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BMS1230S2 intermittent charging issue

Manager30 BMS1230S2 model intermittently not charging


For Fixed Voltage or Temperature Compensating alternators


When the DC Input Trigger is set to “12V”, the BMS1230S2 turns off at 12.7V but operates down to 9V. The reason for the two figures is that, while it is running, the current drawn by the BMS1230S2 causes a voltage drop along the line from the start battery. It is designed to still charge effectively even when the voltage drop is so extreme as to produce only 9V at the BMS1230S2 input  even though the actual start battery may be well above the “turn off” voltage (allowing for a very long cable run). Every 100 seconds, the BMS1230S2 switches off briefly, (eliminating the voltage drop) and checks the input voltage (effectively measuring actual start battery voltage). If it is still above the “turn off” voltage, it switches on again. The cycle is repeated until the battery voltage is below the “turn off”, in which case the BMS1230S2 stays off. It will only turn on again when it senses the start battery is above the “turn on” voltage of 13.2V when is charging.


If there is additional load on the line to the BMS1230S2 (as can be found for example, when a 3 way fridge and the BMS1230S2 are both being supplied via the same cable and Anderson plug from the start battery) the fridge alone can cause enough voltage drop to cause the BMS1230S2 to see less than 12.7V when it does its 100 second check. In this case it will not switch back on unless (in this example) the fridge cycles off.


To alleviate this problem, first check:


  • Check that the alternator is keeping your start battery to at least 13.5V.
  • Cable thickness. For the BMS alone we recommend at least 6AWG (13mm2) wire for more than a 3 metre run (see page 17 of the BMS1230S2 Instruction Manual). If the cable is being shared with another load, you should have at least 4AWG (21mm2) wire.
  • Connectors. Ensure that all connections (terminal to wire, wire to wire Etc) are cleanly crimped and/or soldered.
  • Plugs. Anderson plugs are not recommended for long term connection. They are meant for applications where they will be unplugged & reconnected occasionally, as this process makes the contacts “wipe” each other clean of any contamination or tarnishing that can build up in this style of connector. If an Anderson plug has been plugged in for an extended time, unplug it and spray with WD40 Etc. before reconnecting.
  • Fuses. We strongly recommend the use of MIDI style fuses. Other types are prone to poor contact and can eventually cause overheating and melting even well below the fuse rating. For more on this, see Bad fuses? Or bad fuse holders?


If none of these points apply, it is possible to ignore the voltage drop by rewiring as if you had a Variable Voltage “smart” alternator. To do this, change the input trigger to “Ignition”, and either run a line from vehicle ignition to pin 4 of the BMS green plug or from park lights to pin 4 (and remember to keep park-lights on when you want he BMS charging) OR by adding a link from pin 4 over to pin 6 of the green plug on the BMS and an ignition controlled relay or solenoid to supply power from the start battery to the BMS (see How to wire a BMS1230S2 from caravan to vehicle with Variable Voltage alternator).  


For a Variable Voltage “smart” alternator


The BMS operates in a similar way to the other alternator systems except that it turns off at 11.9V (still works down to 9V), with the same 100 second test cycle. With a “smart” alternator, the input trigger must be set to Ignition and wired as described above. (see How to wire a BMS1230S2 from caravan to vehicle with Variable Voltage alternator).  


If this has been done correctly, check the same points as listed above for other alternator systems.


Any further questions please contact the REDARC technical support team via phone or email.