And now for something right out of left field: Battery Desulfating?

[Grimly]

My “smart” Optimate charger wants to see 2 or 2.4V to start charging a deeply discharged battery. The parallel trick is a good hack.

I thought the same about the older so-called smart chargers I have, the one I used at first seemed to do nothing, but when I put a DVM on the battery there was a slight increase of voltage in the tenths range. I decided to leave it overnight.
Following day, the voltage had risen to 2.5V and leaving it another 12 hours it rose to 3.5V. At that, I disconnected the charger, then reconnected it, and it recognised the battery as a fairly dead 6V unit, so proceeded to charge it up more rapidly.
I left it overnight again, and did the same - this time it had topped out at 7.5V, and it recognised it as a fairly dead 12V battery, so carried on from there.
It eventually fully charged it, but I wasn't happy with its AGM settings, as it wouldn't take it above 14.2V, when I was looking for 14.8V.
That's when I bought a new charger, one that had a more definite selection of AGM regime on it.
With that, it rapidly banged it up to 14.8V full AGM and I left it to maintain for a couple of days.
That battery is fine now, but I've undoubtedly put a dent in its longevity, so will just have to be careful with it.

 

[bosco659]

I thought the same about the older so-called smart chargers I have, the one I used at first seemed to do nothing, but when I put a DVM on the battery there was a slight increase of voltage in the tenths range. I decided to leave it overnight.
Following day, the voltage had risen to 2.5V and leaving it another 12 hours it rose to 3.5V. At that, I disconnected the charger, then reconnected it, and it recognised the battery as a fairly dead 6V unit, so proceeded to charge it up more rapidly.
I left it overnight again, and did the same - this time it had topped out at 7.5V, and it recognised it as a fairly dead 12V battery, so carried on from there.
It eventually fully charged it, but I wasn't happy with its AGM settings, as it wouldn't take it above 14.2V, when I was looking for 14.8V.
That's when I bought a new charger, one that had a more definite selection of AGM regime on it.
With that, it rapidly banged it up to 14.8V full AGM and I left it to maintain for a couple of days.
That battery is fine now, but I've undoubtedly put a dent in its longevity, so will just have to be careful with it.

My Optimate is supposed to charge regular flooded lead acid and different AGM batteries. There is however, no selector switch do the charger is supposed to figure this out on its own?

 

[Grimly]

My Optimate is supposed to charge regular flooded lead acid and different AGM batteries. There is however, no selector switch do the charger is supposed to figure this out on its own?

Does it have a snowflake setting? That's usually the 14.8V output needed for AGMs.

 

[bosco659]

Does it have a snowflake setting? That's usually the 14.8V output needed for AGMs.

It doesn’t have any settings. All auto

 

[Wingedwheel]

I have one charger/maintainer with different settings that I really like, my other is an auto and sometimes it seems it has a hard time figuring out the battery. I normally just leave that one on the big Harley battery since it’s LA. Everything else I have is AGM so I prefer being able to leave that charger set for AGM all the time.

 

[Grimly]

It doesn’t have any settings. All auto

Put a voltmeter across the battery whilst charging and see if it hits 14.8V at any time.
Off charge and sitting for a few hours, it should be at least 12.6V, preferably up to nearly 13V.
https://www.batteryskills.com/battery-open-circuit-voltage-test/

Before I bought the latest charger, I'd been keeping an eye on the behaviour of the cheapo smart charger(s) I already had. Now while the green light had come on and told me (according to the instruction sheet) that it was fully charged, it wasn't really and was topping out at 14.4V or so. However, the voltage being supplied was very slowly increasing and might eventually have reached 14.8V the AGM needed. It wasn't doing it quickly though, which wasn't a bad thing, but the lack of feedback on the charger was annoying*. I'm not junking the older chargers, they're still perfectly adequate for most uses.
The newest one I bought tells me exactly what it's putting out and what mode it's in.

* Turns out a lot of AGMs are prematurely life-ended because they get under-charged. If they only ever see a charging voltage of 14.5, it doesn't do them any good in the long term.

 

[bosco659]

Put a voltmeter across the battery whilst charging and see if it hits 14.8V at any time.
Off charge and sitting for a few hours, it should be at least 12.6V, preferably up to nearly 13V.
https://www.batteryskills.com/battery-open-circuit-voltage-test/

Before I bought the latest charger, I'd been keeping an eye on the behaviour of the cheapo smart charger(s) I already had. Now while the green light had come on and told me (according to the instruction sheet) that it was fully charged, it wasn't really and was topping out at 14.4V or so. However, the voltage being supplied was very slowly increasing and might eventually have reached 14.8V the AGM needed. It wasn't doing it quickly though, which wasn't a bad thing, but the lack of feedback on the charger was annoying*. I'm not junking the older chargers, they're still perfectly adequate for most uses.
The newest one I bought tells me exactly what it's putting out and what mode it's in.

* Turns out a lot of AGMs are prematurely life-ended because they get under-charged. If they only ever see a charging voltage of 14.5, it doesn't do them any good in the long term.

I have a small LED voltmeter attached to the battery while the charger is doing its thing. I’ve run the charger on two different batteries and get similar results. Peak voltage while charging is 13.7V. Then the charger will switch to test mode and charging current is discontinued and the battery voltage drops to its resting state voltage. On the one battery it’s 12.9V and the other 13.0V. Then after several minutes at these voltages, the charger starts delivering current again up to 13.7V and the process repeats itself. The only time I saw 14.8V was when the charger inexplicably went into desulphate mode. The local Optimate distributor offered to do an assessment on my charger for my peace of mind. I’ll probably take it in this week. Lucky for me they are in town, 15 min away.

 

[bosco659]

My mystery is solved - for my Optimate charger anyway. I at last confirmed that it’s performing as designed. First it charges the battery to 13.7V and holds it at this voltage - this part of the process takes about 1/2 hr. Then the charge is terminated and the battery voltage is allowed to drop down to its resting voltage. This is the test mode that takes 1/2 hr as well. While in the test mode, the charger is monitoring the rate of “discharge” and is determining if the battery is healthy. Once the 1/2 hr elapses, it starts charging again and the cycle repeats itself. All is good as long as the light remains green. Once it goes red the battery is toast.