Charging an AGM Lead acid battery

[cowana]

I have a 12v, 7Ah AGM lead acid battery that I am not able to charge.

I am using a smart Ripmax Pro Peak Constellation charger. When I tell it to discharge the battery at 1A, the battery voltage falls to about 11.somethingV and the charger tells me it is finished. When I tell it to charge the battery, the battery voltage rises to about 14.1v, and the charger tells me it is finished. I am unable to charge it.

Is the battery completly useless now? Or should I buy an overnight non-smart trickle charger for it? Is there any way to get it to take a charge? I'm tempted to get a regular battery charger with no microchips in it and see what that does - my charger is too smart for its own good!

Any thoughts?

Andrew

 

[cowana]

I've just checked the terminal voltage 40 minutes after trying to charge it, and it is 12.46v.

Andrew

Edit - that's after trying it on the smart charger - that is the voltage it is stuck at. It won't charge or discharge.

 

[bychon]

Using a "stupid" charger is a way to help diagnose the problem, just as trying a new battery on the smart charger would be a way to diagnose which is bad. Good idea you had. Save it for the next time a battery looks strangely bad.

 

[cowana]

My last post wasn't very clear - the 12.46v is the rest voltage - the smart charger refuses to do anything. Should I buy a dumb charger? I don't have one at the moment.

However, I do have a 19v/5a max power supply with current and voltage limitting - when I connect the battery, the terminal voltage rises to whatever the input voltage is, and very little current flows. Any ideas?

Andrew

 

[bychon]

My idea is that the battery is D.O.A.

Try discharging it with a resistor, a car tail light, whatever. With the voltage it's showing, it should run the load a good long time because it's pretending to be fully charged. If it empties in a matter of minutes, it just demonstrated that it's not holding power like a good battery should.

 

[MikeMl]

If you have allowed any lead-acid battery (flooded or sealed) to sit around for months to years without recharging it at least every month, the battery's capacity will deteriorate to where it has only a small fraction of its rated A-H capacity. This is due to the formation of Sulphate on the plates. A single recharge does not remove hard Sulphates.

There is only one way to store unused or rarely used lead-acid batteries: keep them fully charged. If fully-charged, the Specific Gravity of the acid is high, which is the only thing that dissolves Sulphates. If the state-of-charge is 75% or less, the S.G is low enough to promote growth of Sulphates. The only thing that moves the S.G. to a range where the Sulphates are dissolved is keeping the battery at a state-of-charge > 95%.

This requires either a continuous "float" charger, or recharging once every two weeks or so. If you put them away discharged, or do not make up for their self-discharge (by floating or periodic recharging), they will self-discharge, reducing the S.G., and promoting Sulphation. Think about this: why does a lead-acid battery in a car that is driven daily last so much more than one in a car that is driven only once a month?

If a lead-acid battery is Sulphated, it is possible to partially restore it. This is done by raising the S.G. and holding it there for a few days. To get the S.G. up to the range where this can happen, charge the battery using a conventional charger. Then use a constant-current power supply set to a low-value (say 2A for a 70AH battery, much less for a small AGM) and leave it applied for up to 72 hours. Under constant-current charge, the battery voltage will climb higher than normal, (~14.8V for a 12V battery). Holding the S.G. at this high value for 1 to 3 days will dissolve what Sulphate can be removed, and the capacity of the battery will be improved (but not as good as new). btw- the name for this is "Equalization".

The infamous "pulse chargers which claim to dissolve Sulphates" floating around on the web are just a pulsed constant-current supply that take weeks to accomplish what a constant-current D.C. supply can do in about 2 days. Caution: Equalization does use up some of the electrolyte in the battery. If the battery has caps, you can add distilled water. If the battery is sealed, there is no way to put back the electrolyte. For this reason, it is better to maintain unused AGMs in a fully-charged state, rather than trying to recover them after years of sitting around discharged.

 

[cowana]

Many thanks Mike.

The batteries are only £10 new, but I'll have a play and see what I can do.

Thanks for the detailed reply!

Andrew

 

[cowana]

The latest:

I tried a load test, putting on a 50W bulb, and the voltage dropped to 9v.

I then connected it to my power supply set at 14.8v. The initial current was 500mA, dropping to 130mA within a few seconds. After one hour, the current it 20mA. I'll leave it like that for a while and see what happens.

Andrew

 

[cowana]

Just to say that after about four hours of this, I noticed the battery was quietly fizzing, so I disconnected it. After letting it stand for a few more hours, I'll stick it back on the charger and see if it can be charged.

Andrew

 

[cowana]

Answer - no.

That battery is on its way to the bin.

Andrew

 

[tvtech]

If you have allowed any lead-acid battery (flooded or sealed) to sit around for months to years without recharging it at least every month, the battery's capacity will deteriorate to where it has only a small fraction of its rated A-H capacity. This is due to the formation of Sulphate on the plates. A single recharge does not remove hard Sulphates.

There is only one way to store unused or rarely used lead-acid batteries: keep them fully charged. If fully-charged, the Specific Gravity of the acid is high, which is the only thing that dissolves Sulphates. If the state-of-charge is 75% or less, the S.G is low enough to promote growth of Sulphates. The only thing that moves the S.G. to a range where the Sulphates are dissolved is keeping the battery at a state-of-charge > 95%.

This requires either a continuous "float" charger, or recharging once every two weeks or so. If you put them away discharged, or do not make up for their self-discharge (by floating or periodic recharging), they will self-discharge, reducing the S.G., and promoting Sulphation. Think about this: why does a lead-acid battery in a car that is driven daily last so much more than one in a car that is driven only once a month?

If a lead-acid battery is Sulphated, it is possible to partially restore it. This is done by raising the S.G. and holding it there for a few days. To get the S.G. up to the range where this can happen, charge the battery using a conventional charger. Then use a constant-current power supply set to a low-value (say 2A for a 70AH battery, much less for a small AGM) and leave it applied for up to 72 hours. Under constant-current charge, the battery voltage will climb higher than normal, (~14.8V for a 12V battery). Holding the S.G. at this high value for 1 to 3 days will dissolve what Sulphate can be removed, and the capacity of the battery will be improved (but not as good as new). btw- the name for this is "Equalization".

The infamous "pulse chargers which claim to dissolve Sulphates" floating around on the web are just a pulsed constant-current supply that take weeks to accomplish what a constant-current D.C. supply can do in about 2 days. Caution: Equalization does use up some of the electrolyte in the battery. If the battery has caps, you can add distilled water. If the battery is sealed, there is no way to put back the electrolyte. For this reason, it is better to maintain unused AGMs in a fully-charged state, rather than trying to recover them after years of sitting around discharged.

Brilliant post and worthy of Rep.

Rep given.