I've just installed a Victron BMV-600 battery monitor into my system. Works great, plenty of numbers to look at, etc - just one niggly problem :
- My 12V/2kW autostart sinewave inverter pulses once a second to see if there's anything worthwhile out there to power.
- The battery monitor seems to sample and update the display about every 1/4 or 1/2 of a second.
- With the inverter on standby, the monitor picks up pulses of approx 7-8 amps and can sometimes synch up with the inverter to read that 7 or 8 amps nearly consistently.
- This makes the overnight discharge (with all AC loads off) measured by the monitor to be in the order of 30Ah or so, which seems rather high.
The inverter manual states that it's standby power is approx 60mA - of course, this is averaged out. Putting my scope across the shunt shows some pretty big current pulses (sharp 20-30A spikes) for 5 or 10ms as the inverter pulses the AC system once a second. Judging from that, it's clearly not drawing 30Ah over the 8 or so hours that I'm in bed.
I presume that the battery monitor has a pretty high input resistance (500kOhms +) so it can sample the 500A/50mV shunt voltage fairly accurately, even after passing it through the 10 metres of skinny 4-core telephone cable that is supplied with the unit.
So to help alleviate this issue, I am considering putting a smallish resistor (1k-10k) in series with the positive side of the shunt connection and after the resistor, a smallish (10uF) capacitor across the positive/negative shunt sense wires. This should (in theory) smooth out the quick spikes a little and get some more sensible readings, whilst still being able to accurately measure the larger, more sedate loads. The smallish-value series resistance should be fairly insignificant compared to the largish value of the monitor's input circuitry, so things shouldn't really be thrown out of whack there.
Unfortunately, I have no idea of the smallish-ness of the resistor/capacitor combo needed to get a good smoothing effect on 5mV pulses that only last 10-20 milliseconds.
So, before I go out and stuff about with this, does anyone have any suggestions?
I did think about going down to the nearest bling-bling car sound shop and getting a 1F capacitor to put across the inverter terminals so it could smooth the spikes as needed, but they're rather expensive.
Cheers
Dave |
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