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Old 16-11-2007, 22:56   #1
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Why as Low as 10V

So if 12.1V is 50% discharged and ~11V is like really flat and detrimental to the health of the battery, why do inverters allow the voltage to drop to as low as ~10V before shutting down or soundign an alarm??
I would have thought it much safer to set that trigger voltage much higher, maybe even user programable. But I so far have seen no inverter with either.
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Old 17-11-2007, 00:00   #2
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An inverter can still convert 10V to 110V. I would imagine that a battery that has dropped to 10V would start a dramatic decline from there.

But I'm no expert on Batteries.
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Old 17-11-2007, 00:21   #3
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Here's the deal

There is no "correct" low voltage disconnect value for an inverter to use as a drop-out voltage and here's why. No existing inverter design utilizes a "real" battery monitor that "knows" the internal resistance, state-of-charge, and state-of-capacity of the source battery. If one did then voltage alone would not be the determining factor; other parameters used in conjunction with a calculating algorithm WOULD.

A "dumb factor" used is a so-called low-voltage-disconnect" value that exists for some predetermined (and always a "dart-board" choice) value that might work for most cases and minimize complaint calls in others.

Regardless, any inverter design is constrained by internal and external capacitance values that can "hold-up" an internal voltage equal to or greater than a value that can deliver minimal currents to maintain specified operational parameters for a delivered ac output voltage and current. Ultimately any design is constrained by advertised performance specifications that must be met (or be invalid).

Long has it been recognized that a marriage of a battery internal monitor that "talks" to an external inverter or inverter/charger would lead to a more optimum disconnect situation regardless of load or source deficiencies yet no existing marriage exists with today's products.
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Old 17-11-2007, 00:39   #4
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The voltages you refer to - ie 12.1 = 50% are open circuit voltages. A battery under load can have lower voltage than that, but still be better than 50%. Large inverters can pull heavy current, so if they were set to cut off at say 11.9 volts they might be doing it all the time, even though the batteries were still OK.
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Old 17-11-2007, 03:08   #5
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The voltages you refer to - ie 12.1 = 50% are open circuit voltages. A battery under load can have lower voltage than that, but still be better than 50%. Large inverters can pull heavy current, so if they were set to cut off at say 11.9 volts they might be doing it all the time, even though the batteries were still OK.
I am with you on that, the 10v assumes the battery voltage is depressed due to the load from the inverter - well that is what I have always assumed .

Other consumers may do the same - our refrigerator, but not the freezer, drops off at a seemingly low voltage too (10v also from memory).
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Old 17-11-2007, 03:13   #6
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On the series of FastCats we use Mastervolt or Victron inverters and both are equipped with programmable alarms both for high and low voltage, for our 12 volt versions these alarms are set by us to 11.6 volts and for the 24 volt version to 23.2 volts
The fridges and freezers are set so they switch off when we reach 11.5 volts.
We use the New capi 2 distribution system and each consumer you can preset to switch of at a preset voltage so first fride freezer and last the bilge pumps is how we set them

Capi2 is a smart, easily accessible bus-based power supply system.
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Old 17-11-2007, 11:47   #7
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preset to switch of at a preset voltage so first fride freezer and last the bilge pumps is how we set them
Sounds great. But why would you set the bilge pumps to cut off. I would want the pump still running till that last bit of energy is eventually squeezed from the battery and I eventually have to "step up" to my life raft :-)
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Old 17-11-2007, 19:24   #8
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Alan,
On my boat the inverter cuts out at around 10 volts and I am glad it does. When I start the engines when the batteries are low the system voltage drops below 10 volts which shuts down the inverter which kills the computers. When the battery is topped up and I start an engine, the inverter stays on and does not kill the computers. If the inverter had a threshold of 11 volts for example, the computers would be rebooting every time I started an engine. I could install a massive a UPS that can keep 3 computers running, but then why not just have an inverter with a low 10 volt (or so) threshold? I know most people do not have a bunch of computers but what about all the other AC loads that have to be restarted when the AC goes out?
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Old 17-11-2007, 20:00   #9
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Now that I understand that this about a voltage rating under a load, it all makes sense. The reason I came to this question is, I have just had a 200Ah battery fail. It is only 22months old, so should be under warranty(24mth). On three occasions I had the inverter go into alarm mode with voltage being at ~10V. I thought maybe I had killed the battery myself, from to lower cycling. The battery passes the load test with flying colours, but will not come out of bulk charge and of course, the entire bank then boils. But maybe it isn't as I assumed after all and it is a case of the battery has simply failed. Which I certainly hope so. I was concerned because my expensive German banks arrive next week and I didn't want a similar incedent wrecking one of those.
Apparently they have to replace both batteries, which is a bonus as I am passing them on to a friend when my new flash German banks arrive next Friday. I have also just installed a Heart interface Freedom 25 charger/inverter. So I basicly have a new electrical system nearly on line.
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Old 18-11-2007, 04:40   #10
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Hallo Alan

the lowest voltage we can set them to is 10 volts and it is my guess that at that voltage the bilge pumps will hardly move at all and long before we have received both audible and visual alarms from both the Victron and mastervolt units and the capi system.
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Old 18-11-2007, 06:33   #11
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design cut point

That 10V point is selected to ensure load shedding & to save the batteries from being completely flat. (Especially where you might have to start an engine.)
Regards Bill
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Old 18-11-2007, 07:21   #12
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That makes sence but i feel it should be a higher voltage since 10 volts is a virtually flat battery and it will last only a few cycles if this voltage is hit often. I feel 10.8 would be around the right switch off voltage leaving around 30 % in a lead acid battery. we now use Lithium phosphate batterys and these switch of automatically at 10.5 volts .Valence Technology | Lithium Phosphate Batteries
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Old 18-11-2007, 07:33   #13
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Hi David M,

I must be misunderstanding something here, but surely your invertor is not taking power from the same batteries you use to start your engines?
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Old 19-11-2007, 11:51   #14
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A battery can get to 10 volts basically two ways.

First the battery can be slowly discharged until it would measure 12.0 or 12.1 volts open cirucit, if a load even a smallish load is then place on the battery it will quickly fall to 10 or less volts.

Secind the battery may be fully charged so it would measure 12.8 or 12. 9 volts open circuit, a large load (100 amps) a kettle on the inverter for example, would then draw the battery down to 10 or 10,5 volts. (depends on sizing etc)

The first way is almost sure to cause quick battery failure, the second while not great for the battery will be tolerated okay.

Since you failed your battery quickly I would suspect that the first way is what is going on.

Check your daily consumption of power, and the daily charge rate. Ensure open circuit voltage before you go to bed is greater than 12.4 volts at least half charged before recharging.

New expensive batteries will fail just like the lower cost ones.
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Old 19-11-2007, 12:30   #15
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Hi David M,

I must be misunderstanding something here, but surely your invertor is not taking power from the same batteries you use to start your engines?
They are, and it is a potential problem I intend on correcting. It's not a problem when the batteries are charged but they could be a problem if they get into a discharged state. Being a power boat that does research, I use my DC system differently than most cruising sailboats do, so this has never been a problem but it could become a problem. I just need to find a place to put a fourth 8-D battery that is isolated and dedicated to starting the main engines.

What I do sometimes is sit on station collecting data with the engines off, so as not to contaminate any hydro-carbon sensitive samples while in the meantime I am running a few computers that do data logging and run and control the oceanographic sampling equipment, combined with pumps and other relatively high current loads. All of this equipment is drawing on the three house 8-D's...so I really do need those three house 8-D's. I start one of the engines if the house voltage drops below 12 volts, which many times kicks the inverter offline. I have just learned to keep an eye on the house voltage when the mains are off and we are drawing a lot of current from the batteries.
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