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Sometimes you need a quick and effective way to stop the car battery from draining overnight if you are in a rush and perhaps going away for a few days.
Is disconnecting the negative battery cable effective in stopping the battery from draining? Is it safe?
Read on to learn whether disconnecting the negative cable is effective, if it is a good idea, its disadvantages, and what alternatives there are.
Quick answer: While you can disconnect the negative battery cable to stop the battery from draining flat from parasitic accessories, it is not a convenient way because of the time and dangers associated with connecting or disconnecting the cable.
It is best used as a temporary fix while you troubleshoot the underlying reason why there is excessive drain on the car battery.
Note that even with the battery cable(s) disconnected the battery power reduces albeit slowly from self-discharge.
Should You Disconnect the Negative Battery Terminal?
Whereas disconnecting the negative battery cable from the battery stops any draw of current from the battery, it is not a convenient way to stop the drain.
.Disconnecting the battery cable safely takes time (up to 3 minutes).
You need to have the right tools and be comfortable working with cars too.
To disconnect the battery cable, you need to open the hood, use a wrench to disconnect the negative battery terminal from the battery post, tape it with electrical tape.
To reconnect the cable, open the hood, remove the tape and reconnect the battery cable to the battery post.
Read on for the disadvantages and dangers involved in disconnecting the battery cables.
Battery Power Savings when the Battery Cable is Disconnected
Disconnecting the battery negative terminal can save an estimated 2.6% or 0.72Ah of a car’s battery capacity over a 24 hour period.
This is based on the assumption that the current drawn by the car’s electronics- clock, alarm, e.t.c is about 30mA and the period is 24 hours.
The onboard electronics consume 0.3mA x 24 hours or 0.72Ah which is about 2.6% of the usable capacity of 55Ah car battery.
How long a Battery Lasts with the Negative or Positive Cable is Disconnected?
As an example, it can take about 38 days to drain a 55Ah battery to 50% of its usable capacity.
The actual time really depends on many variables including the actual battery drain (mA) by the car’s electrical system when the ignition is off, the capacity of the battery, (Ah) and whether the battery is in good condition or its aging and has lost some its capacity to hold charge.
The battery will last a shorter time if the drain by the onboard accessories on the car is higher, the battery capacity is smaller (Ah) or the battery has lost some of its usable capacity.
To check how much usable capacity a battery has, have it tested at an auto repair shop on load.
You can also conduct a quick test of available capacity by charging the battery fully and measure how long the head lamps can light before dimming.
Depending on the reserve capacity of the battery, the runtime can be anywhere from 60 – 90 minutes.
If it is significantly lower say 30-45 minutes then there is a high chance that the battery has lost some of its capacity.
You may also be interested in this post: 5 signs that your car’s battery terminals are loose and how to fix them
The Disadvantage of Disconnecting the Negative or Positive Battery Cable to Stop Battery drain
#1. You should only do this if you consider yourself knowledgeable and competent to work on the car’s charging system
#2. The car battery charging may become unreliable if the jumper cables are not firmly fixed
#3. Removing and reconnecting battery cables can Increase the likelihood of accidents involving short-circuits if not done with care.
Does Disconnecting the Positive Battery Cable Stop Battery Drain?
Removing the positive battery cable can also stop battery drain however it increases the dangers of short circuits.
Should the loose positive battery cable touch the chassis when the battery ground is still connected, there’ll be a large spark and a short-circuit!
For this reason, if you must disconnect a battery cable, disconnect the negative battery cable.
#1. Can a Loose Negative Battery Cable Stop Battery Drain?
A loosely connected negative battery cable can stop the battery from being drained by the onboard accessories if it is not making contact with the battery post.
If it makes partial contact, the draw will be reduced compared to when it is making solid contact.
What are the alternative ways to stop battery drain?
# 1. Disconnecting the battery cable should only be a temporary solution. Aim to identify the fault if there is an unusual battery drain.
You can find more information on: 6 ways Why your Battery Power is being Drained Flat and how to fix them
#2. Use a battery tender to guard against normal battery drain by the car’s electrical accessories such as the clock, car alarm and others. Leave it connected to keep the car battery topped up when the car is parked for a long time.
Disconnecting the negative battery cable from the battery’s post stops the drain on the battery by the car’s electrical accessories.
The battery will still gradually lose its capacity albeit slowly from self-discharge.
Disconnecting the negative terminal should only be a temporary measure as it increases the likelihood of electrical short-circuits in the car if not done carefully.
It is better to invest in identifying and fixing what is causing excessive battery draw or buying a battery tender to keep your car battery topped up when parked for an extended period.