cuanb avatar image
cuanb asked

Smart BMS 12/200 voltage difference

I'm scratching my head with the Smart BMS 12/200, and hoping someone can shed some light for me.

The voltage on the alternator terminal appears to be 0.2V higher than the voltage on the System+ terminal when the absorption voltage of the charging is reached. Eg, when absorbtion voltage of 14.2V is reached on the alternator terminal, the System+ terminal reads 14.0V. I have resorted to installing a DC-DC charger to the alternator terminal and set the absorption voltage to 14.4V, this brings the System+ (battery) voltage to around 14.2V. In the BMS I've set the battery charge limit voltage to 14.5V to keep the DC-DC charger alive in order to provide power to loads like a 12V fridge - otherwise the BMS cuts the alternator charging and the fridge draws down the batteries until the alternator charging kicks back in (lots of micro cycles)

Does anyone have any idea why this is?

I have the "dumb" older version of the BMS 12/200 and the voltage readings on the AB and LB terminals are the same...

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1 Answer
philiplyra avatar image
philiplyra answered ·

Part of this drop may be attributed to the fuse. F.e. for 80A fuse at 75% of rated current (60A) this drop is 0.087V. If you have a smaller fuse the resistance would be higher. See LittleFuse "MEGA® FUSE RATED 32V" datasheet.

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cuanb avatar image cuanb commented ·
Good thought, but I have tested a few different mega fuses as well and a bunch of ATO fuses with the ATO fuse holder but there is still this higher voltage on the alternator input side.

I initially thought it was to "trick" the vehicles alternator and keep the alternator from ramping down if it's a smart alternator, but this seems backwards to me so I'm probably wrong thinking this
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