Battery Discussion

See also our article on
Fault Finding the VW Electrical Charging System.


This article includes discussion of -


Battery Strength

The lead products inside a battery get soft as the battery goes flat (chemical change on the positive (+) side of the plates), and can 'slump' off the plates (which are shaped like a waffle to hold the lead in place). So a car battery (designed for very high starter currents for short periods) needs a high number of thin plates (more plates means more cold cranking amps (CCA), but will suffer if allowed to remain flat for any length of time. Car batteries need to be kept charged to near 100% to get their maximum life.


Battery Drain

Someone wrote with a mystery - The car's been sitting for about two weeks without being driven, and the battery's gone flat again. The "Oil" light barely comes on; the "Alt" light not at all. The interior light was set to be on when the door is open; perhaps the door isn't closing all the way, I don't know. The light came on very dimly, but the ignition wouldn't turn over the starter motor at all.

Not being an electrician, a dumb question: Is it possible to run through the system with a VOM somehow to detect shorts? Although I think if there were a direct short the battery would run down a lot faster, like within just a few hours (or less). I've gone 3-4 days at a time between outings the last little while with no problem, but each time the car has sat idle for on the order of two weeks or more the battery as turned up flat.

"Speedy Jim" responded - There is a relatively easy test for electrical drainage from a battery -

  1. Take the positive cable off the battery.
  2. Set the VOM meter on the DC volts scale (highest to start with).
  3. Connect the meter negative probe to the cable you took off of the battery.
  4. Connect the meter positive probe to the positive (+) battery post.
  5. If set to DC Volts, the meter will respond to the slightest drain. If it shows a drain, switch to DC milliamps. Any reading over about 50 milliamps is suspect.
  6. Note - If you have a modern radio with a memory (remembers channels and settings etc.) then you WILL have a small power drain (some radios are hungrier than others).

  7. If you do have a drain then you can remove fuses one at a time to find which circuit has the problem.

Thanks to "Speedy Jim" for the foregoing. He's a great source for VW electrical information.

Further, from Rob - Make sure that you first test the door switches. If they prove to be okay, remove the bulb from the interior light so you can test the other circuits without having to continually close that door to disable the interior light.

If it's taking up to two weeks or so to drain the battery the current will not be large (maybe as little as 200 milliamps (0.2 amps) or so). So if it's a light bulb that's shorting out it may not even be enough to make it glow.

The headlight switch on VWs is used as a junction box for the main power supply. You could try pulling the thick red wires off the light switch one at a time (there should be three). The one tracking in from the left side of the car is the one from the alternator (comes through the floor of the luggage area just under the left side hood hinge as part of the wiring loom there). If removing that fixes it, the problem has to be in the alternator (that red wire brings all power to the front of the car - and then back to the engine ignition and rear lights).

Still draining? Then put the first red wire back on remove the red wire which disappears down through a hole in the luggage area floor above the ignition switch - this one takes power to most of the operating circuits in the car which work with the ignition on (lights, ignition, wipers, backup lights etc).

Still draining? Then put the second wire back and try the third wire, which leads off the switch to the fuse block. This supplies power to the non ignition switch circuits like the horn, interior lights and maybe radio. If the radio is an after market job it may have it's own in-wire fuse behind the radio - pull that and check for current drain too.

Don't forget to pull the separate fuses for backup lights and rear window demister too. This might help pin down the area if nothing else.

Then remove one fuse at a time on the main fuse block and test check the multimeter - this might narrow it down further. If not, remove each light in the instrument cluster one at a time and repeat the test (the current drain might be through a light but not enough to make it glow).

Someone wrote - The battery has sat unused for almost eight months while the restoration work was going on! Am I in trouble? I'll get a charger on it right away!

Rob responded - If it was a new battery it might be okay. But the plates get coated with sulphate if they are not used, so an overnight charge will do it a world of good. The weather for many of those eight months should help - batteries last longer in cool weather - heat is bad for them. That's why many modern cars have heat shields around the sides of batteries in the engine bay - trying to keep the heat from slowly cooking the battery. VW batteries generally last a long time because they are NOT in the engine bay.

Someone wrote - In my experience my batteries have always given up the ghost in the winter, not in the summer.

Rob responded - Yes - that's when the oil is thicker so current draw is higher to turn the engine over. And although the battery lasts longer in cooler weather, it's performance drops off as the temp drops, which is why they are rated at COLD cranking amps (CCA), rather than warm cranking amps.

The Beetle starter for example, needs about 70 amps when it's spinning the engine, but to GET the engine spinning, the initial current requirement is about 2.5 - 3 times that, which is why a 220 CCA rating is the minimum for a VW - still quite a low figure as it's a comparatively easy engine to start (low compression for one thing).

Interesting thing about electric motors, the "stalled" current can be huge compared to the running current.

Someone wrote - I left my lights on in the Beetle (it was dark when I left home but light when I parked the car), and so I had a flat battery. There wasn't even enough juice to get the warning lights in the instrument cluster to flicker. I had to wait for the Auto Club for nearly an hour for a 20-second job to get the engine started. If I'd had any juice at all I could have clutch-started it easily, since a generator only need 3-4 volts to start self-exciting and generate current, whereas alternators need almost 12 volts and so don't help with a near flat battery).

Someone else wrote - My morning with the Bug was a little disappointing. It started the previous evening when I went out to back the car into the garage for work the next morning, only to find that the battery was dead! I think one of the doors didnít get closed properly and the dome light was left on.

Rob responded - Annoying for sure. Though the dome light (usually 10w or less) SHOULD run on battery power for 40-50 hours before completely flattening a good battery.

Someone wrote - I got in the Bug this morning and discovered that the battery was (and still is!) dead AGAIN! I'm beginning to think I've got a short somewhere that's draining the battery, even when the car is just sitting at the curb. I vaguely recall a test for such a thing -- do you remember? But -- even when you've verified that you have a short, then you still have to FIND it! :-/

Rob responded - Not too hard - you have a multimeter with an amps setting? Just set it to the highest amps setting and remove the positive battery lead. Place the meter between the battery and the lead, so any current flows through the meter (if the needle moves backwards reverse the meter leads). If it just barely moves turn the meter to a lower amp setting (more sensitive). Once you have an obvious current reading (with the ignition off and the interior bulb removed so the readings are not "contaminated" start pulling fuses until you see a change in the meter (might even need to have TWO fuses out if there are two circuits using power). Once you have the offending fuse out, you can work out what's on that circuit and maybe find the culprit. The radio is sure to be part of it, especially if it has a "station memory", but usually that current it tiny and should take a long time to flatten a battery. Hopefully you'll find something pulling a bit more power and that will give you a better clue. 500 milliamps (0.5 amps) would flatten an unused battery in about 100 hours. 50 milliamps would take more like 1000 hours (40+ days).

Dave wrote - There's got to be a reason why this battery keeps going flat. It's not that old! As a matter of fact, it looks brand new -- very pretty there under the back seat, which I cleaned, painted and insulated! Also untangled the wire "spaghetti" and tied things up neatly with cable ties.

I'll get the charger on it right away, then once the battery is charged I'll check for an extraneous electrical draw.

Rob responded - Don't forget that car batteries don't like to be flat for too long - it causes sulfation and shortens the life of the battery. And since you did get a quality battery it would be a shame to lose it.

Generators are more tolerant of the sick/flat battery. Generators self-excite up to full voltage very easily, even with just a few battery volts (dead battery); but with an alternator, a successful push start is less likely since it may not self-excite to start charging. It will still jump start just fine (because it will then "see" 12v from the other battery so will self-excite just fine).


Battery Charging

Dave wrote to ask - I'm going to install the Capacitive Discharge Ignition (CDI) system this weekend, and before I try to start it the first time I'll put the charger on overnight. Should I disconnect the battery cables first? Seems like I recall that VW batteries must be charged with the cables off.

"Speedy Jim" responded - There was some concern about charging batteries when there is an alternator connected or on fuel injection systems. Take the cables off to be safe.

Follow on from Dave - I've had the battery on the trickle charger all day (with the cables removed, on the advice of "Speedy Jim"). It was only putting out nine volts when I checked it last night. I took the charger off a few minutes ago, and it was at about 12.5 volts on my meter.

Rob responded - I guess it's an additional safety thing -- removing the cables, but I've never bothered, and never had any trouble. But I HAVE always left the car door open so any hydrogen can vent out easily. That also means turning off the interior light, just so there's no possible ignition source.

Nine volts is a VERY flat battery. You'd be lucky to get any cranking at that, and certainly not enough volts left to fire the plugs.

An unused battery should be recharged every month or so -- 4-5 hours (or overnight) with a 4 amp charger is fine. If they are left for too long the plates sulphate and the battery behaves like it's old.


Rust Associated with the Battery

Dave asked - Already I'm noticing rust forming under the battery on the new tray I put in there especially designed to hold the battery. I think it's just surface rust - the tray is coated with a very thick coat of plastic-like material. Mid-America Motor Works and others sell a battery mat that is designed to absorb and neutralize acid -- impregnated with something like baking soda (sodium bicarbonate). Does this sound like a good idea to you?

Rob responded - It certainly wouldn't hurt. I haven't heard of these mats (but then I haven't been looking either).


Future Batteries

A comment regarding future batteries from Rob - I was reading some interesting stuff about car batteries the other day. Within five years cars will switch to using 36-volt batteries and 42-volt chargers. This will enable thinner wiring (cheaper cars), and will allow manufacturers to move the battery to the trunk for better life and not suffer voltage drop to the starter motor. Aparently it's better for electronics to operate at these voltages too, I guess the loss of a volt has little effect on 36 volts, but quite an effect on 12 volts.

I find it interesting that they are going to 36 volts instead of 24 volts, which the trucking industry has already established.


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