## Getting the most out of your Optima Yellow Top batteries

### My official Optima Yellow Top cycle statistic

I bought my Optima Yellow Tops (YT) in October 1996. My first cycle was not until February 1997. Since then I have recorded every cycle based on distance traveled. This graph is updated about every month.

During the years I have replaced some of the batteries. I have found that I have murdered a few batteries, because my batterichargers have been shaking apart. Loose conection have given faulty operation. Most of the time I have been assuming that the batteries have been at fault. But, then after replacing a battery the corresponding charger sometimes would still behave strange. I have now added the diagrams below in order to explain what I think is cherger fault and what is battery fault. You may find additional information on this under from the daily life with my EV.

This is where the second set of batteries are reported.

I would like to maximize the total number of kilometers (miles) I can get out of my batteries. Past experience with flooded batteries indicate that a DOD (depth of discharge) of 30 to 70% is optimal. If charging is done accurately, I think it's better to be closer to 30% than 70%. Back to top

### How I record the cycle statistics

My driving pattern has four stretches that I repeat most of the time. Once in a while, after driving one of these stretches, I let the Optimas sit for a couple of hours. Then I measure the OCV (open circuit voltage) of the batteries. One such recording may look like this:

 Bat1 Bat2 Bat3 Bat4 Bat5 Bat6 Bat7 Bat8 Bat9 Bat10 Bat11 Bat12 Average 12,76 12,78 12,74 12,79 12,67 12,77 12,79 12,80 12,78 12,80 12,65 12,81 12,76

It just so happens that battery one often is very close to the average OCV, so frequently I just check this one battery. Also, note that battery five and 11 are the two with the lowest OCV. I probably need to keep an eye on these. Then I plot OCV to km in this graph:

Most of the time I do not measure the OCV of my batteries. Instead I use the above polynomal (used to be linear in 1997) approximation to estimate the OCV. So this is the relationship between kilometers driven and the OCV. Now I need to convert the OCV to DOD (depth of discharge).

In 1997 I used the relationship between DOD and OCV taken from the Optima Red Top batteries (starter battery), as I did not know of available information on the Yellow Top. Now I know that the OCV of a fully charged Yellow Top is 13.20 volts compared to the somewhat lower OCV of 13.02 volts for a fully charged Red Top. I have made my own approximation and adjusted the voltages accordingly.

 OCV (Volt) SOC (%) DOD (%) 13,20 100 0 12,95 95 5 12,7 85 15 12,6 75 25 12,5 65 35 12,4 55 45 12,3 45 55 12,2 35 65 12,1 25 75 12 15 85 11,9 5 95

Data is for 25 C. The OCV for 75, 85 , and 95 % DOD is a rough estimate. I find their values to be of less importance since I try to stay away from such deep discharges. Using the "OCV to km" graph and the table above I come up with my final table for keeping track of what kind of cycles I put my batteries through. Every day I record how far I drive between charging. Then this table gives the DOD for each cycle..

 km miles DOD (%) 2 1 10 9 6 20 16 10 30 21 13 40 26 16 50 31 19 60 32 20 70 34 21 80 40 25 90 more more 100

The data might still change some what as I get more data to average in the "OCV to km" graph.. An indicated DOD of 40% is actually the range of 36-45%.

So, it is my daily record of how many kilometers I have travelled, and the table above that generates the "official Optima Yellow Top cycle satistic". If you visited this page in 1997 you might now notice from the graph on top of this page that my DOD has increased due to the latest corretion of the fully charged OCV. Back to top

### Driving profile

These two stretches are driven in either direction and are driven on roads with speed limit 60 km/h (38 mph) with an occasional 40 km/h (25 mph) and 80 km/h (50 mph). It is some hills up and down all the time with an absolute elevation change of about 150 m (500 ft). All the driving is done with conservation of energy in mind. The car has new wheel bearings, newly adjusted toe in, synthetic ATF oil in the gearbox, and tires that are 10-15 % over inflated. On the down side, the car has wide, low profile tires (185/70R 13) and a big frontal area because of the elevated roof. Back to top

### The battery pack and data from Optima

My battery pack consists of a single string of twelve Optima D750S batteries. The car has the weight capacity to have two strings of Optimas, but I decided that one string will cover my needs for transportation.

This is the performance specification as it is given by Optima Batteries for batteries cycled up to full capacity:

 Voltage 12 Fully charged voltage 13.20 Cold Cranking Amps 750 Reserve capacity 124 min Capacity (C/2 rate) 52 Ah Capacity (C/20 rate) 65 Ah Battery Power 9.0 kW Internal Resistance 0.0028 ohms BCI/SAE CYCLE LIFE 350 Length 254 mm Width 172.7 mm Height 198.1 mm Type Post/Terminal SAE post

### How Optima suggest you charge their batteries

(Optima has changed the upper voltage from 14.8 to 15.0 volts on their web site in fall 1997).

Alternator charge: Voltage regulated, 13.8 to 15.0 volts / no current limit

Shop Charge: 13.8 to 15.0 volts / 10 amps / 8 hours maximum

Constant voltage: 14.7 to 15.0 volts temperature < 50C, no current limits, continue until current falls below 1 amp, then finish with 2 amp constant current for 2 hours.

Constant current: 25 amps, no voltage limit / replace 110% to 120% of charge removed on previous cycle / temperature < 50 C.

Float charge: Voltage 13.2 to 13.8 volts / current 1 amp, time indefinite Back to top

### How I charge the Optima batteries

I use one single three phase 6 A charger for each of the twelve Optima YTs.

1. They all start out charging at a constant current of 6 A. About 80% of the discharged capacity is replaced during this phase.
2. When the voltage reaches 14.7 volt the charger switches to constant voltage. In this second phase the current slowly drops from 6A to 0.7 A as the battery almost gets fully charged.
3. When the 0.7 A cut off current is reached, the charger moves to the float phase of 13.7 volt at 25C. The chargers stay on until whenever I pick up the Ford Express Electric again.

On the picture you can see how the wires that goes to each charger. There is a 10 Amp fuse for each charger. When charging there is a voltage drop accross the fuse, so the voltage sensing is taken directly from the battery terminal. You can see a the very thin, blue wire bypassing the fuse. It is so thin it will act as a fuse itself in case of a short.

The 12 volt chargers are temperature compensated with -24mV for each C increase in temperature. I believe that Optimas are about -15mV (for a 12 volt battery). (My own attempt to measure the temperature coefficient indicated -12mV for each C increase in temperature.) Also, the sensor is in the charger and not located at the batteries. Most of the time this probably means that the chargers believes the batteries are warmer than they are, and thus undercharging them some what. However, when reaching the float phase the current is so low that the chargers cools down to about the same temperature as the batteries. I believe the float phase, after several hours ,will have put in the charge that was perhaps left out previously. At 16 C I measured an average of 13.13 volt for a fully charged battery.

The charger is based on the U-3906 part from Unitrode. Read the application note U-104 "Improved Charging Methods for Lead Acid Batteries using the UC-3906" to find out more how you can build your own charger. Back to top

### Verification of a full charge

It is possible to verify if the batteries are fully charged. Charge the batteries. Then remove the surface charge by waiting for 24 hours. If you don't feel like sitting around for that long, you may instead discharge the batteries with 1500 amp seconds (turn on your 20 A EV heater for 75 seconds) and wait 5 minutes. The voltage of a fully charged battery is 13.20 at 25 C, as you can also see from the previous table showing the relationship between Open Circuit Voltage (OCV) and State Of Charge (SOC). Use -15mV /C to compensate for other temperatures. Back to top

### Temperature management

There is a big wooden box surrounding my batteries in the back of the van. A 820 W standard car compartment heater, heats the batteries to 16 C (60 F). The idea is to keep the batteries from getting warmer than 25 C (77 F). I understand that for every 10 C (18 F) increase in temperature above 25 C, the battery lifetime is halved. 16 C might be over doing this, so I may increase the temperature some what to increase battery capacity and performance. Back to top

### Improvements

If you have any comments, corrections or more data on the Optima Yellow Tops please send me a note on how I can improve this page. You may reach me at svein.medhus@itec.no