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Manual Charging – When is Enough, Enough

Most service bay shop chargers are of the wheel charger variety and the vast majority of those feature manual charging controls. This has both positives and negatives for the shop. The best aspect of manual charging is that it allows the technician to specify all relevant charging parameters. The charger does exactly what it is told to do, with no intervention or overrides on its part. The largest drawback of manual charging is that the technician must take care to ensure the battery isn’t overcharged. Overcharging, and the prevention of it, is a topic that results in numerous calls to our Technical Service support team each week and our goal with this article is to address the key issues related to manual charging.

The first thing to address is the general charging routine you should follow each time you perform this application:

1. Visually inspect the battery for signs of damage. Charging a damaged battery is dangerous and should be avoided. 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.

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 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 – determining 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 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 % of charge of the battery 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 about as fast as should be used as a general rule. 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. But, there is no clear-cut way to read an ammeter and determine exactly when charging is complete – at full charge, the ammeter will still register some current draw. In many cases, overcharging can occur if the charger is not disconnected when the battery reaches full charge – or sooner. Therefore, it is very important that you determine the amount of time needed to reach full charge as outlined above.  You’ll be glad you did.


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