# AGM Virgin - Questions about charging



## mechman600 (Oct 16, 2010)

That's right folks, I just bought a bunch of Deka 8A27s because I can get rebranded ones (Paccar/TRP parts BA27D51) for ultra cheap from my parts department at work ($147CDN/ea).

If I look up Optima charging specs, it says 15.6V @ max possible amps is okay as long as battery temps do not rise too much. MK specs for the 8A27 says a max of 14.6V for "constant charging voltage" @ 20C.

I have 10A cheapo Canadian Tire Mastercraft chargers that shut off once voltage reaches 15.2V (on "conventional setting, 15.6V on AGM setting). Once charged, the voltage rises from 14.5V to cutoff in a matter of minutes and then shuts off. My Schumacher "smart charger" also cuts off at 15.6v (!!!) when on AGM mode.

Why do these automotive chargers cut off AGMs so incredibly high?
Am I going to murder my new AGMs by over volting them, even if for a few minutes?

Here is the MK brochure for reference:
http://www.mkbattery.com/documents/3701MK_AGM_LG_v3_r8.pdf
And the Optima brochure for reference:
http://www.optimabatteries.com.mx/_media/documents/specs/D31T_082104.pdf

Thanks in advance!


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## dtbaker (Jan 5, 2008)

I dunno much about AGMs.... except that if you are using a single charger for the whole string you should consider shunting regulators to keep them balanced as they do not have a 'gassing' cycle like floodies to balance.

I took a set of Manzanita rudman regulators off a vehicle I purchased and am upgrading from lead to lithium..... for sale in the classified section of the Forum.

http://www.diyelectriccar.com/forums/showthread.php/manzanita-rudman-regs-agm-batteries-79452.html


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## mechman600 (Oct 16, 2010)

Many AGMs do have have an equilization cycle. Optimas specifiy a "3 amp constant current for one hour."

My MK/Dekas include equilization in the regular charging voltage, which is a max of 2.43 VPC @ 25F.

I do have a 72V charger on order, as well as all the zener diodes and whatnot to make basic rudman balancers. However, right now I am using individual 12V chargers that cut off at ~15.2V (2.53 VPC, I will watch closely this morning to verify this number), which is well above Deka's spec of 2.43 VPC max for constant voltage charging. It only goes this high for a few minutes, but it still scares be. I don't want to ruin my new batteries!


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## Roy Von Rogers (Mar 21, 2009)

http://www.lifelinebatteries.com/manual.pdf


Roy


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## mechman600 (Oct 16, 2010)

Thanks for the link.

Section 5.5 shows a "conditioning charge" (equilization) of 15.5V @ 25C for 8 hours (!!!). Since my chargers stop at 15.2, and they only there for five minutes or less, I am less concerned now.
But that document is for Concorde batteries and MK Batteries doesn't show an equilization/conditioning charge option, at least in their brochure, and THAT'S what has me worried.


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## Roy Von Rogers (Mar 21, 2009)

mechman600 said:


> Thanks for the link.
> 
> Section 5.5 shows a "conditioning charge" (equilization) of 15.5V @ 25C for 8 hours (!!!). Since my chargers stop at 15.2, and they only there for five minutes or less, I am less concerned now.
> But that document is for Concorde batteries and MK Batteries doesn't show an equilization/conditioning charge option, at least in their brochure, and THAT'S what has me worried.


With @25C I hope you understand that means Celsius, and not 25X the AH of battery..lol

Roy


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## mechman600 (Oct 16, 2010)

Ha! I wonder what THAT (2300 amps) would look like....


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## Sunking (Aug 10, 2009)

I know a fair bit about AGM FLA, and Gel. Been in the telecom sector for 30+ years and Renewable Energy for the last 10 years.

You did not mention what size the battery is (Amp Hours), but typically you want to charge an AGM at the maximum recommended rate. But C/4 is generally accepted by all manufactures. YOu go can lower but understand if you do it is going to take longer. 

OK for AGM there are two recommended methods to safely charge them. For DIY applications the least expensive and safest is Float Charge aka Constant Voltage. However this method also takes the longest, but it requires no timing, just hook it up and forget about it until you see the charge current drop to about 2 to 3 % of the battery rated AH capacity. So for example if th ebattery is rated 100 AH you are fully charged up when the charge current tapers off to 2 or 3 amps. The beauty of float chargers assuming they are set to the correct voltage, you do not have to terminate the charge. You can leave them on forever. FWIW this is the method all utilities use on their station batteries.

The other method is known as 3-Stage. The 3 stages are *Bulk*, *Absorption*, and good ole *Float*.

The bulk charge is a Constant Current charge up until the set poing is reached. So if you have say a 30 amp 3-stage charger it pumps 30 amps continuously until the voltage set point is reached. Depending on temp and manufacture on a 12 volt battery is around 14 to almost 15 volts.

Next is the Absorption stage which happens when the Bulk is complted. It is a Constant Voltage, current Taper just like Float. The voltage is just slightly higher than Bulk. Initially when it start wil be constant current of what ever the charger current rating is like 30 amps discussed above. As the battery voltage begins to reach the set point the current will taper off. The Bulk charge terminates when the CURRENT tapers to about 1 to 3 % of the chargers rating. Absorption voltage varies like the Bulk depending on temperature and manufacturer. On a 12 volt battery around 14.5 to 15.5

An AGM has one real drawback, the need a temperature probe to adjust the output voltage. This is because AGM batteries are suspect to Thermal Runaway. As the temperature rises, the voltage is reduced. What can happen is a cell can short out, when that happens the charger sees the voltage drop and fools it into thinking the battery is low and needs more current. As the battery heats up, the internal resistance goes down taking more current from th echarger and any parallel batteries. So what you get is a positive loop feedback and RUNAWAY. Battery gets super hot, th electrolyte spews out the vents, and it is all over. 

Last stage is the Float which I already discussed.


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## Sunking (Aug 10, 2009)

mechman600 said:


> But that document is for Concorde batteries and MK Batteries doesn't show an equilization/conditioning charge option, at least in their brochure, and THAT'S what has me worried.


You should be because most AGM's cannot be EQ. So unless the manual tells you that you can EQ it, never EQ it. If the manual tells you that you can EQ, then only do it when and how it is specified by the manufacture.


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## mechman600 (Oct 16, 2010)

Thanks, Sunking.
My batteries are 92aH in parallel pairs, so 184ah.
I understand how a proper charging algorithm is supposed to look. Unfortunately I do not have the benefit of a good, proper charger...yet.
I watched the battery voltages like a hawk last night to see what my six chargers are actually doing. They have a setting 1, "conventional/low-maintenance setting", and a setting 2, "deep cycle/maintenance free setting."
My cheap Motomaster 12V chargers are basically internally resistive to maintain a max 10A charge. Once the batteries rise to about 13.5V, the battery resistance (of lack of voltage difference?) starts to slow the current down. On setting 1, they shut off when voltage reaches 14.3V (I don't know where I got the 15.2V that I mentioned above), which is great for my AGMs. On setting 2 they shut off above 15V - I didn't let them get that high when I was testing yesterday. They are pushing 4-5 amps at the 14.3V cutoff, which for 184aH is under 3% of C, so my AGMs should be happy and I can relax - for now....because my 72V charger shipped today and then I will have to mess around with the all over again!


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