Equalising AGM batteries

[RobbieW]

As my AGM bank is now 4 years old, and still looking good, I asked the maker if equalising would be worthwhile. They are 6 x 6v deep cycle connected in 3 pairs to make a 12v bank. The reply I got was...

"...that these batteries do not need an equalisation charge and as long as the they are on a float charge at 13.6V they will equalise by themselves."

Is that a reasonable proposition ?

 

[MikeBz]

I would believe what the battery manufacturer tells you. There is an interesting (subscription only) article here http://www.morganscloud.com/2010/10/05/equalizing-batteries-the-reality/ which talks about equalising Lifeline AGM batteries.

 

[RobbieW]

I would believe what the battery manufacturer tells you. There is an interesting (subscription only) article here http://www.morganscloud.com/2010/10/05/equalizing-batteries-the-reality/ which talks about equalising Lifeline AGM batteries.

Thats mostly what prompted me to ask the maker. I'm questioning the answer because it doesnt fit within the context I have from that article. The chemistry cant be radically different so why would one battery need a charge at 15.5v to equalise and another can be allowed to float at 13.6v?

 

[halcyon]

Thats mostly what prompted me to ask the maker. I'm questioning the answer because it doesnt fit within the context I have from that article. The chemistry cant be radically different so why would one battery need a charge at 15.5v to equalise and another can be allowed to float at 13.6v?

Depends a lot on battery type and charge regime. Back in the 80's and 90's we manufactured chargers and supplied Westerly who had gone over to Delco Freedom batteries. Now due to the battery design they needed a higher charge voltage, so we reset the charger to 15.5 charge / 14.4 float, while normal charge was 14.3 volt charge / 13.6 float.

Normal charge cycle was based on the 10 hour charge rate, this meant that conversion of active material was achieved through the plate thickness. While though the measured DC voltage was 14.3 max, the charge pulse was 15.5 / 16.0 volt allowing a normal high charge level with minimum gassing. The 13.6 volt float then maintained a holding charge and a power supply function.

When you operated a high charge rate /short time regime, you got a rapid conversion of active material in the outer layers of the battery plate, but not the inner. This generated a high a charge voltage to allow conversion of the inner layers, thus the high voltage equalisation charge to complete conversion. Added to this the high voltage generated gassing and bubbles on the plate faces, acting as a insulator and further increase in voltage levels during equalisation

The longer charge regime did not need the equalisation phase, so total times were not that much different in normal use plus it reduced battery stress and battery water top up's.

It was / is more complex with charger types ( switchmode / transformer ) and better charge regimes were developed, but the above may give you a background.

Brian

 

[RobbieW]

Brian, thanks for the reply.
My understanding is that equalisation removes sulfation from the plates of a battery. Doing that requires a sort of shock to the mechanical properties of the battery usually delivered in the form of a higher than usual charging voltage for a fixed period.

I think your description is of a mixed technique using a particular form of charger that delivers pulses at a higher enough voltage to deal with sulfation overlaying a 'normal' charge voltage that allows the plates to retain the power being put in. That should negate the need for equalisation - is that a fair summary?

In a sense both the AAC (Morgans Cloud) article and my reply from the battery maker are 'marketing' statements because they give a single simple answer. It seems the engineering answer would be, as ever, it depends - on my charger as well as the design of the batteries. The AGM batteries I have are very similar in ah, size and weight to the Lifeline 6v AGMs.

I still dont get how applying a slightly higher, non 'shocking' float voltage over an indefinite period might deal with any accumulated sulfation. It may be that I have a huge gap in understanding, wouldnt be the first time

btw, there is no particular evidence of a loss in capacity that might be attributed to sulfation - its more that the AAC article suggests that AGM batteries in a liveaboard setup benefit from monthly equalisation if they will take the process. One difference is that I have a large enough bank to be able to keep discharges under 25% of capacity at present, rather than the 50% more commonly used, and, as far as the Link monitor tells me, fully charge the bank at least weekly.

 

[halcyon]

Brian, thanks for the reply.
My understanding is that equalisation removes sulfation from the plates of a battery. Doing that requires a sort of shock to the mechanical properties of the battery usually delivered in the form of a higher than usual charging voltage for a fixed period.

Sulphation is the opposite, it is due to over discharge and under charge resulting in grain growth in the active material. You also get surface charge ( conversion in material at the face of the battery plate ) and a high charge voltage, thus alternator reg will cut back on amperage to limit voltage, increasing the problem.

The other problem is that during charge you can get a build up of muck on the plate, which acts as a insulator adding to the sulphation problem, or may be is what people refer to as sulphation. All transformer chargers have a hi-voltage charge pulse which coupled to the reverse current flow between pulses tended to break this down and help keep the plates clean. This to a higher level we used in the maintenance phase to get to full charge level and clean the plates.

Brian

 

[RAI]

I thought the reason AGMs should not be given high voltages was because they are sealed, so the gas cannot get out. My one AGM has a overpressure safety valve. The manufacturer warns that if the valve is burst, the guarantee is shot.

 

[Plevier]

Equalisation and sulphation are different matters. Crudely speaking sulphation arises from being left discharged. That's not the whole story!

A battery that is fully charged and then goes onto long term float with no significant discharging generally does not need equalisation charges.
The more you cycle and recharge a battery, the more the state of charge of cells within it tend to become unequal due to minor variations of internal resistance etc. In a flooded battery you can detect this in hydrometer readings as well as voltage.
It does apply to SLA batteries as well. However if you equalise charge most SLAs more than a few times you may well start losing capacity because of water loss. Consequently manufacturers generally will say don't equalise charge them, because some clowns will then do it to excess.
When I was dealing with large SLA industrial battery installations (typically around 360V 400Ah), we did do equalisation charges as necessary on ones we maintained, and occasionally we would authorise customers - especially remote ones - to do a one off, but all the published information said "don't".
AFAIK Lifeline is the only one recommending regular equalisation. I don't know why their batteries tolerate it. Lifeline didn't exist when I was in the industry and their parent company Concord were way behind us in introducing SLA aircraft batteries. I have to assume that their batteries have more acid in and that they tolerate more loss from the pressure relief valves when equalising. In AGMs the acid is absorbed in the separators, and they can only be about 95% saturated to allow nascent gas transport for recombination, so again my assumption would be that they must use thicker separators which in turn will reduce high rate output performance and make charging a bit slower.
It may also be that they use higher specific gravity acid, raising all the voltages a bit, but I doubt it in long life batteries as it accelerates corrosion.

In summary, be very wary of doing an equalisation unless the manufacturer recommends it. Definitely do not do it monthly. Maybe annually. If you could get at individual cells to read the voltage, the key is if there is significant variation after standing for a few hours after a good period of float with no discharge. Presumably you can only read each 6V block, you can't really tell from that.

 

[Plevier]

I thought the reason AGMs should not be given high voltages was because they are sealed, so the gas cannot get out. My one AGM has a overpressure safety valve. The manufacturer warns that if the valve is burst, the guarantee is shot.

The valve is generally a little rubber top hat over a nozzle and will reseal, it's not a one off device.

 

[MikeBz]

I think it is probably useful to separate the terms 'equalisation' and 'desulphation'. I note that my recent Sterling A2B product has a 'desulphation' mode (15.5V approx) but there are (quite rightly) extreme warnings about its use in the manual.

Also be aware that (and this is a generalisation) US-made AGM batteries are different (and have different 'ideal' voltages for the various charging stages) from non-US ones. This will almost certainly affect any desulphation process as well.

 

[RAI]

The valve is generally a little rubber top hat over a nozzle and will reseal, it's not a one off device.

On mine, the only external sign is a pin hole in the top cover.

 

[Plevier]

On mine, the only external sign is a pin hole in the top cover.

Yes, that's where the gas escapes, the valve will be below with the cover holding it on.
The rubber cap system is proven and cheap and definitely used by numerous manufacturers. I can't say for certain that nobody uses a non-resealing device such as a bursting disc but I never heard of one.

 

[halcyon]

Equalisation and sulphation are different matters. Crudely speaking sulphation arises from being left discharged. That's not the whole story!

Is equalisation the same a destratification for batteries on long term float charge, when the electrolyte separates out, the high volt charge acts as a stir to give equalisation of electrolyte.

Remember when we started designing chargers in early 1980 the 'in' charger was Constavolt ( most other firms following spec ) that ran at a fixed 13.6 volt. They had read the float charge section for stationary batteries, but failed to take in the hi-voltage charge phase. Result reduced battery life, but all the marine chargers used the same voltage and all had literature that gave all the sales benefits.

Brian

 

[Plevier]

Is equalisation the same a destratification for batteries on long term float charge, when the electrolyte separates out, the high volt charge acts as a stir to give equalisation of electrolyte.


Brian

Stratification was certainly a phenomenon with Planté telephone battery cells that floated at 2.15VPC and had up to 2ft acid depth (up to 75 litres per cell!) in them. The 10% constant current monthly refresh charge dealt with it though.

In my personal experience, I've never knowingly encountered stratification as a problem needing specific addressing in anything floating at 2.25vpc or above. I doubt if it can occur in AGMs but can't say that with certainty.

 

[Plevier]

Also be aware that (and this is a generalisation) US-made AGM batteries are different (and have different 'ideal' voltages for the various charging stages) from non-US ones.

Don't think I can accept that even as a generalisation - would you like to expand a bit?

 

[halcyon]

Stratification was certainly a phenomenon with Planté telephone battery cells that floated at 2.15VPC and had up to 2ft acid depth (up to 75 litres per cell!) in them. The 10% constant current monthly refresh charge dealt with it though.

In my personal experience, I've never knowingly encountered stratification as a problem needing specific addressing in anything floating at 2.25vpc or above. I doubt if it can occur in AGMs but can't say that with certainty.

We found it with flooded batteries, not been making charger for a bit so not done deep testing with AGM, but the charge regime covered the problem if present.

Brian

 

[Plevier]

Sulphation is the opposite, it is due to over discharge and under charge resulting in grain growth in the active material.

I think damaging sulphation is more due to being left discharged or undercharged than being over discharged (i.e. very deeply discharged) per se.
The discharged state of both the positive and negative plates is lead sulphate PbSO4, so in a sense sulphation occurs in every cycle, and any recharge process is desulphation.
The charged state of the +ve plate is lead dioxide PbO2 and of the negative plate is spongy lead.
On both plates there is a volumetric change of the active material between charged and discharged and particularly on the +ve plate this does mechanical damage leading to loss of contact and shedding of material. The deeper the discharge the more the damage.
If it's recharged promptly you can get effectively total conversion back to charged state (until the plates are so damaged by cycling that you start getting isolated dead areas which won't recharge).
If it's left discharged for some time it certainly becomes harder to recharge. That could be a result of changing grain structure, I don't know for sure.
What's really damaging is if it's left for a really long time, you may see white areas on the plates. Those are PB2SO5, still sometimes called lead sulphate, more correctly lead oxide sulphate. That seems impossible to get rid of.
I'm not a great believer in desulphation cycles. A correctly treated battery shouldn't need it. I suppose it might help in restoring a battery that's been left discharged for a while but I haven't seen a lot of evidence. I have used pretty severe constant current charge regimes for that purpose with some success.

Excessively deep discharging is particularly bad in AGMs because it can turn the electrolyte virtually to water whereas in flooded cells the s.g. will not normally go below about 1.1. This makes the start of recharge difficult because of low conductivity, also lead sulphate has some solubility in water whereas it is negligible in sulphuric acid above a low concentration.

 

[RobbieW]

Thank you everyone for an informative and educational discussion. I was guilty of believing that 3 conditions - sulphation, cell inequality & stratification - had but one remedy called equalisation. I know differently now and understand battery chemistry a little better than before.

I went back to the battery maker and asked for their recommended bulk/absorption voltage as well as the float so will now be setting my charging systems to 14.6 & 13.6 respectively (or as close as I can get). For the record the maker is DMS, whose own brand are RedFlash although they no longer supply the 6v GC2 format deep cycle batteries I have.

 

[MikeBz]

Don't think I can accept that even as a generalisation - would you like to expand a bit?

Apologies if I've got that wrong. I did a lot of reading up on various 'smart' battery chargers and AGM battery products and formed that view bolstered by the fact that my A2B (and I thought some other chargers I looked at) has different settings for 'US' and 'Euro' spec AGM batteries. Maybe it's a marketing gimmick, or is a misrepresentation of differences between spiral wound and non-spiral wound battery requirements? There is a lot of variation on manufacturer advice hence the need for caution before anyone jumps in and has a go at desulphating their batteries.

Optima claim that "Sealed AGM batteries do not sulfate or degrade like flooded batteries do, since they are not vented to the atmosphere" (http://www.optimabatteries.com/en-us/technology/agm-batteries). It seems like a strange claim to make (other types of sealed battery are available which do sulphate) and contradicts the advice that Morganscloud has had from Lifeline.

 

[Plevier]

I went back to the battery maker and asked for their recommended bulk/absorption voltage as well as the float so will now be setting my charging systems to 14.6 & 13.6 respectively (or as close as I can get). For the record the maker is DMS, whose own brand are RedFlash although they no longer supply the 6v GC2 format deep cycle batteries I have.

DMS Red Flash batteries are badged from more than one supplier. I know what the high rate ones are but I don't know where they source the deep cycle range. I note they are offering Lifeline now as well. I believe they only sell good stuff and they are technically competent not just box shifters.