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1. How can I tell if my battery is charged or not?

Simple Answer, Well, maybe not so simple:

The simplest method requires that you use a voltmeter to measure the voltage between the positive (+, red) and negative (-, black) posts (lugs, or terminals). For this method to work at all, the battery should sit idle for at least a couple of hours. When the battery is not being charged and not delivering charge to a load, this is the rule of thumb. For a 12 volt, lead-acid battery, the voltage measured between these two points should be between 11 and 13 volts. The closer the voltage is to 13 volts, the closer the battery is to being fully charged. The closer the voltage is to 11 volts, the closer the battery is to being “dead”. The exact value or the voltage that you measure will be different depending upon the style of battery, and the age of the battery.

More Detailed Answer:

Lead acid batteries are made up of cells. Each cell is approximately 2 volts, so a 12-volt battery has 6 individual cells. It turns out that a fully charged 2-volt cell has a voltage of approximately 2.15 volts. Oddly enough, a fully discharged 2-volt cell has a voltage of 1.9 volts. That’s only a difference of 0.25 volts on each cell from fully charged to fully discharged. So a 12-volt battery will measure at about 12.9 volts when it’s fully charged and about 11.4 volts when it is fully discharged. That’s a total of 1.5 volts that represents the full range of charge on a 12-volt battery. To make a good guess at how much charge your battery has left, you can assign a percentage of charge remaining that is directly proportional to the battery voltage. Let’s see how we can do that.

If the battery voltage is 12.15 volts, how much charge is left? Beginning with 11.4 volts representing no charge or 0% charge available, subtract 11.4 volts from the voltage that you read. So 12.15 – 11.4 = 0.75 volts. Since there are only 1.5 volts above 11.4 volts that represents the full range of charge, we can divide the difference that we just calculated by 1.5 volts to get the percentage of charge remaining. 0.75 volts / 1.5 volts = 0.5 or when expressed as a percentage, multiply by 100 and get 50%.

Here’s the procedure written as a formula that is applicable to 12 Volt Batteries:


% Charge = SOC

% Charge = ((Measured Battery Voltage – 11.4 volts) / 1.5 volts) x 100

Equation 1

That seems easy enough. So what’s the catch? In order for this formula to work, the battery must be in a rest state. In other words, the battery should not be supplying power to any type of load. The experts say that the battery should remain at rest for at least 24 hours to get an accurate measurement, but in a pinch a couple of hours are good enough to make a reasonable guess.

The only way to be absolutely sure that your battery is fully charged is to do a load test. It is best to have the battery dealer do this for you. We only mention it here because it is possible for a battery to indicate a good voltage, but then immediately when you try to use it, it acts like it’s dead. This doesn’t happen very often, but it is a possibility.

2. How can I tell if my battery needs to be replaced?

Referring back to the discussion of how you can calculate the charge level of your battery, we know that about 1.5 volts represent the full range of charge on a 12-volt battery. Now it is possible to over-discharge a battery, well beyond its intended design. It is possible to take the battery voltage on a 12-volt battery down to 3 or 4 volts, or even to almost ZERO under load. That would constitute a severe over-discharge. Many lead acid batteries will not respond kindly to such abuse. Some batteries may recover to 8 or 9 volts without recharging. However that is still a very low voltage and it does not represent any real charge storage, rather only a ‘surface charge’ that cannot do any useful work. Assuming that there are no other usage issues with the battery it may be possible to restore to full charge if it is recharged within a few hours of experiencing the severe over-discharge.

The battery may also not recover, in which case it is safe to say that the battery is defective. If after using a Deltran Battery Tender® battery charger, a battery in this condition does not recover to at least 12.6 volts, then the battery should probably be replaced.

3. What is Temperature Compensation and how important is it?

While a battery is being charged, it is important that the charger absorption and float, maintenance voltages closely match the recommendations of the battery manufacturer. The absorption voltage match is important for quick charging. The float, maintenance voltage match is important for long term, storage charging.

Batteries are sensitive to temperature. Recall the number of TV ads showing how tough a battery is when it can start a vehicle in sub-zero temperatures. Cold temperatures tend to reduce a battery’s ability to deliver current to a load. High temperatures not only increase a battery’s ability to deliver current to a load, but also increase a battery’s internal losses. Temperature compensation is a way to change a charger’s output voltage to maintain optimum compatibility with the battery’s charging requirements. The way it works is that the charger senses the ambient temperature. Then it increases the charge voltage when it is cold and decreases the charge voltage when it is hot. Typical values for temperature compensation for a lead acid battery are minus 0.0025 to minus 0.004 volts per degree Centigrade per 2-volt cell. For a 12-volt battery, that would be minus 0.015 volts to minus 0.024 volts per °C. The reference temperature requiring zero charge voltage compensation is 25 °C or 77 °F.

How important is temperature compensation? Like with most everything else about batteries, it depends on the application. For industrial, critical load, standby power applications, where the batteries may be connected to a live charger for a number of years, then temperature compensation can have a significant influence on battery life. In many consumer applications like SLI, deep cycle marine, etc., temperature compensation will increase long-term battery performance, but it is probably not essential in all applications. Where it is most beneficial is in helping to minimize the negative impact of a battery’s self-discharge characteristics in high temperature environments. Deltran Battery Tender Plus Battery Chargers Overcome the Negative Impact of High Temperature on Battery Performance.

The self-discharge rate of a battery is directly dependent upon the ambient temperature of the battery environment. At higher temperatures, the chemical reaction rates that determine self-discharge will also increase.

When a battery sits idle, its self-discharge characteristics will reduce its ability to deliver power on its next use. If the battery either sits long enough, or if the ambient temperature rises high enough, then the battery may become fully discharged. In fact, it is possible for the battery to be over-discharged to the point where it cannot be recovered.

The Deltran Battery Tender Plus battery chargers overcome the negative impact of higher ambient temperature and battery self-discharge in two ways. First, the Deltran Battery Tender Plus battery charger applies a safe, float, maintenance voltage level to the battery to overcome its internal losses and counteract the self-discharge phenomena. Second, the Battery Tender Plus battery charger automatically compensates the amplitude of its charge voltages for changes in ambient temperature. It reduces the amplitude of the float, maintenance voltage as the ambient temperature increases and it increases the amplitude of the charge voltages in colder temperatures. In mathematical terms, this type of compensation scheme is called a "Negative Temperature Coefficient".

The temperature compensation ratio employed by the Deltran Battery Tender Plus battery chargers is approximately minus 3.67 millivolts per battery cell per degree Centigrade of temperature rise above 25 °C. Stated another way, the output voltage of the Deltran Battery Tender Plus battery charger will drop 0.022 volts, or 22 millivolts, for every degree Centigrade temperature rise, when it is connected to a 12-volt battery.

In the event that the temperature would rise enough so that the Deltran Battery Tender Plus battery charger voltage output drops below the what would be considered a normal operating voltage for a 12 volt battery, then the Deltran Battery Tender Plus battery charger automatically disconnects itself from the battery via an internal solid state mechanism, affording an extra measure of safety in a very high temperature environment.

4. What is the expected life of a H-D battery?

Proper care is the key to battery longevity. Laboratory tests have shown that consistent use of any Deltran Battery Tender can add as much as three to four times normal battery life (five to seven years is not unusual). What the lab tests can not prove is how vigilant one is about battery maintenance (if watering is required) and how much shake, rattle, heat, and cold the battery has been subjected.

If it were me, and I had five years of good service out of my battery, I would be hunting for a new one. What would I replace it with? Our theory is "if you have to fill it, forget it." In other words, stay with sealed maintenance free, most of the new maintenance free AGM (Absorbed Glass Mat) batteries like the new Harley original equipment batteries are excellent. They never require watering and they come from the Harley dealer fully charged.

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5. How long will it take to charge a battery?

We can make a pretty good guess by just dividing two numbers:
Equation 2: Approximate Recharge Time Calculations

(Battery Capacity) / (Charger Current) = Hours
(Amp-Hours) / (Amps) = Hours

Let’s say I have a 50 Amp-Hour battery and a 10 Amp charger. These are fairly typical sizes for an automotive engine start type battery and an automotive battery charger. The dash that normally appears between Amp and Hours has been replaced by the multiplication sign (*) to emphasize the behavior of these 2 items in a mathematical equation.

If the battery is fully discharged then the first approximation for the charge time is (50 Amps*Hours) divided by (10 Amps) = 5 Hours. Truthfully, this is a rough estimate and it usually tells us how long it will take to recharge the battery to about 80% of its capacity.

To complete the recharge of a battery to 100% with a 3 or 4-step charger, it turns out that it will probably take an equal amount of time, or another 5 hours to recharge the last 20% of the battery capacity. A 4-step charger may get to 100% charge a little faster than a 3-step charger, but there is absolutely no guarantee. It actually could take longer. There is no definitive answer to this question.

6. Intro to Lead Acid Batteries

7. Battery Charging Basics

8. What is the difference between Lithium Ion batteries and Lithium Iron Phosphate batteries?

Lithium-ion batteries are rechargeable batteries that use lithium as one of their active components. Lithium Iron Phosphate Batteries are a type of lithium-ion batteries known for their long life and inherent safety.

9. How do I choose the correct Battery Tender® Lithium battery for my application?

Battery Tender® Lithium batteries are available for most Powersport and other small engine start applications. You can find the correct battery for your application from your local Battery Tender® Lithium Battery dealer or online at http://products.batterytender.com/Batteries.

10. How does the “Lead Acid Equivalent” capacity rating compare to Flooded, AGM, and Gel cell battery ratings?

The capacity of lead acid batteries decreases markedly when discharged at high currents such as when starting a vehicle. This effect is much less for a lithium battery. In fact, when discharged at a current equal to its capacity, a 2 A-hour lithium iron phosphate battery will have as much capacity as a 9 A-hour lead acid battery.

11. How do I charge and maintain my Battery Tender® Lithium battery?

See next.

12. Can I use a standard Lead Acid charger/maintainer for my Battery Tender® lithium battery?

As long as the battery charger does not have a desulfation mode and the peak voltage does not exceed 14.8V during the charge cycle it can be used to charge a Battery Tender® Lithium Battery. Of course the best way to charge and maintain a Battery Tender® Lithium Battery is to use a Battery Tender® Lithium Battery Charger.

13. What is the best way to store my battery long term?

A Battery Tender® Lithium Battery has virtually no self-discharge and has a shelf life of 2 years. The best way to store the battery is disconnected from the vehicle in a clean and dry location with the protective covers installed on the positive terminals.

14. How do I prevent water damage or corrosion to my battery?

A Battery Tender® Lithium Battery has a waterproof construction however the battery should be kept clean to minimize corrosion. A corrosion inhibitor can also be used on the terminal and connections.

15. What is the warranty on my Battery Tender® Lithium battery?

Warranty Periods

  1. 0-12 months: Replace free of charge with original receipt.
  2. 13-24 months: 50% off MSRP with original receipt.
  3. 25-36 months: 35% off MSRP with original receipt.

16. How do I recycle a Lithium battery?

There are no specific federal laws requiring the recycling of Lithium Iron Phosphate batteries however local governments may have different requirements. The Rechargeable Battery Recycling Corporation provides free recycling locations throughout the US and Canada. You can locate their nearest drop off location at www.call2recycle.org/locator/.

17. How do I ship my Battery Tender® Lithium battery?

Ship the batteries in their original packaging if possible.

18. What happens if I hook up my battery charger to the wrong terminals?

If you happen to hook up your battery charger cables in reverse (positive cable to negative terminal and negative cable to positive terminal) then you will potentially damage the battery.

19. What happens if I hook up my battery to the wrong terminals on my vehicle?

If you incorrectly connect your battery to the wrong cables (positive cable to negative terminal and negative cable to positive terminal) then there is a chance for spark and damage to the battery.

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1. How is the Battery Tender® Plus battery charger different from a trickle charger?

The BatteryTender® Plus (BT Plus) battery charger has microprocessor controlled power electronic circuitry which enables it to perform and safely control a number of sophisticated charging functions, well beyond the capability of inexpensive trickle chargers.

After connecting the BT Plus to a battery and then applying AC power, it first conducts a number of checks during Initialization Mode to ensure that the battery functioning normally. Then it will deliver its full charge at a constant rate of 1.25 amperes. This is called the Bulk Charge Mode. The battery voltage will rise and when it reaches a predetermined level the BT Plus will hold the battery charge voltage constant at that level, allowing the charge current amplitude to drop. This is the Absorption Charge Mode. The Absorption Charge Mode is complete when the battery charge current drops below a very low value, usually below ¼ ampere. Some BT Plus models have timers to limit the duration of the Absorption Charge Mode.

After the current drops or the allotted time expires (typically several hours), the BT Plus automatically switches to a Float / Maintenance Charge Mode. The purpose of the Float / Maintenance Charge Mode is to maintain the battery voltage just slightly (typically between 1/10 and ½ volt) above where it would be if it were fully charged and sitting at rest. This keeps the battery topped off at voltages well below the gassing voltage of a lead acid battery.

Based on price alone, trickle chargers often appear to be a better economic choice for the typical consumer, but trickle chargers do not have the advantage of sophisticated electronic control. Therefore, as they allow the value of charge current to trickle down to what appears to be safe levels, the output voltage of the charger may very well rise to an unacceptably high level, sometimes even going higher than 16 VDC depending on the charger type and the battery that is connected to it. This magnitude of voltage is far above the gassing voltage of a lead acid battery. If the battery remains connected to this high level of voltage for an extended period of time, extreme damage may be done to the battery. Without Battery Tender® type electronic safety controls, what appears to be an initial cost savings for the charger may actually cost several times the charger price in replacement batteries.

2. Connecting Batteries and Chargers

3. Float/Maintenance Charging: Is It Really Necessary?