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Old 27-04-2021, 18:52   #1
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Getting to absorption but not always float.

One for the battery fanatics.

I’ve got a bank of 6 Trojan T105s charged by solar with Victron controllers and a Victron battery sense.

Back in South Australia the bank would be in float before lunch time, often by 10 am in summer.

But I’ve sailed south to Tasmania and I am now at 43 degrees south, not 35.

I’ve noticed that for last week or so that the bank is not always getting to float. The absorption phase is beginning but not completing, there simply isn’t enough solar to get the job done.

Now, I’ve got another 380 Watts of solar on its way, but I think I am going to have to start using the engine to charge things from time to time.

How long would you let the bank go without completing absorption before you started to worry? A day, a week? I’ll run the engine when I have to, but I’d rather keep that particular joy to a minimum.

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Old 27-04-2021, 19:01   #2
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Re: Getting to absorption but not always float.

My personal number is a week. It’s all a balancing act about the cost of running the engine vs. a reduction in cycle life for the batteries. I suspect everyone has their own analysis.

I’m not too finicky about that number, if I’ve been anchored for a week and I know I’ll be getting underway (and using the engine anyway) in a couple of days then I just wait. And if I’m expecting some good solar weather on a particular day I might charge for an hour or so early in the morning to give me a better shot at reaching “full”.

FWIW in the bad old days when I started cruising the boat had a battery bank of 4 8Ds. The only charging source was the engine alternator, those batteries would have been lucky to get to float once a year, and they lasted five years of live aboard full time cruising.
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Old 27-04-2021, 21:38   #3
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Re: Getting to absorption but not always float.

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My personal number is a week. It’s all a balancing act about the cost of running the engine vs. a reduction in cycle life for the batteries. I suspect everyone has their own analysis.

I’m not too finicky about that number, if I’ve been anchored for a week and I know I’ll be getting underway (and using the engine anyway) in a couple of days then I just wait. And if I’m expecting some good solar weather on a particular day I might charge for an hour or so early in the morning to give me a better shot at reaching “full”.

FWIW in the bad old days when I started cruising the boat had a battery bank of 4 8Ds. The only charging source was the engine alternator, those batteries would have been lucky to get to float once a year, and they lasted five years of live aboard full time cruising.
Good point about how we used to treat batteries. And they did last a reasonable time too. When did we all start pampering our batteries?

Ok, a week feels ok to me. And like you say, good chance I'll want to use the engine at some point in a week.

I guess I'm still not all that clear on what is happening during the absorption phase. Back to some reading for me.
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Old 27-04-2021, 22:45   #4
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Re: Getting to absorption but not always float.

Absorption is a preset length of time (an hour or so) at a preset voltage that is a volt or so higher than float. If you are going to run your engine, best to do it in the morning when you have a high acceptance rate, then let the solar finish it.
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Old 27-04-2021, 23:23   #5
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Re: Getting to absorption but not always float.

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Absorption is a preset length of time (an hour or so) at a preset voltage that is a volt or so higher than float. If you are going to run your engine, best to do it in the morning when you have a high acceptance rate, then let the solar finish it.

??? What sort of batteries are these? An hour or so absorption for lead acid is unlikely to be sufficient.

My absorption time is not based on time, it's based on current.

It is at a set voltage and continues until the current accepted by the batteries drops to a pre-determined amperage (End Amps).
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Old 28-04-2021, 00:37   #6
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Re: Getting to absorption but not always float.

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Originally Posted by Thumbs Up View Post
Absorption is a preset length of time (an hour or so) at a preset voltage that is a volt or so higher than float. If you are going to run your engine, best to do it in the morning when you have a high acceptance rate, then let the solar finish it.


I really don’t think you read the question.
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Old 28-04-2021, 00:48   #7
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Re: Getting to absorption but not always float.

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I really don’t think you read the question.
It's good advice otherwise you are going to be idling your engine for hours with only a limited amount of current going to the batteries because the voltage regulator on the alternator has wound the current output way back.

It makes more sense to push a big charge into the batteries when battery voltage is low and the alternator provides high charge capacity and let the panels do the topping up and soaking.

The above assumes you have a decent sized alternator and are not relying on the 35 amp ones which seem to be fairly common on new engines these days.
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Old 28-04-2021, 01:03   #8
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Re: Getting to absorption but not always float.

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It's good advice otherwise you are going to be idling your engine for hours with only a limited amount of current going to the batteries because the voltage regulator on the alternator has wound the current output way back.



It makes more sense to push a big charge into the batteries when battery voltage is low and the alternator provides high charge capacity and let the panels do the topping up and soaking.



The above assumes you have a decent sized alternator and are not relying on the 35 amp ones which seem to be fairly common on new engines these days.


No, seriously, read the question. I didn’t ask anything about alternators, engine charging etc. I asked about how long it was safe to go without reaching float.
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Old 28-04-2021, 01:38   #9
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Re: Getting to absorption but not always float.

The Rolls battery manual has a formula for calculating absorption time. Time in hours is 0.42 * ratio of 20 hour capacity divided by charge rate. The recommended ratio is ten to one. So 4.2 hours is the average absorption time.
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Old 28-04-2021, 01:48   #10
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Re: Getting to absorption but not always float.

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The Rolls battery manual has a formula for calculating absorption time. Time in hours is 0.42 * ratio of 20 hour capacity divided by charge rate. The recommended ratio is ten to one. So 4.2 hours is the average absorption time.

The problem with any fixed absorption time is that it assumes a fixed sufficiently large charge source output to meet house loads and still maintain the full charge current. . Neither of which are common with solar/wind and a boat with varying house loads.
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Old 28-04-2021, 01:52   #11
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Re: Getting to absorption but not always float.

To take a legalistic viewpoint mate, you introduced the subject of recharging:

"Now, I’ve got another 380 Watts of solar on its way, but I think I am going to have to start using the engine to charge things from time to time."

in your OP.
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Old 28-04-2021, 02:48   #12
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Re: Getting to absorption but not always float.

You must be asking the wrong question Matt

However your question in your second post in the thread gives a clue to what would for me be the answer to your first question.

IMO, we began pampering our batteries as technology gave us the tools to do it with. To wit, solar panels, digital and programable charge controllers, accurate battery monitors and so on.

So back to the questions in the OP 'How long would you let the bank go without completing absorption before you started to worry? A day, a week?'

For me (hypothetically), it would depend a lot on how far into absorption you are actually getting. If you are getting say >97% SOC, I might be happy to go a few weeks (even a month) but if say you are only getting <90% SOC, I would be looking a only a week.
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Old 28-04-2021, 03:16   #13
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Re: Getting to absorption but not always float.

It would be nice to find out what SOC you are typically reaching. With a low charging current the correct absorption time can be very short so not reaching the float stage does not necessarily mean your batteries are not at 100% SOC.

Nevertheless, flooded lead acid batteries do not seem to suffer much reduction in life when they do not frequently reach 100% SOC, and providing your SOC is not gradually reducing I would not run the engine simply to achieve a high SOC.
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Old 28-04-2021, 03:43   #14
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Re: Getting to absorption but not always float.

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To take a legalistic viewpoint mate, you introduced the subject of recharging:

"Now, I’ve got another 380 Watts of solar on its way, but I think I am going to have to start using the engine to charge things from time to time."

in your OP.


No, that was background.

Actually, I was trying to avoid some of the pointless responses by addressing what I guessed I would be told even though I hadn’t asked.

For instance, someone was bound to tell me I needed more solar.

It didn’t work, of course, because no matter what you do, some people just don’t read the QUESTION.
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Old 28-04-2021, 03:48   #15
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Re: Getting to absorption but not always float.

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Originally Posted by noelex 77 View Post
It would be nice to find out what SOC you are typically reaching. With a low charging current the correct absorption time can be very short so not reaching the float stage does not necessarily mean your batteries are not at 100% SOC.



Nevertheless, flooded lead acid batteries do not seem to suffer much reduction in life when they do not frequently reach 100% SOC, and providing your SOC is not gradually reducing I would not run the engine simply to achieve a high SOC.


Thank you, a considered response to the actual question.

Yes, it is hard to be sure what SOC is being reached.

My simple battery monitor usually concludes I am at 100% at about the same time the voltage starts to climb rapidly, so, say 14 volts. So I think I am getting close.

And the Victrons are usually reporting a VMax of 15 volts or close to it, suggesting I really am getting close.

But I guess I’ll not fret too much, and like you suggest, watch the SOC trend over time.
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