Why No Inverters for Deep Cycle Batteries?

[Jack Kessler]

Every inverter I have been able to find shuts off at 10 or 10.5 volts. My deep cycle house batteries will produce usable current down to 5 volts. Have inverter manufacturers just not heard of the existence of RV's and deep cycle batteries? What is the problem here?

Does anyone know of an inverter that WON'T shut down at 10 volts?

[BB.]

What deep cycle batteries are you talking about? (link?).

From my point of view, there are several reason you will not find inverters that will cutoff at 5volts.

First, battery damage. Storage battery cells are not usually matched before connecting in strings (RC model folks will test discharge battery cells and match those in series strings with equal capacity). Standard storage batteries can have cell to cell capacity differences of up to 20% (roughly) between two series cells.

What will happen is when you collapse the voltage in the "weak" or low capacity cells, the other series cells can actually "reverse charge" the weak cell(s)--For most battery chemistry's--that will destroy those cells.

Second, is AC inverters are typically constant power devices... Remembering Power=Volts*Amps -- we see that an inverter that operates at ~15 volts (battery bank under charge) vs one operating at ~5 volts will draw 3x as much current to support the loads.

Generally, when batteries are below ~10.5 volts (for lead acid storage cells), there is little chemical energy to support any substantial currents.

Another reason -- Power is also equal Amps² * R -- To design an inverter that can operate over a 3:1 voltage range (at constant power) will require 3x^2 or 9x as much copper, magnetics, capacitors, switching transistors, heat sinking, etc. to prevent overheating when operating at ~5 volts.

Most batteries are, more or less, constant voltage sources (solar cell/spanels are actually, more or less, constant current sources)--So for standard chemistry lead acid batteries, I would wonder how much useful energy for a 12 volt battery is available at ~5 volts.

My guesses--anyways...

-Bill

PS: I should clarify--the "matched series cells" for RC planes/cars/etc. is to prevent cell damage from reverse charging without having to add the electronics/costs to protect cells against "reverse charging".

I don't believe it is to gain more usable energy from the battery pack.

-BB

[niel]

i'll give you the simple answer. they are considered dead at 10.5v.

i suspect you believe just because they are deep cycle rechargables that they can be abused by running them into the ground. not so and in fact they shouldn't be run below 50% dod or their useful lifespan is more compromised.

[Cariboocoot]

No such thing as a 12 Volt deep cycle battery that can be drawn down below 10.5 Volts and then be fully recharged.

In fact, the "RV" type aren't even true deep cycles and won't take even normal off-grid use for very long.

If you do draw a battery down below its "dead" point you can run in to the very interesting (and unrecoverable) state of reversed polarity.

The current increase would be mind-boggling. 1200 Watts @ 12 Volts is 100 Amps. @ 5 Volts it would be 240 Amps.

[bryanl]

re: "In fact, the "RV" type aren't even true deep cycles and won't take even normal off-grid use for very long." -- bull. (see the FAQ here, for example).

If you can find some objective and available spec or measure I can use to tell an "RV" type from another type or even define "deep cycle" objectively, I'd truly like to know. I have been looking for years and haven't found one. -- keep in mind my context is the typical battery the average consumer can easily find intended for RV and auto service that weighs from about 40 to 200 pounds and is usually 6 or 12 volts.

All lead acid batteries (and many other types as well) suffer severe life degradation with increasing cycle depth.

Lab tests for this class of battery indicate the range is from about 200 up to about 800 cycles down to 80% discharged before capacity loss exceeds 20%. Variances of up to 20% result from many factors and that can complicate things out of the lab.

All of these factors go into the cost analysis that indicates a 50% DoD is a good target for nominal use of these batteries (see smartgauge for a good rundown).

Besides the battery use and cost effectiveness factors in answering the OP, another factor is one alluded to above. Once the batteries get below about 11.0v under load, the voltage will drop quite rapidly. You just don't get much more out of a battery run down that far and you do significantly increase risk of failure.

[john p]

Does anyone know of an inverter that WON'T shut down at 10 volts? YES Its called a malfunctioning one

You cant possibly serious to believe a 12v battery can give usable power output at 5v.THATS LESS THAN HALF A VOLT A CELL?????
THIS HAS TO BE A JOKE POST

[hillbilly]

re: "In fact, the "RV" type aren't even true deep cycles and won't take even normal off-grid use for very long." -- bull. (see the FAQ here, for example).

If you can find some objective and available spec or measure I can use to tell an "RV" type from another type or even define "deep cycle" objectively, I'd truly like to know. I have been looking for years and haven't found one. -- keep in mind my context is the typical battery the average consumer can easily find intended for RV and auto service that weighs from about 40 to 200 pounds and is usually 6 or 12 volts.

from the FAQ here at NAWS:

"Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).
Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. This gives less surface area, thus less "instant" power like starting batteries need. Although these an be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge.
Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The golf car battery is quite popular for small systems and RV's. The problem is that "golf car" refers to a size of battery (commonly called GC-2, or T-105), not the type or construction - so the quality and construction of a golf car battery can vary considerably - ranging from the cheap off brand with thin plates up the true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.
Marine batteries are usually a "hybrid", and fall between the starting and deep-cycle batteries, though a few (Rolls-Surrette and Concorde, for example) are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid. Starting batteries are usually rated at "CCA", or cold cranking amps, or "MCA", Marine cranking amps - the same as "CA". Any battery with the capacity shown in CA or MCA may or may not be a true deep-cycle battery. It is sometimes hard to tell, as the term deep cycle is often overused. CA and MCA ratings are at 32 degrees F, while CCA is at zero degree F. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open - not much of an option."

Not sure if that's what you meant? I'll agree with the overall sentiment that Coot was trying to put across here: that deep discharges down to ultra low voltage settings ARE damaging even to the best of batteries, let alone a cheaper RV type battery. I also think it's appropriate to point out that not all "deep cycle batteries" are built to withstand the same level of use and abuse...

[john p]

Jack Kessler here is your simple answer. I just tested a 225 ahr deepcycle battery that is in good working order..
I put a load on it 30a for 4 hrs it dropped its voltage to 11.2
I then let it cool down for 2 hrs

then put the load back on again in 1hr 42 mins it dropped to 10.3v
35 mins under 30a load 9.1v (273w)
10 mins later max output current 11.6a 8.5v (98.6w)
5 mins later max output current 5.2 amps 7.9v (41w)
3 mins later 7.6v and 2.3a (17.5w)

This shows after it gets below 10.3 v you only have 35 mins of anything useful available from the battery.

battery is now dead and most likely will not fully recover

Now you know why inverters have 10.5v cutoff. the manufacturers are not quite as stupid as you thought they were..

[hillbilly]

Jack Kessler here is your simple answer. I just tested a 225 ahr deepcycle battery that is in good working order..
I put a load on it 30a for 4 hrs it dropped its voltage to 11.2
I then let it cool down for 2 hrs

then put the load back on again in 1hr 42 mins it dropped to 10.3v
35 mins under 30a load 9.1v (273w)
10 mins later max output current 11.6a 8.5v (98.6w)
5 mins later max output current 5.2 amps 7.9v (41w)
3 mins later 7.6v and 2.3a (17.5w)

This shows after it gets below 10.3 v you only have 35 mins of anything useful available from the battery.

battery is now dead and most likely will not fully recover

Now you know why inverters have 10.5v cutoff. the manufacturers are not quite as stupid as you thought they were..

Do you mean to say that you just killed a perfectly good battery in order to provide a thorough answer to this post??? In that case I've been asking the wrong folks here for answers to all my questions That's going the extra mile to answer his question!

[Cariboocoot]

john p has about the greatest job in the world and we're all envious!

He also has on more than one occasion delved into some real-world testing relevant to this forum and come up with some great data, for which I'm sure we're all grateful.