AGM House Battery Charge/Discharge

[MangoMike]

Truckman,

Great post and you didn't even have to buy the guy a beer.

mm

[kmuller]

We have a fairly comprehensive tech article regarding batteries on our website. On the home page, click on Assured Service, then down the left side you will see ZMI LINKS, under that click on Tech Articles. It's about the 5th or 6th article down - intitled AGM Batteries.

[kmuller]

Here is some more food for thought relative to the Marathon discussion at the top of the thread...

Comments from our VP of Operations...

"This flies in the face of both the common wisdom in the marine industry as well as my research and anecdotal experience. I've been selling, installing, servicing (and replacing) AGMs for nearly the entire time I've worked here at ZMI. We wrote an entire article about AGM batteries and why they fail (posted on the website). I would most definitely take with a large grain of salt anything battery manufacturers have to say on the subject of life cycles, they all exaggerate, it’s an accepted fact. I had a number of conversations with two representatives from Concorde/Lifeline and got conflicting info, the engineer was more frank than the front office guy (big surprise). In the marine industry, we use 50% discharge cycles and experience and testing shows this gives a good balance between cycles and longevity and, this is important, the key metric being amp hours delivered over the life of the battery. I wouldn't discharge an AGM below 12.2 (technically, this voltage has to be measured at rest, that is with no load on the battery for 24 hours, so some interpolation is often necessary). If the batteries are at 11.8 or 12 under a load, they may come up to 12.2 or 12.4 when the load has been removed for some time. Additionally, running batteries this low produces unwanted side effects for any motor loads. As the voltage falls, amperage rises on these inductive loads, which causes the motors to run hotter and die sooner, not to mention tripping circuit breakers. Simply put, working a battery, and the motors attached to it, this hard, regardless of battery chemistry, will, in my opinion, dramatically shorten its life as well as the life of motors, fans, pumps etc. 50% discharge has been accepted for quite some time as a standard in the marine business. 13.3, by the way, is the float voltage, the voltage at which a battery is maintained. The charging voltage is typically much higher, on the order of 14.7, depending on temperature, if the charge source is temperature compensated, AND IT SHOULD BE, particularly for AGMs. The leading cause of their failure is positive grid corrosion, which results from floating at too high a voltage."

Hope this adds to the discussion,

[Joe Cannarozzi]

Karl seems to me that where the gen. kicks in and the inverter starts charging "under load" would be important to the relationship between the inverter and batteries.

Well if we run our 12s down to 11.4 under load as Toms are set I'm wondering were that would put them if you shut everything down at that point and check them later, resting?

Let me rephrase.

What is the under load voltage you can go down to and still have the batteries come back to 12.2 resting without recharging?

I'm thinking that both posts just about corolate. Marathon is streatching it but just a little?

[truk4u]

Karl,

Thanks for the posting, this is the stuff we need. It's difficult to sort out the correct way to go with regard to the magic discharge number and the 12.2 still makes the most sense to me, but I'm going to stick with Marathons recommendation and see how it goes.

One of the problems I had using 12.2 was the short amount of time for inverter use. As an example, at float of 13.3 and resting, as soon as I turn on the master light switch with all house lights on (all 24v) the voltage goes to 12.8, but will hold there a long time. We seldom use all the house lights and usually have the 120v lights on.

What I found was I could go from about 9 or 10 pm with normal 120v usage, fridge, tv, lights, webasco and still be good until 8am with 12.2 or better on the gauges. I guess this a pretty acceptable amount of time, but if it was during the day and using the microwave, coffee pot, etc., the time would be much shorter.

I'm concerned about all of this because I spent a pile of Lew bucks on new batteries and want to get the most out of them. I could sure live with shorter inverter time if the batteries would last longer. I'm going to follow up with Trace and see what their take is on all of this.

[merle&louise]

I have a question about the auto start feature on the generators.

After the generator starts up and runs until the batteries are fully charged up, does it shut the generator down or does the operator have to do that?

[Jon Wehrenberg]

Tom,

This is a perfect example of how confusing some of these issues are.

I admit to being too conservative with my batteries. I justify that position by saying there is no downside to using the generator, so I rarely ever have the auto-start switch on.

If we look at the two points of view (doing a deep discharge versus charging the batteries at some higher level such as 50%) to me the choice is obvious.

There is no downside to limiting the amount of discharge on the batteries. There is absolutely no disagreement about the fact that the batteries suffer no ill effects by doing this. A deep discharge may have an adverse impact on battery life, and as Karl points out there may also be some additional considerations.

For someone who must leave the coach for an extended time period occasionally it seems the deep discharge and use of auto-start is acceptable, but other than that type of situation it seems to me that running the generator for extended periods (or often) does no harm, and may possibly be beneficial.

[Joe Cannarozzi]

Tuga It will shut itself down as well. You can also hook an A/C thermostat into it to keep cabin temps. down and program it for timed sequences, daily or weekly or whatever. We had to replace ours and picked one up for 159 bucks. Never did install it however because the low voltage limit is 12.0 at the high end Ind I thought that was too low. After reading the posts in this thread I've decided to install it afterall. When I dig it out of the basement I'll give you a web adress. The big boys units have other options as well like kicking on when park power is insufficiant and probably other things as well but the up front cost of the one we got is uncomparable.

[Jon Wehrenberg]

Imre,

When I talk about being a generator slob, maybe I need to be more specific.

If my batteries are at an acceptable voltage and I have no loads beyond maybe a light or two, my refigerator and the TV I agree and typically do not run the generator, but, as soon as I ramp up loads such as running a lot of lights at night even if I am not running toe space heaters or the AC I crank it up until I go to bed.

It borders on insanity to worry about the fuel a lightly loaded generator consumes, or how heavily it is loaded while we are sitting in an RV that is typically on the extreme high end of the expense curve. I'll go so far as to bet nobody on either of these forums has run a generator to death. These things are good for 30,000 hours or more if maintained and if there are any out there with even 3000 hours I gurantee you that will be rare.

There is only one time when I don't want the generator running and that is when I go to bed, so by running it often I don't have to try to keep track of battery condition or wonder if the available battery power will make it until morning with the refrigerator and Webasto and whatever else is running.

[Jon Wehrenberg]

I can't speak for others on Power PUlse, but for me...no.