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Old 01-08-2020, 12:54 AM   #1
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Battery Behavior on Charger

Just finished putting four Trojan T-105 AGM batteries in a Torklift box on the tongue of my toyhauler.

Previously I was running two Trojan T-105 wet cell batteries which I removed and hooked up to an 8-AMP Battery Minder charger. I had the new batteries on the same charger while waiting to install and they would show 14.4 volts and then down to 13.55+ on float. The Battery Minder is supposed to charge at 14.4 and then reduce to 13.6 volts.

The three year "old" wet cells have been on a Progressive Dynamics 9200 charger-inverter 24/7 when not on the road. I've tried to never discharge below 12.2 volts, but have went to 12 volts a couple times. We have solar and a generator to recharge when out camping.

The "old" batteries are showing 14.2 volts, then dropping to 13.4 volts while on the Battery Minder charger - a little short of the stated charging voltage of the Battery Minder.

1) Can anyone explain why there is a voltage drop between new and old batteries while on the same charger?
2) If anyone has experience with Battery Minder chargers, do you think the desulfating process may bring the voltage up some if I leave the batteriess on the charger for a couple weeks? I'm giving a friend the old batteries and I'd like to be able to accurately describe their condition and/or improve them if possible..

I've not taken the "old" batteries off the charger and rested them for a day to get their base charge level.
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Old 01-08-2020, 09:22 AM   #2
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Having had the Battery Minders for several years (maybe seven?), I can report that my voltage hovers around 13.6 while on the charger. My group 31 batteries are lasting about five years or so. I very rarely let my batteries get below 12.1 volts when I am camping.

I don't know whether the pulsating "desulphation" feature of the Battery Minders really accomplishes anything, since I am not an expert. But my batteries are performing satisfactorily, so I guess the Battery Minder is doing the job.
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Old 01-08-2020, 10:22 AM   #3
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It is more than likely a difference in battery chemistry between Wet Cell and AGM rather than new vs. old. It is withing the margin of error any way.
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Old 01-08-2020, 06:05 PM   #4
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Battery tenders/minders/maintainers I've looked at usually employ a switched float mode, turning the charger on when the battery drops to a given low voltage and turning off at a higher voltage. So depending on what point in the cycle you measured it, you might see a terminal voltage somewhere between those two points.

In my opinion, I think the desulfator/rejuvination mode is mostly a gimmick. Maybe one can show that a battery which has suffered a particular trauma might benefit from it, but in my experience most battery issues are eggs you cannot uncook. Can't hurt, but probably won't help.

You can do a measured discharge on these batteries and can report to your friend exactly what their capacity is.

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