Can a battery drain with the negative cable disconnected

You are here:
  • KB Home
  • Can a battery drain with the negative cable disconnected
Estimated reading time: 7 min

Waking up to a battery drained simply because you left it on can be distressing when the simplest solution could be to disconnect the battery cables from yachts, cars and trailers.

The idea of a battery going flat when it isn’t being used is not new and many boaters will have left an expensive piece of equipment on the battery only to return several days later to find that the battery has gone flat or at best needs charging before anything can be done with their boat.

So, the question most people ask after finding the battery flat is: “If I disconnect the negative cable from my boat, can it still go flat?”

As a general guide; a battery can go flat if you disconnect the negative cable. Though disconnecting the negative cable may seem like the best way to keep your boat or RV from using battery while you are parked, it is in fact the worst thing you can do.

First, let’s look at how a typical 12V battery system works.

A basic 12V electrical system consists of two main parts; The battery and the rest of the electrical devices that are powered by it (lights, pumps etc). It also includes the cables that take power from the battery and take it to the devices that use power.

The battery, as the name suggests stores 12 Volts of electrical energy for later use. This chemical energy is converted into electrical energy when you connect your boat’s electrical equipment (lights, pumps etc) to the battery.

From its very basic, a battery can be looked at as a box of ‘inexhaustible’ energy. Once you connect your electrical equipment to the battery, it starts giving up its charge to power them (lights, pumps etc).

While the battery is providing the boat with electricity, its chemical reaction is ongoing; working hard to provide power for an array of devices operating at the same time. This internal chemical reaction is known as the ‘load’ on the system.

As time goes by, the battery will inevitably provide fewer and fewer amps of power until its chemical energy is exhausted. Once this happens an electrical overload can damage or destroy equipment (lights, pumps etc) connected to it if you don’t disconnect them in time.

When this happens, the battery needs to be topped up using a charger. Boaters will often use a trickle charger, which is the slowest type of recharging system that uses a low voltage and high current to ‘trickle’ charge the battery over time.

Some people wrongly assume that if they disconnect the negative cable from these devices, their boat will not discharge and leave their battery in a better state. This is because they think the battery will not discharge if it is connected to nothing (a ‘floating’ system).

However, this is incorrect; if you disconnect your negative cable from your boat, you create a floating system where the positive and negative sides of the circuit are connected together through electrolyte in the battery.

In a floating system, it is possible to have a current flowing through the circuit because there will be some resistance between the positive and negative sides of the circuit. This current flow can result in a charge of up to 1 Volt being applied to your battery.

This may not seem like a lot but if you leave your lights on for just an hour or two, this can be enough to flatten your battery.

A modern boat’s electrical system is designed so that if something goes wrong, the negative cable will disconnect. This is so it does not create a danger of electrocution by keeping the current flowing through an object (like you) instead of returning it to the battery.

In theory, a boat or RV can go flat if you disconnect its negative cable. In reality however, it is a lot more complex than that and a battery will discharge regardless of whether it’s connected to something or not.

For example: 1) Modern batteries have ‘polarity reversal’ protection which stops them going flat in normal circumstances. 2) In a modern boat or RV, when you disconnect the negative cable with the engine off, it will still go flat after a minute or two.

This is because in these systems there is huge resistance to electrical current flow around the system in normal circumstances, and when you pull the negative cable out of its socket it simply cause a large, brief flow of electrical current which is enough to flatten the battery in a few minutes.

The best advice when you leave your boat or RV in storage for any period of time it always disconnect the negative cable and place it in an area where it cannot come into contact with conductive materials (like metal).

This will ensure that if there is a short, the battery will not be damaged or destroyed by being flattened.

If you are unsure about how to do this, get an auto electrician to fit a switch in the negative line so that you can easily switch it off when required.

Do i need to disconnect both battery terminals

As a general guide, you should always disconnect both terminals when leaving your boat or RV in storage for any length of time. But this depends on a number of factors:

1) The condition of the battery and its performance history varies depending on usage, charging, maintenance etc.

2) Running a trickle charger will overcharge a battery unless the voltage is cut-off at some point. This cut-off voltage is normally either a specific time e.g. 16 hours, or voltage level e.g. 13.8 volts (which shoud be set to match your battery manufacturer’s recommendations).

In most cases it would be recommended that the negative terminal of your boat or RV is disconnected but it’s worth checking with your battery’s manufacturer’s recommendation before you disconnect anything.

This is because some batteries (particularly the small ‘marine’ type) are actually very good at performing even when they are seriously discharged and will not need any further charging or maintenance.

How long will a boat/ RV battery last if disconnected

If your boat’s or RV’s electrical system is working properly it will last for years even if you leave the battery connected.

If the battery has not been used for a long time, then it may go flat even with the negative terminal disconnected – depending on how much electrical current was flowing through it before you pulled the negative terminal out of its socket.

If possible, it is worth starting your engine every couple of weeks to recharge the battery. This will ensure that you don’t end up with a flat battery when you finally go boating or RVing again.

How do I charge my boat/ RV  battery without connecting it to an electrical circuit?

To safely and effectively charge your boat/ RV battery you need to use a battery charger.

There are three types of battery chargers: –

1) Trickle Charger with built in voltage cut-off – This is the most popular type of trickle charger and will safely overcharge your battery if the voltage level is not set correctly. But… it’s important that you don’t leave them connected for too long – read the manufacturers instructions.

2) Trickle Charger without voltage cut-off (ie constant current/voltage charger) – These types of chargers are rarely used in homes, boats and RVs because they could overcharge your battery or damage it if left connected permanently.

3) Smart charger – This type of battery charger is actually a microcomputer with a display and several charging circuits inside. It will automatically switch from ‘trickle charge’ mode to ‘float charge’ mode when your battery reaches full capacity. In most cases these types of chargers are too expensive for use on small boats and RVs.

How do I connect a trickle charger to my boat/ RV battery?

A. Trickle chargers are connected directly to the positive and negative terminals of your boat or RV’s battery. However, some smart battery chargers have built-in connectors which you plug into your boat or RV’s battery clamps – these types of chargers are usually easier to use.

What is ‘Trickle charge’ mode?

If your trickle charger has a built in voltage cut-off, it will automatically switch from ‘trickle charge’ to ‘float charge’ when your boat or RV battery reaches full capacity. This means that the charger will deliver enough electricity to keep your battery fully charged, but not too much to damage it.

Clever Tips For When leaving car battery disconnected overnight

When leaving your car unused for a long period of time, it’s worth disconnecting the battery.  This will ensure that if you leave some lights on or some other electrical component drawing power from your battery then you don’t risk draining and damaging it and finding your car stuck in the driveway!

Remove all cell phone chargers – Most phones these days use only a small amount of power when they are charging. But, if you forget to remove them after using them… well… you know what happens next

Remove any electronic devices like GPS, CD players etc – These also use very little power while turned off but can drain your battery’s power if left in for too long!

Leave the car keys out of the ignition – If you take these with you, then your car’s electronic security system will be activated.

Take the radio code with you – If you leave your radio on for too long, it may drain your battery so much that it can’t unlock itself to let you turn off the radio.

If i disconnect my car battery will it stay charged?

If you choose to disconnect your boat or car battery it will slowly discharge over time.  This is especially true in very cold conditions where the chemical reactions inside your battery slow down.  It’s worth adding that leaving your battery disconnected for too long could damage it – so don’t leave it disconnected for more than a couple of months

Will my radio code be erased if I disconnect my car battery?

Some security models will erase the radio code if you disconnect your car battery – make sure that the radio code is stored on a sticker somewhere on your dashboard.  If you can’t find it, try calling the manufacturer for more information on how to retrieve the code.

How long can I leave my boat/RV battery disconnected?

In most cases, you should always try to re-connect your boat/RV’s battery as soon as it’s safe. Leaving a boat or RV battery disconnected for too long could cause damage to the battery or worse – leave you stuck in an inconvenient location!

cables from your automotive or ride-on lawnmower.

why disconnect negative battery terminal when working on car?

When working on your car, you can disconnect the ‘negative’ terminal first. This is because if you accidentally connect a power source to your car’s electrical system, it won’t have an easy path to follow back out of your vehicle – so there’s less risk of causing damage. It may also be easier for newer individuals to remember which terminal this is!

If disconnecting the negative battery terminal to check the alternator, must the positive terminal also be disconnected?

If you are checking your car’s alternator, it is important to disconnect both terminals. This is because some large electrical components like your battery will show resistance to current flow if they are attached to an active circuit (such as one that uses a lot of power).  By removing both terminals, you will ensure that there is no potential difference between them and minimize the risk of damage to your car.

Can you leave a battery disconnected without hurting it?

As a general guide, leaving the battery disconnected for too long can cause serious damage.  Basically, if it isn’t attached to anything, then it’s like an anchor trying to hold onto the ocean floor.  Over time, this can cause irreparable damage to your car or boat’s electrical system!

Was this article helpful?
Dislike 0
Views: 199