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How To Recharge A Deeply Discharged AGM Battery (Optima)


Optima batteries are well know for their reliability and the fact that they are a sealed battery which means no risk of an acid spill. To us that own and drive antique vehicles that is very important because the battery locations in many of our vehicles are under the floor or on the firewall, which involves lifting a battery over the fender or a running board. In the installation process it would be easy to tip a battery enough during installation to spill a little acid which would almost immediately remove some paint. It was a common event in the old days. Not Good!

I have been selling Optima Batteries since their introduction. They were originally designed by Gates of belt and hose fame. There is four feet of battery plates in each one of those cells. The 6-volt Optima Batteries like their 12-volt counterpart are a thousand cranking amps. Who could ask for anything more.

So... what happens when you deeply discharge an Optima Battery. If you connect a conventional lead acid battery charger to an Optima battery with less than 10.0 volts in the battery, the battery charger will often just "click and turn off! Your first thought is the battery must be defective. However...odds are it is not! You just need to understand a key difference between an Optima battery and a conventional lead acid battery.

The great thing about AGM batteries, like the OPTIMA, is that they have very low internal resistance. That translates to the very high cranking power that we enjoy.

Most battery chargers built in the last twenty years, have a built-in safety feature that prevents the battery charger from recharging a deeply discharged battery. A traditional lead acid battery that's at 10.5 volts or less is seen by the battery charger as being defective, having either a short, a bad cell or some other defect. Because an Optima battery has low internal resistance it too is often seen as defective.

Most analog battery chargers are binary, which means they are either "on or off". If they don’t come on, it is often because the battery charger thinks the battery is “bad.” Continuing to charge a “bad” battery is dangerous we all agree. But the Optima battery may be just fine; it has simply slipped below the minimum voltage threshold of the battery charger turn on, and because of the low internal resistance of the Optima battery, the battery charger simply doesn't know what to do, so it does nothing. Ideally you would want to connect your discharged Optima battery to an AGM rated battery charger.

But...What If You Do Not Have an AGM Battery Charger

You can trick your traditional lead acid battery charger into charging the deeply discharged Optima battery. here is how you do it...

Here's what you need...

Battery charger (under 15 amps)

Jumper cables

A good battery, preferably one with a charge of 12.0 volts or above. (It can be an AGM or flooded battery- it doesn't matter.)

The seemingly dead, deeply discharged Optima battery

A volt meter

Step One - 
Using jumper cables connect the good battery and deeply discharged Optima battery in parallel – positive to positive and negative to negative. Do not have the battery charger connected to either battery while making these connections.

Step Two - 
Now, connect the good battery to the charger. Turn on the charger. The charger will "see" the voltage of the good battery (hooked up in parallel), and start providing a charge.

Step Three -
After the batteries have been recharging for about an hour, check to see if the Optima battery is slightly warm, which is good. Batteries naturally become warm during charging, but excessive heat may be an indication that there really is something wrong with the battery. Stop charging the Optima battery immediately if the battery is very hot to the touch. Also stop the process if you hear the Optima battery "gassing (making a hissing sound coming from the safety valves). 

Step Four - 
With your voltage meter, check back often to see if the Optima battery has been recharged to 10.5 volts or above. This generally takes less than two hours with a 10-amp charger. 

If it has, turn off the battery charger, and disconnect the battery charger from the "good battery" Remove the good battery and the jumper cables from the Optima battery. Now, connect only the once deeply discharged Optima battery directly to the battery charger. Turn the battery charger back on and continue until the Optima battery reaches a full charge, or until the automatic charger completes the charge process. In most cases, the Optima battery will fully recover.

This procedure is the same for either 6-volt or 12-volt applications, but remember you cannot mix battery voltages. So if you are using this procedure to recharge a 6-volt Optima battery... the battery in the middle also has to be a 6-volt.

Battery Color Identification - 
And while we are on the subject of Optima Batteries there seems to be a lot of confusion between the red top, the yellow top, and the blue top Optima batteries and what applications they are designed for. So here is a quick rundown...

Red Top Starting Battery: Use this for normal engine starting where an alternator immediately monitors the state of charge and provides current to the battery whenever it is needed. This would include most automotive applications.

Yellow Top Deep-Cycle Battery: Use this battery when electrical loads are higher than average, or when the discharge cycle is more than typical engine starting, such as vehicles without alternators.

This also includes vehicles with significant electrical loads that may exceed the average alternator output (for example, Police cars, firetrucks, ambulances. Also aftermarket audio systems, winches, snowplows, inverters, etc.

The Blue Top starting battery (dark gray case) is to be used when a dedicated starting battery is required; it should never be used for cycling duty.

The Blue Top dual purpose battery (light gray case) can be used for both starting and deep cycling; it is a true deep-cycle battery with extremely high cranking power. Suggested applications include...

Trolling motors, marine applications with heavy electrical accessories, and RVs should use a dual-purpose Blue top (which is both a starting and deep-cycle battery).

Note: The difference between Blue top and Yellow top deep-cycle batteries is that Blue top batteries have both automotive (SAE) posts and threaded studs, while Yellow tops (other than D31T) only have SAE terminals.

Tech Tip...
If you often get confused about the colored tops, just remember: If it has a dark gray case, then it’s a starting battery; if it has a light gray case, then it’s a deep-cycle (dual-purpose) battery.

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Since 1987, Fifth Avenue owner, Randy Rundle, has been making antique, classic and special interest vehicles more reliable and fun to drive.