Internal Auxiliary Battery

MarcFJ60

Adventurer
Just looking for a little re-assurance here . . .

What I'm planning is really just an extra battery, not really an auxiliary battery that is part of the vehicle charging system.

I have an Edgestar fridge that I run off its own battery (when not using the car's 12V while moving). As much as I'd like an auxiliary battery, I find it is more useful to just bring an extra battery. That way, I can take the fridge and extra battery out of the car and it is more convenient when camping. At some point, I want to make a setup where I can leave the extra battery in the vehicle at all times (except when removing it at the campsite).

So my questions (or my assumptions) -

- If the battery is not being charged and it is an AGM battery, I'm assuming there is no need to vent it while it is inside the car.

- If I decide to rig up some sort of part time battery tender, using it only for brief intervals at a time (couple hours), am I still OK not having it vented?
 

dwh

Tail-End Charlie
AGMs are VRLA (Valve-Regulated Lead-Acid). They have a pop-off valve to release overpressure in the event of overcharging. It takes a fairly persistent overcharge to get that valve to open up. Push the battery up to a high enough voltage and hold it there a while - sooner or later the valve will open up and vent a little hydrogen.

(Exactly what voltage it'll take to overcharge the battery depends on the ambient temperature - a battery might not have a problem being pumped up to 15v when it's cold, but might pop-off if pushed up to 15v at 100 degrees F.)

Some of the pop-off valves are one-time use - once they open up, they stay open and will vent hydrogen whenever the battery is charged after that. Some are spring-loaded and will close up after the over-pressure event and won't open up again until the next time the battery is overcharged.

Either way, at that point, you've now screwed up the chemical balance inside the battery, with no way to replenish the lost water, so as regards the life of the battery - it's all downhill from there.


But no, the chemical reaction which builds up pressure to pop the valve doesn't happen unless the battery is being charged.

So there is no need vent the battery if it's not being charged. In fact, there is no need to vent it even if it is being charged, as long as it's not being OVERcharged.



(But of course, for legal purposes, I should state that no one should ever listen to me nor do anything in the crazy and unsafe way that I do things. Please, make sure that you always have your sealed batteries in a vented situation, "just in case" the valve magically pops and vents hydrogen even when it's not being charged. And always, always wear eye protection - after all, a BB could bounce off that battery and put your eye out.)
 

dzzz

I would worry more about an AGM coming loose in an accident than gas venting. It can only vent if it's improperly charged. AGM have a lower charge voltage than typical starting batteries.
Whatever charger you get should be for the battery type you buy.
Marine deep cycle batteries sometimes have good handles as they may be removed frequently when used in apps like trolling motors.
If you have a garage, I wouldn't necessarily worry about charging in a vehicle overnight from an AC charger. Just leave a window open.
 

dwh

Tail-End Charlie
AGM have a lower charge voltage than typical starting batteries.

Well...not exactly.


Absorbed Glass Mat is just a method of electrolyte storage. The other two methods are FLA (Flooded Lead-Acid, i.e., it sloshes), and Gelled.

There are cranking (starting) batteries which use absorbed glass mat to hold the electrolyte, just as there are also AGM deep cycle batteries, and AGM RV/Marine batteries.

For example, Optima sells both cranking batteries and deep cycle batteries - but all of their batteries use absorbed glass mat to store the electrolyte.

Whether the battery is for cranking or deep cycle use depends on the thickness of the lead plates, not on the method of electrolyte management.


"Smart" chargers, and solar charge controllers, generally have two settings (if they even have settings) - "FLA/AGM" and "Gel". This is because AGM and FLA batteries generally have about the same charge specs, whereas Gelled electrolyte batteries normally have a lower voltage at which they are considered "full".

The exact charge "profile" for any particular battery is specified by the manufacturer. FLA and AGM batteries generally take about the same charge profile, and this is why most smart chargers and solar charge controllers lump them together.

There are some notable exceptions; Interstate, for example, is somewhat notorious for specifying an "absorb" voltage (for some of their batteries) of 15.5v - which is quite a bit higher than what most FLA/AGMs specify for absorb. Many smart chargers and solar charge controllers can't even DO 15.5v. Some can be programmed to do it, but most non-programmable chargers (such as those from Iota, WFCO or Progressive Dynamics) simply won't go that high.

http://www.batteries-faq.com/activekb/questions.php?questionid=1


But in general, FLA and AGM are interchangeable as regards charging voltage. It's Gels that require a lower voltage.

Whether the battery is for cranking or deep cycle is usually irrelevant as regards charge voltage.

For instance, Optima's specified charge profiles/voltages for their Red Top (cranking) and their Yellow and Blue Tops (deep cycle) are essentially identical:

http://www.optimabatteries.com/product_support/charging.php


Lifeline Batteries (AGM) also specify the same charge voltages for cranking or deep cycle type batteries:

http://www.lifelinebatteries.com/manual.php
 

keezer37

Explorer
What you propose it similar to how I use mine. I have an Odyssey in a truck tool box in the back of my truck. It stays in the charging system in the winter as a backup battery and is for the most part out of the loop in the summer. I use a Deltran Battery Tender Plus. The tool box is a gasketed Weatherguard. I've had this setup for about three years now.
 

CliftonSmith

Observer
Hello guys! Speaking of auxiliary batteries. Can anyone suggest me what's the best to have? and where can we find one at an affordable price. any help is very much appreciated. thanks! :)
 

LandCruiserPhil

Expedition Leader
Hello guys! Speaking of auxiliary batteries. Can anyone suggest me what's the best to have? and where can we find one at an affordable price. any help is very much appreciated. thanks! :)

Interstate battery in AZ is the distributer for Optima battery. I purchase refurbish batteries from them for $60 blue/yellow tops with great results.
 

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