A decent one is gonna cost around $30. It's a Low Voltage Alarm, and monitors the battery's voltage and squeals (or winks) at you when the battery voltage is too low. The problem is how "too low" gets defined.
Most low voltage alarms will go off on you when the voltage gets too low to run inverters and other appliances. Same as an inverter with a low voltage disconnect that powers off when the voltage gets too low to power stuff, usually well under 12 volts, most often it's around the 10.8 volts mark.
The way batteries work, the way that lead acid batteries generate electricity is through a process called double sulfate chemical reaction. Battery plates are made of lead plates that is more of a lead grid than a plate, and acts and looks more like a sponge. Many holes are in the plate, which gives it a much higher surface area than plain solid plates, just like a sponge. The holes are filled with a lead paste made up of different formulations depending on the manufacturer, but primarily of "red lead", which is "lead tetroxide", sometimes referred to an "minimum" or simply as lead oxide. (I should note that true deep cycle batteries, like from Rolls-Surrette, Concord, Discover-Energy, Deka, etc., are not lead plate grids, or sponges, but are, in fact, solid lead plates).
The lead and lead oxide, which are the active paste materials on the battery plate grids, react with sulfuric acid in the electrolyte to form lead sulfate. This lead sulfate is in a finely divided, amorphous form, which is easily converted back to lead, lead oxide and sulfuric acid when the battery is recharged.
Over time, lead sulfate converts to the more stable crystalline form, coating the battery's plates. Crystalline lead sulfate does not conduct electricity and cannot be converted back into lead and lead oxide under normal charging conditions. As batteries are "cycled" through numerous discharge and charge sequences, lead sulfate that forms during normal discharge is slowly converted to a very stable crystalline form. This process is known as sulfation.
That's why batteries wear out. Sulfation is a normal process and it cannot be avoided. It can, however, be sped up like you can't imagine.
When you too deeply discharge a battery down below a certain voltage (see below) lead sulfate crystals rapidly form, and they are not broken back up when the battery is recharged. Instead, the crystals stick to the lead plates like iron filings on a magnet. And since lead sulfate crystals cannot conduct electricity, there ya go.
To make matters worse, when you recharge batteries, if you don't fully recharge them (to within 90% or 95% at least), the fine amorphous lead sulfate that is not broken back down into the lead, lead oxide and sulfuric acid promptly crystallizes and sticks to whatever part of the lead plates it can find. Those that can't find an open spot on the plates, fall down and start accumulating at the bottom of the battery case. In addition, and more significantly, the plates start shedding lead (those "chips"). As the paste in the lead grids absorb the sulfate from the acid during a discharge, and as they give up the sulfate during charging, the plates change size a little, bulging and shrinking as they go. With each discharge and recharge, a little lead paste is given off the plates. When thin plates are hit with a very high amp draw, like with a microwave, for any kind of sustained period, they must give up a lot of sulfate in short order, and therefor shrink very fast, and give up a lot of lead paste very fast. (And that's why starter batteries cannot handle deep cycle applications.) Once the accumulation reaches the lead plates, the cells are shorted out, dead. Time for a new battery.
So, when your low voltage alarm goes off when the batteries are too low, it's almost always when the batteries are
way too low, and sulfation has already begun. Then, when you fire up the engine or APU to recharge batteries, and you run it for 15 minutes or an hour or two, that's not nearly long enough to recharge the batteries, and sulfation relentlessly continues.
And it's a vicious cycle. Once sulfation begins, it feeds on itself. That's why batteries die in a year. Or more accurately, that's how their owners killed them.
With traditional wet batteries that boil and you need to add distilled water to them periodically, with those you can force something called an "equalization" charge that purposefully charges the batteries at a higher voltage to break up the sulfation and turn it back into lead, lead oxide and sulfuric acid. Can't do that with maintenance free or any type of sealed battery, though, so don't try unless the manufacturer specifically says you can.
A battery is 100% fully charged at 12.7 volts.
95% - 12.64
90% - 12.58
85% - 12.52
80% - 12.46
Sulfation Begins
75% - 12.40
70% - 12.36
65% - 12.32
60% - 12.28
55% - 12.24
Battery Damage Begins
50% - 12.20
40% - 12.12
30% - 12.04
20% - 11.98
10% - 11.94
As you can see from the table above (which is for AGM, gel and maintenance-free batteries, traditional wet cell voltage states of charge are slightly different), when low voltage alarms go off at 12 volts and lower, your batteries are well on their way to an early death. And if you've got multiple batteries, don't worry, they'll all go down together. Sulfation is ruthless.
Ideally, to prevent sulfation from even happening, and to make your batteries last for many, many years, you should never draw them down below 80% DoD (Depth of Discharge), and then fully recharge them. If you do that, then batteries that are designed to last for perhaps 5 years will easily last 10 or 12, maybe 15 years. But, in most cases, that's not practical. You'd need 10 times the batteries.
So, the best compromise for long battery life versus cost and weight issues is the 50% DoD mark. If you never discharge down deeper than that, and if you always fully recharge the batteries, they will last several years. Good deep cycle batteries will last 5-8 years if they are never discharged down below 50%. Whatever your amp hour requirements are, you need twice that many batteries. Then, you'll always be at 50% or better, and all of the batteries will last a really long time.
As for the discussion of high CCA versus low CCA and plate thickness and thin plates cracking when bumping down the road, that's largely inaccurate, and very misleading. Thick or thin, doesn't matter, sulfation is what cracks the plates. As the crystalline lead sulfate builds up it puts pressure on the plates, as well as on the battery case itself, to the point where the case will bulge. It's the pressure that cracks the plates, not necessarily how thin they are.
True enough, thicker plates will last longer and won't crack as easily when they are sulfated, but that whole thick-plate, thin-plate discussion is based on the assumption that the battery owner is not going to take good care of the batteries in the first place. It's more like, "If you're going to buy batteries and then kill them, you might as well buy batteries with thicker plates so they'll last a little bit longer before they're dead."
A lot of that comes from buying "truck" batteries, which are not designed for deep cycle applications, and then running an inverter off of them. As for thick plates not having as many CCA, that's simply not true. An 8D battery has some of the thickest lead plates of any 12 volt battery, and they usually have more than 1300 CCA. It's because 8D batteries are designed to start very large boat engines, like on a 60 foot yacht. But 8D batteries (and their 4D cousins, 1100 CCA) work extremely well in deep cycle applications, as well. 8D batteries have 255 amp hours and 4D batteries have 220 amp hours.
Golf cart batteries, on the other hand, have much thicker (and much taller) plates than do 8D or 4D batteries, and most have 220 amp hours, same as a 4D battery. But, a golf cart battery may only have 700 CCA, significantly less than the 4D battery. It's because that battery is designed strictly for deep cycle operations, and has more actual usable reserve time than a 220 amp hour 4D battery will. It's not made for starting. But just the same, it'll give enough to start most engines, especially when in a multiple battery bank.
Incidentally, a golf cart deep cycle battery is really only a semi-deep cycle battery. Far better than a starting battery, and much better than a marine hybrid, but the plates are sponges, not solid plates. They are thicker, much thicker, which is what gives them their deep cycle properties, but they are no match for a true deep cycle solid plate battery (the Concord Lifeline golf cart sized battery is, in fact, a solid plate, however, and is why it has a significant reserve time versus other golf cart and/or 200 amp hour sponges).
Most truck batteries are Group 31 batteries, but that's just the case size. There are Group 31 batteries that are designed for starting, some for deep cycles, and some are hybrids that will do either quite well, within certain limits. Group 31 truck batteries have some deep cycle properties, like slightly thicker plates, but they are not, and should not be used as, deep cycle batteries. (I don't understand why the truck manufacturers haven't figured this out, yet.) Small inverter loads and for relatively short duration and a truck battery will handle that kind of deep cycle application with no ill effects, provided you don't discharge the battery down more than 50% and the amp draw is minimal.
Like, if you draw 250 watts sustained, that's drawing about 22 amps from a single 100 amp hour hybrid truck battery. A 22 amp draw is considered a deep cycle load. At 22 amps, you'll drain the battery dead in about 2.7 hours (3.8 hours for an AGM). If you have three 100 amp hour truck batteries, you'll drain it in 11 hours, or you'll be down to around 50% DoD in 5 1/2 hours.
Batteries are rated in amp hours at the 20 hour rate, which means whatever sustained amp draw that will deplete the battery in 20 hours is how many you can draw from it for 10 hours before the battery is too deeply discharged. In the case of a 100 amp hour battery, that's 100 amp hours divided by 20 hours gives you a 5 amp draw. 5 amps for 20 hours to 100% DoD, or 5 amps for 10 hours to 50%.
There are other factors such as battery temperature and the Peukert Effect that will increase or decrease the actual amp hours you have available depending upon battery temperature and the rate at which you draw the amps from the battery (or recharge them back into it, for that matter), but suffice it to say that cranking and hybrid batteries aren't as efficient as true deep cycle batteries. For this reason, if you're using truck batteries for your inverter loads, you should never draw a sustained load (more than 30-60 minutes) of more than the 20 hour rate draw, which for 100 amp hour batteries is 5 amps. And you should not draw from those batteries for more than 5 hours without fully recharging them.
Now, if you have more than one battery, you can multiply everything by the number of batteries. Three 100 amp hour batteries is 300 amp hours, and you can draw a sustained 15 amps for the same 5 hours, or you can draw 5 amps for 15 hours, same thing either way (dismissing the Peukert Effect, of course), and you'll keep your batteries at 50% DoD or higher. So, if you have a microwave that's gonna draw nearly 15 amps, you can do that with at least 3 truck batteries, for a few minutes, anyway, and as long as you compensate later in time or amp draw, you should be OK.
Know your total daily amp hour needs, or you total watt hours, and then go from there. You may need to cut back on your electrical usage, or you may need a separate house bank for your inverter loads. But you can get by with truck batteries, as long as you don't try and use them as if they were deep cycle batteries. They're not.
Your best bet is to not try and use the battery for which is was not designed. Buy starting batteries for starting, and deep cycle batteries for inverter applications. In most cases, your truck batteries already have their hands full in just dealing with the demands of the truck.
And, you have got to monitor the batteries to keep theme from being too deeply discharged. Best method is with a real battery monitor, like a Xantrex Battery Monitor. It will keep track of all amps in and all amps out, and will know (and tell you) when the battery is fully charged and when it's down to 50% DoD.
Or at the very minimum, for starting batteries, anyway, and perhaps for batteries that are used for very light inverter loads, use something like a Battery Minder from UPG.
http://www.batterystuff.com/battery-products/UPGvoltminder.html
It will monitor your battery's voltage and let you know when it's down too low, and it's one that you can
manually set the low voltage setting at 12.2 volts for the trigger, and that will help prevent you from too deeply discharging the battery and keep it alive much longer. Most low voltage alarms are preset to squawk at far too low a voltage and, despite its intentions, only aids you in killing your batteries. It's $30, and is not even close, not even in the same ballpark, as a $225 Xantrex Battery Monitor...
http://store.solar-electric.com/xabamosy.html
but it's way better than not monitoring them at all and then not being able to start your truck.
Keep in mind that voltage testing a battery, at least insofar as trying to determine the actual percent state of charge, is by and large, inaccurate, as it's only good when the battery is a rest, meaning with no loads connected to the battery for at least one hour. And even then, a battery may show 12.7 volts and then as soon as a load is applied (or a load test) it may drop immediately to 11.5 volts or less, because it can't hold the charge (sulfation). Only a sustained load (Or the Xantrex) will tell you how good the battery is (how many amp hours are left in there).
But the Battery Minder works for what it designed to do, at least better than anything I've ever seen or heard of, in that it will almost always prevent you from running your starter down to the point where it won't crank the starter motor. Don't trust high end (high dollar) deep cycle batteries to a $30 Battery Minder, because that's not what is is designed for. Use something like the Xantrex, instead.
But for a starting battery, the Battery Minder should do you OK. Just keep the inverter loads to a minimum, and no heavy inverter draws, like with a microwave, unless you are completely aware of what you are doing. Using high amp draws, like with a microwave, dramatically speeds up sulfation on thin plates on starter and hybrid batteries. You wouldn't hook a trolling motor to a car battery, so why hook a microwave to a starting battery?
Just a couple of quick points... (well, relatively quick)
"Thicker plates are normally found on marine batteries vx. automotive due the the heavy pounding they encounter on the water. Way more than we hammer ours."
Uhm, that sounds like the mantra of a bass boat salesman.
There are two basic types of "marine" batteries, those designed to start the main engine, and those that are designed specifically for trolling motors. Batteries for starting your motor are called cranking batteries, and trolling batteries are called deep-cycle batteries (even though most "marine" deep cycle batteries aren't actually true deep cycle batteries. They are merely deep-
er cycle batteries than are cranking batteries. Still, they work perfectly for what they are designed for, which is a trolling motor.) These cranking and deep-cycle types are distinguished in their construction by the lead plates inside the case.
Cranking batteries have numerous thin lead plates that give better bursts of energy for a fast start. Deep-cycle batteries have fewer, thicker plates that provide better power output over a longer period of time. A marine cranking battery's plates aren't any thicker than an automotive cranking battery, boat pounding notwithstanding. With many battery manufacturers, the only difference between a marine cranking battery and an automotive battery is the terminals and the labels on the case - inside they are identical. (my sister-in-law is a big shot with Johnson Controls in San Antonio and she gave me an interesting, and eye opening tour of the battery plant where several familiar brands and types of batteries are made.)
As for the alpha particle paste and beta particle paste, well, alpha and beta particles are more akin to cosmic rays than they are lead.
You could call the two predominant forms of lead oxide as "alpha lead oxide" (more accurately as litharge) and "beta lead oxide" (more accurately as massicot). Litharge and massicot are the natural mineral forms of lead oxide, with massicot being the crystalline form of litharge (which is lead tetroxide, or red lead). They are used together in different ratios in the paste of the lead plates depending on if the plate is to be a negative or a positive plate, and because each form of the lead oxide has a different specific gravity, they are mixed in different concentrations in order to achieve a specific, specific gravity within the cell.
"One exception to all of this, high quality AGM batteries. They are a true lead acid dry battery. They have very thin plates with fiberglass mats soaking in electrolyte in between them..."
Well, I don't know that I'd call them dry. They're not wet flooded like traditional flooded batteries, where the electrolyte will slosh around in there, but the fiberglass mats are soaking in traditional wet battery acid, just the same. More accurately, they are referred to as SLA batteries (Sealed Lead Acid), and are, in fact, wet batteries.
"They are then put into the case very tight under pressure to give the plates greater strength than all other batteries. They will also function well in either a starting or deep cycle application."
AGM's are sealed VLRA (Valve Regulated Lead Acid) batteries, and are usually (but not always) designed for deep cycle operations. AGM batteries have some of the thickest, and quite very solid, plates in the "consumer" battery industry, with L16 (case size and configuration) batteries being the only battery that will have substantially thicker plates than a standard 12 or 6 volt AGM battery. L16 batteries, both traditional wet and AGM or gel, are more commonly referred to as floor scrubber batteries, and quite honestly blurs the line between industrial and consumer batteries as far as that goes. But for serious deep cycle application, they're hard to beat. For thicker lead plates in a true consumer application, you'll have to go with some seriously high dollar Rolls-Surrette and others of that type that are used in alternative energy off-the-grid applications. (There are some 2-volt batteries that would blow your mind. Like, 2 volts, and you need 6 of them to make a 12 volt battery, 450 pounds each, 3300 amp hours, $3,400 a pop. Thaaaat's 2700 pounds for a 12 volt AGM battery that you can draw a sustained 165 amps for 20 hours. Kewl. Wrap it up. I'll take it with me.)
Automotive batteries have plates that are usually .040" thick or less. Most "marine" and hybrid plates will be around .06" thick.
Most truck batteries, the 100 amp kind (Group 31, Group 27), are .06" to .08" thick.
A typical golf cart battery's plates will be .07" to .11" thick. But they are taller than most batteries which makes a difference.
A typical AGM plate will be .113" and the Concord AGM's are .115" (except their golf cart sized is .117").
US Battery and Trojan wet cell L16's are .090", which is kinda wimpy, but an L16 is much taller than a regular battery.
A Rolls-Surrette wet cell L16 and a Discover-Energy AGM L16 are both a significant .150" thick, and they are very tall batteries (16 inches plus), 6 inches or more taller than most regular batteries.
A typical forklift battery will have plates that are .265" thick, more than 1/4". (The 2 volt battery mentioned above has .52" thick plates).
Thicker means more reserve time, more amp hours, and can be deeply discharged at a high amp draw. You get what you pay for.
The case of an AGM are sealed under slightly positive pressure, not to strengthen the plates, per se, but to prevent them from leaking while inverted, and internally process charging gasses (won't outgas hydrogen gas under a charge), and as a side effect, it makes the cases and everything in them less susceptible to damage from vibration and impact damage.
If you want the best in an AGM you should pay close attention to where these are used in critical applications, like off-the-grid renewable solar and wind energy applications, seaworthy watercraft, ambulance and rescue, aircraft. People's lives depend on those brands of AGM batteries. Brands like Concord Lifeline, Concord SunXtender (same batteries as each other, just different labels and warranties), Discover-Energy (especially for an L16 AGM), Deka (often as MK, they're from East Penn Mfg). Not all AGM's are created equal. There are brand names that you may know and trust, but their AGM's just aren't up to par (Trojan comes to mind, as their AGM's aren't the same as their wet cells are, which is a shame, as their wet cells are nearly cream of the crop).
After exhaustive research and experience (I've had a boat or two in my day) the brands I've mentioned here are the ones that I can confidently recommend. For my money, the best bang for the buck, hands down, is the Discover Energy L16 AGM (assuming that it's properly cared for, of course). After that, a close second is Concord Lifeline. Concord lifelines, even though they are deep cycle batteries, are also designed for use on large watercraft, and they make excellent cranking batteries as well as excellent deep cycle batteries. They may be the only exception to a battery that does both very well. And they handle cold cranking in extreme cold weather extraordinarily well (unlike a gel battery which is useless under 30 degrees, and equally useless when it's over 105 degrees outside, which is why gel batteries are rarely used in solar power applications, but keep them within a certain temperature range and they have some significant advantages over AGM's).
I should note that AGM's (and gels) require some rather strict charging parameters, so make sure you know them and can live up to them before you go out and fork over big bucks for these high end batteries. (for example, AGM's will not properly charge off the Sprinter's alternator. Period. Gel batteries
might but I'm not all that confident, yet. Gels are even pickier than AGM's, but in different ways.)
Your mileage may vary.
Slow and steady, even in expediting, wins the race - Aesop
