AGM overcharge?

[noseoil]

Mtn Don brought up a good point in a recent thread which I hadn't considered. He stated that an AGM battery could be damaged by overcharging. Hmmmm...

I'll be using a group 27 AGM in my build & was planning on having the truck alternator charge it for me while I'm on the road. With the "normal" output from an alternator running to the battery in the trailer, is there a danger of "cooking" the battery if I'm on the road for 8 hours at a time? I don't know the usual voltage the truck runs at now, never bothered to look, but is this a bad idea with an AGM? I will have a 100 watt solar panel to use installed and mounted permanently, so it should make plenty of power on a sunny day to keep things topped off. Is the circuit from the truck even really necessary at this point?

Thanks, tim

[MtnDon]

A lot depends on what voltage the TV charge system is outputting. Check the battery manufacturers recommendations for max voltages. The AGM general rule of thumb is 2.4 volts per cell max charging and 2.25 volts per cell float. Problems could occur when the TD/trailer AGM battery has reached full charge but the TV battery is lower in charge and therefore the vehicle charge system is pushing higher voltage. Hot weather requires the charge voltage to be lowered. This is where a good PV charge controller with a remote temperature sensor shines. The sensor attaches to the battery and permits the voltage to be adjusted by the CC as temperature rises and falls. That can help if the battery is quite depleted and is being charged with "lots" of amps. The battery warms and if in a closed space gets even warmer. The RTS can lengthen battery life.

I presently have a 10 gauge connection between TV and trailer batteries. But I have flooded lead acid which are more forgiving. Sometime this winter I plan on adding a couple hundred watts of roof top PV. When I do that I am probably going to leave the 10 gauge charge circuit unplugged.I do have an on/off switch incorporated so I can disconnect the charge line to the trailer from inside the TV if I want to. It simply interrupts the sense connection that activates the HD constant duty relay.

My mind is drawing a blank on who here has designed a TV to TD charge system in which he makes use of an inverter in the TD to run a battery charger also in the TD. It seems crazy but does seem to eliminate the TV / TD over-under charge dilemma as long as that battery charger is suitable for AGM and has multi stage charging.

[capnTelescope]

My mind is drawing a blank on who here has designed a TV to TD charge system in which he makes use of an inverter in the TD to run a battery charger also in the TD.

My ears were burning, so I stopped by. I believe you're talking about me, MtnDon.

There's a good description complete with photos starting here in my build thread. There's a schematic to be found a little farther down. It's not hard to do, at all, and you get the feeling that everything is working the way it was meant to. Charging directly from the TV apparently works, but not really the "right" way to do it, in my incredibly humble opinion.

It's also discussed in the Charging While Towing thread.

Get a sufficiently smart charger, and the battery should be perfectly happy.

Plenty of good progress in your build, Tim.

[noseoil]

After doing a little more reading, here's what I've found about charging an AGM battery using the standard output from a car alternator.

"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."

"AGM and other sealed batteries do not like heat and should be installed away from the engine compartment. Manufacturers recommend halting charge if the battery core reaches 49C (120F)."

Basically, what I'm thinking now is that an AGM battery is a good choice for a trailer with a solar system, but a bad choice for a vehicle set up with a flooded lead-acid battery and charging with an alternator. Doing a little more reading, a lot of the tests done on a standard car battery are actually an immersion test where the car battery is cycled while sitting in a water bath which is 165 degrees f. An AGM battery would really be cooked in no time if this type of test was done. It just wasn't designed for this type of work.

Here in Tucson, it's possible for the trailer's interior to reach over 120 degrees easily in the summer, if it's left in the sun and closed up. Add it all up and the AGM will be better off running in a cooler climate with a solar panel of adequate size. I may still think about a charging circuit from the truck since I have the parts I need already, but running down the freeway for hours on end in the summer would be a large & costly mistake. I'm also going to add a switch to cut off charging from the solar panel when I'm not using the trailer, something I hadn't considered. Since a good AGM battery will self-discharge very slowly, there's really no need to leave the solar system on charge if there isn't a current draw happening, especially during the summer months where I live.

Don & Cap'n thanks for your feedback on this one. The more I find out, the less I know.......

[capnTelescope]

Good report. I'm a little taken aback at how fussy the AGMs are about temperature.

[noseoil]

Yes, I was surprised too. It seems they're pretty good at fast charging, deep cycling down to 80% without problems, plenty of power for their size & good with hard duty in general. They will run in a position which would leak acid all over the place with a conventional battery and deal with the cold pretty well also.

They don't like heat & they don't like being over-charged in general. As with everything else, it's a compromise. Glad I did some more reading & found out BEFORE I cooked a group 27 Trojan battery. Best price I found was in Flagstaff, Arizona for what I needed. Went to pick one up in July when I was up there from home, but it's not a stock item. Takes about 4 weeks to ship in to your address. Hopefully it will be a good one & a keeper for many years to come. Have to wait & see.

[Dale M.]

Kind of wondering if we don't get a little over sensitive to a batteries environment and charge rates.... I run a a full automotive size AGM in my fiberglass (Manx style) dune buggy (same battery -12 years) and a Odyssey 850 CCA (1500 amp pulse) in my Jeep and have abused the heck out if it having run it down with hard winch pulls and had it down to where voltmeter in dash reads about 9 volts and alternator is putting out full 100 amps to recharge and so far as I can tell in two years of abuse it has not suffered much.... Mainly it states do not go over 15 volt when using a external charger in the owners manual (yes first battery ever with owners manual) ,.... If one thinks about it a battery in automotive application will self regulate on how much charge it will accept due to its chemical state and not always what you read in battery care documents.... If battery is greatly discharged the current inrush will be massive and taper to almost nothing as battery recovers charge..... Once battery reached float voltage and providing charge source (alternator, generator - external charger ) has correct float voltage, once chemical needs of battery is reached the charge rate drops to a point that can be a little as a few milamps ...

http://shop.odysseybattery.com/p/34-pc1500t-a

Dale

[MtnDon]

You can't compare the quality of an Odyssey AGM to other run of the mill AGM's.

[Dale M.]

You can't compare the quality of an Odyssey AGM to other run of the mill AGM's.

Not sure what you are trying to say here?

Dale

[MtnDon]

Odyssey AGM are made with virtually pure virgin lead. Most others are made with less pure recycled lead. They are very fine batteries. Better than many.

[Shadow Catcher]

I have been thinking about this for some time as we have a rather expensive Lifeline AGM. one option is a rather expensive Balmar http://www.balmar.net/regulators.html multistage regulator, a bit expensive at about $300. but some great features. I have been thinking in terms of a PMW to regulate current. Could you use an inexpensive PMW solar controller.
As an aside when we got Compass Rose the wiring to/from the converter (the DC and AC were grounded to the frame) was messed up and it boiled an AGM battery to half-life.

[noseoil]

My main concern is charging in the heat, since I live in the desert. Here'a an evaluation of several types of batteries in the group 27 size. It was done to evaluate the needs of sail boats, but there are similarities which can be found with trailers. Draw your own conclusions....

http://www.practical-sailor.com/issues/ ... 244-1.html

[lrrowe]

Noseoil,
Thanks for this link and to all the others posters, keep up the good work in sharing.
I have been lurking around with this topic. Batteries are not my forte and I cannot offer any insight, but what you and the others are posting helps me very much. I am maybe 3 or 4 weeks away from having to make my expensive decision on what batteries to buy.

[noseoil]

Yes, way too much conflicting data & at some point you just have to bite the bullet & hope for the best. I chose the Trojan AGM based on research, storage capacity & general durability, it certainly wasn't cost. No maintenance to worry about....

Pros: Draw down to 80% won't hurt it. Quick recovery with a charge. Many hard duty cycles available. Long life span IF properly cared for. No worries about gassing during a charge. No spills from acid. Made in the USA.

Cons: It costs more than a good lead-acid battery. Susceptible to heat problems & charging. Limited availability.

All in all, I decided to go with a battery with greater inherent energy reserves for the size & cost, & less trouble over time. An AGM is "cleaner" to use in general. No worries about adding distilled water twice a year. Doesn't really need a vented battery box, although I was planning on a lead-acid type to begin with, so there's a vented battery box in the galley now anyway. If it decides to take a dump, ever, at least the mess will be contained in the box. Maybe not your needs or ideas here, but you get what I was thinking to do the buy on mine. Good luck on your purchase, find the best a little money can afford!

[lrrowe]

Yes, way too much conflicting data & at some point you just have to bite the bullet & hope for the best. I chose the Trojan AGM based on research, storage capacity & general durability, it certainly wasn't cost. No maintenance to worry about....

Pros: Draw down to 80% won't hurt it. Quick recovery with a charge. Many hard duty cycles available. Long life span IF properly cared for. No worries about gassing during a charge. No spills from acid. Made in the USA.

Cons: It costs more than a good lead-acid battery. Susceptible to heat problems & charging. Limited availability.

All in all, I decided to go with a battery with greater inherent energy reserves for the size & cost, & less trouble over time. An AGM is "cleaner" to use in general. No worries about adding distilled water twice a year. Doesn't really need a vented battery box, although I was planning on a lead-acid type to begin with, so there's a vented battery box in the galley now anyway. If it decides to take a dump, ever, at least the mess will be contained in the box. Maybe not your needs or ideas here, but you get what I was thinking to do the buy on mine. Good luck on your purchase, find the best a little money can afford!

I will research this more with a specific focus on AGM's and a PD4045 unit which I have already purchased. I hate to waste the purchase money so much of my decision will be based on whether I can use this charger. For the most part, my CT temps will be under control, being in the East and all that. I just do not see hitting 120 F even while sitting in the yard.

Up til this point, I was convinced that 6v Trojans T105's was the way I would go. I like the AH ratings for them and my ultimate goal was to have four of them with over 400ah's available.

I will have one or two solar panels, but not til spring.