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Inverter Low Battery Cutoff

Our inverter has a settting, where it will automatically discontinue inverting below a set value. It came from the factory, set to 12.2 volts.

When under a heavy load to run a 110v appliance, the voltage drops quickly. This was my scenario this morning.

Woke up, at anchor, and house found batteries at 12.6v and the SmartGuage says 83% State of Charge.

Used the inverter to make a pot of coffee. I saw a load draw over the shunt of 101 DC amps, with only the coffee pot on for about 10 minutes. Voltage was reading 11.9, while under load. I lowered the cutoff, to see what would happen. When the coffee was done, I shut the inverter off. After a minute of rest, the batteries were at 12.5v and the SG read 77% SOC. The SOC drop seems right for our 400 Ahr Gel house bank.

What is a safe inverter cutoff voltage while under such a severe load? Youd never allow a battery to drain to a resting 11.9v, but I understand it is different for a temporary high load. I just need a floor.


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Re: Inverter Low Battery Cutoff

The sag in itself is not harmful to the bank.

Keep tweaking it downward until - under your use patterns - it both

A. meets your needs, and

B. does not allow the SoC to drop below the point you want to use as your floor

As the bank ages, the same high usage pattern will bring it to a lower SoC, maybe also a lower "peak V sag".
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Re: Inverter Low Battery Cutoff

Ive not seen a published floor and would have thought such a thing exists.


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Re: Inverter Low Battery Cutoff

10.5V **at rest** is certainly a standard for "too low" don't sit anywhere near there for long.

But running a very high C-rate load will make even a brand new top quality AGM voltage sag **a lot** for a bit, then it recovers at rest.

Some cases absolutely require that sort of usage, so too high a LVC setpoint would render their system useless, the owner just accepts it is abusive to longevity and replaces more frequently.

Throw in FLA, or aging, or lower quality to start with, to different owner preferences for coddling vs accepting early replacement.

And that's just assuming a single "master LVC" setpoint.

As opposed to putting LVCs at different circuits, Essentials vs Auxiliary low priority, nav gear vs your entertainment systems.

How could a single number apply to all those scenarios?

Get to know the details for your unique setup, everything involves trade-offs, make your decisions in accordance with your goals and preferences.
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Re: Inverter Low Battery Cutoff

Note also many devices not just inverters, have a hard-coded internal LVC, no adjustment available.

They are designed to protect the device, not the bank feeding them.

If you want a guideline, use 11.5V for very high-load scenarios, maybe 11.8V for medium, 12.0V where C-rates are always low.

Now we can argue for hundreds of pages about those too-specific statements. Those valuing longevity more will want to go a bit higher.
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Re: Inverter Low Battery Cutoff

The LVC on the inverter may be there for two reasons. One being to protect the inverter as well as protecting all the loads from too low an output voltage. The other, more important, is to prevent it from consuming your batteries and leaving you dead in the water.

The inverter maker can tell you, if that is adjustable, how low it can do without causing the inverter output to drop below ~105 VAC.

On how low to go to keep starting power in the batteries...I've seen battery "savers" typically set to 11.5 volts, which should leave enough power to crank a healthy engine.

If you follow rough rule of thumb that a new battery is 12.6 volts and 11.6 is effectively dead, you might want to set a LVC closer to 12 volts to give you some leeway in not harming your batteries (by deep cycling) as well as preserving enough power to start. A personal choice.
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Re: Inverter Low Battery Cutoff

The cutoff setting follows voltage under load, which is a different animal than voltage at rest. 11.6 volts is not effectively dead, when taking a 100 amp DC load. Im not sure if its a good idea or not, but the battery was still well above 50% SOC. As soon as the load was removed, the voltage jumped right back up to normal operating ranges.

I know folks who run electric ranges off inverters. They have to do the same as my coffee pot, or more.


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Re: Inverter Low Battery Cutoff

No, not under load but at rest, or under minimal load, not starters or winches.

I still have the table cut out of Practical Sailor many years ago, taped on the side of my hydrometer case, showing the voltage drop from "new" to "useless" 10% drop in power for every 0.1v drop in resting voltage.
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Re: Inverter Low Battery Cutoff

There is often a low voltage disconnect as well as a low voltage reconnect - and these are usually programmable. If yours has these, and they are programmable, then you are likely safe to set the LVD to quite a low value, and set the LVR to a value that you know charge sources are taking care of the batteries.

For example, set the LVD to 11.3V and the LVR to 12.7V, or even 13.5V. This way, a heavy load that pulls a reasonably charged battery down temporarily won't trip the LVD, but if the batteries did get discharged deeply, the inverter cuts out before it is flirting with the dead end, and won't kick back in until charging sources have got the battery back to a reasonable charge again.

Mark

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