inverter low voltage disconnect

System · August 2006

How come most of the inverters I've researched seem to shut off the inverter at a voltage which has already brought the state of charge of the battery to the point where they're damaged. I thought this feature was there to protect the batteries from completely killing them.

An example is one inverter which has a low voltage alarm at 10.7 volts then shuts off at 10.0 volts. A 12 volt lead-acid battery is considered dead when it reaches 10.5 volts.

Am I missing something here?

BB. · August 2006

Re: inverter low voltage disconnect

I am not the battery expert here... But a couple of points to concider.

1) 10.5 volts would be a dead battery with no load. An inverter drawing heavy currents (10 amps of 120 VAC would be 100 amps at 12 VDC) will drop the voltage of a charged batter (plus wiring losses) quite a bit (when measured at the inverter).

In reality, it seems that a controller/inverter that tracks the power used from a battery (I*V) or Watt*Hours is a more accurate measurement than simply measuring the battery voltage. Things like temperature, charge condition, battery age, current drawn, battery resting, etc. all affect the measured battery voltage.

10.5 VDC is probably just a shot in the dark and is, like you state, not really a good way to save an expensive battery bank.

2) The inverters need some set point at which to shut down. Inverters are, more or less, constant power devices in that, as an example: You connect a 100 watt bulb at 125 VAC... or 0.8 amps. The inverter will draw from a 14 volt battery 100w/14v=7.1 amps. When the battery is at 10.5 volts, the inverter will draw 9.5 amps. So, the I^2*R losses are rising in the inverter (and wiring) as the battery discharges--at some point the inverter could fail due to high currents required at low battery bus voltages.

Also, the inverters are designed to only step up the voltages so much--once the input voltage falls below that required, the output voltage will sag and could damage the AC load (brownout). So, another reason to pick an input voltage power-fail setpoint.

Does this help?

-Bill

crewzer · August 2006

Re: inverter low voltage disconnect

A 12 V battery at 10.5 V under load is considered 100% discharged. The voltage will recover somewhat once the load is removed.

The battery suffers a temporary voltage dip (“coup de fouet”, or crack of the whip) when it’s first hit with a heavy load on the inverter. My take is that the low voltage cut-off setting is seemingly low in order to allow the inverter to not disconnect from the battery during this (hopefully) brief low voltage reading.

HTH,
Jim / crewzer

System2 · August 2006

Re: inverter low voltage disconnect

Wouldn't it be great for those who actually know what they're doing, to have an inverter with "consumer adjustable" set points.
Wayne

System2 · August 2006

Re: inverter low voltage disconnect

Hint! Hint! Midnight Solar :-)

brad

System2 · August 2006

Re: inverter low voltage disconnect

I agree that the voltage will drop when there is a load attached, but the size of the battery bank and the size of the wiring will effect the voltage seen at the inverter under load. A more reasonable solution (as I see it) would be to have a second set of leads connected directly to the battery with very little current flowing through them to sense the actual battery voltage.

Anyhow, can I assume the inverter will shut off before damaging the battery bank regardless of the misleading specifications? Would all inverters basically do the same thing, or do I have to shop around for the one that will not take my batteries below 80% discharge?

BB. · August 2006

Re: inverter low voltage disconnect

They make meters/systems that keep track of the total stored energy in your battery bank (needs to keep track of charge energy into batteries and also removed by loads, and is reset to full when charging cycle is completed, etc.)... That is the only (sort of) accurate method to know when you are running your batteries below a safe margin.

-Bill

System2 · August 2006

Re: inverter low voltage disconnect

what inexpensive meters/systems would you recommend?

halfcrazy · August 2006

Re: inverter low voltage disconnect

the trimetric or the xantrex tm 500 are both excellent meters.

crewzer · August 2006

Re: inverter low voltage disconnect

I agree that the voltage will drop when there is a load attached, but the size of the battery bank and the size of the wiring will effect the voltage seen at the inverter under load. A more reasonable solution (as I see it) would be to have a second set of leads connected directly to the battery with very little current flowing through them to sense the actual battery voltage.

Anyhow, can I assume the inverter will shut off before damaging the battery bank regardless of the misleading specifications? Would all inverters basically do the same thing, or do I have to shop around for the one that will not take my batteries below 80% discharge?

Lot,

We’re not communicating very well here. The battery voltage dip is a characteristic of battery performance, and not of “misleading” inverter specs. The dip is caused when a load is placed on a battery and the thin layer of sulfuric acid in direct contact with the plates is diluted. The reduction in acid pH drops the voltage until the acid begins to self mix (like tea steeping) and higher pH acid contacts the plates.

The reverse phenomenon, BTW, is called surface charge. Taking a battery voltage reading right after a charging source is removed will result in a “false high” reading. Allowing the electrolyte to mix will later result in a voltage reading that accurately depicts the battery’s state-of-charge.

Adding sense lines to the battery terminals will just confirm the temporary voltage reduction at the battery, less, of course, any voltage loss in the main inverter cables. The solution is not a different inverter. Instead, the solution is a combination of a well maintained-, correctly charged- and adequately sized battery bank.

HTH,
Jim / crewzer

inverter low voltage disconnect

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