Charging System - Does this sound right?

[supernovatx]

Background: 1976 911 with '83 3.0L. Bout 9 months ago the car wouldn't start. Replaced battery everything is well for 8 months. Then suddenly it won't hold a charge. Headlights dim slightly so I pulled the alt and took it to a reputable shop. I was surprised when the said it tested good but then again it had been replaced about 2 years ago. So I pulled off the ground straps, cleaned and reinstalled. Problem continues.

Unable to think of what else it might be I took the battery out and had it tested. They said it was fine but low on charge. Wanting a clean slate I bought a new Interstate battery of recommended size. I put it in this morning and the damn thing wouldn't start.

Put the trickle charger on the new battery. According to the trickle charger it was on bout 70% charge on it. Not having a multimeter I went out and bought a $40 model at O'Rielly Auto. I haven't used one in years but I think I'm working it right.

Do these numbers sound right :

Charged battery: 12.90V 0.13mA

Battery + connected with - disconnected. Attach multimeter to positive terminal, neg to battery ground and I'm getting -00.1V is this to much loss?

Engine started I get 14V at idle and 3000rpm
Engine started with headlights on 13.75V @ 3000rpm, 13.30V at idle
Engine started with headlights and A/C 13.60 @ 3000rpm and 12.64 at idle.

It seems to crank well but I'm not enjoying the thought of a dead battery again....suggestions and thoughts appreciated.

[ClickClickBoom]

Numbers sound about right. You neglect to say how often the car is driven. Batteries have a self discharge feature, nature of the beast, add some constant discharge features like clock, misc other items and you have a battery killer. Get a battery "maintainer" they monitor the battery and when it draws down to a specific voltage it trickles it back up, it is different than a trickle charger. I have all my vehicle batteries on maintainers, with the exception of, oddly enough, the Porsche, which is the daily driver. My last motorcycle battery lasted 10 years.
eric

[supernovatx]

The car is driven on a 140 mile trip every 5th day at highway speeds with only about 3 stop signs with no real waiting along the way. Thanks.

[HawgRyder]

Everything about batteries is figured in Watts....or Volts X Amps.
When you start a car...you use a certain amount of Watts (amps @ battery voltage).
If we say that is takes 1200 Watts to start....you will need to put that back into the battery to bring it back up to where it was before you started the car.
So.....1200 Watts = 12 Volts at 100 Amps.
If your alt kicks out say 55 Amps per hour, it will take almost 2 hours to recharge the battery (that's 2 hours of driving above idle speeds).
When you run the numbers, you will see that in most cases, the alt will not return the battery to full in normal day to day driving (unless you have a long commute).
These are all approximate numbers....but you will get the idea.
A battery tender or trickle charger is the answer....to have the battery up to snuff every morning...and give you a good sense of comfort about the car during the day.
BTW....in cold climates (like up here in Canada) on really cold morning....we might use 5 times the normal Amps to start the car!!
Good luck
Bob

[supernovatx]

Thank you for the replies. So two more questions. Not to beat a dead horse but:

Battery + connected with - disconnected. Attach multimeter to positive terminal, neg to battery ground and I'm getting -00.1V is this to much loss?

and who makes the best battery maintainer, is a solar option available?

[HawgRyder]

If I understand the test correctly...you are measuring the back flow in the diodes of the alt.
Sounds about right....diodes are low resistance one way....high resistance in reverse.
Alts are normally a 3 phase unit....meaning...you have 3 sets of diodes installed in 3 pairs (wired in series)...with the output connected to the joint in each pair.
This gives you a full wave rectifier network on each phase.
This also explains why sometimes you get a "whirring" noise from an alt...(heard in the radio)...one of the diodes may be leaking a bit.
And....a Solar charger might be OK....but would require the car to sit in full sun all day to charge....not the best for the paint job...LOL.
Bob

[curiousone940]

If you still have the original battery cables on the car, I would check the resistance on those cables to eliminate that as a possibility of the battery failure. I do agree that on a seldom used car that a trickle battery charger would be a good idea.

[lucittm]

You need to check for battery drain. Disconnect the negative cable from the battery. Put your meter on the 10 AMP current scale and make the connection where the battery ground strap was. In other words place the positive lead on the chassis ground and the negative lead on the negative battery terminal. If you get no reading, scale down to 1 AMP or 100mA. You should see some current flow. Don't go too low or you will fry your meter. You should have around 50mA of current drain. More than that and your battery will drain itself over a short period of time.

Report back with your findings. Then you can start pulling fuses and relays until the high current draw goes away.

Mark

[ischmitz]

Originally Posted by supernovatx

Battery + connected with - disconnected. Attach multimeter to positive terminal, neg to battery ground and I'm getting -00.1V is this to much loss?

no idea what you are measuring here and what is connected and disconnected. 0.1V is NOTHING - so you are obviously not measuring with any load. In general the loss needs to be measured while you draw a huge load (cranking). Anything else is pretty much meaningless.

Your voltage numbers in your first post sound O.K. and it seems you battery is charging - or better said; it was charging at the time you took these measurements. I have not seen any clear evidence that your battery was empty. A 70% charge is pretty normal for a car battery. And it should start a car just fine.

What exactly is the nature of the "car won't start"? Does it crank over or not? Does the cranking sound slower than normal when it happens? The reason I am asking this is that you could have an issue (maybe intermittent) with your starter or starter relay (the big one attached to the starter motor)

To reallly get a good idea you need to measure the voltage at the battery while cranking. This will tell you if the battery can supply the current demanded by the starter. If the voltage goes down quite a bit it points out a bad battery (high internal resistance).

Next check the voltage at the starter motor while cranking. And if the loss is more than a couple of volts compared to the measurement at the battery you either have a bad relay or bad cabling.

Then you can measure the voltages across the relay while cranking to find out if it is the cause for the drop. In a healthy system the battery voltage shouldn't drop much below 11 volts while cranking and the loss from the battery to the starter motor shouldn't be more that 0.5V.

Ingo