by Paul Higley

How the battery works:
A typical 12 volt car battery is made from 6 cells. Each cell has two sets of lead plates in a bath of sulfuric acid. Filled with the acid “electrolyte” and charged, one set of plates become positive plates and the other set of plates become the negative plates. The positive plates have a porous construction with red lead paste in the plate pores. The negative plates are also porous but are left bare and become spongy and grey in color. The color difference may be the origin of the red and black terminals we come to expect on batteries with the red lead plate being the red positive terminal and the grey plate being the black negative terminal. Opposite plates are held apart by insulating separators. Each cell is in its own compartment so the electrolyte does not mix between cells. Each cell produces about 2 volts. The positive plate from one cell is connected to the negative plate of the next cell and so on to add the 2 volts of each cell up to about 12 volts from 6 cells. (MGB people with two 6 volt batteries can take all the battery voltages mentioned below down by a factor of two for each battery.)

There have been many improvements and variations to the original design worked out in the mid-1800’s. Old liquid electrolyte batteries ( “flooded” batteries) needed to be topped up with distilled water to replenish the water in the acid electrolyte. These batteries used Lead alloyed with about 12% Antimony to strengthen the plates. These batteries tend to release a lot of hydrogen and oxygen gas, depleting the water from the acid electrolyte. The out-gassing was reduced with the change to lead-calcium alloy plates with only 2% Calcium alloyed into the lead. Reducing the gas production reduces the loss of water and makes the battery more “maintenance free”. This is why we no longer need to regularly top up the battery with distilled water.

The separators are used to hold the plates at some distance apart so “+” and “-” plates do not short to each other. The separators also need to be porous so the electrolyte can pass through and be in contact with both plates. These separators have evolved from wood to porous plastics to glass fiber.

Variations on the design of the lead acid battery include the glass wrapped plate, the “gel cell” battery and the “AGM” or the Absorbed Glass Mat battery. The glass wrapped plate uses fine glass fiber wrapped around the plates of the battery. This glass wrap aids in holding the lead plate in its desired shape when deeply discharged and helps to hold the shape of the plates during recharge. This extends the life of a battery that is frequently deeply discharged. These are sold as a “deep discharge” battery for golf carts and marine use. The “gel cell” uses a sulfuric acid that is gelled so it is no longer liquid. This prevents spillage, can operate in any orientation and can extend battery life slightly by limiting the loss of sulfate from the plates. There are some disadvantages to both with reduced capacity or reduced maximum current available for starting an engine. The AGM battery uses an extremely fine glass fiber matt as the separator into which the liquid electrolyte is wicked. In this battery there is no liquid electrolyte except that wicked into the glass fiber. The battery becomes non-spilling and further limits the sulfate migration. The result is they have a uniquely low self-discharge rate amongst lead acid batteries. Manufacturers are quoting 2% to 3% per month and my testing of early AGM batteries has shown even lower rates. An apparent disadvantage to the AGM battery is an increase in the weight of the battery for the same capacity. This may slow the predicted “all cars will soon use AGM batteries” I have been hearing from battery manufacturers.

Causes of battery aging:
The problems limiting battery life center around the fact that as the battery is discharged the lead in the plates changes to lead sulfate and when recharged it does not perfectly return to its original configuration. In the liquid electrolyte batteries, some of the lead sulfate precipitates down to the bottom of the battery. This material is lost from the process and reduces the sulfate and lead available in the battery. Eventually it builds up in the bottom of the battery to the point that it creates a short between the plates. This increases the self-discharge rate of the battery until it will no longer hold a useful charge. The deeper the battery is discharged and the longer the battery is left in a discharged state, the more sulfate builds up and the shorter the battery life. If the battery is completely discharged, the battery becomes “acid starved” with all the sulfate from the acid consumed. The electrolyte is now nearly pure water and the battery cannot easily be recharged. Adding a small amount of acid to each cell can restart the charging process but if there is too much acid in the battery, the plates corrode to the point that they disintegrate when the battery is fully discharged. Lead acid batteries also tend to self-discharge. This means even if fully charged they will eventually discharge themselves. This discharge is typically 5% to 15% per month even without any use.

Another aging problem is a result of the loss of water from the acid electrolyte in the battery. Although the water loss is now greatly reduced, it is still possible to have a low level in the battery. This leaves more concentrated acid and if the level is low enough to expose the plates, the exposed part of the plates are no longer available to function as a battery. Refilling the battery will only partially restore function to this part of the plate. If you should find a need to top up a battery use only distilled, de-ionized water.

The connections inside the battery can be weakened by shock or over-charging of the battery. This can lead to a very weak connection which fails when a heavy load is placed on the battery. This load can be either due to charging the battery at a very high rate or discharging a battery at a high rate such as when the starter tries to start the car. Either way it can cause the battery to literally explode. The high current flow in the weak connection causes the connection to fail and heat up or spark. This ignites the hydrogen and oxygen that inhabits the inside of your battery. I can vouch for the fact that pure hydrogen in perfect combination with pure oxygen is very explosive. The gas inside the battery can have exactly this mixture. When it explodes, it is particularly nasty as it not only throws bits of the battery around at high speeds but also coats everything with the battery acid. Lead, Lead Sulfate and Lead ions in the electrolyte are not good for you either. The potential for an explosion is a good reason not to have your battery rapidly charged as some garages will still do and a reason to stand clear when someone puts jumper cables on a weak battery and tries to start the car. The battery may be weak due to a failing internal connection.

So what can we do to extend the life of our batteries in our collector cars?

First, keep the battery fully charged.
Use of a trickle charger or a “float” charger on an infrequently used car battery can keep the battery fully charged and reduce the degradation of the plates, thus extending the battery’s life. Leaving a battery discharged can ruin a good new battery in weeks to a few months. Keeping a battery with a float charger on it may extend the life of an infrequently used battery from 2 or 3 years to 6 to 8 years.

There is a debate going about which is best for a battery, recharging weekly or float charging. In theory the float charge minimizes any change to the plate dimensions and maximized the battery life but some argue that continuous float charging continues to cause some out-gassing and drive off small amounts of water from the electrolyte. Since a battery will not self-discharge significantly if held above 12.9 to 13 volts I believe the best approach is a slightly lower float charge voltage. The difference in battery life is probably insignificant compared to leaving it only partially charged for just a few days. The longest life I had from a battery in one of my old cars was one that I charged every few weeks. That battery still started a big V8 after 9 ½ years of service so maybe there is something to not float charging. I find that I forget to charge the batteries and I leave them for 3 or 4 months between charging. This is much worse than any difference in float charging versus charging every few weeks, so I opt for float charging. I reduce the float charging voltage by putting a diode from Radio Shack on one charger lead and therefore dropping the commercial float charger from 13.6 to 13.8 volts down to 12.9 to 13.1 volts. If you try adding a diode, make sure you put the diode in so the current can flow. Putting the diode in backwards will stop the charger from float charging the battery. You can check by measuring the battery voltage with and without the charger. Whether this is beneficial is still an unanswered question. Battery manufacturers all call out a slightly different float voltage level with liquid electrolyte the lowest around 13.4 and gelled batteries the highest at around 13.8 volts.

The major “no no’s” on batteries include:
Over charging or continuous charging – this is death to a good battery. This is easily done by leaving an older charger on the battery longer than needed to fully charge the battery. It drives the water out of the electrolyte, increasing the acid strength and lowering the electrolyte level. The exposed part of the plates dry out and are destroyed and the wet parts of the plates are damaged by the higher acid concentration. The new “automatic” chargers prevent this by charging to the 14.4 to 14.6 volts needed to charge a battery and then when the charging current (or charge rate) reduces, the automatic chargers will reduce the charge voltage to a lower “float” charge voltage, thus preventing over-charging.

Too fast discharge – This can heat up the connections between plates or in extreme conditions heat up the plates and cause warping which causes shorts and kills the battery. Usually you will only have this problem if the battery is too small for the application. Size the battery to your car.

Too fast a charge - This causes faster gas production inside the battery than the battery can burp out and causes uneven charging in the battery. The result is irregular plates, faster plate degradation and a drastically reduced life of the battery. A good rule of thumb for most lead acid car batteries is to recharge at no faster than a four hour rate. For a 60 ampere-hour battery as in the typical small car battery, this is 60 amp-hours divided by 4 hours or 15 amperes. So a 6 or 10 ampere charger is fine. Don’t put a 200 ampere car starter on a battery and try to recharge it in 20 minutes. You can put a lot of charge back in a battery in a hurry but you won’t have much of a battery left when you are done.

Over heating - A hot battery is a short life battery. Keep any heat shields or add some to prevent the battery from absorbing radiated heat from an exhaust manifold. This is one reason some car manufacturers removed the battery from the engine compartment.

Contamination of electrolyte with non-sulfuric acid, salts and minerals – almost any impurity in the electrolyte will shorten the life of the battery. This includes most “battery additives”. Use only distilled or de-ionized water to top up.

Low electrolyte levels - Too low and the exposed part of the plates will dry out and become worthless, too high and you will weep or burp acid and corrode your battery terminals and mountings. Also be careful not too add too much acid. Doing so and deep discharging will completely deplete the lead in the plates and destroy the battery when recharged.

Complete discharge - OK this is hard to avoid if you leave the lights on overnight. Just don’t wait days or weeks to recharge the battery, do it right away. Complete discharge causes electrolyte conversion to water. If this happens you may recover the battery by extended charging or in severe cases add just a very small amount of acid to each cell to get the electrolyte to start functioning again.

Freeze damage to your battery. A reasonably charged battery has electrolyte with a high specific gravity indicating a high acid concentration. High specific gravity electrolyte will not freeze at most temperatures to which we will ever expose our cars. However, since a discharged battery is left with nearly pure water as the electrolyte, it can freeze. As you might imagine this is death to your collection of plates, separators and battery housing. Keeping your battery charged during the cold months will prevent this.

A few Hints and Myths

The Concrete Myth:
There is a popular belief that leaving a battery on concrete will cause the battery to be ruined. In all the research and readings on batteries I cannot find one valid reason why this has any scientific validity. I have even tried it by storing identical batteries, one on concrete and one up on a bench. There has been no measurable difference in performance or life. I believe this myth comes from leaving a battery on the floor and forgetting to recharge or float charge the battery. A battery not float charged is a battery that self-discharges. A discharged battery does not live long. It should make no difference if on concrete, plastic, in your car or strung up from the nearest tree limb. If you let it sit, it will go dead and will not recharge. Keep you batteries charged!

A Painfully Learned Lesson
Don’t connect a new “automatic” style battery charger to your car without a good battery also attached. Not even for a second. I found out, somewhat painfully, that some of these new chargers, will produce voltage spikes of 50 volts or more without a battery attached. Once connected to a good battery, the spikes are gone. Putting this high voltage on your older car will probably not do much unless you have an electronic ignition or possibly an electronic clock. On a more modern car, connecting one of these chargers without a battery attached can destroy some car computer “brains” or damage “always on” circuits for digital clocks or alarm sensors. It does not take much of a load on the charger to reduce this to a safe level but modern car electronics have such low drain that they don’t create enough load to eliminate the voltage spikes. If you disconnect your battery and don’t want to lose the computer memory, don‘t use an automatic charger. While a small battery or other drain on the charger will help hold down these spikes, it’s best to wait until you have re-connected your battery before connecting the charger.