Gelcel charging voltages

[kcj]

I have a new to me lagoon cat with four 183
Amp-hr gelcels that are four years old. I have three ways of charging them. A victron charger which runs off a Genset, solar and two alternators.

I have been on the hook for a month and can never get them to charge at over 13.2v. The victron has a stated absorption voltage of 14.3, but it only gives full amps (100) for 20 minutes at 13.2v, then it drops to 25 amps at 13.0v, atwhat I presume is float. We motored across the stream and the voltage under alternator charging reached a max of 13.7v. At peak solar output the voltage is 12.9. My overnight house load takes them down to 12.1 or so. Per the specs this is 50% Soc.


Why can't I get them to charge at 14+v?
Is this a battery issue, Genset or charger issue? Or no issue at all? How do I test
The alternatives?
How does "active" voltage that I can measure in real time differ from a Voc/ocv ? Can't take my batteries offline for a day to get Voc....

Many thanks in advance! John

[a64pilot]

It does sound like your Victron is tripping into float soon, if you turn it off and back on, what happens?
Of course what you should see is it running at full output but lower than absorption voltage, voltage slowly climbing until absorption voltage is reached, then slowly decreasing amperage until it drops into float.

Using voltage to determine SOC with batteries under a small load is conservative, meaning your on the safe side doing that.


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[kcj]

Great idea! Thx. Never crossed my mind as the is no on off switch. Had to use the battery disconnect, but it does stay a high amp absorbtion levels now.

Any thoughts on why the voltage is in low 13s not 14+ as per the specs?

Cheers, john

[a64pilot]

Originally Posted by kcj

Any thoughts on why the voltage is in low 13s not 14+ as per the specs?

Cheers, john

What is the amperage when it is low 13's? If the charger is maxed out, then of course the batteries are at a SOC where they can absorb more than the charger can put out.
Think of it this way, when batteries are low and you first hook up say a 100 amp charger, it's output is 100 amps, and stays that way as voltage slowly climbs to absorption voltage, then voltage stays the same at absorption, while amperage decreases until it drops into float.

I don't know the size of your bank, size of your charger or your amp load on the bank, but would suspect that your amperage should be as low as 10 amps or so when it drops into float, if it's dropping into float with a high amp reading, then it's dropping too soon.

[kcj]

There are about 730 ah batt capacity. Post reset the 100amp charger delivers75 amps (as opposed to the previous 10h) at 13.2v, as measured at the battery. The battery specs allow 14.2 and the charger spec says that 14.2 is their absorbtion voltage. Thus my confusion...

Are you saying--- if my batts are low they will start accepting high amps at lower voltages, and then as the batts get to a higher soc the amps will drop and the voltage will rise, eventually approaching the absorbtion 14.2v?... There by suggesting that my batts are in need of a day's worth of generator charging... Not surprising given three months on the hook...

Cheers, j

[a64pilot]

I think something is flaky with your charger, but I am not familiar with a Victron.
Surely it is programmable? Have you checked the programming or battery program?
Check to see if it is not set just for 13.2 V, I know my Magnum can be set for constant voltage. Incorrect programming would explain it.


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[ericoh88]

Originally Posted by kcj

The victron has a stated absorption voltage of 14.3, but it only gives full amps (100) for 20 minutes at 13.2v, then it drops to 25 amps at 13.0v, atwhat I presume is float.
Many thanks in advance! John

Can't you what is right or wrong.

My charger would go to 14.3v when switched on. You may want to check the settings. Mine is Pro Sterling - with many many modes and custom settings. Usually these units are forced to float after a pre-set time. The mode selected is GEL.

A good digital multi meter at battery terminals to compare the readings at the charger to see if the cables are too small. Touch the cables when the charger is on to see if they are hot. If so, consider changing to a thick gauge.

I am using mmpt for the solar and this unit would give 14.3 v reading as well.

[smac999]

what size are your charger cables? what is the voltage at the charger output?


you may be getting 14v at the charger and 13v at the batteries with a 75a charge and too small and too long cables. but the charger will see the 14v and be happy.

[kcj]

Upon further review...
Notwithstanding the poor info in the victron users manual--which does not mention the word " bulk"--- I found a charge curve in their marketing material which depicts a bulk charge period where amps remain near max of unit with voltages rising from low 12s to 14.2, an absorbtion phase where voltage remains at 14.2 and amps decline, and a float phase with lower voltage and amps..

My theory is, now that I've managed to reset my unit (thx a64), I have not gotten out of the bulk charge phase. Sounds like I need a day on the Genset or a long cord...

Thoughts?

Thx to all.

[a64pilot]

You may be correct, your description mirrors how I think chargers work, you need to be on external power long enough for a full charge cycle to occur, you need to keep power applied on the charger, 730 amps is a big bank, and will take a long time to fully charge.
While I am not familiar with Gel cells, my gut tells me your not full charged until you see less than 10 amps output at absorption voltage. Key here is I do not know your average draw of your DC system, if it was zero, then I think you would want to see less than 5 amps, but I am basing that on AGM batteries, which may differ greatly from Gel.
See if you can find a manual online for your batteries and or look up Mainsails knowledge base, I'd be surprised if he does not cover Gel cell batteries

If possible I'd buy a day at a Marina and leave the charger on for 24 hours and see if it follows that charge curve you spoke of, if it does, then your problem is your not charging your batteries fully, nothing wrong with charger.

[a64pilot]

Look around here, lots to learn
Compass Marine How To's Photo Galleries at pbase.com

[a64pilot]

Read this, it's a good explanation, it's an AGM battery ,but I think it is relevant, particularly notice that although it appeared the battery was fully charged in two hours, it actually took five and a half, and that was at a charge rate that nobody can match

How Fast Can an AGM Battery Be Charged..? Photo Gallery by Compass Marine How To at pbase.com

[Maine Sail]

Originally Posted by kcj

My theory is, now that I've managed to reset my unit (thx a64), I have not gotten out of the bulk charge phase. Sounds like I need a day on the Genset or a long cord...

Thoughts?

Thx to all.

If charging off a genset I often advise a custom charge setting that eliminates "float" where absorption & float are set to the same voltage or 0.1V apart. Too many so called "smart chargers" enter float far too prematurely. If the charger does not offer custom programming options then you get stuck with what you have..

Once back at the dock you simply switch back to the absorb>float program & it's best to use a sticky note so you don't forget... Course if your Victron is not programmable then you will just need to keep doing re-boots.

Even some of the most healthy AGM batteries require 5.5+ hours at absorption voltage to fully re-charge. With sulfated batteries and chargers that float prematurely this time to 100% can easily extend to days not hours...