Frequently people working on their first off grid power system will get poor information about how to obtain their batteries free or nearly free. I have been there before, where we obtained old sealed calcium cloride batteries that were used originally in a telecommunications site, then briefly in a network center. They seemed to test good, but in the end I turned out to be the donor’s the free disposal company to get rid of 
thousands of pounds of junk! Why not use inexpensive “used” batteries (forklift or some other source)?? This solution for your off grid energy storage is very problematic for the following reasons.
- Unless you bring a hygrometer to measure each cell in each battery before you take these heavy beasts, you will not know the charge state of each cell. All too often these are offered as a lot, that must be moved quickly.
- Typically each deep cycle RV battery will have 3 to 6 cells, as they come in 6 and 12 Volt varieties. The quality of each cell can vary too much in a used battery. The over battery quality is only as good as its weakest cell.
- Similar to the point above, the quality of a battery bank is equal to its weakest member when they are connected together. A common misconception first time folks have is that, “I can always add more batteries later in a year or so”. Wrong! New batteries would be discharged to meet the level of the old batteries.
- The charging history of the “used” solar batteries is unknown, and its charging history is critical in determining the unit’s future live. You can’t see thin plates inside the batteries, yet brand new 6yr deep cycle batteries have been rendered useless in as little as a year with poor charge and discharge management!
See the series, Deep Cycle Batteries [1] Living in End of Life for real details on the subject
Deep cycle batteries have thicker internal plates that allow them to be discharged deeper than average car batteries. Car batteries are designed to be discharged 4-6% because they are alwaysbeing charged when the engine is running. Deep cycle batteries are designed for 50% discharge and a certain number of discharge cycles. The bigger the battery bank, the shallower the discharge and longer life for the battery bank.
I suggest always starting your off grid power design with 1) inverter/charger that will meet current and future needs and 2) the largest deep cycle battery bank that you can afford. Add an inexpensive and temporary generator for the charging the system, and you’re nearly ready to move! The extra investment in a larger battery bank means the depth of discharge will be smaller thereby extending the life of the system.
The next phase would then be to invest in a more permanant flavor of charging system like solar panels, diesel or biodiesel generator, or propane generator, or get that wind project started. Anyway, don’t skimp on storing your energy, and you will have more years of useful production before re-visiting energy storage!
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I find it strange that so few off-grid users have heard of desulfating technology despite publicity in media such as Homepower magazine. Originally developed in 1987 as a military product complete with patents etc. the product was released for consumer sales in 1997 and i have been using it myself since 2000.
Experts have estimated almost 80% of all so called scrapped dead batteries are only choked by sulfation and much of the storage capacity can be recovered. Yes it is true you do not get 100% recovery but so what when you can get scrapped batteries for free? Pulsetech was the original patent holder but there are now several competitive brands available. There was at one time ever an website illustrating how to DIY build one for $20 worth of parts. Why bother when the factory made product now sells for under $100 complete with warranty?
The key to keeping batteries from getting sulfated is to monitor the specific gravity of each cell. When cells begin to vary some, its time for an equalization charge.
A problem with using ‘used’ batteries is that even after you desulfate them, they’ll all come back to different “normal” specific gravities, and even more so if all the used batteries come from different sources. You have no way of estimating the remaining plate thickness and/or charge cycles to expect. It becomes difficult to save or prepare for the replacement when you don’t know if that will be 6 months or 4 years.
The real wear out mechanism in the lead acid batteries is the destruction of the lead plates in normal use. Desulfating them may bring back a little more spunk, but does not add to the lead plates.
One issue in this discussion is terminology. Lead sulfate is the natural result of discharging a lead acid battery. Recharging that battery reverses the process. In a perfect world you would recharger the battery as soon as you had discharged it and the revesal process would completely eliminate all of the lead sulfate. In the real world there will always be be some residual lead sulfate. this is why equalization has always been recommended. Unfortunately applying an equalization to an sealed AGM or GEL battery is not recommended since the limited over charging is damaging to the battery and if you are not very careful will destroy the battery by blowing the seals.
A second issue has to do with the fact lead sulfate crystallizes and hardens over time. A normal charge cycle will often have trouble reversing the oldest formed lead sulfate thus leaving a residue. A third detail about charging has to do with the inability of a low current flow to adequately reverse all the lead sulfate. with a strong enough current flow the same sulfate can be reversed. Solar systems often have insufficient panels to deliver a charge current equal to 25% of the amp hour capacity of the bank. If the current flow is less than 10% of the amp -hour capacity then a larger portion of the lead sulfate will not be reversed. It is in effect insoluble. Think in terms of a water analogy. A trickle flow versus a strong stream of water will flush away less dirt.
The electronic ‘de-sulfators helps keep the sulfate from hardening and makes it easier to reverse even residual amounts.
A third issue with solar power installations has to do with the fact these batteries sit still and the electrolyte tends to stratify over time. Dense and heavy electrolyte tends to settle at the bottom and you end up with under strenght electrolyte near the top. Vigorous charging creates enough bubbles to stir up the electrolyte but slow charging from a small sized solar panel will not.
I have a set of telecommunication batteries that are equipped with special air tubes reaching down to the bottom of the cell case. Low pressure air is fed into the tubes to bubble the electrolyte to prevent stratification. Very large batteries such as those used on submarines also have this feature.
Under normal use and following manufacturers recommendations concerning discharge and recharge rates a good quality deep cycle battery contains enough lead in the plates to last 10 years.
As you probably already know, the best system is where all the cells are wired in series. Systems where multiple batteries are wired together in parallel to increase the capacity are prone to various problems.
Thanks for your comprehensive comments on the subject Arild. Your background in the industry and comments are welcome here
When you explain the use of electronic de-sulfators, it sounds like you assume that solar power systems in question never use a generator/charger for periodic equalization charging. A good equalization charge holds each cell at a higher voltage and gets the electrolyte bubbling to prevent the sulfate build up. Could you explain the electronic operations that a desulfator does, or how it is different?
Thank you kindly,
Marshall
Equalization is a controlled overcharge at an elevated voltage of approximately 15.5 volts and with limits on the current. One brand I am familiar with limits the current to 5 amps. This technique is not suitable for sealed batteries because it does cause excessive bubbling and heating of the electrolyte. Flooded cells where you can replace lost electrolyte are fine with this but increasingly we see sealed batteries on the store shelves because those batteries are manufactured over seas and cannot be shipped with spillable electrolyte.
The electronic desulfating technology injects extra electrons into the lead sulfate crystal lattice without the high current associated with the conventional equalization charge. When you now add a conventional charging voltage to the mix the lead sulfate is more readily disassociated back into the sulphuric acid and lead oxide of a fully charged battery. The original patent was issued back in 1987 and I did at one time read those papers with the very involved scientific engeering jargon. After the original patent expired a number of clone products have appeared on the market place. Evidently their manufacturers have found as I did, that the technology does work. Electronic desulfation will recover batteries that failed because the plates were covered by non conductive lead sulfate but they will not repair damaged or eroded plates. Plates do shed material over time. Lead sulfate expands just as ice will expand to larger volume than the same amount of water and if the sulfation is extensive this expansion will damage plates by cracking off bits of the surface. The effect is very similar to what you see on ice weathred rock faces. Water will get into minute surface cracks and when it freezes tiny flakes of rock breaks off. The same thing happens with lead sulfate.It is the natural result of discharging the battery. Depleting the electrons causes the molecules of sulphuric acid to combine with the lead oxide to form lead sulfate. The longer you leave it the more it grows and the greater the amount of plate shedding that takes place. If you have ever seen a battery that was left sitting uncharged for a long time you will see the sides of the case has bulged. That is caused by the expanding lead sulfate. Lead sulfate is not a conductor so if the whole surface of a plate is coated then the battery will not accept a charge. By raising the voltage as is done during an equalization charge
Real deep cycle batteries have thicker plates and are thus better able to resist the force applied by expanding lead sulfate but every plate has a limit beyond which it cannot be bent without permanent damage.
Pulsetech has combined their pulse technology with a charger and offers a superb product for deep cycle battery storage and recovery. A client of mine bought one ton of 2V deep cycle cells because they were on sale at a great price. He was not ready to use them and put them in storage for over one year. Normally this would have ruined the battery bank but he added a Pulsetech desulfator and after a year the bank was as good as new when we finally put them into service.
I only know of Home Power magazine doing a product test with published results and that was done back in 2002 or 2003 I think.
One draw back to solar power installations is that the panels seldom have the current available to really flush out and reverse the lead sulfate back to the original lead oxide and sulphuric acid. To be able to reverse all of the accumulated sulfate you need to recharge the battery at 25% of the amp hour capacity of the bank. with an 800 amp hour bank this would require a substantial and very expensive solar array to deliver that much current. Here is where a Genverter setup is also very useful. The generator can provide a much stronger charge current and reverse a lot more lead sulfate in a short time. This makes more sense and if you have sealed Gel or AGM batteries this is really the only way to overcome accumulated lead sulfate.
One thing I forgot to mention and Marshall pointed out is the fact the batteries should not be mixed by age use or type. I overlooked mentioning that whenever I do salvage batteries it’s usually from a bank containing multiple batteries, and all from the same batch age and usage. That way I have reduced the number of variables somewhat. At least I know these batteries saw the same kind of charging usage, depth of discharge or simply sitting idle. If I can only salvage one from a batch I use it in a stand alone application or give it away tp someone needing only one battery.
At what age do you not want to mix differently aged batteries. I’m using the T-105 style and have a few that are about 2 years old, I watch them and don’t take them down past 75% and do the equalizing process on them, usually at about 2 month intervals though. I just got 8 more new ones and haven’t put them into service yet, have them fully charged and ready, just getting all of my location setup first.
Thanks for any assistance.
JakTrak
I have 4-12V (245 AH)AGM batteries wired to create 48Volts which run through a Xantrex 300-watt inverter. I have a question about the current voltage levels of these batteries. Batteries 1 and 2 which are the closest to the positive lead are now putting out less than 1 volt each, while batteries 3 and 4 which are closest to the negative lead are each putting out 12.56 volts.
These batteries were all purchased new and at the same time and same manufacturer and were within .1v of each other when installed. I have a spare that is not connected to the system and had not been charged for at least 8 months and it was reading 12.45volts (I charged it this past weekend to 13v).
I understand that I have bad cells in the 2 batteries, but why are these two so bad while the others seem to be OK and still take a charge just fine?
Thanks
Sorry, I meant 3,000watt inverter