How do I charge AGM batteries?

[Yantai]

Hey! I have a problem, my AGM battery is discharged to 0, I never thought about how to charge it, I have a regular battery charger, but as I read in this post, it doesn't suit me, really they can only charge 75-100% of the battery? What should I do? Need a new device?

 

[telectrix]

How do I bring my battery back from the dead?

help.summitracing.com

 

[brianmoooore]

This is something I found out the hard way recently. Fancy modern chargers won't work on a battery that has dropped below a threshold voltage.
Never had that problem with my old and recently deceased charger.
Had to resort to connecting up my adjustable bench power supply to bring it up to the threshold.

 

[Strima]

It was quite common with old chargers to recharge completely dead batteries only to find the polarity was reversed...

 

[EricMark]

I use another battery in parallel and a smart charger, the smart charger I use is a Lidi 3.8 amp, and under 7.3 volt it assumes battery is 6 volt, and will not charge at all if under 3.8 volt, however it is the upper limit which also causes a problem, over 15 volt it assumes battery disconnected and auto switches off.

The major problem when charging a sulphated battery is we don't know if it has a shorted cell. So if it does have a shorted cell once it starts to take charge it will go to maximum and stay there, this completely wreaked my mobility scooter battery, as 3.5 amp even with a 35 Ah AGM battery will as found out hard way, distort the case due to over heating, and in the case above with 24 volt (two batteries) the bad battery wrecked the good one.

So a discharged 45 Ah car battery with a charged 7 Ah battery in parallel with the Lidi charger which once it reaches the 0.8 amp charge rate will not auto return to 3.8 amp charge rate is reasonable safe, OK it will discharge the 7 Ah and it will charge for an extended time at 0.8 amp, but unlikely to damage either battery more than already damaged with shorted cell.

However with the reverse, a 45 Ah in good condition and 7 Ah sulphated then there is a danger of over heating. So careful monitoring required.

I found it took a lot longer than expected, a good and a sulphated 7 Ah in parallel was one of the first pairs charged, to keep tabs on them I used an energy meter connected to the PC. So set for under 12 Ah so max 0.8 amp, the charger started at 0.8, quickly dropped to 0.1, then switch to stand-by (off) for nearly 14 days, by this time I was about to scrap the sulphated battery, but then it started to charge, at first a few pulses, then 0.8 amp for around 9 hours then off again, and I could not find anything wrong with battery after.

This was repeated with other batteries, and it seems as if a switch has been flicked, nothing for days, then suddenly starts to take charge and seems to fully recharge. So two caravan batteries and two stair lift batteries all recharged with no sign of damage to battery, not done it with a car battery in the same way, however I have noted although the car battery shows fully charged on the Lidi charger i.e. all 4 bars showing, the energy meter still shows taking charge 7 days latter.

With the 95 Ah AGM battery in the Jaguar XE it did in the end drop to zero charge rate, until 11:40 am each day when charge rate would jump up again, clearly some thing in the car switches on 11:40 every day.

The 95 Ah flooded in the Kia Sorento never switched off completely, likely the oldest battery I have, it ends up swapping between 0.1 amp and 0.8 amp around 10 times per hour, the 45 Ah flooded in the Honda Jazz does go to zero charge rate, it is however less than a year old.

The caravan batteries only seem to reach 0.1 amp, I consider 0.1 amp and holding over 13 volt then fully charged.

I had one battery which was used in caravan with shorted cell, and two VRLA found in draw when moving house have failed to recover, plus the two mobility scooter batteries, I do have an advantage two energy meters so I can from my bedroom computer top is Jag bottom is Jazz so I do not need to go out to check on them. Both using Lidi 3.8 amp smart chargers.

 

[Mike Johnson]

Not that I would recommend this, but flashing a 12v car battery across a below threshold cell can sometimes bring them back to life, so I have been told. ?

 

[EricMark]

Not that I would recommend this, but flashing a 12v car battery across a below threshold cell can sometimes bring them back to life, so I have been told. ?

Not really a flash, it took 10 days, yes if for example lights left on then jump start leads for around a minute may work, but when down to zero volts, looking at days not minutes.

 

[Mike Johnson]

I did say cell not battery.

 

[EricMark]

You can't access cells in a AGM or VRLA, the point I am making is all about time, and it takes time to soften the sulphur in the battery, and both under and over voltage is a problem, battery voltage needs to be between 7.3 and 15 volt. Under 7.3 it is seem as 6 volt, over 15 and it assumes leads not connected and switches off.

So putting a battery in parallel temporary as the donor battery is removed the voltage goes over 15 volt and it switches off, So donor battery has to stay connected until sulphated battery start to take charge.

On flooded batteries one drop of acid in each cell would start process off, but you don't have that option is VRLA and AGM I understand some smart chargers doing auto switch off, and will start charging as a lower voltage, the ctek mxs 3.8 does not charge 6 volt and will pulse at 15.8 volt, and min battery voltage is 2 volt. The ring rsc404 will start charging at 0.5 volt, and has a series of error codes looking at the instructions, and there are many which are similar, but few battery chargers will charge from zero today.

 

[Mike Johnson]

You can access each cell, but not a very safe procedure, so will not be explained if you don't know how to do it.

 

[EricMark]

You can access each cell, but not a very safe procedure, so will not be explained if you don't know how to do it.

I know you can rip the battery apart and remove the valves (rubber caps) designed to allow gases out but not back in, however the whole idea of valve regulated lead acid or absorbed glass mat which are two names for same device is that a very measured amount of acid and water is used so the battery can be used at any angle.

The earlier sealed lead acid battery was still flooded in design, and could not be used on their sides, but the AGM or VRLA are often used on their sides, as with stair lifts etc.

Although poor care can result in the battery drying out, and lifting the valve chamber cover will allow topping up, due to how and where the batteries are used this is dangerous, early mini had battery in the boot, and I know from bitter experience what happens to other items in the boot if over charged. Today it is common to find the battery in the boot, as the engine compartment can get too hot, but there is no real chance of acid escaping, unlike the old mini.

We have seen over the years many methods to top up batteries, I remember the Lucas balls method, where the vent was blocked with balls once the cover was removed so one can easy top up, the replacing of the cover displaced the balls and opened the vents, but most battery chargers said on the instructions remove battery cover, and if you did this with the Lucas then all the acid spilt out.

So over the years we have needed to modify our methods, the old idea of a 12 amp battery charger has gone, in the main 3.8 amp is good enough, and the modern smart charger has started to reduce charge rate within the first hour, and we leave batteries on charge for days to complete the process of turning sulphur into sulphuric acid, back in late 60's I would not have dreamed of leaving a battery on charge for longer than a day. Things have changed.