checking '73 battery - slow turning

billwagnon · 08-29-2002, 05:24 AM

My car still starts ok, but cranking seems a little slower than it should be. I've got the original two battery set up with MSD ignition and Webers.

What equipment do I need, and what's the process of checking the batteries?

911pcars · 08-29-2002, 08:31 AM

You'll need a multimeter and a hydrometer as minimal equipment.

Connect the meter to the battery (doesn't matter which one). Check the open circuit voltage; should be around 12.8V or better. Start engine and rev to about 2000 rpm. Meter should read higher, maybe 13.5V. Turn on high beams and other elec. accessories. Charging voltage should be at least this amount. If not, suspect the charging system or the integrity of the elec. system (loose, corroded connections).

If the charging voltage is okay, it might be the condition of the batteries. Put a trickle charger on it overnight to fully charge them. Turn on lights for about 20 seconds to bleed off any surface charge, then check one of two ways:

1. Check specific gravity of each battery cell (six/battery) with the hydrometer. Should be 1.220 or better on all and even.

2. Disable engine so it won't start (i.e. disconnect the coil wire from the dist. and ground it). Crank the engine for 20 seconds and observe the voltmeter. It should stay above 9 volts. If it doesn't, the batteries might be getting old.

This is important. Disconnect all the battery cables and clean the connections until they're bright and shiny. Do the same with the ground connection to the chassis. There is a ground cable underneatth (trans. mount to body) that should be checked as well.

You could also have what is called "Parasitic Current Drain" that is constantly draining the battery when ignition is OFF. This could be caused by a circuit constantly ON like a glove box light or a short circuit in a system wire (anywhere). It doesn't take much, especially if the car sits for a few days.

There could also be some other voltage drop situation somewhere in the system, but try this first.

Sherwood Lee
http://members.rennlist.org/911pcars

pwd72s · 08-29-2002, 09:03 AM

Great answer Sherwood, thanks! Of course, one could eliminate the hydrometer if one ran totally sealed batteries.

billwagnon · 08-29-2002, 10:27 AM

Thanks - I'm hoping it's not parasitic. It does seem slower after the car sits a couple days.

I'll let you know what I discover!

Early_S_Man · 08-29-2002, 11:21 AM

Bill,

Sherwood's tips are all good, but I suggest 'rotating' the batteries from left to right every six months, assuming they are still good. This also has the benefit of giving you the opportunity to clean battery connectors every six months!

The reason for my suggestion is that I discovered long ago that the left battery in a dual-battery car is 'used' and 'exercised' more than the right battery! How did I make that determination, you ask? Because, even if fully charged to the same Voltage and specific gravity ... the left battery electrolyte level was always slightly lower than the right ... ALWAYS ... after a month of use!

The reason for the disparity is the one extra connection in the right battery's positive lead in the second 'slot' of the left battery positive connector where the heavy lead is 'multipled' over to the second battery!

If both battery positive cables are routed to a 'common' junction point ... the disparity in loading the batteries during starting is eliminated! I tried this with a short jumper from the left battery and a heavy-duty insulated welding cable connector ... shortly before my last 'set' of twin batteries died.

911pcars · 08-29-2002, 12:48 PM

Warren,
This is an interesting observation. You are implying the two batteries are not wired in parallel and that cranking is provided only by the driver's side battery?

Sherwood Lee
http://members.rennlist.org/911pcars

Early_S_Man · 08-29-2002, 01:00 PM

Sherwood,

No, they are wired in parallel, but ... there is an additional connection between the right battery and the main cable to the starter solenoid! The additional resistance in the second 'slot' connection is finite, and significant when the drain is 200 Amps or so! For 'normal-sized' drains on the battery with engine off ... lights, radio, etc. ... I'm sure the aqdditional resistance is immaterial, but for starting, it makes a difference in the loading of the two batteries.

When I tried swapping batteries from side-to-side a couple of times ... the electrolyte level always was lower on the left battery after a month or so ... so it wasn't a difference in the capacity of the individual batteries of the pair.

Fishcop · Fishcop's Garage

This really interesting, I too "thought" I noticed a difference when I swapped batteries around (by accident) a couple of months ago. It was just a subjective observation, seat of the pants really.

Warren, it sounds like you have switched to one battery? Has this been worthwhile, and what is the process? I assume you'll recommend an Odessy battery or simailar?

Can anyone else recommend those cigarette lighter trickle chargers for cars that sit around?

Cheers

pwd72s · 08-29-2002, 05:24 PM

Warren? Thanks again. The side to side swapping procedure begins tomorrow. After all, I can't see even a bit of damage, and I'm willing to bet it'll help!

304065 · 08-29-2002, 07:56 PM

Converting to a single battery is simple. You disconnect BOTH dual batteries, then undo the clamp that holds the cable to the starboard side. With it loose, you remove it from behind the fuel tank, then from the clips going down to the battery in front. Then you remove both batteries. Be careful, you can easily weld watches, jewelry etc. to the car through inadvertent contact.

I have a single "Stinger" which is pretty light and has sufficient capacity to start my car immediately every three weeks or so. I don't use much electrical equipment however.

As for the wire, see if you can pass it off as an ultra-high end speaker wire and sell it to an "Audiophile" for $2000. Mine is currently keeping the mice out of my hangar: they think it's a snake.