Do You Need to Disconnect a Car Battery to Charge It?

If you’ve ever had a low battery or one that you planned on not driving for a while, it would be a great idea to get it hooked up to a charger.  After you pop the hood, you might be wondering if you can just hook the battery charger right up to the battery without disconnecting the battery from the car. 

If you’re new to battery maintenance, it’s a very valid question.  Will it fry my electronics? Will I overheat any of the wirings? 

Disconnecting your car battery to charge it is not necessary when using a modern smart charger that is microprocessor-controlled. If you have an old charger without a microprocessor it is best to disconnect your car battery.

I asked a trusted friend who happens to be a 25-year mechanic this exact question just to make sure I wasn’t missing anything and he pretty much confirmed my suspicions from my experiences with batteries. 

We’re going to cover the two basic chargers for this process, my favorite tool for the job, and how to properly charge a car battery. 

Let’s get started!

Charging a Car Battery with a Smart Charger vs. Old Regular Charger

Why Your Battery Can Stay Connected with a Smart Charger

The reason you can leave your car battery hooked up to the car without fear of frying your electronics is that modern smart chargers are basically mini-computers which carefully regulate the entire charging process.

They emit the proper amount of current in every phase of the charging process so that it lines up with the amount of current your battery can actually absorb.

My personal favorite out of all of the smart chargers I’ve tested. I wish I had the 5-amp model, but it wasn’t sold at the store that I bought this at.

If your battery is low and can absorb a lot of amps, the battery provides it. As the battery nears a full charge, a smart charger will dial back and only give the battery what it can take.

Remember, that during the charging process you are literally turning electrical energy into chemical energy within your battery.  It can only do this at a certain rate without overcharging.

Some smart chargers will simply turn themselves off when the battery reaches a full charge, and some will enter into a maintenance or float mode.  The latter are my personal favorites.

They are the true “set it and forget it” chargers and you can rest easy that you’ll have a strong battery the next time you need it. 

In the float mode, a charger is basically keeping the voltage to the battery just above the voltage of the battery and a small number of amps are always primed to be given to the battery as it naturally self-discharges or loses some charge due to parasitic draws (the car clock, anti-theft system, etc.).

Smart chargers also shine in the reverse polarity protection they provide (hooking the cables to the wrong battery terminals) and protection from even starting the charging process (sending electricity down the cables) until they recognize that they are properly connected to a battery and not just any object. 

So, if you were to clamp the battery cables of a smart charger onto something other than the battery, this would lead to nothing happening.  The microprocessor must recognize the resistance from the battery (its internal voltage) in order to start the current. 

This can be a problem at times if your battery is completely dead since even though you’re hooked to the battery it won’t be able to detect it with so little or no voltage reading.  You can bypass this by jumping the bad battery with a good vehicle for a few minutes to introduce an artificial spike in voltage that the charger can pick up on. 

Old Chargers are Best Used with the Battery Disconnected

So, why is it best to avoid the old-school chargers?

Quite simply, there are no safety or computer controls within them for a proper charge. 

Once you plug an old charger in, the cables are live.  They will try to put current into anything you touch them to. 

An old Schumacher brand charger that will keep delivering current after your battery is fully charged. Use this type with caution!

They also do not sense the amperage that the battery is able to accept and there is a high risk of overcharging your battery.

When your battery is fully charged, the old chargers will not turn off.  They will continue to pump in the amps. Since your battery can’t convert any more electricity into chemical energy, it will go through electrolysis which is the boiling off of your battery fluid as electricity passes through it and heats up since it cannot be converted.  It has to go somewhere! 

Reduced battery fluid through evaporation means a weaker battery.  Keep it evaporating and heating up too long and you could end up with warped plates, a bulging battery, or exposed lead plates which will be permanently damaged.

There is also no reverse polarity protection with old chargers.  If you hook them up to the wrong terminal you will know it, and it will be one of those cases where bad things happen to good people.  

Essentially, old chargers work, but you need to know what you’re doing. You need to understand the rate at which batteries absorb amps, you need to know the math behind how long you’ll need to charge the battery, and you’ll need to be monitoring the charging process for safety reasons.

Leaving an old-style battery charger connected and unattended is not the best of ideas. If you forget about it, you are likely going to have an overcharging battery which will reduce its lifespan.

What is the Best Charger for Car Batteries?

If you’re looking for a smart charger that won’t overcharge your car battery, and one that you can leave hooked-up indefinitely or over a winter season, I love the Battery Tender® by Deltran.  

I’ve used this thing for 5 years without a single problem and trust it to keep my battery bank for emergency power topped off, and I’ve brought some weak car and lawn mower batteries back to a healthy state over the years as well with it. 

You can usually find these for a fantastic price on Amazon and they offer 3, 4 and 5-amp models. I recommend the 5-amp one simply because you’ll get a faster (but still safe and gentle) charge for a car battery but any of them will suffice.

They will charge your car’s battery at a max of 3, 4, or 5 amps (depending on which one you get) and then automatically scale back as the battery reaches 100%.

Once it is charged, these go into float mode and can be left on the car battery indefinitely.

Also, if you’re interested in how long it takes to charge your car battery with any given charger, I have an entire article on that here that you can check out.

How to Charge a Car Battery that’s Connected to the Car with a Charger

  1. Clean your battery terminals of any debris with a damp cloth or wire brush.
  2. With the battery charger UNPLUGGED (assuming it’s a smart charger), connect the red cable to the positive terminal of the car battery.  
  3. Then connect the black cable to the negative terminal of the car battery.
  4. Now, plug your charger into the wall or extension cord that’s plugged into the wall. 
  5. When the charging process is complete, unplug the charger.
  6. Wait 10 seconds for any residual power to dissipate, and remove the black cable first. 
  7. Remove the red cable.

A 5-amp charger like the one I recommended above will charge a fully dead car battery in about 10-11 hours (overnight), and 2-amp maintainers will take 1-2 days depending on the size of your battery.

You might also be wondering why I said to connect the black cable to the negative terminal of the battery and not the body of the car.  

You can do either one since the negative terminal of the battery is grounded to the frame of the vehicle so current will still flow from the point of connection into the battery.

The reason why people say to connect it to a part of the car that is grounded is to keep you from making sparks directly above the battery when you touch the negative terminal.  Those sparks could (under the PERFECT conditions) ignite any trapped hydrogen gas that is built up around the vents or under the cap for the cells.  

The logic is that when you connect to the body of the car you’ll make spark at that location instead. 

The way I do it eliminates the spark altogether since I don’t plug in the charger until it’s fully connected to the battery, and I don’t disconnect it until I unplug the charger and wait for 10 seconds.  

In the end, do what you feel most comfortable with and always wear safety glasses when working around car batteries. 

Can you Leave a Car Battery Charger on Overnight?

If you’re using a smart charger that will default to a float mode, maintenance mode, or turn itself off when the battery is fully charged, then it is safe to leave your car battery charger on overnight. A charger without a microprocessor runs the risk of overcharging your battery if connected overnight.

Is it OK to Leave a Car Battery Charger Plugged In?

If you drive regularly it is perfectly fine to leave your phone’s car charger plugged in while not in use. The energy drawn by the charger is negligible to the total capacity of the car’s battery. When not driving for extended periods of time it is best to disconnect the car’s battery to prevent parasitic discharge.

What Happens if you Leave a Car Battery Charger on for too long?

If you are using a smart charger that defaults to a float mode or turns itself off after the battery is fully charged, then there is no concern. Leaving an old charger without a microprocessor on for too long will overcharge, boil-off of the electrolyte, cause off-gassing, and can physically damage the battery.

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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