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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.

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    \$\begingroup\$ Inverters tend not to be light loads so setting 10.5V cutoff makes sense for them. That doesn't imply it's the right cutoff voltage for anything else. So use a more appropriate cutoff for your 100 hour load. \$\endgroup\$
    – user16324
    Jan 26, 2020 at 19:35
  • \$\begingroup\$ Those are not usually adjustable on inverters, they are set at 10.5V or maybe somewhere between 10.5V and 10.8V. They should have a 3 way switch on those to handle low voltage cutoff for light, medium, and heavy loads. The voltage cutoffs could be 11.5V for light loads, 11.0V for medium loads, and 10.5V for heavy loads. That would be pretty slick and probably wouldn't add much to the cost of the inverter but would make it MUCH more versatile and protective. Even better would be if it would autodetect the load and adjust the low voltage cutoff automatically (and have that be a 4th setting). \$\endgroup\$
    – David
    Jan 26, 2020 at 20:12

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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.

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    \$\begingroup\$ My main point of this post is to confirm that low amperage loads on batteries can be dangerous if the low voltage cutoff on an inverter is your only line of defense. I would like to see inverter manufacturers allow the user to tell the inverter the intended load, and thus effectively "program" the low voltage cutoff which would then be MUCH more effective battery protection. Why not just have a low/high load switch or even better, low/med/high load switch? low could be 10A or less, high could be 100A or more, and med could be everything in between. For low/high only, maybe 25A crossover. \$\endgroup\$
    – David
    Jan 26, 2020 at 20:18
  • \$\begingroup\$ I suppose variable loads would have to be set either at the lowest expecting load setting (for example, a load that varies between 55A and 110A would be set at medium, not on high. I realize there is a tradeoff here but I would rather bias the protection of the batteries vs. getting more runtime at a variable load. If the batteries used are deep cycle rated, then perhaps in the 55A 110A scenario, the high load setting could be used for "extra" runtime (vs. the medium load setting). \$\endgroup\$
    – David
    Jan 26, 2020 at 20:23
  • \$\begingroup\$ The low loads are "dangerous" only in the sense that your inverter might shut down even if there is considerable capacity left in the battery. The low loads are not inherently dangerous to the battery. The voltage under load is also influenced by the battery's ESR, which changes over time and will vary by manufacturer. Trying to estimate capacity by measuring voltage is just not a good idea. That's not how your phone and laptop do it. \$\endgroup\$ Jan 26, 2020 at 21:48
  • \$\begingroup\$ So if cellphones and laptops use a different method to determine SoC, namely Coulomb counting, why then don't they have something similar for 12V inverters? For example, why couldn't I just program my 12V inverter that I have a 12V 230Ah battery fully charged and to count 115Ah coming out of the battery and then turn off the inverter? That way it would NOT matter much what the load is, and it could even vary (which many times it does). There could be something like an intermittent and widely varying load such as a 2000 watt well pump that cycles 1 min on every 20 min \$\endgroup\$
    – David
    Feb 4, 2020 at 11:23
  • \$\begingroup\$ @David, they don't use this in inverters because it's not the inverter's primary job to keep the battery from overdischarging. There are separate 12V/24V Battery Management Systems available (for the marine and/or RV markets) that can do the "Coulomb Counting" for you and raise an alarm (and maybe switch off the load) when the State of Charge reaches a predetermined minimum. \$\endgroup\$
    – StarCat
    Feb 4, 2020 at 12:31
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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?

Yes, you are correct. Lead-Acid batteries should not be left for a long time below full charge because it encourages sulfation.

If so, why do inverter manufacturers set the protection at 10.5V and not consider the load?

They assume the inverter will be used with a reasonable load. An inverter is not a battery management system, and its job is not to keep the battery charged to an appropriate standby voltage.

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?

Expense is always a factor, but also the manufacturer cannot control usage. The same inverter might be used on a large deep-cycle battery, a car battery or a small SLA, and though it could calculate cutoff voltage based on its own current draw, it cannot know if another load is present.

Setting the cutoff to 10.5V covers the vast majority of uses where the inverter is drawing high power that could quickly kill the battery if the user wasn't watching. Low power draw implies a long time period, where the user should have other systems in place to ensure battery health (eg. not leaving the car in the garage for a week with a 0.25 Amp load connected!).

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  • \$\begingroup\$ The 0.25A load for a week could be something the person doesn't know about such as a light from the glove compartment left on by accident, a mirror light left on by accident... It doesn't take much load to ruin a battery if the vehicle sits for a week. I keep my unused car on a trickle charger just for that reason since I have 2 vehicles and alternatively drive them 6 months each out of the year. \$\endgroup\$
    – David
    Jan 26, 2020 at 20:05
  • \$\begingroup\$ Having a single low voltage cutoff is like having a 1 speed transmission on a car. It is very simple, but not very versatile and far from optimal (unless it is a top fuel dragster). \$\endgroup\$
    – David
    Jan 26, 2020 at 20:40
  • \$\begingroup\$ Sure it's not very versatile, but as I said an an inverter is not a battery management system. How is it going to know that the car is unused? What good will shutting off do if something else is draining the battery? If it sounds an alarm and nobody is there to hear it, won't it just drain the battery faster? \$\endgroup\$ Jan 27, 2020 at 3:06
  • \$\begingroup\$ " It doesn't take much load to ruin a battery if the vehicle sits for a week. I keep my unused car on a trickle charger" - trickle charging an unused vehicle is a good idea, as is checking on it occasionally. I also have a spare battery that I keep topped up in case the one in the car dies, though it isn't so important now that I have a Nissan Leaf. \$\endgroup\$ Jan 27, 2020 at 3:13
  • \$\begingroup\$ I think you are confusing float charging with trickle charging. Float will let the battery self discharge some before topping it back off. Trickle charge is a constant charge that counteracts the battery's self discharge and may only be milliamps for a healthy battery. \$\endgroup\$
    – David
    Jan 27, 2020 at 3:45

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