Batteries and why they fail

Rick Blanyer of Austin Texas – icky on rv.net seems to know what he is talking about when it comes to batteries and it does make sense.

As a long time designer of batteries (particularly AGM) and holding 5 patents in this technology, I am always tickled by the mystery surrounding this subject. … The agm technology was invented in 1965 but was used mostly on military stuff due to cost until about 10 years ago. It uses a recombinant oxygen cycle. That is why the glass separators are about 95% full, so the O2 produced at the positive plates during overcharge can pass thru and recombine on the negative plates and reform H2o instead of escaping as gas. This technology uses the same materials as a flooded cell, except the recipies for active materials are slightly different, and the cells are under compression to facilitate recombination. The main characteristics of agm are ability to suffer more deep discharges than flooded cells, lower internal impedance that produces substantially higher currents if needed, ability to accept very high charge currents without damage, lighter, works in any position, no venting (except during overcharge) means almost no post corrosion. The max charge voltage is 2.40 volts/cell at 77F. This is 14.40 volts at 77F. (Flooded cells require up to 2.60 volts at the same temp.) This voltage must be compensated +/- a couple of millivolts/F. … . in RV/marine use when a diode isolator is used, the diodes normally have a forward drop and this reduces your total charge voltage. Generally this means that charging on the alternator alone will never fully charge an agm battery. Good reason to plug in to shore power once in a while to top it off, but don’t leave it permanently plugged in because if you overcharge and lose water you will be up the creek.

Incidently an agm battery can be charged at the same max amps as its rated discharge. In fact the battery works better when slammed with high charging currents (100+ amps if you are below 14.4 volts), because this causes slight internal heating which promotes formation of a particular crystal structure on the positive paste (Beta PbO2). They like it like candy.

There is a difference between deep cycle and SLI (starting, lighting, ignition) batteries in agm just like flooded batteries. Mainly plate thickness, acid gravity, and paste density (porosity). Unlike starting batteries there is no significant difference between a rv or marine deep cycle battery. The rv agm is true deep cycle while the agm marine sacrifices a little life for thinner plates at lower density to achieve higher amps for starting a big engine. …

After performing thousands of autopsies on failed batteries,… the failure mechanisms are on most deep cycle batteries. It’s the same as Dunlop’s disease where when you eat too much, your stomach done lopped over your belt! The culprit is usually the positive plates. When charged they are PbO2, but when you discharge the cell the positive paste goes to PbSO4 which is 40% bigger volumetrically than the PbO2. This stretches the metal grid (the conductor that transports the electrons out of the cell). Now unfortunately the grid is made of lead, a metal which has a very poor electrical conductivity but is the only economical conductor that will survive in sulfuric acid solutions. To make matters worse, lead has only about 1000 psi of tensile strength even in its best alloys. This is about as strong as your ex-wife’s pancakes. So when the positive paste (attatched to the grid)expands by 40%, it stretches the lead grid in the x-y direction.

Now when you recharge and the PbSO4 in the positive paste reconverts back to PbO2 and shrinks, that stubborn lead grid now possesses an extremely high compression strength and doesn’t budge. So the grids curve in the z axis (ever notice the bulge on the ends of a wore out battery?). This separates the plates over time and causes internal resistance to rise. What is even worse is that the paste pellets that lie within the grid spaces become cracked during shrinkage and no longer touch the grid metal and that pellet can never be recharged again. It stays lead sulfate (PbSO4) and the battery is said to be “sulfated”. This is bad and is like having your black tank clogged up, can’t live with it. The capacity of the battery gets lower and lower.

This problem is much worse on thin grid cells like starting batteries and is why they are only good for about 10 deep cycles before they are trash. Keep this in mind when you are deep discharging your rv batteries, it will happen to them eventually if you overdo it. I wouldn’t go past 80% discharge, about 11.5 volts. Of course the voltage can only be accurately checked if several hours have passed since the last charge or discharge to allow the acid to equilibrate.

What a big help this is right?

Well actually if you see the voltage go down to 11.5 while you are under load, that is a good place to quit. Get back on charge as soon as possible. Why? Because when you fully discharge a lead-acid battery, you drive the gravity of the acid down to almost water. Lead dissolves in water and basic solutions, but not in acid solutions. So if you let it sit dead, then the lead in the grids and paste will start to dissolve in the water (it only takes a couple of hours). And when you start recharging, the gravity starts to rise and the water slowly becomes acid again. Now the lead precipitates out of solution (remember it only dissolves in water or basic solutions), and to everyone’s surprise it doesn’t go back to where it came from. It usually forms a dendrite which is a tiny short between the + and – plates. Drat. What a disaster. Back to the store.

Got that? Now you know more about AGM batteries, Why deep cycle batteries are different from starting batteries, why deep discharging causes battery failure over time, why you need to recharge promptly and how to know when you’ve pulled enough from the battery and need to recharge.

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