Hi, which dual battery converter module you chose? As I know, there are two kinds of module, one for discharging from A to B battery, and another for discharging from both batteries in the same time. Even for the latter, it is suggested to choose a second battery with on/off switch to still let the battery discharge from main battery to second one. Lithium ion battery is ususally not suggested to be connected in parallel, and two batteries, new/old, or in different voltages or from different manufacturers, will cause problems in parallel connection, especially when the BMS charging & discharing in common port, the higher voltage battery will charge the low battery.
Reention Polly or Shanshan Hailong series are most polular down tube type for second battery. Below are all Polly battery options with 21700 available.
DP-5/5C, 368x90x96mm, 48V 14Ah max for DP-5
DP-6/6C, 368x90x113mm, 48V 17.5Ah max for DP-6
DP-9/9C, 368x90x141mm, 48V 24.5Ah max for DP-9
DP-2170-4/4C, 368x95x106mm, 48V 15Ah max for DP-2170-4
DP-2170-5/5C, 368x95x125mm, 48V 20Ah max for DP-2170-5
The C version means there is controller box on base plate, which takes up some space of battery body causing smaller capacity.
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