Based on a previous post , if you observe similar malfunction, that may be caused by the wrong type of battery. Another cause is the limited output ability of step down chip . You may need to do tests to determine which cause is your bottleneck. 14500 means 14mm in diameter and 50mm in length. The kit is using the same battery holder for normal AA cells (It's hard to find dedicated battery holder for 14500 Li-ion). They are 49mm high. Most 14500 rechargeables with protection circuitry are 52-54 mm high and do not fit the supplied battery holder. This post intends to take a survey on the batteries available in different regions of the world. Please report your batteries by commenting below to help others find the right solution for Nybble. "14500 3.7V Li-ion" are common words so I'll omit them. I'll integrate your responses into the following list: Brand Location Note Link Works: EBL 800mAh USA Walmart SANYO 800mAh Enerpower 14500 850mAh 2.4A Netherlands NKON Sony US14500VR2B Austria Conrad 18650 unprotected don't fit battery holder Floureon Lipo 7.4V 35C 1500mAh don't fit battery holder Tenergy 900mAh, 25C Lipo USA change battery holder & board Rexant unprotected 750mAh ART 30-2050 Russia -- Doesn't work: XCELL 139988, 750mAh Germany reichelt.de TrustFire Nitecore 850mAh NL1485 Keeppower 14500 840mAh Austria akkushop 3.2 V 14500 LiFePO4 600mAh too weak ROBITON 900mAh protected Li-ion ICR Russia To be fair, it's not necessarily the fault of batteries. They just don't fit with Nybble.