Have I damaged my AGM Batteries ?

[castaway]

Very cleverly I managed to leave my domestic batteries on charge for about 3 weeks...

They are 2 x 120ah 6volt AGM and the charger is a cheap 8ah car charger.

They were not hot ( or even warm) when I found out what i had done.

Will I have I bu88ered them?

Many thanks, Nick

 

[capsco]

I would be prepared for the worst, i.e to replace all.

 

[mikejames]

Probably not. I think the fact that the on-charge voltage of AGM is higher than the normal lead-acid on-charge voltage will help.

That charger sounds like a trickle charger as well- not a fast charger so it probably was pushing a lot less than 8 amps into the batteries by the end.

The AGM batteries will recombine the gas given off when the charge drops to around one amp current so they will survive long charging better than a wet cell without attention.

I have had a spare 120AH AGM on a float charge (to about 14.2volts) since 2003 and I could still draw about 1kW from it for half an hour when I connected a big inverter to it a couple of months back.

If the batteries were at all distressed by the experience, expect to see the sides of the cells bulging and a smell a nasty smell.

 

[npf1]

Depends on what voltage the charger cuts off at. As long it cut off at sub 15V, they might well be OK.

If I recall correctly, the specs for most batteries are based on a battery temp of 25C, but your batteries will only be at 6C at present. That means that to be fully charged they would have required the voltage to reach around 15.1V (half that for 6V batteries).

The only way to check is to test them, so take out, say, 30% of their rated capacity by leaving some consumers on for a few hours, leave the batteries unloaded for four hours and then read the voltage. Bear in mind that for a 12V battery at 6C, a measurement of 12.0V after being unloaded for 4 hrs is about 50% charged. Also bear in mind that at 6C, the batteries will only hold 85% of their rated capacity.

All the info above is taken from the varoius graphs availble for Lifeline AGMs. I did a capacity test last week on mine.

 

[William_H]

Much depends on the style of charger. The older style usually with a meter and possibly a high/low switch are just a basic rectifier and transformer.
The voltage hence current fed to the battery is a series of pulses (half sine waves) rising from zero volts to a peak then down again. 50 or 100 times per second. The voltage at the peak is usually around 17 or 18 volts but the time % at that voltage is relatively small. As the battery voltage rises the current will fall but not cut off. Unlike a regulated charger or alternator where current will fall rapidly as battery reaches the regulated voltage.

So if this is the kind of charger you left on the batteries they may have received an average several amps even after being fully charged.
Of course if it is a dern regulated charger the current would drop to quite a small value so there should have been no probs.

However the batteries are large and may be OK. You need to try both a large load (engine starting) and a small load for a long time to get an idea of their usefulness (condition). good luck olewill

 

[Marsupial]

I doubt it! 8amps isnt going to do much and that is only available for short period of time on a fully discharged battery, as the voltage comes up the cheap chargers have a hard time putting anything in.

 

[Anonymous]

You're probably fine. I would do a discharge to 50% -- i.e. 12.0V on a C/20 load (in your case C/20 = 120/20 = 6Amps). Charge up with your proper charger overnight then turn on a few lamps to draw 6A and see how long it takes for the volts to fall to 12.0V. It should be 10 hours. If you are using these for engine cranking, check that too, but not when half discharged. Don't forget to turn off intermittent electrics - e.g. fridge when doing the discharge test /forums/images/graemlins/wink.gif

 

[castaway]

Okay ...thanks all for the reassurance !

Nick

 

[Plevier]

William_H has got it pretty well right.
Charging voltage is really critical for AGM batteries. It is just too risky, false economy, to use a cheap car charger. What did your batteries cost? A lot.
Anything over about 15 volts WILL cause water loss from an AGM battery. They start off with only just enough liquid in and you cannot top up.
Many unregulated cheap chargers get up to near 20V at the end of charge. If yours is one of these, then yes your batteries are probably damaged or at least reduced in capacity. 8 amps capacity is enough to do damage even with your size battery. The fully cahrged float current at correct voltage is only a fraction of 1% of capacity.
You must get a decent charger if you are going to use AGM batteries. A cheap charger will shorten the battery life.
This is not old wives tales. I was involved with AGM battery applications for several years working for a manufacturer.
Mike

 

[Anonymous]

He said he did it by accident. Something any of us could do -- pop it on charge for a few hours, which would be fine, then forget to turn it off before going home. I think it is VERY unlikely that the peak (rectified) voltage will be as much as 20V and if it is, the conduction period will be a tiny fraction of a half cycle. Chances are he should be fine -- though he knows, and knew before posting, that it was a mistake.

 

[Plevier]

I'm afraid the battery doesn't care whether it's accidental or deliberate. Unfeeling things!
You don't only get the peak voltage briefly and the current doesn't just follow it instantaneously and in phase as you appear to suggest. The battery acts pretty much as a capacitor and smooths it virtually to DC. Just try it with a DC voltmeter, an AC voltmeter, and/or an oscilloscope.
I've got a very similar cheapo car charger, rated (optimistically) at 10A, and on two 12V 60Ah wet batteries in parallel, i.e. the same capacity as the OP's two 6V 120Ah ones in series, it goes up to 18 volts. I wouldn't take it anywhere near an AGM battery!
I'm not attacking the OP, we all make mistakes, but the fact remains that an unsuitable charger is false economy with AGM batteries. It WILL shorten the life even if you don't realise it's happening, and even if you don't accidentally leave it on for long periods.
Mike

 

[Anonymous]

[ QUOTE ]

You don't only get the peak voltage briefly and the current doesn't just follow it instantaneously and in phase as you appear to suggest. The battery acts pretty much as a capacitor and smooths it virtually to DC.

[/ QUOTE ]No. In the case of an unsmoothed charger (i.e a cheap charger) this is very basic electronics -- full wave rectifier currents and voltage waveforms. The battery clamps the volts to between 14.5 and 15.5 depending on the type. The current is what matters. Put a current probe in the line or measure the volts across a small series resistance and you will see just how short is the time of conduction after 14V. I've searched for a suitable website for you but can't find one. Think of it this way...the current only flows when the voltage is greater than the battery volts.

You mention capacitors...that's entirely different as the volts rise to the peak level.

 

[castaway]

I paid £30 each, for the 4 x 6v units. All ex security back up batteries.

One has failed (after about a year) which is a pain as I can't get hold of another one to 'pair' it with. You experts will hate me for this but it is currently (forgive the pun) paired with a similar lead acid 120ah 6v similar sized batt.

I have had it on test in the garage all winter, and it has been charged and discharged and holds at 12.6 v almost indefinately .

I guess at £30 each, one can afford to take a chance.

Thanks for all the info..Nick