Dumb q: is it possible for the battery tender to indicate fully charged but for the battery actually be unhealthy / near end of life?

Or will the tender indicate something less than fully charged as the health of the battery degrades over time? Ie - time to replace the battery as the tender can no longer provide service.

The cheaper battery-tender type devises are usually poor judges of battery health - they can tell if a cell is completely shorted, that's about it.

It's possible for a battery to come to full voltage, but not handle a load very well. That will confuse most chargers that can't do a load test and also some that do a load test will give 'charge battery & repeat test' results multiple times.

quote:

is it possible for the battery tender to indicate fully charged but for the battery actually be unhealthy / near end of life?

Yes

As others have said, yes a Battery tender can show a green light but the battery is done. Many of the better "tenders" will do more testing and are better at maintaining a battery. I like the stuff from CTEK.

You get what you pay for. The dumbest of tenders are basically trickle chargers that don't do anything else. At the high end, some of them are wifi connected and have pretty good analytics and logging.

I have both Battery Tender and CTEK branded tenders. At least the CTEK (which seems a little more sophisticated), if it shows fully charged, can I assume that the battery is still healthy?

Typically, no.

Think of a battery as having 4 parameters that vary with use and age.

First is state of discharge, or how "full" a battery is. For most batteries, voltage serves as a good proxy to measure the state of discharge on a battery. A trickle charger uses voltage to determine when the battery is full and the cutoff for charging, if any.

Second, how much current that the battery can provide, or how much "oomph" a battery can kick out. To test this, you need something that can pull the current until the battery suffers some threshold voltage loss. For something like a car battery, which is designed to provide many hundreds of amps, you measure the current while the battery is attached to something that can pull a correspondingly large amount of current. For example, you measure the current while the motor is cranking.

Third is capacity, or how much storage the battery has. To measure this, you have to charge the battery to it's max and then drain the battery under a constant load and read how much electricity has been consumed when the battery hits empty.

Lastly is self discharge or how long the battery can hold a charge.

A healthy car battery should have good performance for all 4 of those parameters. I am unaware of any consumer car battery trickle charger that is able to monitor and verify all 4 parameters above. And certainly not for less than a couple hundred bucks. Most chargers only look at voltage to determine a cut off for charging. Some "smarter" chargers will measure resistance (likely, I've not really looked into it) to determine if a battery needs a "disulfication" cycle (pegging the battery with ultra fast bursts of high voltage to shake the sulfide? ions back into the electrolyte). But to determine battery health, you'll need something that can also measure max current and capacity.

I have smart chargers for my AA and AAA batteries that can do current and capacity testing, but we're talking little batteries with discharge in fractions of an amp.

For a car battery, I think the best single indicator for health would be to measure the cranking amps it can provide. This is what the auto parts stores test when they test your battery. AGM batteries have a gradual reduction in cranking power as they age. Regular lead acid batteries have a steeper drop off, so you can really only get a go/no-go status from those.

Sometimes, if it's a major issue they will show a warning, but not always.
If it isn't reaching full charge but has been on for enough time to be full charged, disconnected and check with a voltmeter. Leave the charger off and see if it continues to drop below 12.6 or so a few hours later.
Also, how old is the battery and how many months was it rated for? That's usually a pretty good indicator of how long it will last.

Tenders/chargers do go bad, too. Had a green light on a bike one winter, tender wasnt functioning right. Battery had frozen and was stone dead. That tender WAS probably at least 10 years old.

What type of battery? Traditional Flooded or AGM?
There is a trick to charging an AGM battery....

Thanks guys. Okay - so answer is no and if I want to better understand battery likelihood of failure at the most inopportune time, get a CCA test at a local auto parts store.

Usually my batteries last about 2 years (before tenders). This is one going on 4-5 years (w/ use of tenders). But it seems like it's getting more difficult to start the car -- it starts but seems like it takes more cranks / effort.

That's probably your sign time to replace

I generally have a good record with batteries. With the cycle & power sport types I rotate in a maintainer at times, depending on frequency of use.

My Wife drives a Honda CRV, averages shorter trips. The original battery lasted the shortest I’ve ever seen a factory battery go bad, 2.5 years or less.

It has a better battery now. If it’s in the garage at times I may just put the charger on it for a handful of hours.

Just talked to my sister today, she has zero mechanical ability, about a 3 y/o Subaru. I kid you not, she just had the 4th factory battery installed. She has some kind of extended warranty. She has some type of parasite draw, may or maybe not her doing.

She lives by herself 5 States away, usually doesn’t want my advice anyway.

If you don't have one, get an inexpensive digital voltmeter.
When it seems hard to start, check the battery voltage. Your problem could be the starter, a ground, cables or some other issue.

If a battery is left on a tender long term and seldom used, the charger needs to be at least smart enough to stop when the battery is fully charged, restarting when the battery gets below a certain level. Simply leaving even a very low amp charger on will cook the battery in a month or less.

 
I prefer the BatteryMinder brand, as they have built in desulfation.

quote:

Originally posted by henryaz:
 
I prefer the BatteryMinder brand, as they have built in desulfation.

These are good, but pricey! Many folks at Our Little Airport use them for aircraft batteries. I used one for the V-Tail, needed to keep the battery in top shape because the engine was extremely difficult to re-start when it was hot, until after 30 years of ownership, I found a maintenance guy who diagnosed a bad sniffle valve. Inexpensive fix, and never again had a hot-start problem.

quote:

Originally posted by V-Tail:

quote:

Originally posted by henryaz:
 
I prefer the BatteryMinder brand, as they have built in desulfation.

These are good, but pricey! Many folks at Our Little Airport use them for aircraft batteries. I used one for the V-Tail, needed to keep the battery in top shape because the engine was extremely difficult to re-start when it was hot, until after 30 years of ownership, I found a maintenance guy who diagnosed a bad sniffle valve. Inexpensive fix, and never again had a hot-start problem.

BatteryMinder not too bad for car batteries. $29.95

https://www.northerntool.com/s..._200332201_200332201