AGM BATTERY FLOAT VOLTAGE

[jves]

Just purchased an EVERSTART AGM Platinum Group 65 battery for the chassis starting battery. The house
batteries are two INTERSTATE deep cycle standard flooded batteries.
I have a stand alone duel station multistage charger on board when MH is in storage.
After absorption stage, the float voltage for the house batteries is 13.2 and the chassis AGM float voltage is 12.7.
I have used three different multistage chargers, one of which is my roof top solar system, and they all
settle on 12.7 float volts for the AGM battery
Is it possible that 12.7volts is the proper float voltage for the AGM battery?
Many thanks for any advise.

 

[solarman]

float charge voltage for AGM is in the range of 13.5 to 13.8 Volts.

resting or idle voltage after several hours will be 12.6 to 12.7 Volts.

 

[Mark_K5LXP]

I guarantee that a float voltage of 12.7V is too low. Minimum 13.2, maximum 13.4 or so, datasheet would be the final word. My guess is that it's not the charger, but a parasitic/persistent load that is overcoming the float charger's current capacity.

Mark B.
Albuquerque, NM

 

[Ray-IN]

Mixing LA batteries and AGM batteries on the same charger gets different results due to internal resistance. Each battery mfgr. has their own charging recommendations.
For instance, Renogy AGM batteries are recommended to be float-charged at 13.3V. Most are higher.

 

[John From Detroit]

Mixing LA batteries and AGM batteries on the same charger gets different results due to internal resistance.

At FLOAT internal resistance is not an issue for there should be ZERO current flow
And AGM is subset of LA (Meaning agm's are Lead Acid.. just not as much acid)

 

[Gary RV_Wizard]

Don't make this over-complicated. A float voltage in the 13.3-13.6 is widely recognized as a good float value for common automotive & industrial 12v batteries. And usable for AGM as well as flooded. A decade or so ago 13.3v was the consensus, but now it's 13.6v. I think the difference is the vastly improved regulation in modern multi-stage digital chargers rather than the batteries themselves. Lead-acid chemistry didn't change (and AGM is the same Lead Acid chemistry in a different package).

IMO it's not worth quibbling over a 0.1v difference, but 13.2 and 12.7 are just plain too low.

 

[Ray-IN]

Don't make this over-complicated. A float voltage in the 13.3-13.6 is widely recognized as a good float value for common automotive & industrial 12v batteries. And usable for AGM as well as flooded. A decade or so ago 13.3v was the consensus, but now it's 13.6v. I think the difference is the vastly improved regulation in modern multi-stage digital chargers rather than the batteries themselves. Lead-acid chemistry didn't change (and AGM is the same Lead Acid chemistry in a different package).

IMO it's not worth quibbling over a 0.1v difference, but 13.2 and 12.7 are just plain too low.

Within the last 5 months Renogy has revised their spec sheet (pg bottom) to reflect that. When I bought my Renogy batteries float was 13.3. Today float charge is 13.5-13.8V. Now I need to reset my Xantrex charger to reflect that change.
Thanks Gary..

 

[jves]

Many thanks for all your responses to my float voltage issue.
While checking voltages I came across another confusing situation.
When placing my VOM negative to the chassis battery negative terminal and
placing the VOM positive to the house battery positive, the VOM
reads 12.8 volts! I would have expected the VOM to read zero.
I thought that the chassis battery should be isolated from the house
batteries.
Again, many thanks for your input.

 

[John From Detroit]

Many thanks for all your responses to my float voltage issue.
While checking voltages I came across another confusing situation.
When placing my VOM negative to the chassis battery negative terminal and
placing the VOM positive to the house battery positive, the VOM
reads 12.8 volts! I would have expected the VOM to read zero.
I thought that the chassis battery should be isolated from the house
batteries.
Again, many thanks for your input.

"Isolated" means different things to different people (kind of like happiness).
Both the chassis and house batteries negative leads are usually "Grounded" That is connected to the chassis The POSITIVES are isolated. It's simply easier to make the chassis negative for all DC systems.

 

[Tulecreeper]

"Isolated" means different things to different people (kind of like happiness).
Both the chassis and house batteries negative leads are usually "Grounded" That is connected to the chassis The POSITIVES are isolated. It's simply easier to make the chassis negative for all DC systems.

Yep, people do it all the time in aluminum boats. All the positives lead back to the battery, but the battery's negative terminal is grounded to the hull and all the individual components - lights, bilge pump, etc. - are just grounded to the hull. Saves a lot of wiring.

 

[jves]

My understanding of sulfation in a lead acid battery occurs in
a battery that is less than 100% charged.
If 12.7-12.8 volts indicates 100% charge via multistage charger and is maintained at that voltage, why is 13.3-13.6 volts recommended as a
maintainance voltage?

 

[Ray-IN]

My understanding of sulfation in a lead acid battery occurs in
a battery that is less than 100% charged.
If 12.7-12.8 volts indicates 100% charge via multistage charger and is maintained at that voltage, why is 13.3-13.6 volts recommended as a
maintainance voltage?

That is a maintenance charge rate. When a LA battery is not being charged it is discharging at a rate of 3% per month, or more depending on condition. Storage batteries have a finite number of cycles, discharging(regardless of depth) then re-charging is one cycle. A maintenance charge rate prevents the battery from cycling.