Electric Vehicle Conversion Help - Battery Service Life

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Battery Service Life: How long will the battery bank last?

This is a tough question. There are many variables involved in the service life of a bank of batteries. I have heard some people say that they should last about 3 years or more, "if they are well taken care of" – the following will give you an idea of what that means.

The service life of lead acid batteries depends on many factors. . .

Charge Cycles

Service life is often expressed as the number of discharge and charge cycles. I have read figures ranging from 300 to 1000 cycles for lead acid batteries. The reason for the wide range of number of cycles is because depth of discharge (DOD) and operating temperature directly affects the number of discharge and charge cycles, along with other factors. Many battery manufacturers specify the number of charge cycles at a DOD of 80%, which is quite deep.

Note: I go through 6 charge cycles per week, or 312 times per year. If I can only get about 300 charge cycles, I have to replace the battery bank every year. However, DOD does factor in here. I should get many more than 300 charge cycles because I operate the batteries in the 0 to 50% DOD range.

DOD

The service life, and number of charge cycles, can be extended if the batteries are not repeatedly deeply discharged. Many experts recommend that the batteries be used in the 0 to 60% DOD range. The rule of thumb is: the deeper the discharge (on a regular basis), the shorter the service life will be.

Note: More batteries is better than less. Here's why. More batteries in series creates a higher bank voltage. Power to move the vehicle comes from voltage X current. For a given power requirement, a higher voltage means a lower current drain. Lower current drain translates into discharging the batteries less for a given range, which reduces DOD and extends the life of the batteries.

If you want to add more batteries, make sure that the controller and the motor ratings are not exceeded.

Charge Rate

Do not exceed the manufacturer's maximum charge rate specification. Most sources say that the maximum charge rate should be no more than the battery's 20-hr capacity (C) divided by 8 hrs. So if C is 200 Ahrs, the maximum charge rate should be 200 Ahrs / 8 hrs = 25 Amps. Lead acid batteries cannot be quick charged like NiCad or Lithium batteries. Lead acid batteries should not be charged any more quickly than 8 hours. Exceeding the maximum charge rate will deteriorate the plates, reduce capacity, unbalance battery voltages in the series string and shorten the life of the battery bank. Also, charging will actually take longer than 8 hrs because it takes more energy to recharge them than what was given up my them and because the charge current decreases as the batteries charge.

Temperature

Battery service life is also related to temperature. Service life for lead acid batteries is usually specified at 77^oF or 80^oF. If the battery operates in an ambient temperature higher than this, the service life is shortened. Summer high temperatures are hard on lead acid batteries, but cooler seasonal weather brings relief and may equal out.

Water Maintenance

Another factor that affects service life is water maintenance. The water level (electrolyte level) in the batteries must never fall below the top of the lead plates. If this occurs, the exposed plates will corrode and degrade the capacity of the battery. Always use pure distilled water to replace what has evaporated from the cell. Using any other type of water will degrade the cell. (See the Battery Watering System page.)

ADD DISTILLED WATER ONLY AFTER THE BATTERIES ARE FULLY CHARGED.

If you add water to a discharged cell, the water/acid will actually overflow while the cell is being charged by expanding and coming out under the cap seal.

Charge Maintenance

Good Charger - A good charger makes a big difference in the length of service life. Charging at too high a voltage will deteriorate the lead plates and charging at too low of a voltage will allow sulfates to build, increasing internal battery resistance and preventing full charge. Good chargers have three stages of operation: (1) bulk stage during which a constant charge current is applied as terminal voltage increases, (2) constant voltage stage during which charge current decreases while terminal voltage is fairly constant and (3) finishing or soak-in stage during which the voltage is decreased and charge current continues to decrease. The charger should have either manual or automatic equalization capability for flooded lead-acid batteries. Some manufacturers recommend that you do not use any more than 7.2V/14.4V for gel and AGM batteries as higher voltages will damage these types of batteries.

Equalization

Use a digital voltmeter to measure the terminal voltage of each battery in your bank. Ideally, all terminal voltages should be the same and remain the same over time. In practice, terminal voltages will not be the same as the batteries age. This causes some batteries to get a higher charge than others. To combat this, either each battery must be individually charge-managed using electronic circuits in a method called ‘active equalization’ or the overall bank charge voltage must be increased for a period of time to force a higher charge voltage across the low-terminal-voltage batteries in a method called ‘passive equalization’. In each case, equalization forces a higher charge voltage on the batteries to ‘melt’ away the sulfation and mix the acid to restore the cells of the batteries to ‘normal’.

Recommended Link: Equalization - Trojan Battery Company

You cannot constantly charge the batteries at the high equalization voltage level. Not only will they dry out, but the plates will erode from the frantic chemical activity. Over time, at normal charge voltages, sulfates will slowly buildup, requiring an equalization charge session.

Active equalization requires an electronic circuit for each battery whose purpose is to even out the terminal voltage for all of the batteries as they charge. This method ensures that stronger cells are not overcharged and weaker ones are not under charged. However, use of active equalization does not ensure that all of the sulfates are being melted away from the plates in the cells. Brute-force 'passive equalization' may still be needed occasionally to desulfate the plates.

We can kill two birds with one stone here, so to speak. Both great charge maintenance and great equalization can be accomplished at the same time by providing each battery with its own three-stage charger. Actually, I would compromise a little bit here and use one 12-V charger for every two batteries. It's what I have been using since October 2008. I love it. Take a look at the Distributed Charging page.

During brute force passive equalization, a great amount of bubbling and some water evaporation may take place, especially if the cell caps are poor, so the water levels in the cells must be monitored. Also, during equalization, some hydrogen gas may escape the cells, so make sure this process is done out in the open, not in a closed area.

Steps for Passive Equalization and Desulfation for Flooded Lead-acid Batteries (not for gel or AGM)

Using the normal charge cycle, charge the batteries to what appears to be all they will take up to a battery terminal voltage of 7.2 V.
Check the water levels in all cells of the batteries to ensure that the level reaches the bottom of the fill ring. Add distilled water as needed.
Apply the equalization voltage to the battery bank. The batteries' terminal voltages should rise to between 7.5 and 8 V per battery (assuming 6-V batteries).
Monitor the terminal voltages with a digital voltmeter during the equalization process. As a general guideline, when the terminal voltages of all batteries closely match, equalization is complete.
The length of time for equalization may be several hours. Some manufacturers recommend a 3 to 6 hour equalization period once per week if the batteries are used nearly every day.
Anytime that you add water, apply the equalization voltage for about 30 minutes to allow the bubbling action to mix the distilled water into solution, evening out the density of the sulfuric acid throughout the cells, top to bottom. The acid has a tendancy to stratify on its own anyway - becoming more dense at the bottom than at the top.

Learn more about your batteries by clicking here: http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Charging

Battery size chart: http://www.evdl.org/pages/bcigroup.html

- Mark