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I have a pair of 6V wet cell golf cart batteries (for house power), a 12V wet cell battery (for starting) and a Zantrex 3 stage smart charger on my sail boat. The 12V wet cell is buried in the bottom of the lazarette, where it is very difficult to access, so its fluid level tends to get neglected. A clear case for switching to an AGM for the starting battery!

But wait a minute.......the optimum charging voltage for a typical AGM is different to a wet cell. My Zantrex even has a switch to choose the battery type. What to do? One theoretical option might be to fiddle around with adding an extra resistor on the charging side of the battery that prefers the lower charging voltage.

Yesterday a battery sales guy told me that some AGMs (including his) are far more tolerant of wet cell charging voltages than others, something to do with the design of the lead plates. Was this just a sales pitch is there a generic difference in some AGMs that I need to know about?
 

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My understanding is that AGMs will not be harmed by using the wet-cell charging profile. However, you will not get all the benefits of owning an AGM (for instance, faster charging) if you take this approach. But if your objective is simply to avoid the battery maintenance issues, then this would be a good solution -- for a price.
 

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Discussion Starter #3
AGM Charging

However, you will not get all the benefits of owning an AGM (for instance, faster charging) if you take this approach.
I can see fast charging as a great attribute for house batteries, but surely this is only a very limited benefit for a starting battery?
 

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I can see fast charging as a great attribute for house batteries, but surely this is only a very limited benefit for a starting battery?
Probably. I was speaking more generically. Although, I would prefer to have any battery charge as quickly as possible.

But no, the slower charging should not be a big issue for your starting battery.

As for this suggestion,

One theoretical option might be to fiddle around with adding an extra resistor on the charging side of the battery that prefers the lower charging voltage.
I would not go to the bother, but instead would accept the lower wet-cell voltage for charging the AGM since it won't harm it.
 

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There should be no problem using an AGM with the Xantrex in flooded mode. Be sure to get a starting or dual purpose AGM for that role to maximize current. Actually charging algorithms for Wet cells are 1/10 of a volt higher than for AGM's in Xantrex's algorithms. For the minor amounts of amp-hours to be replaced this should be inconsequential. Suggest you remove the AGM from the circuit however when you go into EQ mode on your 6V's or it will be toast.
 

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You're right Cam, I stand corrected. The "warm" and "cold" voltages are slightly higher for the flooded batteries than the AGMs. At "hot" temp settings the charging voltage is the same.

These figures from our Xantrex/Statpower charger manual (Truecharge 20):

Absorption voltage Hot Warm Cold
Flooded 14.0 14.4 14.8 VDC
Gel 13.8 14.2 14.6 VDC
AGM 14.0 14.3 14.6 VDC
Float voltage at no load Hot Warm Cold
Flooded 13.1 13.5 13.9 VDC
Gel 13.4 13.8 14.2 VDC
AGM 13.1 13.4 13.7 VDC
 

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...What to do? One theoretical option might be to fiddle around with adding an extra resistor on the charging side of the battery that prefers the lower charging voltage...

Others have covered this better, but it probably needs to be said, to think twice about the resistor approach. A resistor will only really slow down charging initially. It won't lower the voltage much when the current slows down, when the batteries are approaching full charge.
 
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