How Fast Can I Charge My Car Battery?

On May 16, 1986, the iconic movie Top Gun was released. Part way through the best-on-best flight school training Maverick turns to Goose and says (with Goose joining in to finish the line) “I feel the need—the need for speed!” This immortalized string of words seems to personify American thinking. Our insatiable need to go faster, work faster, be faster is intertwined into our DNA. The underpinning of this mindset brings us to an often-asked question, how fast can I charge my battery?

Yikes! That sounds like a simple question, right? But there is so much more wrapped into those eight words that makes answering harder than one may think! Not harder in the sense of complicated, but harder in the sense that you need to understand what type of battery you actually have before connecting the Godzilla of chargers to your car. A basic understanding of battery type and chemistry will go a long way to help prevent catastrophic battery failure.

Safe Rule of Thumb for Charging Your Battery

If you don’t have the time or desire to figure out what type of battery you have in your car, SUV, pickup truck, bike, boat, or RV then speed of charging should be the least of your concern. Simply telling me you have a certain CCA battery does not cut it. If that is all the information you bring to the table, we must default to a safe rule of thumb. After all our goal, as ‘Battery Doctors’ is to do no harm.

For most all lead acid based batteries—Gell, AGM, Conventional—you can safely select a charger with a maximum charge current that is no greater than 20 to 25% of the batteries capacity. I know this article is about fast charging but I should also mention that you do not want to use a charge current of less than 3% of capacity (think trickle chargers and maintainers).

Try and Remember This Safe Maximum Charge Current Jingle: Rule-of-Thumb is 5 to 1

Now I can see the look forming on some of your faces. Most consumers have no idea what the amp hour (AH) capacity is for their car battery, so how are you supposed to apply the safe rule of thumb jingle? To that I would say your right. Many car battery manufacturers do not display this metric on the top sticker. They often seem to singularly and prominently display the CA or CCA. So how in the world can you convert CCA into AH when that's the only number you have at your disposal?

That is a great question. Sadly, there is no magic formula to convert CCA into AH. They are two entirely different valuation metrics that do not relate to each other. The best option is to pull out your smart phone and look up the battery part number online to try and see if you can find the amp hour rating.

Sometimes you may get lucky and the Reserve Capacity (RC) is provided on the battery or notated online. If so, you can generally multiply this number by 0.5 to get a sense of where the AH resides. (Some say multiply by 0.6 but I find this variable often has the AH coming out much larger than the manufacturers stated AH).

If you still cannot find the info you are searching for or just don't want to take the time to find out, you can use some [very] rough estimates. Smaller cars will have batteries that are 25Ah to maybe 45Ah. Larger vehicles like SUVs and pickup trucks will have batteries that range from 40Ah to 70Ah. Smaller RVs, fifth wheels and bass fishing boats often have at least 2 batteries that are 70Ah to 100Ah. Larger camper RVs and trucks could have 4 or more batteries ranging from 100Ah to 250Ah each.

To help put that in to practical terms a 50Ah battery (i.e. a midsize sedan to perhaps a 1500 Chevy truck) could safely use a 5A charger to as high as a 10A or 12A smart charger. Obviously the 10A charger will be 2x times faster than the 5A charger so if speed is your goal do not select the smallest option...that would be dumb.

Remember, we are applying broad stroke logic here for when we don't know specifics regarding your battery chemistry or specs. This safe rule of thumb will extend the longevity of the battery and help keep you safe. Let me put it like this, I would think having a car to come back too outweighs the extra hour or two you would save in charge time. Do you get what I am saying? The charred metal skeleton parked in your former home will not prove useful in the end.

Below is a short list of widely used Impact Battery approved car battery chargers that tend to fall within this safe-rule-of-thumb profile stated above and will allow you to safely charge your car battery fast (listed alphabetically):

Battery Minder model 128CEC1
Noco Genius G3500 and G7200
PulseTech Xtreme XC100-P
Schauer Charge Master CM6A and CM12A

Battery Theory and Maximum Charging Rates

We have covered some safe charging rates that will help you maximize your speed of charge when you are unsure of what you have. But obviously, we can charge faster if we know more information.

Knowing the type of battery you have makes it much easier to determine the maximum charge current you can use on your lead acid battery. Many of you seem to understand, at least fundamentally recognize, that there is a difference between what we commonly call deep cycle batteries and starting batteries. Or another way to state it is thick plate vs. thin plate batteries.

When it comes to charging your battery, knowing this vital detail may be half the battle. However, there is more to it than simply knowing if you have a starting battery or a deep cycle battery.

The Path of Least Resistance

A batteries internal resistance is of paramount importance when it comes to understanding its charge capabilities. We could not have a thorough discussion about fastest charge times without touching on some theory. To do this we need to mention, dare I say understand, Georg Ohm’s work in this area. Ohm’s Law is the mathematical relationship between current or amperes (I), voltage (V) and resistance (R). The basic equation being I=V/R.

Not wanting to dive deep into the theory and lose all my readers, I think it would be safe to make the following blanket statement:

Thick plate deep cycle batteries typically have a higher internal resistance than their thin plate starting battery cousins.

You may also find this short 6 minute video helpful in understanding the implications of resistance found in within your battery and battery system. James Dann does a good job at presenting basic theory in a very hands on practical manner.

Deep Cycle vs. Starting

Generally speaking, you can charge thin plate batteries at a higher current rate than what should be used on a thick plate battery. The internal resistance is far lower in starting batteries as they are designed to provide a burst of energy in a short span of time. These types of batteries need to magnify the rate in which energy can transfer. Whereas deep cycle batteries, generally with higher internal resistance, are designed to maintain a constant load over a longer time span. I guess you could say they need to slow down the rate in which energy is released.

A typical Group 31 sized battery is usually around 100 amp hour and are used in deep cycle and starting applications. A 100-amp hour thick plate [deep cycle] conventional battery should not be charged with the same amount of current as a 100 AH thin plate pure lead [starting] battery. The results could be messy!

Did you know that a deep cycle battery may actually charge quicker using a slower charge rate? It is true. By slowing the current flow you have removed the 'bottle neck' created by the increased resistance. The resulting heat and friction from trying to charge too fast is gone and the battery can actually absorb the charge faster at the slower speed.

A Painful Fast Charging Story

The premium pure lead AGM batteries such as NorthStar and Odyssey that use thin plates and are highly compressed can actually accept a charge rate equal to their stated amp hour capacity! That’s right they can be charged at 100% of their amp hour rating!

Don’t try doing that with a conventional starting or deep cycle battery, the results could lead to catastrophic failure. Case in point:

A couple years ago I took my Suburban to have some work done on the rear brakes. While in the mechanic’s care, he decided to place a 40A charger on my battery. About 15 minutes later, as he explained, he sniffed a terrible sulfur smell and disconnected the charger as he observed a pile of acid on the floor. With that, I no longer had a working battery. He had managed to warp the plates and shorted the cells. The Group 34/78 conventional battery I had in there was only rated for 45 or 50Ah.

Understatement of the Year: NOT ALL BATTERIES ARE CREATED EQUAL! Don’t assume what you did with your battery is what will work for your buddy’s battery.

Should there have been a Northstar NSB-AGM34/78 installed (As I now have), I would have been fine. This pure lead battery not only has a higher amp hour rating (65 AH) but could have withstood the 50A charge without causing harm.

Final Thoughts

Trying to push too much amperage on a battery that cannot accept such a high current rate will create tremendous heat and will damage the battery or worse. Tell tail signs of this type of abuse is a battery case that no longer has flat sides or is discolored. ABS plastic will start to flex and melt at 221 degrees Fahrenheit. It will ignite at 416 degrees. In case it was not obvious, if you are experiencing those types of temperatures, the battery has not failed you; YOU FAILED THE BATTERY!

There are some premium sealed AGM deep cycle batteries, such as Concorde's LifeLine Marine Battery, that have a really low internal resistance. Like the North Star's and Odyssey's these batteries use high compression. These batteries can be charged at higher rates of current compared to other deep cycle batteries. I was told by one LifeLine Battery rep that they can be charged up to 5x their C rate. In truth I am still trying to wrap my head around that one and have yet to suggest such an extreme charge rate to anyone. What I do know is that these Concorde batteries are indeed an industry leader and highly sought after.

There are two charger companies recommended by many in the industry including Impact Battery, that produce high output chargers. The first and aptly named, Quick Charge, is made right here in the USA and have 12V to 48V units that can produce up to 100A. The other, Iota, is a charger/power supply manufacturer capable of producing up to 90 amps on their 12 volt units. That is some serious speed!

To recap, remember these four Fast Charging nuggets:

• If you don't know what type of battery is in your passenger vehicle, selecting a 10 amp charger is probably your safest bet.
• The safe maximum charge current rule of thumb is 5 to 1.
• Pure lead thin plate starting batteries can be charged the fastest, up to 100% of their rated capacity.
• To a certain extent, deep cycle batteries can actually charge faster at a slower current rate.