Tolga and Fatin contact you on the radio and order you to come to them - order you! As if they have the authority?! Really?
You meet them in the Safe Zone by the school - which has its own little community of survivors who are not very friendly.
When you speak with the boys they explain that they need batteries - bus batteries with marine grade plates. These guys really think that they are pretty smart - and you are very very stupid. They want you to retrieve three batteries for their project.
When you reach the Bus Depot there are plenty of infected to deal with first - once you clear enough of the faster ones away - including the three Toads up top - you can pick into the three buses with batteries in them and grab them, then head back to the Safe Zone.
Once delivered that completes the side-quest and gets you yet another rude attitude from the geek twins.
Collectibles
Note #26 - in the Trailer just outside of the zone.