This is a set of two 4,000 mAh batteries made by BergHeat for its heated socks and heated knee socks. Each sock gets its own battery, which slots into the cuff pocket, so both feet heat evenly instead of one running weaker. Output is 3.7 V / 2 A, and a charge lasts up to 10 hours in the socks on the low setting. The pair charges over USB with the included cable and has a protection circuit against overcharging, overcurrent and short circuits. It is a dedicated accessory: it does not fit BergHeat's vest, jacket, hoodie or gloves, and the 5,000 mAh power bank for those garments does not work with the socks.
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Buyers rate the pair very highly, and the recurring point is that having a battery for each sock keeps both feet warm at the same level. Owners also value the long low-setting runtime and the simple USB charging.
This set of two batteries is made for BergHeat heated socks and heated knee socks. Each battery holds 4,000 mAh and delivers 3.7 V / 2 A. One battery powers each sock, so both get their own power.
Each battery goes in the pocket at the sock cuff. A short cable connects it to the sock. In the heated socks, one charge lasts up to 10 hours on low. On higher settings and in very cold weather, the battery runs out sooner.
The batteries charge over USB at 5 V with the cable in the set. A protection circuit guards against overcharging, overcurrent and short circuits. They are not meant for the BergHeat vest, jacket, hoodie or gloves.