For　the　popularization　of　constant-current　charging　to　onboard　energy　storage　hybrid　systems　(ESHS)　and　requirements　of　higher　voltage　and　higher　current.　A　novel　bidirectional　three-level　inductor-capacitor-inductor-T　type　structure　(LCL-T)　resonant　converter　applied　to　onboard　ESHS　based　on　quasi-constant-current　(QCC)　is　proposed　by　combining　the　LCL-T　resonance　with　the　three-level　cascaded　neutral-point　clamped　active　bridges　(3L-CNPC).　Each　sub-NPC-arm　of　3L-CNPC　can　work　solely　or　in　tandem　with　a　coupling　transformer,　which　is　adopted　to　design　different　modulation　methods　to　control　the　input　voltage　of　the　resonant　tank　module　(RTM)　that　can　establish　once　and　double　QCC　modes.　While　a　characteristic　of　unified　power　transmission　in　two-way　is　achieved　by　a　symmetrical　design　of　the　RTM　parameters　that　can　access　an　LCL-T　resonant　QCC　output　open-loop　configuration　only　depended　on　f(n)　and　Q　.　Besides,　an　approach　to　QCC　for　getting　operating　frequency　f(s)　that　meets　the　restricted　given　judgment　criteria　of　QCC　is　designed　under　reactive　power　control　and　realizing　ZVS　of　switches.　At　last,　the　simulations　and　prototype　experiments　both　prove　that　the　proposed　converter　almost　implements　the　QCC　output　within　judgment　criteria　in　each　mode.