Batteries! How weird they are...

[JWR]

Well, sort of. Again the context here was one of ordinary lead-acid batteries of the tractor and auto motive normal variety. I have no idea what ohms law has to do with it but on further study it may become clear or you may clarify.

On small mowers (e.g. Briggs and Stratton) there are often just a few turns of wire on the rotor that (via a cheap voltage regulator/diode) provide similar outputs to that of a trickle charger . Again, out of context for this overall discussion.

In automotive / tractor context a trickle charger works fine regardless of the CCA capacity of some big battery. Has nothing to do with internal resistance. Just takes longer for the small trickle charger to put enough energy back into the big battery to have much of an effect. You need a larger current capacity charger to reduce the time it takes to charge up a battery, not to overcome internal resistance. You are not matching anything other than your desire to achieve a charged battery condition in a reasonable period of time.

 

[Fuddy1952]

Lots of ways to explain the importance of things need to be properly matched to work correctly. In the mid 70s wife & I took auto mechanics at a local college, different instructors for major parts (suspension, brakes, carburetors, electrical, transmissions, etc). The electrical instructor said (I know it's overly simplistic) the battery is for starting. Once running you could throw it away, it runs on alternator or generator. To make it easy to understand compared it to water, battery is a tank, alternator a pump, wires are pipes, etc.
Over the years I believe things are designed a certain way, when you change things that's when you have trouble.
If something worked when new and now doesn't troubleshoot it don't modify it.

 

[Williy]

An alternator puts out so many amps and if you
install a very large capacity battery your alternator
may not be big enough to fully charge it. So you
want to get a battery that your alternator can fully
charge and that's why when you go to the battery
shop they ask you what are you driving or what does
the battery go in so you get the correct size battery!

willy

 

[Grumpycat]

An alternator puts out so many amps and if you
install a very large capacity battery your alternator
may not be big enough to fully charge it. So you
want to get a battery that your alternator can fully
charge and that's why when you go to the battery
shop they ask you what are you driving or what does
the battery go in so you get the correct size battery!

willy

Wrong. If your battery is 100 Ah and alternator is only 1 A then (overly simplistic) the battery takes 100 hours to charge. If 10 A alternator then 10 hours.

Back to the real world there is a minimum current before the battery starts storing but that is somewhere around C/100 hour, really low. C is amp-hour capacity.

Some chemistries such as NiMH prefer a charge at the C rate.

But in tractor and lawnmower application the alternator only has to output enough to meet electrical load plus a safety margin. Any size battery you care to install will charge on that excess.

 

[JWR]

An alternator puts out so many amps and if you
install a very large capacity battery your alternator
may not be big enough to fully charge it. So you
want to get a battery that your alternator can fully
charge and that's why when you go to the battery
shop they ask you what are you driving or what does
the battery go in so you get the correct size battery!

willy

No Willy, as Grumpycat so compactly stated: Wrong. There is no such thing as an alternator not big enough to charge a large capacity battery. A tiny 1 amp trickle charger will fully charge the biggest lead acid car battery made. It just takes forever. The reason
"when you go to the battery shop they ask you what are you driving or what does
the battery go in" is they know nothing about batteries and were hired at minimum wage to look up things in a book. As cantakerous as I usually am, I tell them it is none of thier business what the battery goes in. I came to buy a battery with certain physical dimensions and terminal locations and am looking for the best specs I can find on that physical size battery in terms of cold cranking amps, etc. The "correct size" battery in the typical battery store is a table look up by folks completely ignorant of the battery characteristics.

 

[Grumpycat]

The reason
"when you go to the battery shop they ask you what are you driving or what does
the battery go in" is they know nothing about batteries and were hired at minimum wage to look up things in a book.

Ditto. Recently had to force an autoparts store to sell me a vacuum hose. "What does it go on?" "Doesn't matter, I want to replace this hose." "What does it go on?" "If you must know, an Omron Blood Pressure Monitor." "We don't sell those."

Most recent, try browsing online autoparts batteries by BCI Group size. Nope, none let you view batteries that way. Must enter your vehicle so they can tell you what fits. Then they won't let you out of that selection. If they say you need Group 25 then you can't see anything else. But if the P/N is ABC25 then you can guess a Group 24 is ABC24 which a search will find, and will even suggest other Group 24 batteries... Argh! But you can not search for "group 24" for some reason.

My Subaru Outback comes with a Group 25 battery yet the battery tray easily fits a Group 24 which is 720 CCA and 120 minutes reserve vs Group 25 580 CCA and 90 minutes, same price. Same size but 24 is maybe an inch longer. Which would you rather have?

 

[WranglerX]

Ditto. Recently had to force an autoparts store to sell me a vacuum hose. "What does it go on?" "Doesn't matter, I want to replace this hose." "What does it go on?" "If you must know, an Omron Blood Pressure Monitor." "We don't sell those."

Most recent, try browsing online autoparts batteries by BCI Group size. Nope, none let you view batteries that way. Must enter your vehicle so they can tell you what fits. Then they won't let you out of that selection. If they say you need Group 25 then you can't see anything else. But if the P/N is ABC25 then you can guess a Group 24 is ABC24 which a search will find, and will even suggest other Group 24 batteries... Argh! But you can not search for "group 24" for some reason.

My Subaru Outback comes with a Group 25 battery yet the battery tray easily fits a Group 24 which is 720 CCA and 120 minutes reserve vs Group 25 580 CCA and 90 minutes, same price. Same size but 24 is maybe an inch longer. Which would you rather have?

Same BS on buying online tires, was told tires I wanted don't fit my truck... But but they happily accepted my money and put tires I wanted on my truck...

 

[Fuddy1952]

Yesterday I go to an O'Reilly's 5 minutes away, I needed a top gasket for a Carter AFB (later Edlebrock "Performance" series carb.
No one knew what I was talking about...not even the oldest guy I could find!
Finally I spotted a rebuild kit faded, dust covered on the back wall. "There it is"! I said...and it worked fine.
We have cars with Carters, Holleys, Rochester, even Zenith.
Most workers there were texting on phones.

 

[Williy]

I tried to buy some spark plugs, what model, what make, what
size engine, how many doors and I also told them what plugs
I wanted in the first place! You would think it would be easy to just
get the plugs off the shelf and sell them to you but no gotta go
threw the computer! When I go to purchase a battery I just put
one in a cart and hit the checkout saves me a lot of time and blah
blah blah like whats it for etc

willy