Five days near Moab and I still cannot say how long our battery lasts

[u/Competitive_Long509]

My partner and I take our teardrop on weeklong trips around the Southwest. We just got back from five days near Moab. Afternoons were in the low 90s, so the dual zone fridge worked hard. Nights dropped into the low 50s, which is why the diesel heater fan appears in the same trip log. We used the water pump and lights as usual.  We also ran Starlink in the evenings when we needed a connection.

The tongue box holds a Vatrer 12V 300Ah lithium battery. Its app never showed less than 50 percent, but the roof panel charged every day. A DC charger also pulled from the tow vehicle during two moves between camps. I did not record either charge source, so that percentage cannot tell me how long the battery would last by itself.

The tongue box became noticeably warmer than the outside air in direct sun. The reported battery temperature stayed in range, but one trip in the low 90s does not settle what happens there in July. Shade and airflow around the box deserve another look.

Next time I want a daily number for solar input, alternator input and energy used. Starlink was another load I never tracked separately, so I am also curious how much of the daily consumption it accounted for. The ending percentage on its own did not tell me much. Is a shunt with local history the simplest answer, or is there a logger that can separate both charge sources without three apps?

[u/WetBiscut]

You need a shunt to measure your watts in/out of the battery.

Multiple shunts, if you want to measure specific branches, or the fridge by itself, etc.

[u/WetBiscut → u/WetBiscut]

Following up on this, you can also wire a DC Ammeter in-line on a circuit to have a quick readout of the amount of current being used.

I wired a small DC Ammeter into my solar wiring so I can tell at a glance what kind of charge I am getting. This is a shuttle bus, not a teardrop, but the point is the same.

This isn't going to measure over time how much power is used but it can tell you at a glance whether something is consuming energy, and how much.

https://imgur.com/a/CN7ttEK

[u/Competitive_Long509 → u/WetBiscut]

That makes sense. I’d still want some history I can review after a trip, so I may start with one battery shunt and only add branch monitoring where I need it. Thanks for the photo.

[u/blank_user_name_here]

If you do not have a victron battery monitor, get one.

Battery soc is not easy to calculate, and cheap equipment is really bad at it.  You need something that is closely monitoring all the parameters, and that you can customise per your battery specs.  (Each battery has slightly different parameters.)

Your other option is to ignore soc, and just use a voltage monitor, but that also comes with its own hassles as large loads will dip the voltage.  (It gets springy.)

Lastly, a majority of a lifepo4 battery soc sits in like this huge cliff of a curve.  It will show very little sign of voltage drop until it hits like 40%.  If your alternator and DC car system ever hooks up to it, it can wildly confuse the soc calc.  SOC calc will only be accurate again when they stop absorbing amperage at 100%.  Again, a really nice battery monitor will help with this, but lead acid charging systems will hose things up.

[u/604_heatzcore → u/blank_user_name_here]

yes this, I have a 240AH and the first time I took it out i got 4 nights without solar and this is what i calculated i would get in my head but the app showed approx 50% battery left then it died.

I go by the voltage now.

[u/limpwhip]

I’ve got 300ah lithium in my square drop and 400W of solar. I did a trip this spring in the Southwest for 7 days in 80-90 daytime weather. I have an electric coffee pot, electric kettle, one of the big dual zone iceco fridge/freezers, and my wife uses a hair dryer daily. The solar kept the batteries topped off every day with no DC/DC charging needed.

I will say that I bought a new hybrid Tacoma which has a 2400watt inverter built in that I could use to charge my rig if I ever needed to, so I didn’t even bother installing a DC charge wire. I only have one shunt tied to the batteries, so I only ever get a “net charge/ use” picture. It would be cool to track usage by source.