Charging AGMs with truck alternator?

That was not my experience with a new lifeline GPL-31XT at 14.56v, not when it was drained to 50%. 40 amps got it to absorption voltage in about an hour but tapering to 0.65 amps took nearly 4 more hours.



http://www.pbase.com/mainecruising/effect_of_charge_rate_on_agm_battery

My Northstar AGM 27 has a spec'd lower resistance( 2.6mOhm) than a Lifeline 27(3.54mOhm). When new it should be able to charge faster.

31T is 3.62 m ohms
31Xt is 3.03 m ohms

http://www.northstarbattery.com/product/nsb-agm-27m

http://lifelinebatteries.com/wp-content/uploads/2015/12/Internal_Resistance_and_Short_Circut_Current_of_Lifeline_Batteries.pdf

Reason # 594 why I prefer Concorde Lifeline. They do top off <2 hrs @ 14.4 volts absorbsion.

I think 2 hours from 80% to 100% is being very generous and 3.5 hours at ideal voltages is more likely.

My 90AH AGM battery at is requiring about 5 hours at absorption voltage these days with 200 watts of solar as the ony charge source.

https://forums.sailboatowners.com/index.php?threads/agm-batteries-making-the-choice.124973/

Thanks everyone, I appreciate the input. I will check out the Sure Power relay.

You need at least a Two Hundred Amp Charge relay like the Sure Power. The inrush amperage of AGM batteries can be cruel. Don't try to Ohm's Law this. From the school of hard knocks.

AGMs charge quickly to 80% state of charge then taper by virtue of rising impedance. Even with optimum charge potential it takes an AGM two full hours to fill from 80% to 100%. Full being 14.4 volts and one half amp per 100 ampere hour capacity. = 5 100 amp batteries = 2.5 amps.

The penalty starts after AGMs are filled to only 80% for three weeks consecutively. Then a modified charge regimen is called for. Bump the absorbsion voltage up .3 volt and use the same amperage value to determine a filled battery. One proper recovery starts the process fresh.

Severely mistreated batteries need conditioning as per the Concorde Lifeline PDF battery maintenance manual.

What I am thinking is adding 2-4 batteries in the bed connected to a charging circuit from my second alternator, I will run as large of wire as needed. I would install an isolation relay like one of these.

https://www.bluesea.com/products/7601/m-Series__Automatic_Charging_Relay_-_12_24V_DC_65A

This will power a 3000 watt inverter on my commercial canopy for work and add battery capacity for the truck camper when I use it. So I want to set it up with an umbilical cord to plug into both.

WNYBob wrote:

This is an OEM setup and did not come with an AGM battery. The altinator is controlled by the computer! It looks to me that GM has it set to charge an AGM or ruin a wet cell.

Not true at all.
With a LARGE battery that is not prone to "boil" out the water rapidly,
there is absolutely nothing wrong with those voltages.....especially if the battery was down a bit when you started your measurements.

S Davis wrote:
Ok, thanks. Now to figure out how many I can put in front of the wheel wells.

What exactly are you trying to accomplish here ?
I can't help the feeling that you asked the wrong question.

Being able to charge--and getting the job done may be different things.

The charging control system "sees" the starter battery. Once the starter battery is full, charging to the house bank may be quite limited.

It may be possible to improve the charging rate a bit with thick wires to the house bank. It may be a good idea to wire the bank in a balanced manner.

If the plan is to do most of the charging from the engine, look into adding an external diode that fastens directly to the alternator.

S Davis wrote:
Ok, thanks. Now to figure out how many I can put in front of the wheel wells.

I put a volt meter on my 2011 Silverado and here is what I get.
After start up 15.1 or .2 for at least 20 min.
And then slowly goes down to 13.8 after a hour or so.
Then goes down to 13.2 after another hour.

This is an OEM setup and did not come with an AGM battery. The altinator is controlled by the computer! It looks to me that GM has it set to charge an AGM or ruin a wet cell.