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It is critical to monitor the voltage levels on every battery packs to prevent voltage levels from dropping too low which can cause system failures and battery damage. More importantly, Lithium batteries are unlike other batteries and discharge on a non-linear curve, therefore it is important not to drain any Lithium batteries to it's minimum capacity before charging as it can cause permanent damage to the battery cell.
Western Robotics' battery monitors can prevent battery damage by giving you an early indication of your battery strength by visual LED technology feedback.
Spectrum LiPo Battery Monitor
7s Cell LiPo Battery Monitor [...]
Spectrum LiPo Battery Monitor HV
13s Cell LiPo Battery Monitor [...]
Spectrum Single Cell Battery Monitor
1s Single Cell Lithium Polymer Battery Monitor [...]
Spectrum NiCd/NiMH Battery Monitor
4/5 Cell NiCd/NiM Battery Monitor [...]
Sentinel Rx Volt Monitor
Monitors receiver voltage from 4V to 5V [...]
Sentinel Rx LiPo Battery Monitor
2s LiPo 8.4V to 6.2V Lithium Polymer Battery Monitor [...]