Battery Information

There are many technologies for rechargeable batteries. Lead- acid is by far the least expensive.

Automotive-style lead-acid have very thin plates, and not much lead paste, for very high power. Usually, all you need to start an engine is 800 amps for 10 seconds. Then, after the engine is started, the alternator recharges the battery. So, it has only discharged a few percent of it's total capacity. Therefore, the capacity of an auto battery is not important in its designed use. If you discharge an auto battery below 50%, it is not going to survive well because it wasn't designed for it.

Deep-cycle batteries are designed to be discharged to 80%. They have thicker plates, and more dense paste, that causes less power. They typically have a higher energy density (watts per cubic metre) than auto batteries, and in a deep-cycle application, they have a long life.

There are two advanced lead-acid technologies, that improve on the 35WH/Kg typical of traditional batteries. There is a lot of lead in conventional batteries that acts as electrical conduit: top strapping, cases, separators. Removing that lead reduces the weight of the battery, which is important in electric vehicles. One way is quasi-bipolar technology - the positive plate and negative plate are butted together to make their connections. The joined plates are alternated, which removes most non-energy lead in the battery. The true bipolar technology puts a positive plate and a negative plate together with a thin membrane between them. This allows ions to pass between them but not electrons. This removes even more non-energy lead. These batteries are capable of both high energy and high power.

Within traditional lead-acid technology, there are two main types of construction: flooded and sealed. Sealed doesn't mean completely sealed; really, it is valve regulated. As gasses reach a certain pressure, the valve releases some so that the battery doesn't explode. Under the flooded category, there are two different types, maintenance-free and maintenance (water addition needed throughout its life). The maintenance-free batteries will leak if tipped over. There is a lead-calcium in the paste which reduces the water consumption.

The battery engineers, when making a 36 month battery, put in exactly how much electrolyte you will need for 36 months of typical use. So, it is not surprising to have that 36 month battery fail on the 37th month. The lead-calcium in the paste makes the deep-cyclablity not as robust.

Maintenance batteries (most deep-cycle) are usually made of lead-antimony. Because of this, the battery is capable of going down to 80% discharge, and you can get from 300 to 700 cycles, depending on the battery design.

In the sealed batteries, there are two types: absorbed glass mat (AGM) batteries use lead-calcium. There is a glass mat in the middle and there is just enough electrolyte to dampen the middle. In gel technology, there is a gelled material in the electrolyte to immobilize the electrolyte. Because the electrolyte is immobilized, it gives lower power.

Flooded		Sealed (valve regulated
maintenance-free	maintenance	AGM	gelled
lead-calcium	lead-antimony	lead-calcium	lead-calcium
deep-cycle 50%	deep-cycle 80%	deep-cycle 50%	deep-cycle 50%
100-300 cycles	300-700 cycles	100-300 cycles	100-300 cycles

All lead-acid batteries are made up of 2-volt cells. Large batteries are made as 6V batteries, really large as 2V, so you can lift them. This is the voltage you can expect per cell:

Here is a table to help you determine the state of charge of a 12-volt battery:

*Voltage*	*Specific Gravity*	*Percent of Charge*
12.73	1.277	100
12.62	1.258	90
12.50	1.238	80
12.37	1.217	70
12.24	1.195	60
12.10	1.172	50
11.96	1.148	40
11.81	1.124	30
11.66	1.098	20
11.51	1.073	10

Add .17 volts for every 10 degrees below 80F.
Subtract .17 volts for every 10 degrees above 80F.

The only accurate measurement of open-circuit voltage is after the battery has rested for 24 hours, with no charge, recharge, or load of any kind on the battery. Note the big dropoff at the right side of this graph. At about 80% depth of discharge the battery is running out of lead dioxide, so potential drops off very quickly. If you discharge beyond that point, you can even get voltage reversals, when the battery gets very hot and essentially self-destructs.

When a battery is fully charged, the positive plates are lead dioxide, the negative plates lead, the electrolyte sulfuric acid. When it discharges, both plates turn into lead sulfate, and the electrolyte changes to water. This is not hazardous to the battery. The problem comes when you discharge your battery down to about 80% and let it sit in that state for an extended period of time. That is when lead sulfate crystals will grow. When you finally do put a charge on your battery, it is difficult to get these large crystals to break down. When a cell charges and discharges some lead sulfate flakes off and drops to the bottom of the cell. This flaking is good because it exposes more lead and keeps the cell from deteriorating. A deep cycle cell has a large space below the plates to hold the flakes because when the flakes fill the space and touch the bottom of the plates they short the plates and kill the cell. If you charge and discharge the cell really slowly there is not enough action to cause the flaking This increases the internal resistance of the cell until it will not take a charge and will produce little external power. Thus to keep a cell working properly you need to drain it to something like half or less charge and recharge it at a high rate (30A for RV-size cells) until each cell is gassing freely. Note that the gas that is escaping is hydrogen, it is flammable and will set off your propane gas detector if it is not vented safely.

As you charge and discharge a lead-acid battery, the electrolyte is changing from water to acid to water. Sulfuric acid weighs more than water, and goes to the bottom of the battery, and the water goes to the top. Over time you will see a difference in specific gravity depending on the height of the electrolyte of the battery. This specific gravity gradient results from ineffective over-charging. Where you would normally see, straight from the factory, 1.265 gravity after a full charge, if you have not been adequately over-charging the battery, you may find as high as 1.400 in the bottom of the plate, and close to water at the top of the plate. Pure water does not conduct electricity well. If the only conductivity is in the lower half of your plate, you are only able to use half of your battery, or half of the capacity. Also, the extra-high acid content at the bottom will start breaking down the active material. So, it is very important to equalize the battery. Don't cut charging back to avoid the gassing state. It is very important that you allow the battery to gas vigorously, the gassing actually stirs the electrolyte. Allow gassing for 2 hours at 40 amps once every 10 days or every 10 cycles. And, fully charge deep cycle batteries, to 2.58V per cell.

The capacity of a battery is the number of amperes-times-hours you can get from it until the voltage drops below 10.5V for a 12V system. This is usually specified at a current draw that will discharge the battery in 20 hours. If you discharge it faster than that, the capacity you actually can use is lower:

(If you see a "Reserve Capacity" rating in minutes, multiply it by 0.6 to get the approximate ampere-hour rating.)

The optimum temperature for a battery is 77F. For every 15 degrees F below 77F., there will be a 10% loss of capacity. So, at 30F, you will only have 65% usable capacity. Also, when the battery is fully charged, to above 1.200 specific gravity, the electrolyte freezes at about -10F. But, if your battery is not fully charged, and your specific gravity is 1.100, then it freezes at +20F. This stresses the cases, the plastic gets brittle and may crack. High temperatures cause a reduction in life because of the increased corrosion of oxygen and lead, but not of capacity.

The Battery Council International) standard lifetime of a battery is the number of times it can be discharged 80%, recharged, and retain 60% of it's peak capacity. If it is discharged less than 80%, the lifetime is longer. The less you stress the active material, the longer the battery will survive. A brand new battery has only 80 to 90% of it's peak capacity. It will reach its full capacity after about 50 to 100 cycles. And then for wet cell batteries it pretty well stays constant until it passes 3/4 of it's life, then decreases. At about the half way point of the battery's life, the water consumption starts to go up, dramatically. Chemically, what is happening is that some of the Antimony on the positive plates starts plating on the negative plate. So just expect this, it doesn't mean the battery is failing.

If you connect cells in parallel then try to charge the combination, the cells will have different internal resistance and the current will not divide evenly. The good cell (with low internal resistance) will get overcharged and flake off a lot of lead while the weak cell will not get enough current to cause any flaking thus the cell continues to deteriorate. Big battery users hook identical batteries in series when they are being charged and in parallel when they are in use. RV semiconductor battery isolators work, but the voltage drop across them is so high that the charging system needs to be readjusted upwards in voltage so that the batteries fully charge, and many automotive systems are not adjustable. You must have at a minimum 14.0V at the battery while charging, 14.66V is optimum. If you spend a lot of money on batteries and need more than one bank, it's worth getting separate chargers for each.

* Overcharging (voltage greater than 14.8 volts) for longer periods than described above.

* Undercharging (voltage less than 13.8 volts) resulting in sulfate crystal growth and/or freezing. A battery has internal leakage which causes it to slowly discharge. Charge of batteries in storage should be checked once a month.

This info is mostly extracted from the transcript of a talk given by Kitty Roden of Trojan Battery Ltd. and posted to electric vehicle groups.