need help with AGM charging.

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need help with AGM charging.

Postby nemoskull » Fri Dec 29, 2017 6:50 pm

im half way through building a solar charge controller, and im at the point where i need some help understanding the charging profiles for a agm battery.
aside from just setting for 13 volts, i really have no idea.

thanks.
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Re: need help with AGM charging.

Postby featherliteCT1 » Fri Dec 29, 2017 6:56 pm

nemoskull wrote:im half way through building a solar charge controller, and im at the point where i need some help understanding the charging profiles for a agm battery.
aside from just setting for 13 volts, i really have no idea.

thanks.


If it were me, I would start by looking at the manufacturer's web page to see if they specify the charge parameters. Hope this helps.
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Re: need help with AGM charging.

Postby Bill n Robi » Fri Dec 29, 2017 7:11 pm

Yep, see above.
My controller has a setting option for either 'Wet" of "Agm' if it is any good controller then there should be an option.
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Re: need help with AGM charging.

Postby MtnDon » Fri Dec 29, 2017 7:12 pm

For a battery mfg example seepage 19 of this link
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We have a small off grid cabin we built ourselves in the NM mountains; small PV solar system; 624 watts PV, Outback CC & inverter/charger ... http://countryplans.com/smf/index.php?topic=2335.0
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Re: need help with AGM charging.

Postby Trebor English » Fri Dec 29, 2017 7:41 pm

See page 19 in the prior post. You should find the same stuff for your battery. It should be the same give or take a few tenths of a volt. Temperature compensation might vary. If you can't find data for your battery this should be close enough. In any case, 13 volts is way too low for an absorption voltage.
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Re: need help with AGM charging.

Postby nemoskull » Fri Dec 29, 2017 8:03 pm

im still haveing trouble understanding the switch off between bulk and absorption charging; why it happens and when it happens and how to know when to switch.
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Re: need help with AGM charging.

Postby MtnDon » Fri Dec 29, 2017 9:53 pm

The change from bulk charge to absorption charge occurs when the battery voltage builds up to the preset voltage; current constant. I don't know the nuts and bolts of how that is achieved by the charge controller, but it is a user selectable voltage on the CC I use (midnight Solar products). Charge current should be as high as practical to get as much energy replaced as quickly as possible, but not to exceed the manufacturers specs. Once the battery voltage has hit the set level it changes to the absorb profile. That voltage level also usually has a time requirement to prevent system fluctuations from causing unwarranted changes.

Once the voltage in the battery has risen to the set point, then the charger changes to the absorb stage profile and holds the voltage constant, or nearly so, while the current decreases. The current tapers lower and lower and I believe is a function of the lead-acid battery internal resistance. I could be wrong on that. Low resistance cells such as lithium don't need an absorb stage. Lead-acid need the absorb stage for the lead plates to slowly soak up the remaining charge. The duration of the absorb stage is often set to be a timed event. Some charge controllers allow the end of absorb to be set by how low the current flow into the battery falls. That is determined by total battery capacity. Using "end amps" allows the absorb charge time to be 'just long enough' to ensure a full charge and not unnecessarily long as can be with a timed charge.

Then the float takes over.

Is your design an MPPT or a more simple PWM?
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Re: need help with AGM charging.

Postby John61CT » Fri Dec 29, 2017 11:43 pm

Each mfg have their documented charge profile specs easily obtained. If they don't call tech support. If no go, don't buy that brand.

IMO the only quality mainstream deep cycling AGM mfg in the US are Lifeline Odyssey and Northstar.

All setpoints, absorb times endAmps etc should be user adjustable. Include ability to set Absorb time to 0 and a "just stop" option Float=No, mainly for LFP

Best algorithm for determining 100% Full - never drop to Float until that point - is trailing endAmps, e.g. .005C so 8A for a 400AH bank. That requires a shunt on the bank.

The transition from Bulk to Absorb is handled by the bank chemistry, charge rate and SoC/resistance, not the regulator, that's seeking Bulk V.
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Re: need help with AGM charging.

Postby nemoskull » Sat Dec 30, 2017 2:11 pm

its a mppt charger.
and its a HF 35 amp /hr batter, so i guess ill just have to guess.
i might just to lifepo4, those are more straight forward.
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