A problem I have with 12V power inverters is that they are set to give a warning beep and/or shutoff at about 10.5V (1.75V per AGM cell). This is supposed to protect the battery from overdischarging, and thus help it to have a longer service life. However, the effectiveness of this protection is load dependent. For example, if I put a very heavy load on a 12V battery such that it would hit 10.5V under load very quickly, that battery would likely "spring back" quickly to 12.0V or even higher when the load is removed, thus the 10.5V low voltage cutoff is effective in that scenario. However, imagine I put a very light load on a battery (such as a small parasitic load) and takes 100 or more hours to get the battery down to 10.5V. Now we have a BIG problem! The load is so light that once removed, the battery will NOT spring back to 12V or even any voltage close to that. If we are lucky it will be maybe 11V with no load but that is way below 50% SoC! That battery will be discharged WAY too much! So it seems the 10.5V low voltage cutoff is assuming at least a moderate load such as enough to get the 12V battery down to 10.5V in perhaps 20 hours or less.
So to me it seems very dangerous to the battery's health if the load is very light and the entire setup is not monitored carefully. For example, someone has a small 0.25A load in their car (that happens to have an AGM battery under the hood), and they don't drive that car for a week so the load has been draining for 168 hours so that would be about 42 Ah drained from the battery.
So am I correct in saying that small loads on a battery can be more detrimental to the battery's health than large loads for the reason I explained here? If so, why do inverter manufacturers set the protection at 10.5V and not consider the load? For a light load, it should be more like 11.5V for protection. Is it too difficult and/or expensive to do a load dependent low voltage cutoff so they just "cut corners" and set it at 10.5V assuming a "reasonable" (20 hour max) load down to 10.5V?
I suppose "worst case" might be turning on an inverter with NO load on it (other than the inverter itself), and relying on the 10.5V (or something close to that) low voltage cutoff when using an AGM battery for example.
Best Answer
Taken to its extreme limit, you are suggesting that leaving a battery in storage, with no load, will harm the battery is some way that a heavy load would not cause harm. That is clearly not the case.
The key to your question is "the entire setup is not monitored carefully". If you let a battery discharge too far, whether from self-discharge or under heavy load, it may be damaged. Your problem is that you are using the voltage under load to estimate the remaining capacity, and that is a very poor method. It seems that the inverter manufacturer's intent is to protect the battery from damage, rather than trying to extract as much energy from the battery as possible.