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Old 01-09-2018, 03:21   #31
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Re: Adjustible Solar PV Controllers

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Originally Posted by conachair View Post
Not so sure "so many" - add house loads and a few clouds and a bit of shadow I would guess "so many" actually struggle to get back to 100% every day even with no float.
If you are not reaching absorption voltage, the absorption time is unimportant for that cycle, it will not come into play so the setting is immaterial.

If the solar output is commonly not meeting the requirements, many assume that a long absorption time is appropriate, but the opposite is true. On the less frequent occasions when the solar output is exceeding the requirements, the ideal absorption time will be short.

This is especially true if you select agressive voltage setpoints which I think is good idea (although stay within the manufacturers recommendations) to squeeze the maximium amount from the solar panels.
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Old 01-09-2018, 03:48   #32
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Re: Adjustible Solar PV Controllers

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If the solar output is commonly not meeting the requirements, many assume that a long absorption time is appropriate, but the opposite is true.
How does that work? If solar is struggling to get the batteries back to full you want to set it to put less power back?

Don't understand that, how can putting less power back be appropriate?
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Old 01-09-2018, 03:59   #33
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Re: Adjustible Solar PV Controllers

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Good point. It is not commonly appreciated that the lower charge level is, the shorter the absorption time needs to be. People often get this wrong and assume a lower charge level will need a longer absorption time.

Solar outputs, especially the net output into the batteries, even from larger arrays, are typically lower than battery chargers. It is therefore often incorrectly assumed the absorption time with the solar controller will need to be much longer than their battery charger. The opposite is true.

This is why so many solar installations are better with a quite short absorption time, rather than the very long, or even continuous absorption times that are frequently recommended in the forum (but take the time to measure the voltage end points over a few cycles). This is especially the case when aggressive voltage set points are chosen. The latter is beneficial (although I would stay within the battery manufacturer’s recommendations) if you want to cram the most power out of the solar array in as short a time as possible.
Hi, but we don't get to determine the absorption time, battery acceptance rate does, well it does if we want our batteries full.

I realise that we can set absorption time but this isn't what your talking about?

I remember reading once before you mentioned your absorbtion time is quite short, I don't understand this, with 500w solar on a sunny day the reality is my absorption time is all day if I go by trying to achieve approx 1% tail-end amps which is 6-9a depending on whether I have one or two banks online.

If your using a lower absorption voltage then achieving the required end amps will be easier (although not really full) but still will take more than a couple of hours.

What am I misunderstanding here?
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Old 01-09-2018, 04:27   #34
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Re: Adjustible Solar PV Controllers

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Originally Posted by daletournier View Post
Hi, but we don't get to determine the absorption time, battery acceptance rate does, well it does if we want our batteries full.

I realise that we can set absorption time but this isn't what your talking about?

I remember reading once before you mentioned your absorbtion time is quite short, I don't understand this, with 500w solar on a sunny day the reality is my absorption time is all day if I go by trying to achieve approx 1% tail-end amps which is 6-9a depending on whether I have one or two banks online.

If your using a lower absorption voltage then achieving the required end amps will be easier (although not really full) but still will take more than a couple of hours.

What am I misunderstanding here?
As a hypothetical example, If you had a charger that put out 9 amps into a 900 ah battery continuously for a day or two, it would reach end amps at the same time as it finished bulk, requiring zero absorption time.
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Old 01-09-2018, 04:34   #35
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Re: Adjustible Solar PV Controllers

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Hi, but we don't get to determine the absorption time, battery acceptance rate does, well it does if we want our batteries full.
The absorption time is determined by the solar controller. The battery (together with the input current) will determine if the voltage reaches and stays at the acceptance voltage, but the time that this voltage is held is determined by the setting on the controller

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Originally Posted by daletournier View Post
I remember reading once before you mentioned your absorbtion time is quite short, I don't understand this, with 500w solar on a sunny day the reality is my absorption time is all day if I go by trying to achieve approx 1% tail-end amps which is 6-9a depending on whether I have one or two banks online.
If your batteries are being held constantly at the absorption voltage (say 14.7v for a flooded battery) for more than a couple of hours before reaching 1% acceptance current then that unusual. Certainly not impossible, but unusual.

Remember that battery end amps is only the current going into the battery not the total current produced by the solar panels. Also keep in mind if you are using the popular Victron controllers, the true absorption time will be less (and often much less) than the reported number due to the unusual way the controller counts “absorption time”.
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Old 01-09-2018, 05:17   #36
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Re: Adjustible Solar PV Controllers

Hi Noelex, understand and agree with your first comment, i think its just a communication thing there.

But not sure i agree with, "its unusual for end amps not to be reached in a couple of hours", I would say its "unusual for end amps to be reached in a couple of hours". Absoption voltage for my t105's is 14.8v, at that voltage (manufacturers recommendation) im struggling to get end amps prior to the end of the day without a generator run first up in the morning or a very large solar array in order to get bulk out the way early. Sure the reg can finish absorb and hit float based on a preset time but that dosent mean end amps was achieved. Its been disscussed many times here theres just not enough time in the day due to Lead acid batteries acceptance rate dimishing as SOC increases, therefore im not sure how you can fill the last 5-10% in a couple of hours?
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Old 01-09-2018, 06:27   #37
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Re: Adjustible Solar PV Controllers

No one is saying reduce total energy.

At a lower amp rate, the CC-CV transition point is just at a much higher SoC.

Say total charge time is 7hrs.

A high amp rate: Bulk 2hrs, Absorb 5hrs

Low amp rate: Bulk 6hrs, Absorb 1hr

Obviously simplistic to make the point
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Old 01-09-2018, 06:44   #38
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Re: Adjustible Solar PV Controllers

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Originally Posted by daletournier View Post
But not sure i agree with, "its unusual for end amps not to be reached in a couple of hours", I would say its "unusual for end amps to be reached in a couple of hours".
It is not a couple of hours of charging, but a couple of hours where the voltage is held at 14.8v (in your case). It is not unusual to be struggling to fully charge the batteries and meet the end amps on solar alone, but it is unusual if the battery voltage has been held at 14.8v for two hours or more.

To put some typical numbers that might be seen in your case, you have indicated that you have 500w of solar and an 800 AHr battery bank. On a sunny day your solar panels are likely to be putting out in the real world say 20A if you are using say 5A with 15A going into the battery. At the very start of the absorption time (not the start of charging but when you have reached 14.8v) your end amps are already below 2% so the batteries are at a reasonably high charge state. Two hours with the battery held at 14.8v is a long time for the end amps to drop from under 2% to 1% or even 1/2% (if you want to use an agressive 1/2% setting). End amps reduce reasonably rapidly.

On a cloudy day it is even harder to understand. If your solar panels are putting out for example 13A or less and you are using 5A you have already reached 1% end amps as soon as the absorption voltage is reached. The ideal absorption time is zero (if using 1%).

These numbers are only examples, but I think they are realistic and show why a two hour absorption time is often on the generous side. A slightly generous absorption time is not terrible for flooded batteries, but many posters are advocating far longer than two hours. I do not think this is good general advice especially when combined with aggressive end amp and voltage set points. It is especially poor advice for some types of lead acid batteries such as Gel, but this distinction is rarely made.
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Old 01-09-2018, 06:55   #39
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Re: Adjustible Solar PV Controllers

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Originally Posted by noelex 77 View Post
It is not unsual to be struggling to fully charge the batteries and meet the end amps on solar alone, but it is unusal if the battery voltage has been held at 14.8v for two hours or more.
At a guess that would be unusual most days on a large majority of cruising boats with shadows and house loads going on and off. Which is why staying at absorption can make more sense, from recent measurements if your down to 1% A/Ah acceptance there is still a fraction to go which takes a long time, dependent on temperature as well, so is going to float at 99% charged better or worse than 99.9% with an hour or 2 at absorption instead of float?

I'll stick with no float or maybe try a few days logging data with different settings & see what's what once the solar gets put back on.
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Old 01-09-2018, 07:11   #40
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Re: Adjustible Solar PV Controllers

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At a guess that would be unusual most days on a large majority of cruising boats with shadows and house loads going on and off.
The absorption time should only be counted when the battery voltage is at the the absorption voltage.

Therefore if a shadow reduces the solar output, or the load is increased, so the voltage falls below the absorption voltage, the countdown is paused.

So a shaddow or an increased demand will lower the ideal absorption time if the net battery current is positive and increase the ideal absorption time if the net battery current is negative.


Not all solar controllers have a smart and correct absorption countdown. Some simply report the time from when the absorption voltage is first reached. This type of controller needs a much longer “absorption time” which complicates the discussion. As the true absorption time is not measured the relationship between battery SOC and the reported “absorption time” is less precise. This type of controller needs a different mechanism to estimate the battery SOC and the correct time to drop down to a float voltage.
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Old 01-09-2018, 07:25   #41
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Re: Adjustible Solar PV Controllers

The actual endAmps should be as spec'd by the batt vendor.

Certainly with GEL or Firefly I'd stop at a higher point, be more careful not to let Absorb Hold Time go on too long.

And AGM more so than FLA.

But still the goal of **actually getting to endAmps** a few times per week is the priority

which means AHT will be longer than necessary some days.

If you are that concerned this constitutes harmful overcharging, then either sit there with an ammeter to manually control things, or spend the extra to buy the shunt/BM controlled charge sources that transition to Float based directly off a reading of trailing amps.

Note also that as a bank ages, it will take longer to get to endAmps, at some point you may need to choose between changing your definition or with solar just eliminating Float while cycling.
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Old 01-09-2018, 07:49   #42
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Re: Adjustible Solar PV Controllers

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Originally Posted by noelex 77 View Post
The absorption time should only be counted when the battery voltage is at the the absorption voltage.

Therefore if a shadow reduces the solar output, or the load is increased, so the voltage falls below the absorption voltage, the countdown is paused.
"Should" sounds great. How accurate are the regulators?

Plus when it comes to end amps a tiny difference in voltage at the battery can make a substantial difference, below from just now after turning the charger off and on again, when the fridge cycles 60mV means a difference of 0.2A going in. These are also T105's - looking at 1% as a "charged" A/Ah then your batteries aren't completely charged, just really close. Getting down to 1% may be possible in an hour or 3, getting down to 0.6% takes longer. So back to is charging to not quite full better or worse than an hour or 3 not at float.



After restarting, ramps down quickly to well below 2A >

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Old 01-09-2018, 07:54   #43
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Re: Adjustible Solar PV Controllers

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The actual endAmps should be as spec'd by the batt vendor.
Disagree. Trojan told me "The finishing rate for a fully charged battery is 1-3% of the total capacity of the battery bank (2.25A - 6.75A)" - on my batteries at the temperature today there's still power going in at 2.25v, lots still going in at 6.75v. Power still going in must mean they aren't completely there yet. Better to measure carefully your own batteries in your own setup. As built lab figures may or may not agree.
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Old 01-09-2018, 08:58   #44
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Re: Adjustible Solar PV Controllers

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Disagree. Trojan told me "The finishing rate for a fully charged battery is 1-3% of the total capacity of the battery bank (2.25A - 6.75A)" - on my batteries at the temperature today there's still power going in at 2.25v, lots still going in at 6.75v. Power still going in must mean they aren't completely there yet. Better to measure carefully your own batteries in your own setup. As built lab figures may or may not agree.

I think this a common misconception. When the battery is 100% full, by any definition you can pick, it will still accept current at the absorption voltage. The current is mainly going into heating up the battery and evaporating the electrolyte.

The battery monitor will often report that the AHrs are increasing but this is not the case.

The difficulty is finding when the battery is at true 100% SOC. This can only be done by measuring the capacity of identical batteries charged with different algorithms. However, just because a battery is accepting current does not mean its SOC is rising. At some point extra charging is not raising the SOC. The trick is identifying this point. Manufacturers produce guidelines to help.

Exceeding this 100% point, especially occasionally, is not always harmful (an equalisation cycle deliberately does this). Many cruisers exceed the manufacturers specifications erring on the agressive side of voltage setpoints and battery return amps. In mild degrees this is not a terrible policy. More batteries die from undercharging rather than overcharging. Squeezing the maximum power into the batteries in a limited time from sources like solar is a worthwhile goal.

However, when using agressive settings the charging parameters need to be carefully chosen and monitored. I do not agree with the blanket recommendation commonly posted on this forum, of very long absorption times or effectively disabling the float stage. More is not always better.
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Old 01-09-2018, 09:14   #45
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Re: Adjustible Solar PV Controllers

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I think this a common misconception. When the battery is 100% full, by any definition you can pick, it will still accept current at the absorption voltage. The current is mainly going into heating up the battery and evaporating the electrolyte.
Yes, of course it does - obvious from the data plots posted, point is the current acceptance is still lowering until it gets below the 1% you are considering as full - my point is I don't think 1% is full, maybe very nearly but not quite. In these trojans anyway.

Quote:
However, just because a battery is accepting current does not mean its SOC is rising. At some point extra charging is not raising the SOC. The trick is identifying this point. Manufacturers produce guidelines to help.
Again it's not the current - it's the change in current - why not look to when the current acceptance stabilizes? It's not that the battery is accepting charge, it's that the charge it is accepting is continuing to lower, it's not done yet. Temperature may be a factor but is a few hundred millamp really going to increase the temperature of a battery already above 30degC that's been on charge at absorption for a few hours?

So test for someone with a temperature bath and accurate test kit - discharge then capacity test a battery on charge until the acceptance rate reaches 1%, then do the same when the acceptance rate has stabilized. Strongly suspect the latter will have more charge in it.
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