Battery Size?

"And guys, I've already figured out you can't drill holes in a battery..."

Spoilsport.  We could have an entire thread on that...

Ok, Bob, you just made my evening!                                                                                 And I'm not going to tell you how I know...

While I hate to interrupt this frivolity with a serious answer, I'm a strong advocate of Odyssey gel-cell batteries in Speedsters (and dune buggies).  They're small (7.2"L x 3"W x 6.6"H), light (~14lb), and can be placed in any orientation except fully inverted.  As they are AGM they do not outgas toxic fumes so they can be placed in the passenger compartment.  I had great luck with them lasting years.  What's not to love?

@Lane Anderson posted:

What's not to love?

The price tag?

I believe @DannyP uses a U1 lawn-tractor battery. They're tiny and dirt cheap, and available in Lithium Ion for about $125 (if that rocks your world). They're all over with 300+ CCA.

Use a gear reduction starter and it'd be more than enough for whatever you're trying to do. I'll be buying this one next time I need a battery. It weighs 3-1/2 lbs.

If you can find something lighter, Al -- go for it. If you're dead set on spending Odyssey money on a battery, this one has 900 CCA, weighs 9 lbs, and costs $280. The general rule of thumb is that you need about 2 CCA for every cubic inch of displacement, so this one should turn over a 12:1 BBC with a tired starter.

i remember buying a high torque starter that fits into this thread somehow.

A very good point, @Stan Galat and @barncobob- a gear reduction starter brings the starting power requirement down considerably and and as such allows a smaller battery to be part of the package.  I'm guessing a stock Type 1 alternator as well?

@Lane Anderson- that's alright, we had to get to the task at hand sooner or later!    That's exactly the kind of info I'm looking for.  Were you running driving/fog lights or anything drawing extra power in your Speedster (which still comes to mind when I think of the classic red Speedster)?

Stan- that Ionic battery you linked to is wicked!

Any one running an agm battery- is a diode for slightly higher charging voltage necessary?                                                                                                                                  And any one running driving lights- halogen or led?  Any issues with power draw on halogen bulbs?

I guess I’m lost what you’re looking for, Al.

The total (constant) current draw on any electrical system should never exceed the capacity of the alternator. If lights or any other constant load draws less that the alternator can put out, then it really doesn’t matter how tiny the battery is, because the battery is just acting to absorb the peaks and fill in the valleys of load and capacity. It’s just an energy sink, it obviously needs to be able to crank the car over, but as long as it’s big enough to do that and absorb peaks and valleys, it’s fine. An analog car will run the ignition on the alternator, without any battery at all  

If loads exceed what the alternator puts out, you better have a battery big enough to carry the loads without the ability to recharge for as long as the loads are on.

Big ol subwoofers (only drawing peak load occasionally): fine, get a big battery. Headlights (drawing continuously more than the alternator can put out), no battery in the world will work well.

@ALB posted:

.........  Were you running driving/fog lights or anything drawing extra power in your Speedster (which still comes to mind when I think of the classic red Speedster)?

I have an Odyssey PC680 (680 CCA) same as Lane.  I'm running 65 watt H4 headlights (10 AMP fuses), plus a pair of 65 watt (10 AMP fuses) H3 driving lights, all Halogen.  I'm also running a gas powered heater with a 10 amp glow plug that is on a 20 amp fuse, only during 60 seconds of start-up.  Add that all up, and you get something way less than a true 50 amps (probably closer to 30 amps, really ).  The Odyssey battery handles all of that, no sweat, BECAUSE the alternator is capable of charging at 55 amps, so I probably have 15+ charging amps to spare (ignition load is almost negligible).  If everything is "on" and the engine is running at anything above an idle, nothing dims and the charging light never lights.  Last time I went to buy a new Odyssey, IIRC it was around $160 but if they have any kicking around that are beyond their shelf date (they wait until they have a dozen to return), you might get one for $50 and it will work just fine.

You can see how much different wattage devices draw for current on your system by plugging in values on this calculator - It's probably less than you think:

https://www.rapidtables.com/ca..._Amp_Calculator.html

is a diode for slightly higher charging voltage necessary?

No.  You're looking for charging current, not increased voltage.  If you're really worried, you could spring for a 75 amp alternator, but the battery only accepts a charge at its' own rate.  Adding a higher current alternator won't make the battery charge faster, but it will power more stuff (like a bad-ass audio power amp) once the engine is running.

So what is the concern for a physically smaller battery (other than fitting it into a tight space)?

I buy the U1(lawn tractor size) from Tractor Supply. Last time I got it, it was a Briggs and Stratton brand and 410 CCA(Cold Cranking Amps). It weighs 17 pounds. I peeled the stickers off so it didn't shout "lawn tractor" from the engine bay. They came off easily when new. Why a U1? They are available absolutely everywhere, and are relatively cheap compared to a full-size car battery or light lithium. They last me a minimum of 5 years, I think I went 8 seasons on  a particularly good one.

Al, and AGM battery is still a conventional lead acid battery. The acid is just suspended in a mat so they won't leak.

Most automotive Lithium batteries have charge circuitry built-in to control the charge rate. Lithium cells are 3.7 volts, so 4 cells is 14.8v, very much in line with a modern alternator/transistor regulator.

And as to rated alternator current, they can't put out max for too long. A typical 55 amp can put out 25-30 for long periods, but will overheat and burn out if you draw max current for more than 20-30 minutes.

On a typical Speedster or Spyder, the current draw is pretty low. Especially if you have all LED lighting(especially headlights). I think my car draws less than 15 amps engine running(EFI ECU and pump), gas heater, seat heaters, and high beams all on.