Is Lead Acid really this sluggish?

CALLD · October 2014

Another observation that makes my head spin:

Just got back from a 2 week holiday during which I left my supposedly 100% SOC batteries on float charge voltage of 27.6v (24v system).
First thing to note is I have never bothered to use a proper hydrometer to measure SOC out of shear laziness and the fact that they all have those little built-in "magic-eye" thingy's that go red when the batteries are supposedly flat, white when they are somewhere in the middle and green when you've been a good boy.

In daily use no more than half of the batteries ever show "Green", they just seem to take turns changing between white and green. Even after once attempting to get them all green by leaving them on float charge for 3 days I never managed to get all of them to say "Good Boy!".

After returning from my trip 2 weeks later they had all gone green - my reward for not making any use of them for 2 weeks! There was a noticeable improvement in the discharge profile for a few days after returning them to service, holding their voltage under heavy loads improved, but has now returned to the profile I was seeing before the trip more or less.

So the question is what happened?

Was there hard sulfation that broke loose over the two weeks on float charge?
Do my batteries really need 2 weeks to get to 100% saturated charge?

By the very nature of my system charging is always at a gentle current.
Bulk charging current averages at C/20, peaking at C/15 under the best PV conditions. I have my daily absorb voltage set at 29.6 volts.

I'm a little confused - I thought batteries needed no more than 8 to 16 hours to achieve 100%

D

stephendv · October 2014

Re: Is Lead Acid really this sluggish?

CALLD wrote: »

I have my daily absorb voltage set at 29.6 volts.
I'm a little confused - I thought batteries needed no more than 8 to 16 hours to achieve 100%

How long is the absorb period?

CALLD · October 2014

Re: Is Lead Acid really this sluggish?

stephendv wrote: »

How long is the absorb period?

2 hours factory set on the charge controller, cannot be changed

Cariboocoot · October 2014

Re: Is Lead Acid really this sluggish?

Deep cycle batteries don't have a "magic eye". This is a built-in hydrometer used in automotive batteries. If you follow the instructions you see it actually tells you to "gently rock" the batteries after charging if it has gone down to red in order to better mix the electrolyte and get the "eye" to read correctly. It is in fact the eye that is sluggish, not the battery.

It sounds like you are using the wrong sort of batteries. If so, good luck ever getting the system to work right - or last. You haven't got enough PV for 400 Amp hours @ 24 Volts either.

CALLD · October 2014

Re: Is Lead Acid really this sluggish?

Cariboocoot wrote: »

Deep cycle batteries don't have a "magic eye". This is a built-in hydrometer used in automotive batteries. If you follow the instructions you see it actually tells you to "gently rock" the batteries after charging if it has gone down to red in order to better mix the electrolyte and get the "eye" to read correctly. It is in fact the eye that is sluggish, not the battery.

It sounds like you are using the wrong sort of batteries. If so, good luck ever getting the system to work right - or last. You haven't got enough PV for 400 Amp hours @ 24 Volts either.

True, they are semi-deep cycle batteries, been in daily operation since April this year. I have my eyes set on 4x Sonx 12v260Ah AGM deep cycle batteries the moment these batteries kick the bucket. The new setup will then be 24v 520Ah.

How much PV will I need for that?

Cariboocoot · October 2014

Re: Is Lead Acid really this sluggish?

CALLD wrote: »

True, they are semi-deep cycle batteries, been in daily operation since April this year. I have my eyes set on 4x Sonx 12v260Ah AGM deep cycle batteries the moment these batteries kick the bucket. The new setup will then be 24v 520Ah.

How much PV will I need for that?

520 Amp hours @ 24 Volts? Using the standard 10% rule-of-thumb you want 52 Amps peak current so: 52 * 24 / 0.77 = 1620 Watts minimum. Of course you have to adjust to suit available panels, preferably by rounding up.

CALLD · October 2014

Re: Is Lead Acid really this sluggish?

Cariboocoot wrote: »

520 Amp hours @ 24 Volts? Using the standard 10% rule-of-thumb you want 52 Amps peak current so: 52 * 24 / 0.77 = 1620 Watts minimum. Of course you have to adjust to suit available panels, preferably by rounding up.

I hear you, and have seen the C/10 charging rule of thumb many times before but that sort of goes against much of what a good off-grid system is all about doesn't it? I've seen advice that calls for 5 days of reserve capacity. My 880watt system can provide me with 7kwh on a clear mid-summer's day. That's nearly 300Ah or 75% of my 400Ah battery bank. It could be more if I had an MPPT charge controller. If I had double the PV as you advise that would be more than the 520AH need to charge from 80%DOD. So what is the point of having so much PV? Would I not be better off getting a powerful battery charger to run off the grid or generator if the batteries need so much current?

D

Cariboocoot · October 2014

Re: Is Lead Acid really this sluggish?

Forget that "5 days reserve capacity" nonsense. It is economically unsound. You would be buying a huge battery bank and massive array that will sit there doing nothing most of the time. Waste of money!

Instead go with this simpler concept: 25% average daily Depth Of Discharge. Since you can take standard Lead-acid deep cycle batteries down to 50% if need be, you have two days capacity right there. On the third day start the generator. They are a much cheaper source of "when needed" power than solar. If you have a location that suffers from habitually poor insolation, increase the array size but not the battery capacity. You may have to limit the maximum current, but PV is a lot less expensive than batteries these days.

You also size your array for your minimum solar day, not your maximum or even average. Up here that's impractical because we get down to 6 hours of daylight in Winter and only 2 of it usable. In Summer it's 16 hours of daylight, but not all of that will fall directly on the panels.

Batteries do not charge on a basis of "just put back the Amp hours used". In the old days when PV was expensive and batteries were cheap the suggestion was that a small amount of solar would recharge a large amount of battery "eventually". What really happened was you bought new batteries fairly often as the old ones sulphated prematurely. You can't afford to do that with batteries these days. So minimum net charge rate needs to be met and SOC needs to be brought up above 75% daily if possible in order to insure enough battery life to make it worth the expense.

Of course if you've got grid, why have batteries at all? They are not an economical back-up source in most cases.

BTW your equation of 7 kW hours to 300 Amp hours is inaccurate as it excludes charging losses. In terms of useful AC production 880 Watts is capable of 1800 Watt hours on a typical battery system due to the rather high losses involved with such a system (based on 4 hours of good sun which most people can count on). This can be increased with load management if the otherwise unrealized PV potential available as the batteries are at or near full can be tapped.

CALLD · October 2014

Re: Is Lead Acid really this sluggish?

Ok, without straying too far off topic - I understand what has been put so far. Critiques will no doubt enjoy having a go at my amateur setup and that's absolutely fine because it was made by me, a complete novice at the time of conception, but as time passes and my knowledge and first-hand experience grows it will evolve and improve as long as it doesn't kill me in the process... But it's all a lot of fun!

The thing I'm trying to get my head around here is how these batteries behave under different circumstances. They are cheap and nasty for sure - 1 dollar per 12v amper hour! So far they have held up ok, doing their job without a fuss, never having failed to provide whatever the inverter has demanded from them. So far so good. I don't expect them to last me forever, they are my training wheels. The purpose of this thread was to try and find out why they took so long to accept a full charge. I suspected that maybe my charge controller's 2 hour absorb stage is too short, but then what more can a solar charge controller accomplish when "4hours of useful sun is all one can expect on most days" Here in SA we can get a lot more in summer - I see 20 amps at 08:30, 26 amps at 12:30 and 20 amps at 16:30 this time of year (spring). That's 8 hours of useful sun in spring. Overcast with thin clouds it's about half that. Overcast with thick clouds it peaks at a dismal 4amps. But those days are rare here, most of the time is sunny for 70% or more of the day.

Back to my battery observation, I suspect that if it's not a shortage of time on absorb stage it's the type of battery - silver calcium which requires higher charge voltage than lead antimony, or that they have permanent hard sulfation which has increased internal resistance and cut their capacity. Strangely though if that is the case then why are they still going strong and steady? Shouldn't their sudden demise be imminent?

zoneblue · October 2014

Re: Is Lead Acid really this sluggish?

Thing is mate, your using a bunch of sub optimal gear, and getting a sub optimal result. Only one way to fix that....

Any charge controller that can't have the absorb setpoints and ending amps set correctly, belongs in the trash.

CALLD wrote: »

They are cheap and nasty for sure - 1 dollar per 12v amper hour! So far they have held up ok, doing their job without a fuss, never having failed to provide whatever the inverter has demanded from them. So far so good. I don't expect them to last me forever, they are my training wheels. The purpose of this thread was to try and find out why they took so long to accept a full charge. I suspected that maybe my charge controller's 2 hour absorb stage is too short, but then what more can a solar charge controller accomplish when "4hours of useful sun is all one can expect on most days" Here in SA we can get a lot more in summer - I see 20 amps at 08:30, 26 amps at 12:30 and 20 amps at 16:30 this time of year (spring). That's 8 hours of useful sun in spring. Overcast with thin clouds it's about half that. Overcast with thick clouds it peaks at a dismal 4amps. But those days are rare here, most of the time is sunny for 70% or more of the day.

Back to my battery observation, I suspect that if it's not a shortage of time on absorb stage it's the type of battery - silver calcium which requires higher charge voltage than lead antimony, or that they have permanent hard sulfation which has increased internal resistance and cut their capacity. Strangely though if that is the case then why are they still going strong and steady? Shouldn't their sudden demise be imminent?

Cariboocoot · October 2014

Re: Is Lead Acid really this sluggish?

I've fixed a lot of systems. Many of them seemed to work well for years, then just drop dead suddenly. This is because battery function is not linear, and once you go past the "tipping point" it's all down hill - rapidly.

For example you can get away with deficit charging for years and nothing appears to be wrong. But one day the amount of Amp hours you use pulls the batteries consistently below 50% of whatever remaining capacity they have and suddenly they have no usable capacity at all.

If you are only cycling them lightly, they will last a long time and will actually die of age sulphation before misuse sulphation. In the meantime you're not getting your money's worth, because you could be utilizing a greater percentage of the batteries' capacity and still get the same lifetime (providing charging is correct). This is the basis of the 25% daily average DOD. When you start going above that the lifetime starts dropping off more rapidly, despite proper charging.

The current levels you see are quite normal, but that 26 Amps represents a peak charge rate of only 6.5%: barely above manufacturer's recommended minimum. And that is without any load. If the PV were increased then the current would be increased at all points during the Bulk stage, meaning they would reach Absorb level more quickly and thus have more time to spend at that level and a better chance of achieving full charge.

The only way you can test a battery's true capacity is with a controlled current discharge, and that is not a practical thing for the average person to do.

It is always a good idea to start out with cheap batteries. If they fail for whatever reason, you're not out much. And if they last you're money to the good.

mike95490 · October 2014

Re: Is Lead Acid really this sluggish?

two things

1) If you are able to maintain & refill your current flooded batteries, why switch to expensive AGM that don't last as long ??

2) you will need a new charge controller for the AGM, it has very different voltage requirements than flooded.

Is Lead Acid really this sluggish?

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