretty hard to come by. While searching for vehicle batteries, remember to check out the following:
The added in patch 0.16, Customs' industrial plant is actually the best spawn for the Car Batteries. In general, this plant will be really good for farming not just Car Batteries but any technical item since there's a bunch of places there with technical loose loot spawns.
You want to be up in those two rooms on this upper floor of the industrial plant. Pictures up there of spawns lying on the ground. On ground near shattered windows on first level, near Wi-Fi Camera position, it is occasionally able to spawn in the Provide Viewership assignment.
There is one interchange after this point. This is, in my opinion, the best place to farm car batteries. There are so many different missions that can require the spawn of electrical items, such as light bulbs, capacitors, power cords, and many more; getting Car Batteries isn't the only option. These can be searched for on Interchange, and if you're extracting via the Emercom Checkpoint you can always run by the back of Goshan to check if any of the vehicle batteries have spawned in onto the shelves.
On the metal shelving circling the image above, you can find the Car Battery. As it can also spawn on the other side, you may want to try that too.
Lighthouse and the Streets of Tarkov are two other potential locations to obtain car batteries; however, this is dependent on your familiarity with the maps. It is quite difficult to escape, particularly with that much weight, from the water treatment facility in Lighthouse or the camp in Streets of Tarkov, both of which have good odds of spawning Car Batteries.
Four "found in raid" car batteries are the reward for the Therapist's Car Repair quest, meaning this is the second in a chain of quests given by the Pharmacist. You will most likely also need additional Car Batteries beyond the Car Repair task because hideout upgrade requires, as of patch 0.16, most items to have the "found in raid" tag in order for you to upgrade your hideout, meaning you'll have to farm additional Car Batteries every wipe.
Vents can be upgraded to level 2 with a single Car Battery and to level 3 with four more. That's nine car batteries over the course of a month. If you want to get these before you get to the hideout improvements or the quest, start saving up some stash space.
Now that you know How to Get Car Batteries in Escape from Tarkov, just apply this guide and perform this search without any complexity.