Battery types

19 Jun 2004 Battery types

–> AeroElectric-List message posted by: Brian Lloyd

SportAV8R@a…. wrote:

> Brian, I predict you are about to hear how a starved electrolyte RG battery will serve well in both applications. Okay, I just fulfilled my own prediction, didn’t I? 😉

Guess what, a gel-cell is also a “starved electrolyte RG battery.” I remember someone telling me that AGMs were common and when one got a “gel-cell” it was really most likely an AGM battery. I have since learned that this is not the case and there really are some substantial differences between the various types of lead-acid battery.

All these batteries are variations of the venerable lead-acid rechargeable battery. The most common is the type with an excess of liquid electrolyte. This is known as the flooded-cell battery. The plates are either made up of lead-antimony or lead-calcium. The former is often found in “deep cycle” batteries but uses more water and must be topped up regularly. It also suffers from a greater level of self-discharge (it slowly goes dead by itself). The latter is used in “maintenance free” batteries used in cars. They don’t use as much water but they don’t like deep discharge.

At some point in time someone got the idea to add a binder to the electrolyte that turns it into a jelly-like substance. That formed the basis of the gel-cell. The problem with gel-cells is that ions do not travel quite as well through the jelly so you cannot charge or discharge these cells as rapidly without damage as you can the flooded-cell batteries.

Later on someone got the idea of using a fine fiberglass separator between the plates of the battery and adding just enough electrolyte to fill the spaces in the fiberglass separator/insulator. Capillary action keeps the electrolyte trapped between the plates. This became known as the Absorbed Glass Mat (AGM) or “starved electrolyte” battery. They have some of the characteristics of both gel-cell and flooded-cell batteries and make a good compromise between the two.

Both gel-cell and AGM batteries are recombinant gas (RG) batteries. This means that the hydrogen and oxygen gas created by electrolysis at the end of the charging cycle (think about the bubbles in a flooded-cell battery when it is almost fully charged) recombine back into water inside the battery. This means the batteries can be sealed and will never need you to add water … as long as you don’t abuse them by overcharging. (More on this later.)

Flooded-cell batteries can take a lot of overcharging abuse before they fail. Gel-cell and AGM batteries cannot take much abuse. If you overcharge a flooded-cell battery, it just converts more of the water into gas, a problem solved by simply adding more water to the battery. If you overcharge either a gel-cell or AGM battery the gasses cannot recombine into water fast enough so the pressure in the cell rises until the overpressure valve pops off letting the gas escape (and thus keeping the battery from exploding). That is water needed by the cell to function and it is never replaced. Your battery is on its way to an early death when this happens.

Remember also that lead-acid batteries can get too hot during heavy charge and discharge cycles. Again, flooded-cell batteries have lots of liquid that can circulate and help carry the heat away from the plates. Gel-cell and AGM batteries can’t do that. The hot plates can warp and cause the cell to fail early. This is more of a problem with gel-cells than with AGMs.

And speaking of heat, there is also the problem of thermal runaway. As you warm up a lead-acid battery it will accept more current from its charging source. If you have a constant-voltage charger such as we have in our airplanes (the output voltage of the alternator is kept constant by the voltage regulator) then when the battery warms up it will take more current. Consider what happens on a hot summer day as you crank and crank your fuel-injected engine during a hot-start and give the battery a deep discharge as well as let it get hot from all the starting current passing through it. The battery may be very hot after that and then you throw 50A or 100A at it from that big alternator you have up there. The battery is going to be damaged by this and can even catch fire. This is very high on my “things you don’t want to have happen while flying” list.

So how many life cycles can you expect out of your batteries and how do you charge them safely? I don’t know what batteries you have but here is some data for the AGM and gel-cell batteries from Deka (East Penn Manufacturing), the company that supplies the 225AH AGM batteries I use on my boat (I have four of them). Deka has a very comprehensive white paper on the care and feeding of their “Valve Regulated Lead Acid (VRLA)” sealed batteries. You can find this paper at http://www.eastpenn-deka.com/products/pdfs/0139.pdf.

Looking at page 10 of their white paper there is a chart of battery life vs. depth-of-discharge for the two types of battery. I reproduce it here:

Typical VRLA Battery Cycling Ability vs. Depth of Discharge

Typical Life Cycles
Capacity Withdrawn Gel AGM
100% 450 150
80% 600 200
50% 1000 370
25% 2100 925
10% 5700 3100

What is apparent from this chart is that if you are cycling your cells deeply, gel-cells will last a lot longer than will AGMs. That is why you want to get gel-cells to power things when they must often run on battery power and the battery will be significantly discharged.

How Improper Charging Kills Batteries

As I said before, it is easy to damage a sealed battery by overcharging it. It can give off gas faster than it can recombine into water. It is also easy to damage it by undercharging. In that case the plates develop a sulphate coating that prevents the plate from delivering all its charge thus leading to a steady degradation of the battery. (Does this sound familiar?)

With flooded-cell batteries you just overcharge them a bit. OK, they outgas a lot and you have to add water but they seem to last pretty well. This is what the average aircraft charging system does to your battery in the summer. In the winter the electrical system does not have a high enough voltage to properly charge a battery so these batteries end up perennially undercharged in the winter. The plates suffer from sulphation and battery capacity slowly goes away. In summer they come back a bit by overcharging them but this kind of abuse still leads to a shot battery after about 4 years.

Now if you have a VRLA battery, either AGM or gel-cell, the summer overcharging kills them very quickly. I remember being so happy when Concorde and others started making sealed AGM batteries available for aircraft. I was actually excited by the prospect of no more cleaning corrosion out of my battery boxes. The problem was, the batteries went bad after a little over a year in the airplane. I think one went two years before failing. I was not a happy camper. Now I know why the problem occurs. The standard aircraft alternator/VR combo kills sealed batteries.

How to properly charge Lead-Acid Batteries

The problem stems from the fact that the voltage needed to charge a battery properly is too high to keep the battery from overcharging. If you drop the voltage to a proper maintenance level to prevent damage, the battery won’t charge. Add to that the fact that the proper voltages change with temperature and you can see why these batteries die an early death.

To properly charge and maintain a battery you need to do it in three steps. They are:

* Bulk charge

* Absorption charge

* float charge

When a battery is seriously depleted, it needs to go through the bulk charge phase. This is usually a constant current that puts back most of the charge in the battery. The bulk charge phase gets the battery to about 80% of full charge.

As the battery goes through the bulk charge phase its voltage will continue to rise. When it reaches a certain voltage the charger needs to switch to constant voltage charging at the absorption voltage. In your airplane this is the regulator setpoint. At this point the voltage stays constant and the current drops until the battery is fully charged. The battery will be fully charged when the charging current drops to about 5% of the battery’s rating. For example, for a 20Ah battery this would be 1A.

But what happens now? If you leave the battery on this charge voltage it will now begin to overcharge and generate too much gas. Keeping the voltage at this set point will now start to destroy the battery. What needs to happen is to drop the charge voltage low enough that the battery will neither charge nor discharge. This is the float charge phase of the charging cycle. Keeping the battery at this float voltage will ensure it stays charged but won’t be damaged.

Here are the values for absorption and float voltages for different temperatures for the Deka AGM and Gel-Cell batteries:

AGM Charge and Float Voltages
at Various Temperature Ranges

Temp. Charge Float Temp.
F Optimum Maximum Optimum Maximum C
>120 13.60 13.90 12.80 13.00 >49
110 120 13.80 14.10 12.90 13.20 43 49
100 110 13.90 14.20 13.00 13.30 38 43
90 100 14.00 14.30 13.10 13.40 32 38
80 90 14.10 14.40 13.20 13.50 27 32
70 80 14.30 14.60 13.40 13.70 21 27
60 70 14.45 14.75 13.55 13.85 16 21
50 60 14.60 14.90 13.70 14.00 10 16
40 50 14.80 15.10 13.90 14.20 4 10
<40 15.10 15.40 14.20 14.50 <4 Gel Charge and Float Voltages at Various Temperature Ranges Temp. Charge Float Temp. F Optimum Maximum Optimum Maximum C >120 13.00 13.30 12.80 13.00 >49
110 120 13.20 13.50 12.90 13.20 44 48
100 109 13.30 13.60 13.00 13.30 38 43
90 99 13.40 13.70 13.10 13.40 32 37
80 89 13.50 13.80 13.20 13.50 27 31
70 79 13.70 14.00 13.40 13.70 21 26
60 69 13.85 14.15 13.55 13.85 16 20
50 59 14.00 14.30 13.70 14.00 10 15
40 49 14.20 14.50 13.90 14.20 5 9
<40 14.50 14.80 14.20 14.50 <4 Three things to notice right away: 1. the proper charging voltages for AGM and Gel-Cell batteries are very different; 2. the proper charging voltages vary greatly with temperature; 3. there is a big difference between the absorption charge and float voltages. The approach taken by automotive and aircraft electrical systems is to pick an average value that will charge the battery but not overcharge it too much and will be good for the average temperature in which the battery will operate. This works after a fashion for flooded-cell batteries. Still, you can make your battery last longer by adjusting the voltage set point of your voltage regulator depending on the temperature. The B&C alternator controller that Bob designed does this and is a big win over standard voltage regulators. In my not-so-humble-opinion the temperature compensation option is a necessity, not a luxury. But even that is not enough. You really need a regulator that will switch to a lower voltage once the battery has been recharged. The only saving grace is that people usually don't fly long enough to let the battery seriously overcharge. Long cross country flights in summer down low where the battery gets warm are real killers unless you turn the charge voltage down to the proper float voltage. Unfortunately most regulators in cars and aircraft do NOT offer this feature. This feature *has* become standard in aftermarket external voltage regulators for the marine and RV markets. I happen to have chosen Ample Power's Smart Alternator Regulator V3 (SAR-V3-24P) for my boat since it will drive dual alternators (I have two engines) and properly charge my battery bank. I will probably use this regulator in my all-electric CJ6A also. When the alternator fails I want the batteries to be healthy. And with regard to AGM vs. Gel-Cell, I made the mistake of choosing AGM for my boat's battery bank. They won't last as long in deep-cycle service. Still, that is what I have and I try not to discharge them too deeply. When they finally reach the end of their life they will be replaced with Gel-Cell batteries. OTOH they are 2 years old and showing no signs of losing their capacity. I am hoping to get 6-7 years of good use out of them before having to replace them. And at $400 apiece, I don't want to have to replace them very often. So now to get back to the original comment of AGMs vs. Gel-Cells, I think I can safely say they are NOT the same. Still, both can serve very well if treated properly. -- Brian Lloyd