Why Does My Car Battery Drain Down Over Night
Reader Question
My wife drives a 1991 Honda Accord. Great little car. Still runs great. It has one serious problem which it has had for several years now.
The battery rapidly drains when the car sits for a couple of days. I replaced the alternator and battery each more than once to no avail. I used an amp meter to check drains on various cables while removing fuses and relays, one at a time.
I discovered a drain going to the alternator on a cable that is supposed to provide current to the battery when the alternator is spinning.
I figured the alternator diodes were going bad. Why they kept blowing on new alternators I couldn’t figure out.
I bought a couple of high-amperage, high-voltage diodes and put them in-line on the alternator cable.
The idea was to replace the functionality of the bad alternator diodes and prevent current from draining back through the alternator.
It didn’t help but it didn’t hurt either. They’re still in-line and the car runs fine. The only thing I could find that would stop the battery from completely draining was a device called a “battery brain”. Battery Brain.com It monitors battery voltage and disconnects the battery before it gets too low. There is a key-chain wireless module with a button you press to connect the battery when you want to star the car. Works pretty well.
I noticed one other thing about the car. When I’ve been in the garage I’ve heard a buzzing noise from the Honda’s engine compartment. Checking under the hood indicated the noise was coming from the fuse box.
Opening that, I discovered it was coming from a relay which controls the radiator fans. I think I may have replaced that relay once before but am not sure.
Any thoughts?
Hi there Gregg,
You are on the right track, just keep investigating. I am not sure what you see as a drain going to the alternator, or if that is really a drain at all, but the buzzing noise under the hood is definitely going to be a drain on the electrical system and something you should keep investigating.
An electrical “drain” can be anything that is drawing power (draining) from the battery with the engine/key OFF. Every vehicle will have some amount of electrical drain which is normal. Radio and clock memory and engine computer memory will drain a small amount of battery power with the key off, and that is normal.
Again I am not real sure what the buzzing noise could be coming from, but any electrical noise/buzz/vibration etc. should not be there, and is probably the source of your problem.
If you had told me the battery drains down while the engine is running, I would suspect a problem with the battery or the alternator, but I think you have already ruled them out.
If you get stuck trying to locate the drain I would take the vehicle to the dealership or a qualified electrical repair shop in your area and at least pay them to track down the source of the drain for you. You can always perform the repair yourself to save some money once you know what it is.
Tracking down an electrical drain can sometimes take hours! Make sure the shop you choose to help you has a qualified electrical mechanic so you don’t needlessly pay for inaccurate diagnosis time.
Some causes for an electrical drain
1. Trunk, hood, or interior light or glove box light that is staying on when the engine is OFF
2. Aftermarket radios, CD players, amplifiers, cell phone chargers etc. etc. are notorious for drains
3. Electric seat motors and electric antennae motors that are stuck in the ON position
4. Relays or components under the hood that are ON or running with the engine OFF – for instance electric cooling fan motors, anti lock brake pump motors, air ride suspension pump motors.
Thanks for the link to the batterybrain.com, I was unaware of it until now.
Blessings,
Austin Davis
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