Sealed Lead Acid Battery Application

Battery Voltage

There is much confusion about "battery voltage" because a battery has more than one voltage and often the literature is lax in defining which voltage is being discussed at the time. Also, measurements of a batteries voltage, particularly float voltage, require time (that is often not allotted) for the battery to stabilize.

• Float Voltage - Batter voltage at zero current (with battery disconnected). You must wait about 20 minutes for the battery to stabilize at this voltage.
• Nominal Voltage - The Voltage that a battery is referred to. i.e. a 12 Volt battery is made of 6 cells and has a Float voltage of about 12V.
• Charge Voltage - The voltage a battery goes to while being charged.
• Discharge Voltage - The voltage of a batter while discharging. Again, this voltage is determined by the charge state and the current flowing.

In a similar manor Discharge voltage is usually specified at a current that is capacity in Ah divided by 20. A 10Ah battery would have a load of 0.5A on it while measuring this voltage. 

Battery Efficiency

Energy efficiency is calculated on the amount of power used from the battery while discharging divided by the amount of power delivered to the batter while charging, multiplied by 100 to yield percent.  P_out x 100 /P_in  . A lead-acid battery has an efficiency of only 75-85%. The energy lost appears as heat and warms the battery. Keeping the charge and discharge rate of a battery low, helps keep a battery cool and improves the battery life. 

The above losses don't include losses in the charging circuit which may have an efficiency of  anywhere from 60% to 80%  - thus the overall- total efficiency is the product of theseefficiencies and ends up being 45 to 68%. (To further this example and to show why physics and not some corporate conspiracy is the reason we don't have electric cars - suppose the controls and motors on a car were 85% - the over all efficiency is now only 38 - 58%. You can see that an electric car would use about twice the energy than a conventional car - not to mention the great cost of the regular replacement of batteries. This is why batteries are best used where only intermittent, or very low power use is required.) 

To further explain - If the electricity is generated from a gasoline engine - and that energy is converted to electricity, and then sent through power line transformers and power lines, and then converted to DC, and then converted to chemical energy, and then converted back to electrical energy, and then converted to rotary mechanical energy - it is clear that many losses have occurred. If the same gasoline motor was providing the rotary energy directly to the drive train it is much more efficient.

Battery Capacity

Battery capacity refers to the total amount of energy stored within a battery. Rated capacity is in Ampere-hours (AH), which is the product of the current times the number of hours to total discharge. The capacity is normally compared with a time of 20 hours and a temperature of 68F (20C). There are five factors that dictate the capacity of a given battery:

• Size
The volume and plate area of a battery increases with capacity
• Temperature
As a battery gets colder, its capacity decreases.
• Cut off voltage
To prevent damage, the cut-off voltage should be limited to 1.67V or 10V for a 12 Volt battery.
• Discharge rate
This is the rate of discharge, measured in amperes. As the rate goes up, the capacity goes down.
• History
Deep discharging, excessive cycling, age, over charging, under charging, all reduce capacity.

Choosing Battery Capacity

Specifying battery capacity involves a bit more than multiplying the load current by the backup time in hours. You must first de-rate the battery for capacity tolerance, temperature, and discharge rate.

• First, multiply the average load current by the backup hours of operation you need.
• Next, add 15% to cover loss of capacity from tolerance and UN-cycled batteries.
• For every 10C (18F) below room temperature (72F) your worst case low temperature is add 10%.
• If your back-up time is less than 20 hours, add 10% for every time you have to double your back-up time to equal more than 20 hours. An example may help: 20 minutes would have to be doubled 6 times to equal more than 20 hours. Thus you would have to add 60% on to your required capacity. 
• Finally, add 40% to provide for an economic life cycle. A battery with 60% of its capacity left is considered worn out.

• 10 hour back-up time at .2 Amperes 2.0 AH
• 15% from step 2 .3 AH
• 0C add 20% from step 3 .4 AH
• Add 10% from step 4 .2 AH
• add 40% from step 5 .8 AH

Total 3.7 AH

Deep Discharge

Discharging a battery even slightly below its fully discharged voltage shortens its life. Letting a batter sit and self discharge to 0 destroys the battery. 

The Transtronics Battery Voltage UPS (BVUPS) disconnects the battery at the fully discharged voltage to protect the battery from damage. It has a time delay of ten seconds, so as not to disconnect on power drain "spikes". Fully discharging a battery to zero volts, just once, will render a battery unusable.

Also, consider that some equipment does not stop working "gracefully" as its voltage supply gets lower and lower. Some electronic equipment may work erratically and cause outputs to turn on and off randomly.

Temperature

Store batteries at a low temperature if possible - 5deg C is ideal. Although capacity goes up with temperature, the life of a battery goes down.

The self discharge rate goes down with temperature. At room temperature, recharge  stored batteries every 6 months; storage at 5C lets you wait 18 months before recharging.

Battery Charge Cycling

Pick a power supply that can provide for all of your maximum load. Using the battery to pick up the load difference between a small supply and a large, but intermittent load, will keep the battery constantly charging and will ruin the battery in a short time. It is much less expensive to buy a larger power supply than to replace batteries repeatedly.

The Transtronics BVUPS supports a variety of charging methods each with a different trade-off. Let us look at the different methods.

Constant Voltage -- Taper Charging

Taper charging is the default charging mechanism of the BVUPS. Adjust your power supply to  13.8 VDC (27.6 VDC for the 24 volt model). Your battery will charge rapidly at first and then slow down as it reaches full charge. After charging the battery fully, you should see a charge current that is equal to the capacity in Amp hours divided by 100 to 200.

An Example:

Using an 8 AH Battery ~ 8/200 = .040; therefore the final charge current should be 40 MA

The advantage to a Constant Voltage system is simplicity. On the other hand, it is a slow way to recharge a battery. Increasing the voltage to speed up the charge rate will cause the battery to overcharge and fail. Instead, speed up battery recharge with a constant current charge system. 

Constant Current Charging

This is the preferred method of charging Lead acid low maintenance batteries. Charge current is limited to a maximum Current AND voltage. The table below is based on I = Ah/4

Modeling Lead-Acid batteries

The ones available in SPICE simulators are just a bunch of capacitance and with resistance in between � quite different from a real battery . A real 12V battery will show 13.8V on a trickle charge and drops to 12V with just the slightest load � with a practical load it drops another .05V per cell or about 0.3 volt to put a 12V battery at 11.7V Thus there is a dead band in the battery that has nothing to do with internal resistance. 

What is interesting is that the no load voltage stays pretty much the same until they battery is really discharged � only the battery drops farther upon applying a load. The reverse is true for charging a battery � the zero current voltage is always very close to 12V but as the battery reaches full charge the jump in voltage on application of a charging current increases. 

I always worried about the Temperature coefficient of lead-acid batteries. dE/dT turns out zero  in a fully charged battery. There is a temp-co for a discharged battery but dE/DT = -0.000 43 V/C (per cell) so it can be safely ignored. I think others have confused batteries dE/dT with dI/dT which does have a noticeable temp-co. 

A batteries internal resistance is not linear with Ah capacity, but can be assumed linear at low power levels. A graph of it is asymptotic to both the y and x axis in the first quadrant. The slope is pretty linear at the power levels we work with � but much of the literature on batteries came from submarines work where the systems Ah rating is magnitudes larger.

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