You are right:Doesn’t make sense to me because it’s an oversimplification. Early in my career there were many “rules of thumb” as they were called then that often led to bad designs because they were taken out of context.
This would be OK if all AGM and LiFe batteries were around 100 Ah, and granted most are, but all are not which opens door to mistakes. An 80 Ah AGM is far from a 255 or 330 Amp-hour AGM. I know what you mean, but the new guy may not.
For that matter, we need to get people to stop thinking of battery capacity solely in Amp-hours, but that’s a different issue; and one that may take years to change if it ever does.
- I purposely did not want to confuse people by bringing in the need to look at battery discharge curves into the discussion. Yes, I have those plots in my computer and have played with these curves and various power implementations.
- I simplified the discussion by focusing on 12 volt, size 24 - 31 AGM and LiFe batteries in the 80 - 110 amp-hr range because that is a common size in van conversions (and what I use in projects).
- People can physically lift this size range of batteries without too much trouble. Larger sizes are challenging to handle, at least at my age. An 8D battery is amazing, but I can't lift one so it doesn't make sense for me to recommend this to anyone else.
- In size 27 you can buy both AGM and LiFe from multiple suppliers, with reasonable specs, so IMHO, that is a reasonable size to use. I fact, that is what I use and do recommend it to people.
- The only AGM battery that I use is Lifeline but there are a couple of other similar ones out there. I am not a dealer, just very familiar with their performance and fit. If someone thinks that they can buy a $50 battery and get similar performance, not much that I can do to help them. I have lowered my expectations that people will or will not accept my suggestions and don't enter these types of discussions about battery selection. Either they are willing to read a data sheet - or not.
- I am not sure if size 27 is the perfect intersection of price / performance / ease of use, but it isn't so far from it that people will say "you are an idiot - size xxx is 10 x better".
- For me personally, I much prefer to use the term "usable capacity of xx kW-hrs" instead of amp-hrs, partially because I prefer to use 48 volt and 24 volt systems, but that is a different matter.