A consolidation of notes from Kitty Rodden's presentation to the Peninsula Chapter of the Electric Auto Association, by David Coale
Care and Feeding of Lead Acid Batteries
A consolidation of notes from Kitty Rodden's presentation to the Peninsula Chapter of the Electric Auto Association.
Now that you have chosen a battery for your EV, how do you get the most mileage out of your investment? That is the question I hope to answer in this article, whether you have 6 volt or 12 volt batteries the answer is the same.
The first thing to do is to make sure you do not order your batteries too soon and then have them sitting around the garage while you are finishing up your EV. If you must store your batteries for a long time make sure that they are fully charged and are inside and covered to reduce the poss- ibility of accidental short from a dropped tool or other conductor. Remember, batteries are always "on".
Lead acid battery background information:
The basic chemical equation for the lead acid battery is shown below. It is instructive to look at this and understand what is happening to a battery under discharge and charge. Under discharge; when an external circuit is made; electrons flow and the positive and negative plates react with the electrolyte to form lead sulfate and water (read equation from left to right). On charge; an external current source drives electrons through the battery causing the lead sulfate to change to lead oxide on the positive plate and the lead sulfate on the negative plate to change to lead; while the water is changed back to sulfuric acid (read equation right to left).
Discharge
Pb02 + Pb + 2H2SO4 ----------> 2PbSO4 + H2O + 2E
+plt -plt electrolyte <---------- +/-plt electrolyte
Charge
In the care and feeding of your batteries how do you know what state your batteries are in? There are two ways to determine this. One way is to measure to Open Circuit Voltage (OCV or Voc) or the specific gravity of the electrolyte of the battery when there is no load on the battery and it has been with out a load for at least 6 hours (24 hours officially). The table below shows state of charge, Voc and specific gravity for a given battery.
State of Specific Voltage Voltage Voltage Charge Gravity (12 Voc) (6 Voc) per cell --------------------------------------------------------------- 100% 1.265 12.68 6.3 2.11 75% 1.225 12.45 6.2 2.08 50% 1.190 12.24 6.1 2.04 25% 1.155 12.06 6.0 2.01 Discharged 1.120 11.89 5.9 1.98
The above table is useful in determining the state of charge of your EV when it is at rest and has not been used for some time. Since Voc measurements are not possible on a signal cell, specific gravity measurements can be taken to determine if there is a bad cell in a battery.
The second way to determine the state of charge is to measure the voltage of the battery or battery pack when it is under a load. This method has the advantage that it can be done while the EV is in use; thereby giving the driver useful information on the remaining range of the EV. The chart and table below show the state of charge or discharge as a function of cell voltage.
Charge Volts/Cell Discharge (75 amp load) -|----------------2.58V--|------------------------- : | .\. | . `. | ------------------2.38V--|------------------------- . . `. | . . ` . | . . ` - . _ | . . \ | ---------------2.10V--- `-. ----------------------- . . | \ _ . . | ` - . . . | ' . . . | . ------------------1.75V--------------------- . ---- . . | . \ . . | . ' . . | . ' -------------------------------------------------|- : : : : : 100% 80% 0% 80% 100% Charge Discharge Charge Volts/cell Comments ------------------------------------------------- 100% charged 2.58 vigorous gassing \ Charging 80% charged 2.38 gassing starts / 0% equilibrium 2.10 charged battery not in use 80% discharged 1.75 end of useful usage \ 75 amp 100% discharged ---- not recommended, damage may occur/ load
To determine the state of charge while driving, simply hold the accelerator so that the current meter reads 75 amps (from the batteries, not the motor) and then observe the battery pack voltage. A table can be made up for your battery pack showing the state of charge at various pack voltages based on the table above. After a while you will get the "feel" of your car and will be able to gauge the amount of juice you have left in your battery pack. If you suspect you have a weak battery you can observe the voltage of just that battery with a separate meter and see if it's state of charge is below the rest of the pack.
Battery Types
We must first know what type of lead acid batteries you have before we can talk about proper care and feeding. There are many types of LA batteries, but as far as maintenance goes we will group them as follows:
Lead-acid Battery types
Flooded Sealed (valve regulated)
================-=================-===============-===============
Maint. Free Maint. Absorbed Glass Gelled
(Don't add- Lead Antimony Mat AGM Lead Calcium
water) Deep Cycle 80% Lead Calcium immobilized-
Lead calcium 300-700 cycles High power electrolyte
Deep Cycle 50% Robust Deep Cycle 50% Deep Cycle 50%
100-300 cycles good service- 100-300 cycles 100-300 cycles
ablity No extra-
electrolyte
In General, the flooded cell lead antimony batteries have greater cycle life and can be discharged to 80% of full charge. The maintenance free lead calcium batteries have less cycle life and can be discharged to 50% of full charge. The lead calcium batteries use less water and do not gas as easily. That is why they are used in maintenance free and valve Regulated (sealed) batteries.
I will talk about flooded cell batteries first since these are the most commonly used (6 volt and 12 volt) batteries for the EV hobbyist.
Charging
Lead acid (LA) batteries like to be left in a charged state. Unlike NiCads, LA batteries do not have any "memory" and when ever you can charge them up, LA batteries will be happier for it. As you can see from the equation above, lead sulfate is a normal product of discharging. The problem with lead sulfate is that it will start to form crystals if the battery is left in a discharged state. During charging these crystals are broken down, if they are not too big, and the battery is charged up. Over a period of time these crystals will effectively remove lead sulfate from the reaction and/ or grow large enough to short out the plates. So always keep your EV charged up when not in use.
A typical charging profile will start with a high (10 to 20 amp) constant current, variable voltage for the first part of the charging cycle until 80% charged and then switch to a low current variable voltage until next part and finishes with a constant voltage for the last part of the cycle. The graph below show the voltage and current for a battery being charged as a function of time.
time -> time ->
|cccc | vv
c | cc v | v
u | c o | v
r | c l | vvvvvvvvvvv
r | c t | v
e | c a | v
n | cc g | v
t | cccc e |v
+------------------ +---------------------
time -> time ->
There are a few points to understand about charging. At the beginning of the charging cycle the battery will readily accept a lot of current with out gassing. In fact, LA batteries can take on 80% of there charge in six minutes if one has the proper charging equipment. Near the end of the cycle, the charging is much less efficient and any excess energy is turned into heat and causes gassing. Normally excessive gassing should be avoided, as this is a waste of energy and will use up water, requiring more frequent watering and cleaning of the tops of the batteries. However, an equalizing charge should be done about every 10 cycles. This equalizing charge is to 100% of charge (2.58 volts per cell) and should be maintained for about two hours. This equalizing charge will bring up any weak batteries and help mix the electrolyte on all the batteries.
Temperature and Charging
The above numbers are for charging at 77 degrees F. The end charging voltage changes with temperature. At lower temperatures the end charging voltage must increase to maintain the same level of charge at higher temps. The equation is given below:
voltage per cell = 2.58 ((T + delta t)/T) * XXX
The table below give some common values for this equation.
Temp. Cell voltage 96v pack 120v pack 144v pack ---------------------------------------------------- 100 2.4 1 85 2.5 77 2.58 65 2.6 55 2.7 40 2.8
A common mistake is to leave the end voltage the some for winter charging, as is used for summer charging. As can be seen by the table, this will not charge your batteries completely and will reduce your range. There are two solution to this: ether adjust your end charging voltage or some how heat your battery pack. Parking your EV in a heated garage is a nice way to go. Battery heaters, that run off of the AC line are another solution.
Maintenance
You should always be aware of any changes in the range of your EV. This can indicate several possible problems. If you notice your range is declining, the first thing to check is the battery connections. Check for any lose or corroded connections. If you have just driven the car, check for any warm connections. Use only one hand to reduce the possibility of getting shocked, with the other hand behind your back. Warm or hot connections indicate high resistance and should be cleaned, tightened or replaced depending on the condition. Bad connections can cause a terminal melt down if not fixed. When you are checking the connections, also check for moisture around the filler caps. Leaked electrolyte can cause shorts to the car body, increasing the possibility of shocks and can cause the GFI circuit that the charger is hooked up to, to trip before the batteries are completely charged. Also check the water level in the batteries and make sure it is at the full line. Always fill the batteries when they are fully charged as this is the highest level for the electrolyte. All of the above checks should be done once a month as a matter of maintenance. Also check to see that the ending charging voltage is correct for your battery pack and that the circuit breaker to the charger is not tripped.
If you suspect that you have a bad or weak battery, check the voltage of each battery just after a drive that is near the limit of your range. If you have heater that runs off of the main battery pack, turn this on. The extra load will help to show up a weak battery (1.75 volts per cell or less). The weak battery will also be warmer then the other batteries. Charge this battery separately with a battery charger and let it gas for a while. This should bring it back up to par, if it is not damaged. If the offending battery continues to be a problem, remove the battery from the pack by jumpering the main cables around it. Be sure to disconnect it before jumpering it. A bad battery will put a load on your system and reduce your range substantially.
Sealed Batteries
Sealed batteries are a nice way to go with an EV if the range and cycle life is acceptable to you. With sealed batteries the maintenance is much less and there is no cleaning of the battery tops. There is one main difference when charging these batteries that must not be overlooked. Sealed and valve regulated batteries, have a valve to release pressure if gassing occurs. Care must be taken not to over charge a sealed battery. Since the battery is "sealed"; that is you can not add water; any charging that produces gassing will lose water and shorten the battery life. For this reason, you should not do any equalizing charging and should closely set the ending or float voltage on your charger. Check with the manufacture for the recommended charging algorithm. To "equalize" a set of "sealed" batteries, it may be necessary to charge each battery individually in the pack when ever you notice that the pack is out of balance. You can check for pack balance by measuring the voltage of each battery at the end of the charge cycle and make sure the variance is not too great, +/- 0.25 volts. Check batteries about once a month.
The information shown here is by way of Kitty Rodden formally of Trojan Battery Company. I do not have any affiliation with Trojan Battery Co. This is just the information I have at hand.
David Coale ___o\____ (650) 493-4503 =)----/()_____()\