cmat

Alex....Thanks for the good explanation and actual measurements. I don't know my current draw but I don't have any aftermarket accessories either so I imagine it is similar to yours. My experience is that after a couple of weeks, there was a noticeable slowdown of my starter motor. My battery is a couple of years old and it is possible that it was not getting fully charged during my usual 30 minute or so warm-up drive. I am using a trickle charger now to be safe.

vincer77

Here is another datapoint. I have a 2 month old Optima in my car. I let it sit for approx 3 weeks and when I went to start it, it cranked very slowly, but amazingly started immediately. I am running equivalent of 7W45 oil, and have not gotten a battery tender yet. It has been very cold here in middle Tennessee.

It did not occur to me that this would be a problem as I started my Honda which sat almost as long, without incident.

Vince

IXLR8

Hi Chuck,

There may be a couple of reasons for that.

A battery is a voltage source and with resistance (called the internal resistance of the battery). Normally, this is extremely low...in the less than 10 milliohm range (<0.010 ohm). On a battery that hasn't been looked after or charged properly, both the capacity decreases and the internal resistance increases; both working against you.

The only thing you can do is replace the battery, make sure your charging system is at peak operating condition and make sure your battery is always fully charged during winter lay-away. A partially charged or drained battery is what causes sulfation, the #1 cause of battery failure and replacement.

It is imperative that the connections at the battery and at the other ends where the cables terminate; the frame for the ground cable and the starter for the positive cable, are clean and offer practically zero resistance. The one at the starter, being exposed to the elements, is usually suspect. I clean them with a Scot-brite pad and then coat them with a silicone grease.

BTW, if you disconnect the positive cable at the starter, disconnect the negative cable at the battery or you'll instandly become an electric welder by trade if it shorts to ground.

Unless you are stuck in bumper to bumper traffic or driving very slowly with lots of loads on, a 30 minute drive should be plenty.


I've just finished putting together an Excel spreadsheet of available batteries for the 993 (group 48 and 91 batteries). I'm checking with the manufacturers as to what chemistries they use.

I always get at least 10 years out of a car battery which has to start down to -30°C on some days; even my seasonal motorcycle batteries last 9 years. One even lasted 16 years and I believe it is still going in the car that we sold. The 993 battery...well it came with the car when I bought it...I had not control over its past maintenance.

IXLR8

Hi Vince,

The Optima according to their spec sheets needs a higher charge cut-off voltage. Most car voltage regulators cut-off at 14.4V which is the magic number for flooded lead acid batteries (FLA).

I take it you have a Red Top Automotive Starting battery?

Optima Red Top Specs


The problem with the Porsche is the long positive cable going to the back of the car and possible slight corrosion on the starter lug. If Porsche picked the right gauge, the voltage drop should be minimal. Of course, its a function of current draw and the colder it gets, the more your starter is going to draw.

The Honda battery to starter cable is only about 3' long.

vincer77

Good explanation! Did not think of the long battery cable - makes sense.

I have a Yellow Top. I looked at the specs and only saw an alternator spec of 13.65 to 15.0 V for the Yellow and 13.3 to 15.0 V for the Red. Is there another spec listing the 14.4V? or is the 15V upper limit what you are referring to?

Cheers,
Vince

IXLR8

I haven't check the gauge of the cable in my 993, but somehow I doubt Porsche would cheap out. Its probably OK, but the connection at the starter would be more of a concern, especially if it hasn't been looked at in ages.

Optima is providing a range, but the lower end will most likely shorten battery life.

With the engine running and up at say 2000 RPM, measure the voltage across the battery.

This is when it is nice to have a voltmeter on board while driving. I am installing one like the one I have on my BMW motorcycle. It'll be mounted above the center console and held in place with Velcro to the carpet.

The 14.4V I mentioned refers to the commonly accepted final charge voltage of a flooded lead acid (FLA) battery (ie: 2.4V per cell x 6 cells). FLA batteries tend to gas above that value. The alternator in the old Toyota Tercel cut off at exactly that voltage. Maybe that is why the original battery was 16 years old when we sold the car and I've heard it is still starting it, three years later. Incredible at these temperatures. The OEM battery is made by Panasonic.

TrackDays247.com

Pull the battery and give it a warm(er) home -

IXLR8

Hi Chuck,

I just noticed that the current draw measurement that I took and posted in post No. 30 of 22 mA was with the immobilizer disabled. That means that the flashing LEDs on our door trim panels and immobilizer circuit were not draining any current.

Trying to measure a blip of current (as the LEDs flash) with a digital meter is difficult without peak hold. Plus the LEDs are on a short duty cycle.

I would estimate the overall standby drain current to be well under 50 mA though. This is still too much and if the battery isn't on a battery maintainer, little wonder they discharge so quickly during storage.

Warpig

Most led's with 100% duty cycle run about 10-20 ma. each. With the low flashing duty cycle they're at some math would have to be done to tell you how that averages down to ma/hours. Figure out the on/vs off ratio. etc. it's a very small number of ma/hours.

Nick

IXLR8

Well they are quite bright and old tech LEDs, so lets say 20 mA each x 2 = 40 mA.

At about 10% (maybe more) duty cycle; 4 mA plus the drain of the immobilizer (usually 15 to 20 mA for most alarm systems of past).

And what are we up to: 22 + 4 + 20....under 50 mA. (Maybe my original 22 mA measurement includes the immobilizer in standby mode...I don't know what all is ON)

Even with the immobilizer de-actived, one needs a battery maintainer.

Rinty

Alex:

Perhaps when you're in there sometime, could you get a voltage reading at the starter, so we know what the drop from the battery is?

TIA.

IXLR8

Since the load varies as the engine turns over, the voltage varies as well making it difficult to take a reading with your average digital meter.

I'm going to have to invest in a fancier DMM or maybe even a scope.

I'd like to put a shunt inline and measure the current...again, I need a scope for that.

rsr91128

I have one of these from a previous life many years ago, don't laugh at the manufacturer.

http://www.manddsmallengine.com/briggs/tools/19468.html

It works good and will allow any digital VOM to read high current draw. The highest I remember reading personally was around 200 amps on a tractor starter, with no damage to my DMM. You leave it set in DC volts (reading millivolts if you want to adjust range).

If you want to borrow it sometime let me know and I'll mail it to you if you are in North America now. Or I can check amp draw on startup on my car but it won't be for a few weeks, parts removed and if I can't drive it I ain't startin' it.

Let me know,

vincer77

Just thought I would add my recent experience for all you newbie 993 owners. After a drive about two weeks ago, I went to start the car last Thursday to take it for a spin before the big storm here in Nashville. Had a pretty "spirited" drive with our local RL members so I figured I had a pretty good charge on the battery. Well, I got in and turned the key and didn't get even a click! I looked around, and the lights were all out - not dim, out.

Turns out that I had left the trunk ajar for about a week and a half and did not know that there was a trunk light. Turns out that the trunk light and the light above the ignition are on the same circuit and are switched by the trunk-latch mechanism. There are a couple good threads on this - usually started by a newbie. The lights apparently draw enough current to drain the battery in a few days.

Anyway, hope this helps someone.

Cheers,

Vince

IXLR8

Thanks Erik, but I should invest in one of my own. I also have access to shunts of various amps that I might be able to borrow. I'd still need a better meter or a scope.

By the way, how did you manage to measure 200 mV (200 A)? Does your meter have a peak hold feature?