Absorbtion Time for AGMs

[john61ct]

Stateside shorthand for "best quality cheap / standard flooded golf cart 6V"

[belizesailor]

Originally Posted by a64pilot

My plan is to get them to 100% twice a week, and equalize once a month.
...

Interesting that LifeLine makes a point of differentiating between "Equalizing" and what they prefer to call "Conditioning" for AGMs.

Havent read that section in detail, but at first glance they look basically the same.

Whats the difference?

[john61ct]

Originally Posted by a64pilot

My plan is to get them to 100% twice a week, and equalize once a month.
I figure we will be making water, washing clothes, making ice and whatever else twice a week, so that will be generator run time.
Generator and two chargers can throw 185 amps at the bank, then after about 20 min. It hits absorption and of course amps begins to walk down, so maybe 30 min I can start making ice and a few small loads, and maybe after an hour begin making water, make water for a couple of hours and then shut down gen. By that time Solar ought to be coming on strong and will I hope finish charge to 100%.
If it works out that is 6 hours generator run a week and about two gallons of Diesel.

Good plan. Do you know what the bank max current acceptance is at say 50% down?

So do burn the fuel in the AM, once Absorb acceptance gets down to what panels are putting out may as well stop the other sources.

And depending on how close solar gets you to 100% anyway, you may find very short runs most mornings will be best, for the bank anyway.

[a64pilot]

Originally Posted by belizesailor

"LA bank"?...as in Lead Acid I assume?

Yes, sorry

[a64pilot]

Originally Posted by john61ct

Good plan. Do you know what the bank max current acceptance is at say 50% down?

So do burn the fuel in the AM, once Absorb acceptance gets down to what panels are putting out may as well stop the other sources.

And depending on how close solar gets you to 100% anyway, you may find very short runs most mornings will be best, for the bank anyway.

Acceptance rate at 50% SOC may be as high as 5C, that would be over 3000 amps for my bank and I'd assume that would last for a very short time, but it will hold 185 amps for 20 min. 185 amps taxes my 3.5 KW gen pretty well so even if I had bigger I couldn't use it, not without rewiring the boat for 50 amp service and installing a larger generator.
Maine Sails has a very good article on AGM charging and charger size, going bigger does shorten charging time, but maybe not all that much as vast majority of charge time is at ever decreasing amps.
Yes plan is to stop gen when water tank is full and clothes washed, that ought to be plenty of charge time.

It's all theory so far, we leave in Jun., but not for real until after Fl Hurricane season, have to make sure youngest is comfortable in College before we go for real.

[belizesailor]

Originally Posted by john61ct

Good plan. Do you know what the bank max current acceptance is at say 50% down?

So do burn the fuel in the AM, once Absorb acceptance gets down to what panels are putting out may as well stop the other sources.

And depending on how close solar gets you to 100% anyway, you may find very short runs most mornings will be best, for the bank anyway.

So...if one week between 100% SOC charges is abusive and twice a week is good...that raises the question of what is the ideal max time between 100% SOC charges for AGM (and of course data/references to back that up)?

Per a test run by Main Sail that I just read, charging an AGM from 50% SOC to 100% SOC takes about 5 hours at 0.2C. 1 hour just to get to 71% SOC. He paints a pretty bleak picture about adequate charging of AGMs in typical cruising use.

Ive yet to read anything that addresses my question above.

[StuM]

Originally Posted by belizesailor

So...if one week between 100% SOC charges is abusive and twice a week is good...that raises the question of what is the ideal max time between 100% SOC charges for AGM (and of course data/references to back that up)?

No data/references, but I'd go with having a large enough battery bank and solar to reach 100% SOC at some point every afternoon/evening (with early morning bulk alternator/generator charging when necessary to achieve this).

[belizesailor]

Originally Posted by StuM

No data/references, but I'd go with having a large enough battery bank and solar to reach 100% SOC at some point every afternoon/evening (with early morning bulk alternator/generator charging when necessary to achieve this).

Stu, Im disappointed...YOU are just gonna wing it with that answer? 😆

[john61ct]

Yes, occasionally missing a day is likely NBD, but 100% full every cycle should certainly be the design goal.

But if that's simply not possible, then the consequence is some incremental loss of bank longevity.

It's a question of priorities and costs. Most people don't put getting the max number of years out of their bank right up at the top. Using a microwave and dishwasher may be what it takes to make liveaboard bearable for a partner. Upgrading the Alt setup or carrying a dedicated little genny may be more expensive or difficult than buying a new bank sooner.

But at least make your choices consciously, most banks are probably murdered out of ignorance.

[john61ct]

Originally Posted by belizesailor

Per a test run by Main Sail that I just read, charging an AGM from 50% SOC to 100% SOC takes about 5 hours at 0.2C. 1 hour just to get to 71% SOC. He paints a pretty bleak picture about adequate.

The solution is a bank chemistry with a higher amps acceptance rate, a daily morning run of a larger charge source, get the 50-85% done in say one hour, and then a full day of enough solar to finish the low-amp long tail.

With more solar and/or less usage, less burning of fossil fuel just to charge. Obviously weather can be a significant variable.

Good SoC monitoring gear helps you get a good handle on things.

[Maine Sail]

Originally Posted by a64pilot

A
Maine Sails has a very good article on AGM charging and charger size, going bigger does shorten charging time, but maybe not all that much as vast majority of charge time is at ever decreasing amps.

The time from 50% SOC to 100% SOC is not shortened by much (about 12 minutes in this test) when doubling the charge rate from .2C (20% of Ah capacity) to .4C (40% of Ah capacity), but the attained SOC does change up until we hit about the low to mid 90's as a percent SOC. The last 8% +/- of stored capacity is just slow to absorb.

TESTING SUMMARY:


1- 50% SOC to 100% SOC at .2C = 5:42 - Exited Bulk Charge at 1:16:00

2- 50% SOC to 100% SOC at .4C = 5:30 - Exited Bulk Charge at 19 Minutes

3- 50% SOC Charged at .2C For Exactly 1 Hour = 71% SOC - Never Exited Bulk

4- 50% SOC Charged at .4C For Exactly 1 Hour = 85% SOC - Exited Bulk Charge at 19 Minutes

5- 50% SOC Charged at .2C For Exactly 2 Hours = 87% SOC - Exited Bulk Charge at 1:16

6- 50% SOC Charged at .4C For Exactly 2 Hours = 96% SOC - Exited Bulk Charge at 19 Minutes

I've yet to cut open more than a couple of AGM's that were suffering from dry-out.. The ones that were had unregulated voltage pushing the batteries over 15V on a regular basis. One lacked a solar controller and two were banks where the battery charger failed in a state of no voltage regulation. It is almost always sulfation that is killing them on cruising boats.

Absorption time / duration will depend upon charge rate and DOD that you started at. The higher the charge current the longer you'll need to spend at absorption or CV (constant voltage/absorption) to hit 100% SOC. For example, in the above test, the batteries were in bulk then absorption for:

.4C - Bulk/CC time 50% SOC to CV limit of 14.4V = 19 Minutes
.4C - Absorption/CV time to 100% SOC 5:11:00

.2C - Bulk/CC time 50% SOC to CV limit of 14.4V = 1:16:00 Minutes
.2C - Absorption/CV time to 100% SOC 4:26:00

Despite the difference in total time to 100% SOC only being about 12 minutes, the .4C charge rate hit higher SOC's in the same duration leaving less work for solar or alternative energy to do. 1 hour at .2C left the battery at 71% SOC and 1 hour at .4C at 85% SOC. If you can hit them with .3C to .4C+ it won't hurt anything but it also won't drastically shorten time to 100% SOC just the time to the upper 80's to lower/mid 90's which leaves less work for alternative energy to absorb them as high as possible...

Also remember that this was a used battery testing at 91.1% of rated Ah capacity. It represents really only this battery and was intended as a glimpse into how charge rate affects charging and storage in an AGM battery. For slightly used Lifeline AGM's this should scale fairly well for temps between 72F & about 85F... At 91.1% of as new rated capacity the battery was well within the range of acceptable but not necessarily new.

As the batteries age or temps change these numbers would also change. Seeing as batteries don't stay new for very long, doing a test on a brand new battery would not be as realistic when related to what you might expect to see on your own boat.

[john61ct]

Excellent, thanks MS!

So when minimizing engine runtime in the morning, the higher charging amps increases the odds limited solar will get to the full 100%.

Cloudy days, maybe run the gen/alts an extra 15-30 minutes.

[Maine Sail]

Originally Posted by john61ct

Excellent, thanks MS!

So when minimizing engine runtime in the morning, the higher charging amps increases the odds limited solar will get to the full 100%.

Cloudy days, maybe run the gen/alts an extra 15-30 minutes.

The greater the odds yes, but to hit a true 100% SOC in a solar day is going to depend upon your available solar charging amperage, minus DC loads, and at what SOC you shut down the Dino sources.

If your array amperage can pick up right where the Dino sources left off, then attaining 100% SOC in a day can be doable. However, it's difficult for most boats, even with AGM's, to hit a true 100% SOC in one day while also using the boat. The solar absorption time will need to be played with on an as necessary basis for the specific boat. In many cases float can simply be programmed to absorption, or close to it, for an active live-aboard cruising boat, just don't forget to re-program it if you leave the boat unattended for a few days.

[john61ct]

Yes besides panel wattage, the biggest variable would be ongoing loads.

Conserving usage is a magic bullet, no dino consumption and fully charged banks every day, but few people are willing to make "extreme" sacrifices.

[belizesailor]

So, given that PSOC service is an issue for AGMs and that most cruising boat charge configurations (least most Ive seen) really are not up to delivering 100% SOC on a daily basis...what is the realistic point at which PSOC charge cycles become a pragmatic issue (1, 10, 100...)?

And, since there are cruisers out there using AGMs successfully, what are the configurations/best practices that actually work (=not murdering batteries) on a long term liveaboard cruising boat (away from the docks)? I find it hard to believe that everyone with AGMs are running their gennies 5+ hours per day...might as well just live off the gennie and a small cheap FLA bank.