Electric Vehicle Frequently Asked Questions

BATTERIES

What batteries are available?

These batteries are listed in order of their availability and popularity. when selecting a battery type, one must consider many factors, including (in no particular order):

1. Weight
2. Size
3. Cost
4. Capacity (Amp Hours)
5. Voltage
6. If a management system is required to avoid overvoltage or for balancing. this usually leads to a mroe expensive charging system.
7. Maximum Rate of Discharge (Flooded - OK. AGM - Great. Gel - Not so great. Lithium - not so great to good, and Nicad - OK.)
8. Maintenance Required
9. Expected Life Cycles

The flooded lead acid battery is cheap, highly recyclable, and available in a wide variety. These batteries are virtually unbeatable in cost per mile. A deep cycle type must be used. The typical choice is a golf cart battery.

Next, Adsorbed Glass Mat (AGM) sealed deep cycle lead acid batteries deliver high currents without as much voltage sag as floodeds. They are lower maintenance and do not require watering. Sealed lead acid batteries require careful attention when charging, and may require the use of a battery balancing system or regulators. They also cost more up front.

Gel-Cell sealed lead acid batteries do not have the high current capacity of AGM's, but are maintenance free. They are advertised as not needing chrge regulators or a management system. They are typically used in high voltage EV's such as AC conversions where the high voltage keeps the current under their limits.

Flooded Nickel-cadmium batteries require water, but can be discharged deeply without damage, and can be cycled about 2000-3000 times while retaining 80% of their original capacity. Compare to 1000-1500 for flooded lead, and 300-1000 for sealed lead. They also have the advantage of not losing capacity in cold weather, as all lead acid batteries do. They are not affected nearly as much by the Peukert effect, that is, having a lower effective capacity when being used at higher currents. Nicads do require a special charging regimen, they have strict maximum temperature and current limits, and have a high up front cost. They are second behind flooded lead acid in cost per mile, thanks to their exceptional cycle life.

Several types of lithium-ion and lithium polymer batteries have been made available by companies like

metricmind.com
kokam
valence technologies

and others. These offer great weight advantages, but require the use of a battery management system. Also, availability is dependent on the manufacturers. Up-front cost is high, maximum current draw is limited on some types, and the number currently in use

What is the best battery for my EV?

Drag racers use Hawkers, Optimas or SVRs (brand names of popular AGM batteries). All of these cost more than flooded batteries but make more watts per pound. They are used by EV'ers who are looking for light weight and zippy EV's (high performance, short range EV's).

For commuting over longer distances, the flooded golf cart batteries are most popular. These are made by both Trojan and US Battery and come in 6V or 8V versions, all the same physical size. evparts also offers an AGM the same size and amp hour rating as the most popular flooded golf cart batteries.

See also question on what types of batteries are available. Advanced chemistries are available, but are not widely used yet.

What is the proper way to maintain batteries? (Cleaning and filling batteries)

The following EVDL post discusses how to get the most out of your batteries. I have reproduced it here and in the charging procedures section, as it is very valuable information. Information on filling and cleaning follows.

Let's anthromorphize a bit, and consider lead-acid batteries as alive; like the family dog.

1. They need exercise; it's good for them. You get the longest life when they are worked to about 50% of their capacity at moderate loads. After they have been loafing for weeks, you will notice a distinct improvement just from giving them moderate exercise.

2. But don't work 'em till they drop! If you drive an EV until it barely moves, the batteries are having a near-death experience! This is outright battricide, and a leading cause of early death.

3. They need to be be fed regularly (charged). Feed as soon as possible after a workout; they don't like to sit around starving after use. Batteries left sitting for days in an undercharged state develop a condition called sulfation.

4. Don't overfeed, or they get fat and have cumulative health problems and so die early. Chronic overcharging is a major cause of early death.

5. Don't underfeed, or they can starve to death. Chronic underfeeding also leads to a weak sickly battery and an early death.

6. Batteries can sit unused for months (hibernate) without needing to be fed. You don't need to put them on a trickle charger; just be sure to feed them occasionally so they stay near full charge.

7. They need fresh, clean water occasionally. Sealed batteries have a built-in watering system, but flooded batteries do not. Be sure to check water levels, and fill with distilled water as needed (dirty water poisons them!)

8. They need to be kept at reasonable temperatures, that you would find comfortable. Not too hot, and not too cold. Lead-acid batteries are "cold-blooded", so the lower the temperature, the slower they get. Likewise, they can't "sweat", so high temperatures cook 'em to death.

9. Batteries can't talk. They won't whine when they're hungry, or cry when you hurt them. You have to check their state of health with instruments, like voltmeters ammeters and hydrometers.

10. There are different "breeds" of batteries, each with its own good and bad points. Slow plodding workhorse floodeds, but long lived. Racehorse AGMs that are fast and powerful, but short lived. Using the wrong breed of battery for the application, or unrealistic expectations leads to disappointing results.

11. And some is just the "luck of the draw". For no obvious reason, identical batteries in the same vehicle will have some die young, and some seem to live forever.

The usual reason you see used EVs that say "needs batteries" is because the previous owner treated the batteries cruelly. Whether by ignorance or laziness, some or all of the above guidelines were violated. But batteries are replaceable, and it usually means you can get the EV "cheap". But such problems can be cured. A little detective work to fix the problems, and then some tender loving care will go a long way toward getting the longest life possible on the next set of batteries.

Flooded batteries tend to form an acidic mist on top of the batteries. This won't hurt the case, but can cause ground faults when charging. Clean batteries with a mild soap and water solution. With flooded batteries, keep the caps on so as not to get any of the solution in the battery. Like the family dog, batteries should be kept clean and dry (although sometimes it is hard to keep them that way).

When filling a flooded battery, first fill only enough to cover any exposed plates (by the way, if any plates are exposed, the battery may be damaged). Then fully charge. Then fill to the manufacturer's recommended height or just beneath the bottom of the filler neck. As batteries charge, the electrolyte heats and expands, so if you filled them before charging, electrolyte could spill out.

How do I design and build a battery box?

A battery box usually consists of the structure (usually a cage-like construction of structural metal, a liner, and insulation. The only one that is absolutely required is the structure. The others are a good idea if you want to keep the batteries clean and warm. The structure is usually a cage-like construction of angles and supports, or it is sheet metal, which also serves as a liner.

The structure used to restrain the batteries must be capable of restraining them and keeping them from entering the passenger compartment in an accident. It is either welded or bolted to the structure of the car. Both the structure and the attachment of the structure to the car must withstand crash loads, and restrain the batteries during an accident. NEDRA (National Electric Drag Racing Association) recommends designing the cage to withstand a load of of 8G's (8 times the total weight of the cage and all its contents) forwards, backwards, and to each side, as well as 4 G's upwards and downwards. It is acceptable if the structure deforms during a crash, so long as it restrains the batteries.

The liner of a battery box must be resistant to battery acid (sulfuric acid). It can be sheet metal. 304 or 304L stainless is ideal, but expensive. Regular sheet steel can be used if it is properly primed and painted. Some EV conversions feature powder coated metal components. Polypropylene can be welded (using plastic welding techniques) into a very nice battery box liner. This is what the battery cases themselves are usually made of. It also provides some insulating value. Plywood may also be used, although acid can attack the glue, causing delamination. On the other hand, plywood is cheap, and can be replaced as needed.

Insulation should be resistant to battery acid. Usually plastic foam available in home stores is used. It is always a good idea to test small samples for acid resistance when the material is unknown.

Sometimes a battery heating system is installed for use in cold climates. Various schemes have been used for this, but it usually consists of some sort of tape or pad heating unit, with a remote temperature probe and a thermostat.

Other considerations include keeping acid fumes out of the passenger compartment by venting the battery enclosure while charging. Seal the top of the battery enclosure off from the passenger compartment, and install a brushless fan - one that does not produce sparks - which is activated while the the charger is plugged in (not just while it is on, as batteries can gas for a while after the charge cycle is through and the charger shuts off). The fan must be brushless because the exhaust vapors contain flammable hydrogen

What is a battery management system?

A battery management system is a computerized system to monitor battery health, keep them balanced (at the same state of charge), and control charging and discharging to some extent.

This is done because all batteries have very slight differences. Over time, they tend to drift apart in their state of charge. Eventually, after a normal charge, some are completely "full," where any further charging would be bad for them, and others are relatively "empty." The higher current needed to fill up the emptier ones would harm the fuller ones. Since the capacity of a series string is limited to the capacity of the "emptiest" battery (or else it reverses, seriously damaging it), the range of the whole vehicle is limited. To prevent this, some need more charging, and some need less.

A battery management system keeps the operator informed of the state of health of the batteries to some extent and keeps them balanced to some extent.

Functions of such systems can include gathering such information as temperatures and voltages. (When this is all a system does, it is usually referred to as a battery monitoring system.) It then applies this information to balance and properly charge the batteries.

Some batteries are more tolerant than others of inbalance. Flooded lead acid batteries in particular may be rebalanced by applying a controlled overcharge. This is called equalization. Other battery types, however, are less tolerant of equalizing charges, and require management systems.

See also Section 13 of this FAQ regarding chargers, charging, and regulators.

What about battery heaters?

Battery heaters are used to keep the battery temperature at or above room temperature when the ambient temperature drops. Since lead acid batteries can lose up to half of their capacity at 32 degrees F., heaters can maintain the range of an EV even in very cold weather. Heaters can either be powered off of an AC line or by the batteries themselves, and are most effective when used in conjunction with insulated battery boxes. Other battery chemistries (like NiCad) are not affected as much by cold weather.

Usually battery heaters take the form of mat-style heaters that are placed under the batteries.

Which is better, flooded or sealed lead-acid batteries?

Floodeds come in a wider variety of sizes and are low cost. They have a long life, if they are well cared for. They can tolerate a lot of abuse. They are generally lower in voltage and larger in size than the available sealed batteries.

Sealeds come in fewer sizes and cost more. They usually do not live as long as floodeds, given the same level of care. To care for them properly, regulators or some sort of battery management system is required. They can also tolerate a lot of abuse (just not the same kind).

Floodeds can be slightly undercharged, overcharged, etc., and rebound quite well if the problem is corrected. Even a recurring problem like having an out of balance pack will not affect their performance too much, although it will shorten their life. The severity of the shortening depends on how bad the problem was and how long it went on. Sealed batteries have very little tolerance for this sort of abuse. Their lives are shortened quite a bit by even just a couple of these types of episodes.

Sealed batteries, though, can belt out several hundred amps at a time without having very much voltage sag or becoming damaged. Floodeds subjected to the same treatment would sag to a useless voltage level, and potentially boil their electrolyte and even explode.

Usually, a newbie EVer is wise to choose floodeds as their first pack, so their education in lead-acid battery care is less expensive and lasts longer. This "training pack" can then be upgraded later, or if the same type are used, the next pack will last much longer.

How about Ni-Cad batteries?

This question has not yet been answered

What are the correct charging procedures for a battery?

The following battery care and charging tips were posted on the EVDL. They are an excellent overview on battery care. General charging procedures are discussed following this post. For specific charging recommendations, refer to your battery manufacturer's literature.

Let's anthromorphize a bit, and consider lead-acid batteries as alive; like the family dog.

1. They need exercise; it's good for them. You get the longest life when they are worked to about 50% of their capacity at moderate loads. After they have been loafing for weeks, you will notice a distinct improvement just from giving them moderate exercise.

2. But don't work 'em till they drop! If you drive an EV until it barely moves, the batteries are having a near-death experience! This is outright battricide, and a leading cause of early death.

3. They need to be be fed regularly (charged). Feed as soon as possible after a workout; they don't like to sit around starving after use. Batteries left sitting for days in an undercharged state develop a condition called sulfation.

4. Don't overfeed, or they get fat and have cumulative health problems and so die early. Chronic overcharging is a major cause of early death.

5. Don't underfeed, or they can starve to death. Chronic underfeeding also leads to a weak sickly battery and an early death.

6. Batteries can sit unused for months (hibernate) without needing to be fed. You don't need to put them on a trickle charger; just be sure to feed them occasionally so they stay near full charge.

7. They need fresh, clean water occasionally. Sealed batteries have a built-in watering system, but flooded batteries do not. Be sure to check water levels, and fill with distilled water as needed (dirty water poisons them!)

8. They need to be kept at reasonable temperatures, that you would find comfortable. Not too hot, and not too cold. Lead-acid batteries are "cold-blooded", so the lower the temperature, the slower they get. Likewise, they can't "sweat", so high temperatures cook 'em to death.

9. Batteries can't talk. They won't whine when they're hungry, or cry when you hurt them. You have to check their state of health with instruments, like voltmeters ammeters and hydrometers.

10. There are different "breeds" of batteries, each with its own good and bad points. Slow plodding workhorse floodeds, but long lived. Racehorse AGMs that are fast and powerful, but short lived. Using the wrong breed of battery for the application, or unrealistic expectations leads to disappointing results.

11. And some is just the "luck of the draw". For no obvious reason, identical batteries in the same vehicle will have some die young, and some seem to live forever.

The usual reason you see used EVs that say "needs batteries" is because the previous owner treated the batteries cruelly. Whether by ignorance or laziness, some or all of the above guidelines were violated. But batteries are replaceable, and it usually means you can get the EV "cheap". But such problems can be cured. A little detective work to fix the problems, and then some tender loving care will go a long way toward getting the longest life possible on the next set of batteries.


Generally, lead acid batteries are charged by applying a voltage higher than the resting voltage of the battery. The general procedure is to bulk charge (charge at the maximum current the charger can deliver) until the battery reaches acceptance voltage (usually around 2.4 volts per cell, or 14.4 volts per 12V battery, though this can vary depending on the battery type, size, etc.). Then, the battery voltage is held constant while the current is allowed to taper. Charge is discontinued when the current is reduced to a set amount. A more accurate way to determine the cutoff point is to cut off the charge when the voltage change is less than 0.1V per hour, but few chargers implement this algorithm.

Again, for exact voltages and currents for your specific battery, refer to your battery manufacturer's charging data.

My batteries seem weak, how can I correct this?

This question has not yet been answered.

11  How do I determine the battery state of charge?

usually a gauge mounted on the charger/dash will tell you .

12  What is the proper way to mount and secure batteries?

The structure used to restrain the batteries must be capable of restraining them and keeping them from entering the passenger compartment in a crash condition. The structure usually consists of structural steel or sheet metal cages, with some sort of acid proof liner and insulation. The cage structure is either welded or bolted to the structure of the car. Both the structure and the attachment of the structure to the car must withstand crash loads, and restrain the batteries during a crash condition. Recommended loadings vary, but NEDRA (National Electric Drag Racing Association) recommends designing the cage to withstand a load of of 8G's (8 times the total weight of the cage and all its contents) forwards, backwards, and to each side, as well as 4 G's upwards and downwards. It is acceptable if the structure deforms during a crash, so long as it restrains the batteries.

13  Can I mix old and new batteries?

It's not recommended. The problem is that the new ones have a different capacity than the old ones. They also charge to a different voltage, and draw different currents when charging. Thus, if you charge a mixed pack with a simple series charger, the old ones will be a bit overcharged, and the new ones will be a bit undercharged. This brings down the life of the whole pack.

The difference usually shows up as an imbalance in the pack, which requires active management of some sort to keep in check. Sealed batteries are much less tolerant of this kind of treatment than flooded batteries.

The older batteries will likely have less capacity than the newer ones, which translates into less range because the range is limited to the capacity of the weakest battery, in order to avoid reversing that battery.

The only time you would want to mix batteries is if some of your pack was damaged or defective, and had to be replaced. In this circumstance, used batteries of a similar age and usage history can be better, if you can find them.

To sum up, avoid it if you can, but if you can't, some sort of bi-directional battery balancing system or careful individual monitoring is necessary to get the most life possible out of you pack.

14  Can I use two different types or sizes of batteries?

Not unless they are charged separately. Even then, your total capacity is limited to the capacity of the smallest, weakest battery in order to keep from reversing it. This means that the larger battery is basically just taking up extra space and weight. Also, different types of batteries have different maximum currents available, so the maximum current you can draw without damaging the battery with the lower current limit is also limited.

The only exception to this is if you are designing a "hybrid battery pack". This is a battery pack consisting of strings of two or more types of batteries, each having certain advantages and disadvantages, intended to maximize the advantages while minimizing the disadvantages. The separate strings are connected either using electronic voltage converters or by carefully matching the voltage levels during driving. The strings are charged separately.

What is Peukert's Effect, and why should I care?

Lead-Acid batteries have a peculiar property; the amount of amp hours you can get out of one changes depending on how much current you are drawing.

This is called the Peukert effect (named after the gentleman that discovered it). For example a 6V golf cart(GC) battery might have an amp hour rating of 225 AH, but this is at the 20 hour rate (11.25 amps for 20 hours) kinda useless for EVs since they typically draw far more than 11 amps. The rule of thumb is that GC batteries produce about 60% of their 20h rating for 1 hour, or about 125 amps for 1 hr (until it's dead, 80% is about 100 amps).

So a given 6V GC battery can produce 67.5 watts (6V x 11.25A) for 20 hours (or 1.35 KWH) or it can produce 0.75KWH for 1 hour. A 96V string would therefore produce 21.6KWH over 20 hours or 12.96KWH for 1 hour (again these numbers are until the battery is dead). So it all depends on the battery and how fast you drain it.

Peukert has two parts. The Peukert Exponent (usually called Peukert's Number or PN) and the Peukert's Capacity (PC). PC equals how many hours a battery can produce 1 amp, many people just use the 20 hr rating but that's usually going to be wrong(way wrong for GC batteries).

The formula is A^PN*T=PC, where A=Amps and T=time in hours. (The ^ means "raised to the power of.")

I've been using a T-105 battery in my examples, a 6V GC battery made by trojan with a 20hr capacity of 225AH, a PN of 1.24 and a PC of 400Ah. So with this information, first determine how many amps you are going to draw, raise this to the power of 1.24 and then devide 400 by the answer and you will get how many hours it can produce that much current.

Let's say we are going to draw 150 amps. 150^1.24 = 499 400 / 499 = 0.8 hour or about 48 minutes. About 38 minutes if we want to keep our discharge below 80%. If it takes you 150 amps to go 60 mph then you have a range of about 38 miles (at 80%).

Drag racers and high performance EV's typically use AGM batteries with very low Peukert numbers, so the capacity doesn't drop much while they pull large currents. Flooded batteries have higher peukert numbers, but are available in larger sizes and are much less expensive. So it is a trade-off, with the choice made dependent on what the vehicle's "mission" is.