Battery monitor (shunt-based)
Monitoring
A battery monitor counts the amp-hours going into and out of your house batteries, so it can show state of charge, current draw and time remaining, which a voltmeter alone can't do reliably. It works through a shunt, a precise resistor fitted in the battery's main negative cable.
How it works
Every amp of charging and every amp of load passes through the shunt. The monitor measures the tiny voltage across it, counts amp-hours in and out, and compares that with the bank's capacity. Small errors add up over time, so it resets itself to 100% (synchronises) when it sees the battery is full: voltage above a set level while charge current has dropped below a set tail current. Settings for capacity, charge efficiency and Peukert exponent refine its estimate.
Using it
- Watch the current reading: a minus figure is being drawn from the battery, a plus figure is charging it. Switching loads on one at a time shows what each one uses.
- Use state of charge and consumed amp-hours to decide when to recharge, and set the monitor's low-charge and low-voltage alarms.
- Give the batteries a full charge regularly so the monitor can re-synchronise to 100%; without that, its reading drifts.
Looking after it
- Enter the correct battery capacity, charged voltage and tail current for your batteries, and the lithium settings if you have lithium batteries.
- After replacing batteries, update the capacity setting and resynchronise.
- Keep shunt connections clean and tight, and keep the fuse in the monitor's small positive supply wire.
- Update the monitor's firmware when the maker releases updates.
Troubleshooting
State of charge is wrong or drifts over days
- Check that every load and charger negative goes to the load/charger side of the shunt. Anything wired straight to the battery's negative terminal is invisible to the monitor.
- Check the capacity, Peukert exponent and charge-efficiency settings.
- Check the charged-voltage and tail-current settings used for synchronising.
- Fully charge the batteries and let it synchronise, or synchronise it by hand.
Charging shows as discharging, or the reverse
- The shunt's two negative cables are swapped; exchange them as the maker's manual describes.
Monitor always shows 100%, or never reaches 100%
- Always 100%: check the negative cables at the shunt are wired correctly.
- Never 100%: make sure the charger actually completes a full charge, then check the charged-voltage and tail-current settings.
Shows a small current when everything is off
- Physically disconnect everything from the load side of the shunt before doing the monitor's zero-current calibration, as the maker instructs.
Buying a boat that has one
- Check the shunt is in the main negative with nothing else on the battery negative terminal; gear added over the years often bypasses it.
- Check the settings match the batteries actually fitted now, not ones long since replaced.
Sources (1)
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.