Supreme Auto Total Car Care - Naples, Florida

Charging Your Battery

At the heart of your car is its battery. Keeping it charged and running at its best means a longer life for it and less trouble for you. One of the most common mistakes made is over-charging the battery. One thing I recommend is to charge the battery overnight once a month. This helps keep your battery fully charged, and an overnight charging is just the right amount of time without risking overcharging the battery. I recommend that this be done with a 10 amp charger.
If you decide to charge the battery yourself, here’s what you need to do:
1. Visually inspect the battery for signs of damage. Check that there isn’t liquid collecting on the top of the battery (a possible sign of leaking) or mushrooming corrosion forming on the battery posts. Both are bad signs and should be cleaned off prior to attempting to charge the battery. Charging a damaged battery is dangerous and should always be avoided.

2. Determine the battery type, if possible. For instance, is it a traditional flooded battery, an AGM battery or another type of battery? This will be important for subsequent steps.

3. If the battery is a traditional flooded battery, check the electrolyte levels in each cell and adjust to meet the manufacturer’s specifications as needed.

4. Test the battery. Is it a properly functioning battery that simply needs a charge? If so, proceed with charging. If the tester indicates the battery is shorted or otherwise damaged, do not attempt to charge the battery.

From here, we’ll pick up with our main focus –how to determine the battery’s current state of charge, identifying the best charging rate for different battery types, calculating the length of time of reach full charge and finally, charging the battery.

A key step in manual charging is determining the battery’s state of charge prior to connecting the charger. This will enable you to accurately calculate the time required to bring the battery to full charge. This can be easily done with a digital tester that provides a state of charge assessment or using a hydrometer.
Note: Batteries that have 25% charge or less can easily freeze and should be charged at once, but you should NEVER attempt to charge a battery that is already frozen.

Next, determine the size of the battery in Amp-Hours or Reserve Capacity (Reserve Minutes). These are the only ratings that can be used to determine required length of charging time. Ultimately, you will want the Amp-Hour rating of the battery. So, if the battery is rated in Reserve Minutes, convert to amp hours using the formula below:
(Reserve Capacity/2) + 15.5 = Amp-Hour Rating.


A battery rated at 75 Reserve Minutes would have an approximate Amp Hour rating of 53 Amp Hours.

With the percentage of battery’s charge and the battery’s size determined, we can easily calculate the amount of time required to bring the battery to full charge. Here comes another formula, but it is straightforward:

((Amp-Hour Rating of Battery x % of Charge NEEDED)/Charging Rate)) x 1.25 = Hours to Full Charge

Here is a quick example: Let’s say you have a 60 Amp Hour battery with a present state of charge of 25%, so the % of charge NEEDED is 75%. Assuming you will charge at a rate of 10 amps, your simplified formula would be: ((60*.75)/10) x 1.25 = (45/10) x 1.25 = 4.5 x 1.25 = 5.625 hours. So, full charge should be reached in just over 5.5 hours.

The higher the rate of charge, of course, the less charging time required. But the old adage “Low and Slow” still applies to today’s batteries, meaning that lower rates of charge are generally best for the long term health of the battery. For typical passenger vehicle batteries, a 40 Amp charging rate is the maximum. For larger batteries used in heavy duty vehicles and implements, higher rates of charge, such as 60 or 70 Amps, are acceptable.

Also, if your charger is equipped with an ammeter, which indicates the charging current being drawn from the charger by the battery, that can be helpful in assessing your charging progress. As the battery becomes more fully charged, the current drawn decreases. The only drawback: even when the charge is complete, the ammeter will still show some current draw. This is where determining the time needed is critical so you avoid overcharging your battery. You’ll be glad you did.

Johnny Nocera has been on the radio airwaves in Southwest Florida with “Dr. Johnny’s Car Clinic” WGUF New Radio 98.9 FM every Saturday morning from 9-10 for over 27 years. He also owns, along with his sons Jr. and Jimmy, Supreme Auto and Collision located in downtown Naples, and has been in business for more than 38 years.