Battery Monitoring
March 16, 2020
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Something is not quite with the batteries, but it might not be the batteries!

And so the investigation starts, why do we seem to have trouble charging the batteries off our Solar panels, even when we have a full day of sun? It starts with understanding how much electricity we use, how much we generate and will likely end with a battery monitoring system and maybe even new lithium batteries.

This blog-post is a work in progress, and we are just at the beginning.

Step 1 – Understand our Electricity Usage

There are many things in Molly that use electricity, but most of them are off most of the time. There are some things that are on all the time and here are their energy consumption

  • The Cradlepoint Modem. at idle is 4 watts and goes to 9 watts with WiFi. Let’s call it 9 watts.
  • The Mintbox computer is 5 to 15 watts depending on connected devices. Let’s call it 10 watts.
  • The fridge is rated at at instantaneous consumption of 72 watts, but average consumption is 440 watts per 24 hours. Or only 20 watts per hour.
  • Lastly although the inverter is off, it has a little parasitic drain. This is called “searching” mode and consumes less than 8 watts.
  • Total draw is 47 watts, or close to 4 Amps.

So, doing a little math, the total draw is around 47 watts which at 12.7 volts is around 3.7 Amps. Of course, when we turn on lights, make a cup of coffee or basically do anything, this value will go up.

We have 2 Mastervolt AGM batteries each rated at 270 Amp Hours (Ah). But AGMs should not be discharged below 50% of their total capacity, so instead of 540 Ah, we really only have a combined capacity of 270 Ah. Without running anything, this means we would get around 69 hours of life or almost three days.

Step 2 – Cruse Measurement

Fortunately we have a BEP Marine DC System Monitor (DCSM) which is wired up to the house batteries and can display the voltage of the house batteries as well as the current consumption (or charging) in Amps. After fiddling with the setup menus, we also configured the monitor to track the State of Charge, which is calculated using the instantaneous voltage and amps integrated over time, with some compensating factors.

We have 2 Mastervolt AGM batteries each rated at 270 Amp Hours (Ah). But AGMs should not be discharged below 50% of their total capacity, so instead of 540 Ah, we really only have a combined capacity of 270 Ah. We wanted to find out how long 270 Ah would last. So, with the house batteries fully charged (at 540 Ah), we monitored how the battery was being drained as follows

  • With no Sun and no Alternator charge, and after three of driving in the dark, the batteries have dropped from 540 Ah to 528 Ah. During these three hours the fridge went from warm to cold, the radio was playing and our computer and modem were powered up. Not a great way to work out “normal” consumption, so this will be ignored.
  • During cooking and dinner, which lasted an hour, another 3 Ah were consumed and the batteries were now down to 525 Ah.
  • Waking up early and to catch the battery state before the sun appeared, showed the batteries at 504 Ah, meaning they dropped 21 Ah over a 9 hour time frame. The fridge was on, as was the computer and modem and the iPhone was charging, but everything else was off (technically the inverter was on but nothing was connected). The average energy consumed was 2.5 Amps per hour. Not bad.
  • It was a dark day, and I was working on the truck during the day, and let’s just say at the end of the day and the end of the sun the batteries were at 500 Ah.
  • One hour and 30 minutes later the batteries were at 496 Ah, which is aligned with the 2.5 Amps per hour. As I headed to bed, 2 hours later, the batteries were at 491 Ah.
  • Getting up late and with the Sun already up, some eleven hours later, the batteries were at 482 Ah, which is good, but guessing this includes about 2 hours of Sun.
  • Another 2 hours later, with about 13 Amps coming from the Solar panels, the batteries were at 489 Ah. This is when I started to think that there is a mystery here. The capacity should have been around 499 Ah. This is calculated by taking charge efficiency of 80% of 13 Amps for 2 hours giving 21 Ah. but subtracting background discharge of 2.5 Amps per hour, or 5 Ah gives a total of 499 Ah. So 489 Ah was way too low and probably the cause for the why it is difficult to charge the batteries off solar.

This first experiment showed that consumption was ok, but charging was not. More equipment would be needed to isolate the problem. The leading theory was that the battery monitor was no correctly configured. Off to amazon to order a DC amp meter.

To be continued….