AGM Overcharging?

[classic72]

We recently made a week long trip.... the first with our dual battery setup. Some background on the setup:

-Starting (Stock Lead Acid) Battery Located under Hood
-Secondary Battery (Exide Group 31 Megacycle AGM) mounted in the Bed inside an ArkPak primarily running Fridge
-Tied Together using a 200 Amp Stinger Isolator with 4 Ga + Wire running from isolator to battery in bed of truck to the wingnut terminals on the Arkpak and 4 Ga - grounded to frame.
-Stock Alternator

Before we left everything seemed to be operating fine for short trips around town. The first day we covered around 6 hours of driving. At hour 4, I pulled into a gas station late at night and (fortunately) looked in the bed to check our the gear. I smelled a funny smell and the arkpak was beeping its overheat warning. I climbed in the bed and saw a sleeping pad roll had fallen and blocked the left vent on the Arkpak and their was condensation down in the bottom of the arkpak and steam coming out of the vent holes. It was so hot it killed the display on the arkpak. I then disconnected the arkpak but left my fridge plugged in to drain the battery back down.

I nursed this setup along the rest of the trip by only having it connected and charging for around 1-2 hours at a time which would power the fridge for a good 16 hours or more. That said, I don't want to have to keep climbing inside, unloading gear, then disconnecting the battery manually etc. The alternator should prevent the AGM from overcharging just like it does the primary battery correct? I have had zero issues with the primary battery. Am I missing something in my setup like a charge controller? Could it be this was just because of a blocked vent hole on the left side of arkpak. The other vent holes were still open. How hot do these AGM's typically get during charge and discharge cycles?

Any thoughts? Thanks in advance.

 

[john61ct]

You need to actually measure the volts and amps at battery terminals, and compare to mfg charging specs.

Stock alternators very often don't fo the right thing.

Ideally the alt VR is adjustable voltage, with separate (from output) voltage and temp sense wires direct to the target battery.

Or a DCDC charger, Sterling b2b.

If you need to replace AGM, better true deep cycle brands include ​Lifeline, Northstar, Odyssey and Firefly Oasis.

That last is G31 and handles PSOC very well, but $500 per.

 

[jonyjoe101]

I would check the voltage at your agm, it should be about 14.4 volts or lower when the engine is running. It should be lower since its further from the engine. If its less than 14.4 volts, then something else is causing the overheating because no agm will vent at that voltage unless its bad.

The alternator doesnt control the charging, it just provide power at a certain voltage (14.4 volts). A 100 amp alternator will provide 100 amps all the time. The batteries themselves are what controls the charging, an empty battery will take all the 100 amps, but as it gets full it takes less amps.

As long as the voltage is at 14.4 volts, you can keep it there all day long without overcharging the agm (which is a lead acid battery just like your start battery). The only way to overcharge the agm is too force it to take more amps, you force it by increasing the voltage above 14.4 volts, at 15 volts or higher the acid starts to get hot and it will vent.

You mention you saw steam coming from the vent holes, thats not a good sign for an agm, usually too much voltage will cause that.

 

[classic72]

Thanks for the tips. I will use a multimeter and see what it says. The ArkPak would show the battery at around 11.7 with the truck off. When I started the vehicle and the secondary charging it would slowly increase to around 14.2 and I never saw it higher than that unless something happened while driving and in to the trip. What other causes if its not the voltage issue? Could it just be a bad battery. The cables are fine, brand new and high-end 4 GA wire with proper fusing.

 

[dwh]

As I understand it, the wingnut terminals of the Arkpak connect directly to the battery, so it's not an Arkpak issue.

With the isolator direct from battery to battery, what you have is a plain old "split-charge relay". So the vehicle voltage regulator controls the charge voltage and both batteries should be getting the same charge voltage.

So first question is: What is the charge voltage that your voltage regulator supplies?

Is it computer controlled? Temperature sensing? 1977 Datsun?

 

[dwh]

11.7v is less than 50% charged. If it was resting at that voltage after being charged, then it's a bad battery.

 

[classic72]

The VR would be whatever the stock VR on a 2013 Ford F150 Platinum so we aren't dealing with a '77 Datsun. I am leaning towards the battery myself because you are correct in that with the cables connected directly to the arkpak you are bypassing the actual arkpak charging system and its controlled by the truck. Also the fact that the stock battery has been 100% fine and tested fine at the dealer last time I was their so I am not having voltage related issues on the primary battery and it's not overheating.

I was able to get Amazon to refund me the full amount on the battery (amazingly was within 6 days of the warranty expiring).

I will test the voltage with the current charging system and post with the results. I still want to make sure the new battery will be ok.

 

[classic72]

Okay, here are results. At the primary (starting) battery with vehicle running at idle I am getting 13.6v, at the isolator I am getting 13.6v, at the Battery Terminals in the bed on the ArkPak I am getting 13.2-13.3v. This is using a Klein MM300 Multimeter.

I was testing at each set of the battery terminal connectors and the isolator.... not directly at the alternator.

 

[pugslyyy]

The issues you describe really sound like there was a problem with the AGM. That notwithstanding, AGM and flooded lead acid batteries have quite different charging profiles. You might consider adding a battery-to-battery charger (such as made by Sterling Power) - let you alternator supply power to your truck battery, and let the Sterling control the charge of your house battery. defender.com has the best pricing, at least the last time I checked.

 

[dwh]

Okay, here are results. At the primary (starting) battery with vehicle running at idle I am getting 13.6v, at the isolator I am getting 13.6v, at the Battery Terminals in the bed on the ArkPak I am getting 13.2-13.3v. This is using a Klein MM300 Multimeter.

I was testing at each set of the battery terminal connectors and the isolator.... not directly at the alternator.

Close enough. What's the voltage at higher revs?

 

[Step-Hen]

This may be a silly question, but did something short the battery out? I assume that you have some sort of fuse close to the battery, but I've seen plenty of instances where fuses and circuit breakers don't open when they should.

 

[classic72]

This may be a silly question, but did something short the battery out? I assume that you have some sort of fuse close to the battery, but I've seen plenty of instances where fuses and circuit breakers don't open when they should.

There shouldn't have been a short. It is fused as well. The connections are separated by a decent distance and the terminals have a plastic wall between them, coated wing nuts holding them down that cover the entire terminal securely, and the wires themselves have very thick sheathing and are additionally wrapped inside plastic flex conduit.

As for the RPM's I had my wife rev to around 1800 and the Voltage didn't change but more than .1 or so.

 

[dwh]

As for the RPM's I had my wife rev to around 1800 and the Voltage didn't change but more than .1 or so.

Ah. Well One possibility would be a modern computer controlled charging system which was just float charging the one battery (because that battery was fully charged).

Another possibility might be a system which deliberately keeps the battery less than fully charged, so that when braking it can fire up the alternator to load the engine a bit to help slow the truck down. Sort of a poor man's regenerative braking system and terrible for charging (actually, not charging) an aux battery. Sterling has a special battery to battery charger for that:

https://cdn.shopify.com/s/files/1/0..._ef287d8b-71f6-4ac3-90ba-951240713d83.pdf?751

"Boost / Reduce Charging. The B2Bs ensure batteries get the correct charging profile irrespective of high or low input voltages."

My old '76 Ford van holds 13.5v at idle, and 14.5v at any rpm above idle. To get a "12v" lead acid battery fully charged, you need to get it up above 14v and hold it there for some hours.

If your charging system isn't doing that, then that could be a reason the aux battery failed - chronic undercharging.


You can get by with a lower voltage - as long as you're only charging an engine cranking battery that usally only gets drained 1/5 amp*hour (or usually less) to start the truck, and then immediately trickle charged back to full. But to fully charge an aux battery, you must supply an adequate charge voltage, and 13.6v ain't gonna cut it.

 

[classic72]

Interesting. I will do some more research on the charging system. This morning before any charging I had totally drained the secondary AGM battery with the fridge and it was registering 10.1v.

Today on my way to work I went ahead and hooked the second battery back up. Overall throughout the day it would have been charging off the alternator for around 2.5 hours without anything connected to drain it back down. When I got home after an hour drive I immediately went in the bed to check it out. Battery actually was pretty warm but nothing major. I checked voltages with truck off. The secondary battery was right at 12.5v and the primary cranking battery was at 12.7v.

Now my mind is playing tricks on me. That night with the condensation issue and the "steam" the ambient temp had dropped pretty rapidly from the start of the day (80's to 40's). Inside the topper I didn't have any windows open and yet I had the heat of the fridge and the heat generated by the battery which could have easily fogged the windows. Still... the battery was extremely hot to the touch and I quickly placed my finger on the negative terminal of which I could only keep it there for a few seconds. Also there was a really odd smell. That doesn't seem normal but I have no idea.

 

[dwh]

The lead plates in a lead-acid battery build up electrons when charging or discharging. That's called a "surface charge" and can give a falsely high voltage reading. To get an accurate reading you have to wait and let the surface charge dissipate and then read the "resting voltage".

The normal recommendation is to wait 4 hours with no charging or discharging taking place and then read the voltage.

Also, starting with the aux battery at 10.1v, there is no way it was fully charged after only 2.5 hours (a resting voltage of 12.7v would be damned near 100% charged), even if the charging system had gotten up over 14v.