Leaving for 2 weeks and need a Battery Tender

Please provide a specific recommendation of a charger that can differentiate between AGM vs. liquid lead acid. I would like to understand how it can decide whether a given battery is AGM or liquid lead-acid without user input.

Here is one example of a CTEK charger that purports to automatically decide what the battery type is, however I personally don't like the idea that it may decide to apply charging voltage of as much as 15.7V during its recondition mode.

I agree that a 1.5A charger can charge a good battery if you are patient and give it plenty of time.

The point of having a battery charger with higher current capacity is that it will become evident pretty quickly if the battery will accept a charge or not. If the battery will accept a 10A charge for example, you should see the current drop down from the 10A level within an hour or less if the battery can be charged, as the battery voltage should rise up to the point where lower charging current becomes appropriate.

I recently noticed the 12V Exide AGM battery installed in my daughter's Prius had a problem, because the overhead cabin lighting was dim. By observing the charging current flow over a short period of time, it was obvious that battery could not be restored due to a dead cell, and I returned that battery to the retailer for a warranty replacement.

If you are using a 1.5A tender, it will take you many hours to decide whether the battery is good or bad, time that you could have used to buy a replacement rather than having to wait until the next day.

 

Walmart
Battery Tender Junior Charger - Walmart.com

On the Deltran Battery Tender website, they claim their product was introduced for AGM batteries:
http://batterytender.com/resources/frequently-asked-questions/

Also from the Battery Tender FAQ:

"Based on price alone, trickle chargers often appear to be a better economic choice for the typical consumer, but trickle chargers do not have the advantage of sophisticated electronic control. Therefore, as they allow the value of charge current to trickle down to what appears to be safe levels, the output voltage of the charger may very well rise to an unacceptably high level, sometimes even going higher than 16 VDC depending on the charger type and the battery that is connected to it. This magnitude of voltage is far above the gassing voltage of a lead acid battery. If the battery remains connected to this high level of voltage for an extended period of time, extreme damage may be done to the battery. Without Battery Tender® type electronic safety controls, what appears to be an initial cost savings for the charger may actually cost several times the charger price in replacement batteries."

From: Absorbent Glass Mat (AGM) Battery Information - Battery University

"As with all gelled and sealed units, AGM batteries are sensitive to overcharging. These batteries can be charged to 2.40V/cell (and higher) without problem; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell), and a direct replacement with a sealed unit could spell trouble by exposing the battery to undue overcharge on a long drive."

 

Maybe I worded that poorly.
All the charger needs to do is sense the back voltage or effective internal resistance which really amounts to current flow at a given voltage application. It doesn't really "know" what kind of battery it is, it just knows that when XX voltage is applied and it only draws xx MA of current, that it can be considered fully charged. It might only be 98% but that's pretty meaningless.

A quote from Greg's post, incase you missed it:
" Whether it is a lead-acid battery, including standard and sealed AGM and GEL Cell battery types or any other kind, the 12v Automatic Battery Charger can be used to charge them all."

I fully agree with your assessment of the usefulness of a larger charger. I actually have both kinds but the bigger one isn't automatic so they serve somewhat different functions.

 

Easy Rider 2,

FWIW, if you do not fully charge the AGM battery using the correct algorithm which does specify higher voltages when charging than traditional flooded batteries and Gel type batteries you will in essence shorten the lifespan of said AGM battery.

I believe that is why folks like myself and Patrick insist on chargers that have the AGM setting.

Peace,
Ron

 

That is marketing hype- bull- crap......left over from when charger companies were sucking extra profits from you for a feature that you really didn't need.......and apparenty still ARE from some people.

The important part is "fully charged". The "algorithm" used to get there is pretty much meaningless.
AND maximum life doesn't just occur at 100% charge. Anything over about 95% works just fine.

YOU DO NOT NEED A SPECIAL CHARGER FOR CURRENT AUTOMOTIVE AGM BATTERIES.
If your charger is over 10 years old and is not labeled for AGM use, you might not want to leave it connected all the time but for occasional use even that will be just fine. The charging characteristics are not THAT much different.

YOU can do whatever you want but I won't stand by and let you preach bad information to everybody else who asks about chargers.

State it as your opinion, fine.
State it as though it is an indisputable fact and then we have a problem.

 

I have discussed AGM chargers with you in other threads, and now I'm getting more confused. In other discussions, you have not wanted to accept what battery manufacturers or battery charger manufacturers say as that is "just marketing hype", but now we can accept Battery Tender's statement on your say so because, seemingly, it supports your claims. It seems like you like "bob each way" bets.

State it as your opinion, fine.
State it as though it is an indisputable fact and then we have a problem.

 

op wanted something to keep the battery from dying when out of town for a few weeks. a battery tender is designed to do just that. and it won't jack the voltage up when the battery is charged. its not a charger, but it will if you have time and a good battery.

 

Who is to say whose information is BAD? I am NOT God, but for some reason you are JUDGING here. Lighten up dude, this is a internet forum, not the end of the road and judgment day...

FWIW, Sources here on Prius Chat go both ways. IMO, a true AGM type charger is needed for AGM type batteries. In your opinion ANY charger will do. So be it. Everyone is entitled to his or her own opinion.

Now, on the other hand when you challenge one another as you are doing, you then are crossing the line and actually judging one another. I don't think Prius Chat is the place for it. Think what you wish, that is fine. But please consider others here on this board before posting the way you did.

Peace,
Ron

 

I never said that ANY old charger will do.......but in fact it will if only connected for a short period.

I've been an Electronics Engineering Technician for about 50 years now so I kind of know what I'm talking about (that is not to say, however, that I never make a mistake though.)

The fact of the matter IS that the charging characteristics needed to adequately charge an AGM battery are not that much different than an old wet cell type. And it is fairly easy to design an automatic charger that will do an adequate job on ALL kinds of auto batteries. No "switch" is necessary.

So.....any off the shelf small automatic charger that claims to be compatible with AGM batteries most likely IS adequate.
Probably ANY that are on the shelf now probably is too, whether it specifically says that or not.

Recognizing marketing hype (lies) is a skill acquired over many years.
Even then almost everybody is taken in at one time or another.

 

I also agree these forums offer advice and information as personal opinions only. This is not a professional service where you are given totally complete and correct answers to your problems. It is the opinions of the forum users only. If you wanted a professional service with the correct advice, you would have to pay for it.......and sometimes even those paid services give you incorrect advice and incorrect information (as in dealership diagnostics)

 

Well said JC, you took the words right out of my mouth.

 

I have been using a cheap Harbor Freight automotive charger to charge my 12 v battery; I charge it for an hour or two and disconnect it as soon as the battery reaches 12.8 volts.

The packaging label on the cheap Harbor Freight charger clearly states "Do Not Use This Charger for AGM Battery".

No, my 12 v battery has not exploded so far.

 

As many have said, battery tender not needed for a 2 week stretch.

 

I think you are under-charging your battery in that case. The charging voltage should reach 14.4V and you should maintain the charging current until the current no longer declines as time passes.

If your charger does not have an ammeter then use a digital multimeter set to the ammeter function to measure current. A 12V AGM battery in good condition should have its charging current stabilize at less than 0.1A. If the current stabilizes at a higher value this means the battery is less good.

The concern with using a charger not intended for AGM batteries is not that it will over-charge the battery; rather the reverse. Typically it will stop charging the battery before it is fully-charged. Hence use of that charger is better than nothing; but less good compared to a charger that will fully charge the battery.

 

But cheap chargers especially old ones can have too high output voltage also. If you don’t check voltage with multimeter you may think that battery isn’t full or is leaking if amp meter stays at some higher number. But if charge voltage is too high then you are actually damaging the battery.

 

I think so too but I have a slightly different spin on it.

With a very low capacity charger, the charging voltage likely will NEVER go much above the resting battery voltage as the charge proceeds.
Thus, while monitoring the voltage at the battery, to have a full charge it should go well above 12.8, it might not go above 13.5 or so and still have very close to a full charge.

While a cheap trickle charger might read 15 V open circuit, the voltage goes down REAL quick when a load is applied.

I am saying this not to be argumentative but to maybe prevent someone who never sees 14.4 volts from automatically assuming that his battery is bad; it might not be.