Equalising

[MtB]

Received wisdom seems to be one must fully charge one's batteries prior to commencing equalisation, getting the tail current down to about 1%, or 2%, or 4% (all figures I've seen stated in various places) and stable for 45 minutes or so.

 

My 24V battery bank takes about three hours to get down from say 90% SoC to a stable tail current for 45 mins. Then a further three hours of equalisation adds up to a LOT of generator time with a lot of it at pretty low charging current.

 

Is there any harm (or is it pointless) putting the Sterling Pro Charge Ultra into the equalisation cycle once the charge current gets down to about 8A (the approximate charge current during equalisation)? This will save about two hours of tedious generator running!

[nicknorman]

Received wisdom seems to be one must fully charge one's batteries prior to commencing equalisation, getting the tail current down to about 1%, or 2%, or 4% (all figures I've seen stated in various places) and stable for 45 minutes or so.

 

My 24V battery bank takes about three hours to get down from say 90% SoC to a stable tail current for 45 mins. Then a further three hours of equalisation adds up to a LOT of generator time with a lot of it at pretty low charging current.

 

Is there any harm (or is it pointless) putting the Sterling Pro Charge Ultra into the equalisation cycle once the charge current gets down to about 8A (the approximate charge current during equalisation)? This will save about two hours of tedious generator running!

Probably doesn't matter too much. I think DMR advocates this approach for the reasons you mention. The only thing to consider is that increasing the voltage above the optimum actually reduces the ability of the batteries to absorb charge (due to all the bubbles) so if you start to equalise too soon, you might end up not fully charging - but probably only in an extreme case. Also make sure the batteries aren't getting hot.

 

Anyway, what happened to the plan to use a buck converter to equalise one battery from the other?

[MtB]

Probably doesn't matter too much. I think DMR advocates this approach for the reasons you mention. The only thing to consider is that increasing the voltage above the optimum actually reduces the ability of the batteries to absorb charge (due to all the bubbles) so if you start to equalise too soon, you might end up not fully charging - but probably only in an extreme case. Also make sure the batteries aren't getting hot.

 

Anyway, what happened to the plan to use a buck converter to equalise one battery from the other?

 

 

Thanks for your answer Nick. As it happens I've been out cruising and boozing with DMR recently and we've discussed this already in an unfocused way, but now he's not here so I thought I'd seek opinions from from board about whether there were any negative effects in particular.

 

I've been equalising for an hour now at 31.6V and the current has stabilised at 7.5A. I equalised a few days ago too for about 90 mins. Can these two equalisation sessions be added together for desulphation purposes? Or does one have to start from scratch each time. I have a refractometer now and I'll take some readings when the genny runs out of petrol...

 

I've yet to set up the buck converter as the new Yuasas last a whole week before falling to 50% SoC, so not found the need. Not sure the gash donkey batteries have enough life in them though to do a proper equalise.

 

Curiously the SG gets at quite a wide variance with the BMV-702 in the early stages of discharge. It can fall to 90% overnight after day's cruising yet the BMV-702 shows only 7AH drawn from the batts. I'm hoping this is the SG struggling to estimate accurately rather than my new batts being down to less than half capacity already! This is a reason for my interest in equalising, or rather desulphating.

[nicknorman]

Thanks for your answer Nick. As it happens I've been out cruising and boozing with DMR recently and we've discussed this already in an unfocused way, but now he's not here so I thought I'd seek opinions from from board about whether there were any negative effects in particular.

 

I've been equalising for an hour now at 31.6V and the current has stabilised at 7.5A. I equalised a few days ago too for about 90 mins. Can these two equalisation sessions be added together for desulphation purposes? Or does one have to start from scratch each time. I have a refractometer now and I'll take some readings when the genny runs out of petrol...

 

I've yet to set up the buck converter as the new Yuasas last a whole week before falling to 50% SoC, so not found the need. Not sure the gash donkey batteries have enough life in them though to do a proper equalise.

 

Curiously the SG gets at quite a wide variance with the BMV-702 in the early stages of discharge. It can fall to 90% overnight after day's cruising yet the BMV-702 shows only 7AH drawn from the batts. I'm hoping this is the SG struggling to estimate accurately rather than my new batts being down to less than half capacity already! This is a reason for my interest in equalising, or rather desulphating.

 

So when the SG falls to 50%, what does the BMV show? This is a much better guide to capacity than when the SG shows 90%.

 

I've not found the SG to be that accurate in the 90s%, I think there are too many issues with surface charge, temperature, electrolyte strength (I find the SG drops the first few % much quicker when the batteries have recently been topped up, compared to when they need topping up) and how recently they were cycled (the SG SoC always seems to fall more quickly on the first discharge after the batteries have been on float for a week+). And in your case, wasn't your SG's voltage over-reading?

 

I would say that both sulphation, and de sulphation, are cumulative processes and so desulphation doesn't have to be done all at once.

Edited December 30, 2016 by nicknorman

[MtB]

So when the SG falls to 50%, what does the BMV show? This is a much better guide to capacity than when the SG shows 90%.

 

Dunno, I've never remembered to look before starting charging. I'll taken particular note next time the SG shows around 50%.

 

 

I've not found the SG to be that accurate in the 90s%, I think there are too many issues with surface charge, temperature, electrolyte strength (I find the SG drops the first few % much quicker when the batteries have recently been topped up, compared to when they need topping up) and how recently they were cycled (the SG SoC always seems to fall more quickly on the first discharge after the batteries have been on float for a week+). And in your case, wasn't your SG's voltage over-reading?

 

I was taking the 31.6 voltage reading from the display on the Sterling Pro Charge Ultra. And yes the SG still over-reads. One day I'll sort that out but for now I adjust in my head when I use it for a voltage reading.

 

 

 

I would say that both sulphation, and de sulphation, are cumulative processes and so desulphation doesn't have to be done all at once.

 

 

This is what I thought too and was hoping you'd think the same!

[MtB]

Ok, ran out of petrol after two hours of de-wossnaming. Added to the one hour about three days ago I'm calling them 'done'.

 

SG results as follows:

 

Battery 1:

1.260

1.258

1.260

 

Battery 2:

1.264

1.266

1.266

 

Battery 3:

1.267

1.264

1.254

 

Battery 4:

1.262

1.263

1.262

 

 

Cell 3 in battery 3 looks a bit suspect to me...

[nicknorman]

Did you adjust the gravities for temperature?

[MtB]

No, they are the direct readings. No instructions came with it for temperature compensation.

 

The prism was at room temp (about 22C) at the start of checking battery 1.

 

The battery room is unheated and air temp in there I'd estimate to have been about 15C. The batteries themselves are probably about the same.

 

It took about 15 mins to take all the readings during which time the prism probably fell in temperature to about 17C but stupidly, I have no thermometer on ze bote!

[nicknorman]

No, they are the direct readings. No instructions came with it for temperature compensation.

 

The prism was at room temp (about 22C) at the start of checking battery 1.

 

The battery room is unheated and air temp in there I'd estimate to have been about 15C. The batteries themselves are probably about the same.

 

It took about 15 mins to take all the readings during which time the prism probably fell in temperature to about 17C but stupidly, I have no thermometer on ze bote!

Ok so I think if you are using a refractometer type, the relevant temperature is the prism temperature since the single drop will presumably warm or cool very easily. Also I think those type of hydrometers (refractometer) have a degree of automatic compensation within a reasonable range. So it is then a question of what the original sg was. And there will of course be some variation depending on the electrolyte level.

 

Anyway I only have experience of my Trojans which have an original sg of 1.277 so your figures look a touch on the low side, but not much. Obviously the 1.254 one is lower than the rest, but Trojan only recommend equalising if there is more than 0.030 difference between cells.

 

Anyway, the "acid test" (pun intended) will be to note the AH taken out when the Smartgauge next gets near 50%.

Edited December 30, 2016 by nicknorman

[WotEver]

Just to confirm the 'SmartGauge in the 90's' observation. Yes it can be a bit wobbly up there, I discussed it with Gibbo once and he kinda said "So what?"

 

Once it's in the mid-80's and lower it should be pretty much bang on though.

[MtB]

Thanks Nick.

 

Yes I'm using a proper refractometer not one of those bulb type hydrometers.

 

And yes I'll post the number of AH recorded next time the SG gets down to 50%. This will take a week or so to happen...

Just to confirm the 'SmartGauge in the 90's' observation. Yes it can be a bit wobbly up there, I discussed it with Gibbo once and he kinda said "So what?"

Once it's in the mid-80's and lower it should be pretty much bang on though.

 

 

Curiously I notice the SG 'likes' the value 74%. It will tumble to 74% after charging and sometimes display it for a full 24 hours before the display begins descending further.

[cuthound]

Thanks for your answer Nick. As it happens I've been out cruising and boozing with DMR recently and we've discussed this already in an unfocused way, but now he's not here so I thought I'd seek opinions from from board about whether there were any negative effects in particular.

 

I've been equalising for an hour now at 31.6V and the current has stabilised at 7.5A. I equalised a few days ago too for about 90 mins. Can these two equalisation sessions be added together for desulphation purposes? Or does one have to start from scratch each time. I have a refractometer now and I'll take some readings when the genny runs out of petrol...

 

I've yet to set up the buck converter as the new Yuasas last a whole week before falling to 50% SoC, so not found the need. Not sure the gash donkey batteries have enough life in them though to do a proper equalise.

 

Curiously the SG gets at quite a wide variance with the BMV-702 in the early stages of discharge. It can fall to 90% overnight after day's cruising yet the BMV-702 shows only 7AH drawn from the batts. I'm hoping this is the SG struggling to estimate accurately rather than my new batts being down to less than half capacity already! This is a reason for my interest in equalising, or rather desulphating.

 

Putting it into equalise early will not do any harm other than to gas off a bit more electrolyte.

 

If the batteries are not heavily sulphated, then equalising will occur relatively quickly.

 

However if the batteries are heavily sulphated, the lead sulphate crystals grow quite large and can take a long time to break down (if at all) at the higher equalising voltage.

 

When BT used open cells (they started changing over to VRSLA's from 1980) they used to equalise every 3 months for 3 three hours on top of a normal charge.

 

If your batteries have never been left discharged for long, the lead sulphate crystals will not have grown large, and thus will be easily converted back to lead and H2SO4.

[MtB]

If your batteries have never been left discharged for long, the lead sulphate crystals will not have grown large, and thus will be easily converted back to lead and H2SO4.

 

 

Thanks for your reply. Points duly noted.

 

This bit could do with picking apart a little. My batteries are about three months old or a little less. I can't remember exactly when I bought them but not long ago. Terminal voltage when I got them home was 12.8V (IIRC), all exactly the same so I was happy they were in good nick from the supplier.

 

Having installed them I find they discharge to 50% in about a week according to the SG, so I charge them once a week. This means most of the time they are not 'fully charged'. However they are not being 'left discharged for long', they are in use intermittently the whole time keeping the fridge cold.

 

Even so, my batteries spend approaching 100% of their time not fully charged, as do most liveaboard batteries in constant use. Do the lead sulphate crystals 'grow large' in the week between charging? Or do they continue to grow all the time between equalisation/desulphation charges?

[WotEver]

They continue to grow but in a week won't have grown terribly big or hard. Nevertheless, regular (I'd take a guess at monthly) equalisation would probably benefit them just to get rid of stubborn crystals.

[MtB]

They continue to grow but in a week won't have grown terribly big or hard. Nevertheless, regular (I'd take a guess at monthly) equalisation would probably benefit them just to get rid of stubborn crystals.

 

 

So me not realising the importance of frequent equalising and failing to do it for two months+ might be very bad news?

[WotEver]

So me not realising the importance of frequent equalising and failing to do it for two months+ might be very bad news?

Let's find out next time you get down to 50%. Then we can all learn

[nicknorman]

So me not realising the importance of frequent equalising and failing to do it for two months+ might be very bad news?

Probably not. There is a compromise to be struck - equalising causes plate shedding and corrosion so should be done no more than necessary. Anyway you sg values are reasonably healthy.

[MtB]

Yes, I appreciate I'm jumping the gun.

 

"Patience, Grasshopper", as someone once said...

[cuthound]

Thanks for your reply. Points duly noted.

 

This bit could do with picking apart a little. My batteries are about three months old or a little less. I can't remember exactly when I bought them but not long ago. Terminal voltage when I got them home was 12.8V (IIRC), all exactly the same so I was happy they were in good nick from the supplier.

 

Having installed them I find they discharge to 50% in about a week according to the SG, so I charge them once a week. This means most of the time they are not 'fully charged'. However they are not being 'left discharged for long', they are in use intermittently the whole time keeping the fridge cold.

 

Even so, my batteries spend approaching 100% of their time not fully charged, as do most liveaboard batteries in constant use. Do the lead sulphate crystals 'grow large' in the week between charging? Or do they continue to grow all the time between equalisation/desulphation charges?

They continue to grow but in a week won't have grown terribly big or hard. Nevertheless, regular (I'd take a guess at monthly) equalisation would probably benefit them just to get rid of stubborn crystals.

Probably not. There is a compromise to be struck - equalising causes plate shedding and corrosion so should be done no more than necessary. Anyway you sg values are reasonably healthy.

In the time it has taken me to excersise the Hound, WotEver and Nick has given you answers that I can add nothing to.

 

As long as you periodically equalise, say every couple of months, whilst monitoring available battery capacity you can increase or decrease the time between equalisations if capacity seems to reduce.

 

You won't grow irreversible crystals in a week, provided you recharge once a week to as close to 100% as practicable.

[dmr]

I reckon Trojans can take a lot of abuse. I suspect I have let mine get quite sulphated in early spring believing that they will still come back when we start doing long cruising days in the summer. I half remember needing to do a long hard equalisation (a few hours at 16volts) a year or two ago in the winter when I let them get quite sulphated.

 

Some might remember that one of my Trojans did fail quite early, after about 1200 cycles I think, but the other pair are still going now and are just over 5 years old and do some sort of cycle almost every day. I don't know exactly how much capacity they have left as they are in a mixed bank of two of the original Trojans and 4 more recent ones (tut tut). I will replace this original pair later this winter.

 

..............Dave

[MtB]

Ok I'm still jumping the gun a bit but things don't look good.

 

I've left a few lights on all day to draw a bit from the batteries and accelerate arriving at 50% SoC, and my SG is now reading 54% SoC.

 

The BMV says 47.5ah have been drawn from the batteries corresponding to this fall from 100% to 54% SoC.

 

I'm off out now to fix a boiler and I'm expecting to see 50% SoC on the SG by the time I get back!

[dmr]

A battery is only as good as its weakest cell.

Not really thought about this before, but this is a weakness with a 24 volt system as the whole lot is limited by that single weakest cell, whilst in a typical 12volt system a single weak cell will only degrade half the bank (4 Trojans) or a quarter (4 standard leisures).

 

Your specific gravities are still a bit low. I have no refractometer experience and I suspect Nick is right that the acid will quickly warm to the refractometer temperature (Nick is one of those irritating people who is always right!) but your engine room is quite cold, I bet well below 15 deg C. Battery gravities are quoted at 25 degC so at 15 you need to subtract almost .01 so your 1.26 is actually 1.25 which is only 85% state of charge. Likewise your dodgy 1.255 cell is only 1.245, or even 1.240 if your refractometer is only 10 degC so thats down to 80%

capacity.

Sooo...if you want to stay above that magical and mythological 50% you have lost 2/5 of your available capacity.

 

I think more equalisation to try to get all the cells higher, especially the weak ones.

 

...............Dave

[nicknorman]

Ok I'm still jumping the gun a bit but things don't look good.

 

I've left a few lights on all day to draw a bit from the batteries and accelerate arriving at 50% SoC, and my SG is now reading 54% SoC.

 

The BMV says 47.5ah have been drawn from the batteries corresponding to this fall from 100% to 54% SoC.

 

I'm off out now to fix a boiler and I'm expecting to see 50% SoC on the SG by the time I get back!

I know it is elsewhere on here somewhere, but too lazy to look it up... can you remind us exactly what model of batteries you have and what total AH (at 24v presumably).

[ditchcrawler]

I reckon Trojans can take a lot of abuse. I suspect I have let mine get quite sulphated in early spring believing that they will still come back when we start doing long cruising days in the summer. I half remember needing to do a long hard equalisation (a few hours at 16volts) a year or two ago in the winter when I let them get quite sulphated.

 

Some might remember that one of my Trojans did fail quite early, after about 1200 cycles I think, but the other pair are still going now and are just over 5 years old and do some sort of cycle almost every day. I don't know exactly how much capacity they have left as they are in a mixed bank of two of the original Trojans and 4 more recent ones (tut tut). I will replace this original pair later this winter.

 

..............Dave

Well mine have been abused, went too many weeks without checking the electrolyte, I expected that to kill them but still doing well enough for me at the moment.

[Mikexx]

A battery is only as good as its weakest cell.

Not really thought about this before, but this is a weakness with a 24 volt system as the whole lot is limited by that single weakest cell, whilst in a typical 12volt system a single weak cell will only degrade half the bank (4 Trojans) or a quarter (4 standard leisures).

 

Your specific gravities are still a bit low. I have no refractometer experience and I suspect Nick is right that the acid will quickly warm to the refractometer temperature (Nick is one of those irritating people who is always right!) but your engine room is quite cold, I bet well below 15 deg C. Battery gravities are quoted at 25 degC so at 15 you need to subtract almost .01 so your 1.26 is actually 1.25 which is only 85% state of charge. Likewise your dodgy 1.255 cell is only 1.245, or even 1.240 if your refractometer is only 10 degC so thats down to 80%

capacity.

Sooo...if you want to stay above that magical and mythological 50% you have lost 2/5 of your available capacity.

 

I think more equalisation to try to get all the cells higher, especially the weak ones.

 

...............Dave

 

While an equalisation charge is intended to removed sulfation (or is it sulphation?) a secondary purpose is to stir the electrolyte which otherwise stratifies.

 

There's a chance that the low cell's SG will increase and match the others. I've read an article to specifically apply an equalisation charge when there is a mismatch in SG.