Once again I find myself with new auto experiences .

I am in costal Oaxaca Mexico again this winter .
Most of the Desoto is well worked out for the extremes of temperature it takes to get here and stay here.
I have recently noticed that at temps 37c and over the battery voltage falls to 12.2 volts. Underhood temps are crazy in traffic.

I had the battery tested and it is fine. The alternator output is good and steady.
Could extreme underhood temps cause my battery voltage to fall enough to cause slow starter cranking?
The car starts fine cold....battery 12.6v

Would a 12v 120mm x 120mm computer cooling fan for the battery be silly?

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Welcome back, Sandy
With the current state of batteries, I really can't answer that. Usually, lower temps will have a negative effect on batteries. Over the years, I have found Slow cranking in high engine temps is generally not the battery, but a problem with more resistance in engine cranking. Either mechanical, electrical connection(s), or starter. That said most newer vehicles do have insulation around the battery. I don't know if a battery fan would help or not. If you have low battery voltage to start with, a fan might just lower the voltage more.
PS: 12.2 volts is really not out of range for starting. I would check for starter draw current both when cold and hot.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

Battery internal resistance climbs as temperature drops, and falls as temperature rises. The charge voltage needed to keep up the battery at 37°F (cold) will boil a battery at 37°C (hot). Voltage regulators are temperature-compensated, and the compensation circuit can drift from specs. I had one that would overcompensate for cold ambient/battery temp and send charging voltage up in to the high 15s-low 16s on cold start; I had to not use any lights on the car until it had run for enough minutes to begin to warm the battery, or the lights would pop like flashbulbs. They can drift the other way, too, overcompensating for high temperatures and lowering the charge voltage too much.

I don't think you need a battery fan; I think you need attention to your voltage regulator (presumably built into whatever alternator you're running; I'm guessing a GM item of one kind or another).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for your replies.

The battery is very hot to the touch in the conditions that generate the slow starter. Battery voltage 12.2

The starter is fine when things are cooler. Battery voltage 12.6

Charging voltage is 13.7.. cold. GM alternator

It always starts.....but terrifies my spouse when, after fueling/parking, it hesitates to fire immediately.
The loss of confidence can be an adventure killer.

Considered some insulation for the battery…?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Is it a regular flooded battery or some high zoot thing? Low on electrolyte?

13.6 is not exceptional for a late model charging system. I'm thinking you've got the setup off the Rudemaster.

Starter in good shape? Brushes and bushing/bearings good?

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Official Cookie and Mater Tormentor.

Its a regular NAPA Gold 78 series battery. It is mounted in the stock DeSoto location and has the usual cooling holes in the rad support. When the car is stationary the airflow through those holes is nada. I have room to augment that airflow with a small fan. A 120mm x 120mm (5x5) computer case fan would fit right in. 157cfm . It could easily join the 2 rad cooling/trans cooling fans that are controlled by the engine management puter. I can (and do) turn on the fans manually when the car is not running.

I have thought of insulation for the battery but I am concerned it might make the situation worse. I may make a shield for the engine side especially if I add the fan. I would help keep the airflow from the fan directed around the battery.

The Roadmaster starter is fairly new and has had an easy life. It normally starts with just a touch of the key and has never been cranked for more than 10 seconds.
The slow starter condition is only when the car has been shutoff in extreme temps. The cooling system never overheats.

The battery gets too hot to touch. The rest of the engine compartment does as well. You cant leave your hand on the hood.

Other than that the car has been wonderful. I had a screw come loose on the armrest on the way down and had to add a litre of oil in Oaxaca City (3500 miles) I did Denver to Del Rio (900 miles) in one day. The day started in Denver with 4" of snow....

Sandy, my fine friend. Nice to see you on here, and so glad you are enjoying time with your spouse in a beautiful place.

I also wonder if the it's the hot starter making the slow cranking and not the hot battery. Still the voltage being lower is strange. I wonder if there is a chemical/phase reaction that speeds up at high T (like electrolyte water bubbles forming on or near the battery electrode plates) that could give you those problems. If battery's too hot to touch (about 80 C?), that sounds too hot to operate as intended. I'll contemplate the battery dynamics further... I am more-or-less a bona fide lithium battery researcher now, but I don't think about lead-acid much.

If there are adequate ventilation holes but not enough heat isolation from the engine bay or enough air flow, the fan could be a good workaround.

Let's catch up by phone sometime soon. It would be nice to discuss a number of things with you.

Lou in VA

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