Battery power - how low can I go?

[DavidM]

I think anyone with a DC fridge should consider installing a battery monitor. These work differently from a simple voltage based monitors because voltage is a very inaccurate indication of battery state of charge (SOC) and these measure amps directly.

The ammeter based monitors use a shunt which is a device connected in the battery circuit which produces a tiny voltage which is read by the battery monitor and converted to amps. These integrate amperage over time and give you amp hours consumed or replaced with a charger.

Some also let you set your battery parameters- Ah capacity and type and tell you percentage used.

You can pay anywhere from $16 for a difficult to use one from Drok, about $100 from Renogy and $200+ from Victron. I have used the top and the bottom of these and the Drok is usable but kind of obtuse. The Victron is first class but expensive. The Renogy seems to have all of the bells and whistles so it seems to be the best value.

But if you like playing and playing cheap, the Drok will work.

David

[AJMike]

Originally Posted by DavidM

I think anyone with a DC fridge should consider installing a battery monitor.

I am sorry I waited as long as I did to install the one I now have. I relied on the Zamp solar controller in the RV and while that has been helpful it really has not told me how much battery power I have left or how long it will last at my current consumption rate.

Now it would be nice to actually be able to go somewhere and see how it works ...

[cj_calva]

Hi
Well this is not a topic which could be answered in a one-liner. But let's say this:
If you need too much electric power there are two possibilities:
1. install more battery capacity.
2. reduce the need of electric power.
I start with point 2:
You can reduce the power consumption of a refrigerator by running it at 41F and not 37F. 41F is always good enough to keep food fresh and beer cold. In addition, you shouldn't put a dozen warm beer cans in the fridge all at once to cool them down, but each time you take a can out, replace it with a warm one (ok, we all know that from the older gas driven fridges).
It's also a good idea not to expose the refrigerator side of the RV to the blazing sun, as this will prevent the warm air produced by the refrigerator from being properly vented out (23A floorplan, where the vents of the fridge are not under the awning).

Point 1:
Installing more battery capacity in a fuse is possible due the compartment should take group 31 sizes too.
Replacing the AGM batteries by Lithium Batteries is a good idea but need some investment. As well as you have to know that LiFePO4 Batteries are good and secure (remember those burning Teslas or exploding mobilephones) but the power density is not as good as Lithium Cobalt batteries and looking to the battery dimensions it's not that much better than AGM batteries. But perhaps it's excactly what is needed.

Another advantage of Lithium Batteries is that they shut automatically down when the reach a lower level, so you cannot damage them due to discharge them too much.

In this point AGM batteries are different. You can theoretically discharge them down to 0V. However, it is also the case that the compressor of the refrigerator stops running when the voltage is too low and this will be at 10V. If you follow this principle, you will shorten the lifetime of the AGM battery from about 6-7 years to 5-6 years. But that's just a thing about the lifetime of an AGM battery: It depends the manufacturer, it depends the maintenance (if you don't need it at all, you will shorten its lifetime much faster), it depends the temperatures too. I have AGM solar batteries at home they are 12 years old and still working great, but our neighbour needed to change them after 5 years. Same manufacturer, same product, same location.

Considering the fact that the prices for LiFePO4 batteries will decrease (100ah below US$850, Black Friday 2019 you could get them for less than US$800), it should pay off, as the life expectancy of lithium batteries is longer and they are much easier to care (put in an forget).