Vehicle Dual Battery Charger for AGM 2nd Battery

[birdie]

I have recently purchased a PowerSonic 120AH AGM PDC-121200 battery to use as my 2nd battery in my vehicle and campsite power when we get to camp (for running my fridge and lights).

My question is about charging the battery in the vehicle (2008 MY09 Mitsubishi Pajero). I have had so much conflicting advice as to what is required for the dual battery charging system.

Some people are telling me I must go with a DC-DC charger like a CTEK D250S or Redarc BCDC1220 as they are designed for batteries like AGM batteries with features like bulk, float, etc. So they charge the AGM battery to 100% and then float to ensure maximum cycles...

Other people are saying it would be fine just to use a Voltage Sensitive Relay like a Redarc SBI12 which will allow much more current so charge quicker. The same people say the battery will float itself because of the chemistry in the battery. They also say that the DC-DC chargers are not needed for the older/regular style alternators with a fixed voltage (which my vehicle has) (instead the new ECU/temp controller alternators), and that the "hype" about needing a DC-DC charger for an AGM battery is just advertising hype...

I'm so confused as to what is needed to ensure my AGM battery is charged fully and has the best life.

Can you please advise me which way to go?

[David Hess]

AGM and gelled electrolyte batteries have different charge characteristics than flooded cell batteries.  To get the best life out of them, I would use a dedicated charger rather than relying on the vehicles charging system.

[birdie]

AGM and gelled electrolyte batteries have different charge characteristics than flooded cell batteries.  To get the best life out of them, I would use a dedicated charger rather than relying on the vehicles charging system.

So a DC-DC charger rather than a VSR which basically connects the 2nd battery in parallel with the main starting battery only isolating the 2nd battery when the vehicle is turned off (to prevent accessories on the 2nd battery from draining the starting battery).

[David Hess]

AGM and gelled electrolyte batteries have different charge characteristics than flooded cell batteries.  To get the best life out of them, I would use a dedicated charger rather than relying on the vehicles charging system.

So a DC-DC charger rather than a VSR which basically connects the 2nd battery in parallel with the main starting battery only isolating the 2nd battery when the vehicle is turned off (to prevent accessories on the 2nd battery from draining the starting battery).

Yes.  Some charging systems intended for flooded cell batteries will shorten the life of an AGM battery considerably.

Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell), and a direct replacement with a sealed unit could spell trouble by exposing the battery to undue overcharge on a long drive.

http://batteryuniversity.com/learn/article/absorbent_glass_mat_agm