The tech giant has been e-mailing customers who purchased any of six AmazonBasics power banks between December 2014 and July 2017 informing them of the recall and requesting they ship them back immediately, The Verge is reporting. According to the e-mail, the units are at risk of overheating and could catch fire, melt, and cause damage.

The affected batteries are all Amazon’s low-cost AmazonBasics brand. They have capacities of 16,100mAh, 10,000mAh, 5,600mAh, 3,000mAh, 2,000mAh with Micro USB cable, and 3,000mAh with Micro USB cable. The power banks are all used as external battery supplies for smartphones and other products.

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According to The Verge, the e-mail Amazon sent to customers offers a link that they can click to register the affected products and learn how to return them. Amazon is also offering refunds on the battery packs and has removed their listings from its online store.

Power banks have become a popular way to ensure longer battery life on handsets, tablets, and even computers. If your smartphone’s battery is running low, you can plug the handsets into the power banks and draw power from them to recharge your handset’s battery.

Amazon did not immediately respond to a Fortune request for comment on the recalls.