Battery Chargers and Maintainers: How They Work

Kyle Smith

It’s storage season for those of us in the Snow Belt, and that means great debates are raging online regarding the “proper” way to store a car. I have multiple vehicles in storage right now, each in a different sort of storage, from the 1985 Corvette lounging in my heated garage with a slim black umbilical cord running from its battery into the nearest outlet, to my poor Corvair sitting inside a concrete-floor steel building in nearly sub-zero temps with zero life support.

One item often discussed in storage conversations is a battery maintainer, trickle charger, or battery chargers. Because the batteries of our cars are designed to be used rather than to sit in storage, it makes sense that storage causes problems. Luckily, products exist to address them. But what are those problems and what, exactly, is a battery maintainer or charger doing to help reduce their effects?

Batteries have existed in multiple forms for centuries, but regardless of voltage, they all work in about the same way. So let’s dive into what batteries are, what problems arise when they sit, and how we can solve them.

How Batteries Work

There are three main pieces to a simple battery: Anode, cathode, and electrolyte. The anode and cathode are the terminals, and the electrolyte is a chemical solution or material that allows subatomic ions to move between them. This electrolyte can be free-flowing, as in a traditional lead-acid battery, which will audibly slosh when you move it around, or held in place, as in an absorbed glass mat (AGM) style. When electrons and ions are moving from the cathode to the anode through the electrolyte, the battery is storing potential chemical energy, or charging. That energy is released when a load is placed on the terminals, and the chemical reaction switches direction, flowing from the anode to the cathode.

It’s a pretty simple operation that we have had a long time to figure out—the first car batteries were made in the 1800s, after all. Like most consumer-grade products, however, modern batteries are a compromise. Given unlimited money and access to exotic materials, it might be possible for a company to build a battery that has little to no problems, but it couldn’t be produced at a price point that would make both buyer and seller happy. Therefore, we consumers get batteries that are designed to hit a specific price and perform best in a specific scenario—regular use with multiple charge and discharge cycles per day or week. Doing significantly more or less than that can invite issues.

The Problem

What happens when a battery is charged and then left to sit alone in the corner? Curious about the real details of the problem, I reached out to Carson Clarke, who works in product development for Deltran USA, the company that makes the Battery Tender line of products, to help me understand better what is happening inside the plastic case.

The charge and discharge cycle is less important than maintaining a minimum voltage, Clarke said. If a battery sits in a fully discharged state for too long, sulfate crystals will build up on the anode or cathode plates (sulfation), and the battery will never recover to 100 percent of its rated capacity. The longer it sits in a sulfate-coated state, the higher the odds that it will never fully accept a charge again and will need to be replaced. If you’re dealing with a battery that has sulfated, you may fully charge it only to find that, when you disconnect the charger, the voltage drops instantly and the battery will no longer start whatever vehicle it is in. This is commonly referred to as ghost voltage, and no battery is immune to it.

“Lead-acid and absorbed glass mat batteries are like each other in that the only real difference is the charge voltage and float [maintenance] voltages differ slightly for each,” Clarke clarified for me in an email. “Lithium has its own set of preferred charge voltages, but there are many more characteristic differences in lithium.”

Compared to flooded lead-acid chemistry, lithium-ion brings a host of benefits. A lithium-ion battery weighs less than a flooded lead-acid one of similar capacity, holds a charge more effectively, and provides full power all the way down to 20 percent charge before tapering off. Lithium also holds a charge much more effectively in storage, discharging slower than the 5 percent per month most lead-acid batteries lose.

The root of the problem is that a battery will naturally discharge even if fully charged and completely disconnected from a load or draw. For cars like my ’85 Corvette or ’65 Corvair, which regularly sit parked for months at a time, each long storage stretch could be taking capacity out of the battery in small increments that I won’t notice until it’s too late and I’m left with a car that only clicks and dims the dash when I turn the key. Both of those cars use relatively old-school lead-acid batteries. Batteries with newer chemistry don’t eliminate the problem, though.

The Solution

Automotive batteries, regardless of chemistry, want to have a certain level of charge at all times, and even when left alone they will slowly discharge. What’s the best way to keep them from fully discharging and potentially sulfating? One option is to start the engine regularly and allow the car’s charging system to top up the battery before letting the car sit dormant for another stretch of time. This can be hard on an engine, though, as most who use this method fail to let the car and its fluids warm up to operating temperature, thus allowing condensation to form inside the engine and exhaust that can silently wreak havoc on those systems. As a rule of thumb, don’t run a car you aren’t planning to drive for fewer than 10 minutes.

An external battery maintainer, also commonly called a trickle charger, charges the battery in order to keep its voltage in the ideal window, and it allows the battery to naturally discharge a bit, which keeps the electrons and ions flowing properly. This small amount of charging does not require a large amount of current, though, hence why most battery maintainers are called “trickle chargers.” They are essentially lower amperage chargers, since the battery does not need a large amount of energy to be topped up.

The amperage level of a charger should be regarded as its “speed,” dictating how quickly it will bring your battery back to 100 percent. The larger the battery, the more amperage you would need to charge and maintain it. Batteries are rated by their capacity in “amp hours.” To put that in simple math terms, if you took a 100AH battery that was 50 percent discharged and placed it on a 10-amp charger, it would take roughly five hours to reach full capacity.  

If you are curious, like I was, Clarke also clarified that there is no need or reason to disconnect the battery from the car or chassis when you’ve connected a maintenance charger. Battery Tender products, along with similar options from brands like Noco and Schumacher, also have protections built in to prevent any issues with starting a car while the maintainer is still connected. Be sure to do your research on which models of maintenance chargers are available and match your needs to the features of the appropriate product. Some have easier disconnects or voltage sensing that make them all but foolproof.

Additionally, Clarke pointed out that once connected, a battery maintainer is often best left alone. “Connect it and forget it is what we design our products to do. We recommend keeping the charger on the battery for as long as it is not in use; however, it is recommended to check the connections every few weeks to make sure the battery is still fully connected and receiving its charge.”

Is This Really That Big of a Deal?

It’s highly dependent on specific situations. I personally view maintainers and Battery Tenders as cheap insurance to keep my cars and their parts in the condition that I expect them to be in. Many vintage car owners might not need to be so preventative in their upkeep, especially if they drive their cars regularly. Use is the simplest solution, but if weather, restoration, or other responsibilities conspire to keep you from driving for a bit, a maintainer will help ensure your ride is ready when you are.

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Comments

    I like maintainers on anything stored for long periods. We have 2 Schumacher tenders. One stays on the Corvette all winter and the other gets moved around to tractors and another stored car. The battery in the Corvette is over 10 years old and still works as new. We don’t have to remove them to start the engines, so they are very convenient to use once connected.

    I also have some batteries that get moved to different pieces of equipment as that equipment is needed on the ranch. There’re are also items that are only used sporadically (boat, car-hauler trailer winch, kid’s minibike, etc.) No need to have batteries in several machines when those machines are only used for certain (and relatively short) periods of time. So, I’ve built a place where those “moving batteries” can sit and be places on tenders. Need to fire up the tractor to cultivate a field or go rescue a friend that’s broken down somewhere? unhook the tender and grab the appropriate battery, all charged up – and which may get used for an entirely different vehicle/machine next week!

    Using a tender is needed in any car with a draw.

    Also if you use an AGM battery they will not take a charge below 10.5 volts unless it is a an AGM charger. Some AGM charger also will recondition batteries too.

    This is the cause why so may think their Optima is defective as they are below 10.5 and try to use a older regular charger to charge it. It will fail every time. You can connect it to a good battery to get the charger to work or use the AGM charger.

    That’s good info for those who have older battery tender products. They updated their products a number of years ago to work with AGM batteries in an improved way. But since these tenders last a long time, it’s quite likely some people still have older models.

    I have 6 classic vehicles and 2 lawn mowers . When everything goes into hibernation in eastern Ontario the batteries are all lined up on a shelf in a heated garage. I have 1 battery maintainer. Start at one end of the line-up and charge until full charge. go down the line until done. Repeat. Takes about a week to do all 8. Some of the batteries are 15 years old and still are 100%.

    I do the same with my ATV batteries. You can simply connect them in parallel to the battery maintainer and leave it alone.

    I have had used the Battery Minder and Battery Tender products over the years. It has kept the battery in my Supra happy and helped top off batteries that have sat too long.

    I had a Battery Tender on my 1992 Corvette. I went out in the attached garage and smelled that something-electrical-is-hot smell and found the Battery Tender very overheated and brown, but not yet on fire.
    Changed to a Harbor Freight and no issues.

    I had my fill of battery maintainers, especially Sears maintainers. (Which I made the mistake to own eight of) I found years ago that maintainers that are supposed to kick back in when the battery is low too often don’t recharge the batteries. Even Schumacher maintainers, that a friend owned, didn’t always keep the batteries charged. When any of those lousy maintainers have a green light on them I found that too many batteries had less than 12 volts while the maintainers were still hooked on!!!! The ONLY ones I own anymore are Harbor Freight 1/2 amp maintainers. They put out a constant 1/2 amp. that doesn’t overcharge even my lawnmower batteries. They don’t have a rotten computer in them which fails to do what it is supposed to do. Also I find that large chargers that have computerized “automatic ” features in them sometimes either cut back too soon or don’t even allow you to start to charge your batteries. I don’t have anything but old tech chargers here as quite frankly, all the new tech ones have failed. I have a few chargers that are over 60 years old that still work. Nuff said.

    I’ve never had a problem with the Battery Tender line of products. The only thing I ever needed to do was get a newer version when I had an AGM battery. I’ve got three doing their job now.

    Hmmmm, my experience is that the Schumacher and NOCO high tech tenders are the best I have found over the last 50 years of “finding.”

    DAL. Since I have proven computerized maintainers do fail it becomes common sense that chargers with computers have one more reason to fail. Old chargers which have a history of lasting for decades are the ones to trust. One thing that people don’t relize is that if your battery is capable of starting a vehicle after a winter of storage and the green light is on, they will think the maintainer did it’s job. Some batteries stay charged that long without help. I caught too many of my stored batteries with less than 12 volts while the maintainer was hooked on. I tried to charge a small motor cycle battery that I still have with a Battery Tender. The battery was too low to start the cycle but when I hooked a Battery Tender on it, the charger would light the green light within a few minutes, long before the battery was charged enough. Had to unplug that maintainer to reboot that junk only to hope it stayed charging while not being watched. People trust high tech too much but maybe they will learn to double check their maintainers while in operation. Good luck with your method.

    Sorry, but I find this a half hearted article. If you are changing FLA batteries the fluid level must be correct, use only distilled water, and check it occasionally. If you are changing, even trickle changing, and the level exposes the plates, hydrogen gas will form in the battery. Starting your vehicle with the battery in this condition may cause the battery to explode.

    I have a small compact diesel tractor I keep in an unheated shed Few years ago we had subzero temperatures and the tractor wouldn’t start. The battery was dead and wouldn’t take a charge. Still under warranty I took it back to Napa. They wouldn’t warranty it. The battery froze and the case was all bowed out on the sides. I now keep tenders on all my stuff- boats, hot rod, utv,garden tractor. Cheaper to run and batteries seem to last a lot longer

    Everyone has there own techniques that work well for their applications. Myself I am not a fan of maintainers as they depend on delicate electronics to work properly.

    My technique is to use my simple, low tech, trusty 30 year old 7 amp battery charger on each stored car battery for 30 minutes about every six weeks. All of my stored cars have the negative lead disconnected.

    I’ve had 7 to 8 years service out of each battery in the old cars using this technique.

    At one time, I had 5 motorcycles (I rode a different bike to work each day. :D) and a boat in my insulated garage. I bought six battery maintainers and kept them plugged in all Winter long. The maintainer was the type that just had two LED’s-a red one, showing “charging”, and a green one showing “charged”. In the Spring, I found that all the batteries were dead from overcharging, except for the AGM battery in the Honda Rebel. The next year, I would only plug in the maintainer over Saturday night, leaving it unplugged the rest of the time.

    This also happened to me. What I do is after the battery is fully charged, I plug my Battery Tender into a Christmas light timing. I set it to charge for 30 minutes per day. No issues with this method.

    I have a 6-volt Battery Tender-brand maintainer, which I use for my Honda C70 and my Allis-Chalmers WD. Just be sure to connect the positive lead from the maintainer to the correct terminal on the vehicle; the vehicle’s positive lead going to ground is not an issue in itself. Done correctly, this works fine for me.

    I have been disconnecting both cables to put the tender on, so your saying that I can just put the positive lead to positive ground without any issues with the tender?

    Yes. I also recommend a simple battery disconnect when not in use. Just make sure the charger leads are connected on the battery side of the disconnect. I have a charger on my Model A 6v and have for years.

    I have 5 of them and all are currently in use. Although pricey, from my experience, the best ones are sold at Harley-Davidson. My oldest one has been in near constant use for almost 22 years now.

    At the museum where I volunteer we keep a number of cars road worthy for events. Unfortunately museum staff have the belief that just disconnecting the battery is good enough and wonder why batteries only last a couple of years. A retired electrical engineer volunteer says it is so and battery tenders are dangerous. He even claims using battery tenders is dangerous and, covered by insurance and against fire codes. Perhaps I can get someone to read this article. Replacement 6 volt batteries for these cars are really expensive.

    I can only reply in reference to boat storage. I don’t know of a single boat storage facility that allows ANY 120v electrical connection left unattended. That applies to both indoor and outdoor storage. You might get away with leaving a charger connected outside, but if they notice, they will unplug you. Inside, they will unplug you and issue a warning.
    I know of two facilities that require start and house batteries be removed from the boat if stored inside.

    I would agree. No matter how good the product is, leaving a charger or maintainer plugged in and operating is just asking for a problem, IMO – even if it does not materialize for 10 years or more. It just seems risky.

    The Optima in my classic car is 12 yrs. Old and shows no sign of failing. It is always connected to my battery tender Jr. when garaged. I am convinced this is the reason for my battery longevity.

    Beware of Optima batteries and their tendency to develop high internal pressure while left on maintenance chargers. I’ve had 4 of them fail this way over the last 15 years. Each time the battery leaks liquid acid around the base of the negative terminal post and the negative side vent. They also build up so much internal pressure they push the negative terminal up by 1/8” to 1/4”. Optima are supposedly AGM batteries that supposedly immobilize the liquid acid electrolyte, and they claim to be “leak proof”. From my experience with three red tops and one yellow top, I have doubts about Optima. Now I’ve gone to Energizer AGM batteries from Costco for my vintage cars, and OEM Honda AGM batteries for my daily drivers and have no complaints. And yes, I’ve switched to NOCO brand battery charger / maintainers.

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