Originally Posted by KYAirstream
If damage to batteries begins occurring once 50% discharged, is there any sort of device that will automatically disconnect the batteries at that point?
Flooded cell lead acid batteries have a lot of seemingly mysterious properties. First and foremost is that they are not linear in many of their characteristics.
As an example, if you discharge them slowly, they will have more capacity than if you discharge them fast. So the same battery discharged at say 3 amps per hour will give you much more energy prior to total capacity loss than it would if discharged at 30 amps per hour.
Similarly, a battery which is only used down to 80% of it’s capacity and then recharged will have many more cycles possible than the same battery used down to 20% of it’s capacity and then promptly recharged. A battery never fully recharged will have a far lower cycle life than one which is brought back to 100% of capacity. So, a discharge to 20% and recharge only to 60% over and over will shorten the life of the battery significantly.
The question of “damage” is hard to pin down as we are really talking about cycle life. Do you want a battery to last a long time? Don’t work it hard and recharge it promptly. If you are not as concerned about long life, you can discharge it deeper, and not recharge it as thoroughly or as soon. The “damage” is what you will accept. It is common to say that a battery should not be discharged below 40 to 50% of capacity but what is really meant is that the battery cycle life will be shortened a fair amount if it is done often. Is this “damage”? Only you can say.
A couple of things will shorten cycle life very severely, and should be avoided or never allowed to happen at all. These things would probably fall under the category of damage, if not abuse.
1. Total discharge of a battery to 0% and no recharge for days, weeks or months. You will most likely never recover a battery which has sat discharged for any length of time. So, if you for some reason run your batteries down to nothing get them charged up ASAP or you will probably be purchasing new ones. Even doing this a few times will greatly reduce the total cycle life of a flooded cell battery.
2. Letting a battery sit for weeks or months partly charged. Chemically this allows the plates to become unable to accept a full charge and total battery capacity is then reduced. They may seem to “work” but not to their original capacity. They charge up fast, but discharge equally fast as their capacity has been changed.
3. Letting the tops of the plates show before you add water. It is battery murder. The tops dry out, expand, and even when covered again with distilled water will have become essentially useless. They may even short plate to plate which is instant cell death.
On the question of a device to limit depth of discharge to any specific amount, then automatic shutdown, yes, such things exist but can be annoying or even dangerous in an emergency situation. And if you have a slow battery drain which causes discharge to say 50% and then the device shuts off the power, your battery sits half charged until someone does something about it, and by that time the battery has been “abused” and may not recover anyway. Sitting half charged for long time periods is not good for batteries. See 2 above.
So, considering the cost of batteries, and the desire to use them as fully as possible with out abusing them, in general don’t use them below 40 to 50% of capacity and then promptly recharge them. You can use them harder, just expect a shorter life from them. You can use them easier, and they will generally last considerably longer.
I recommend a well set up battery monitor such as a Tri Metric (and others) to allow you to see just how far down you are drawing your batteries, and when they are fully charged. Only with a device like that will you actually be able to see what is happening to your batteries. Most everything else is guessing.
In my own case, I use a pair of 6 volt golf cart type batteries in series for a capacity of about 220 amp hours at a 5 amp discharge rate. I have a 200 watt solar system which, when out camping, replaces some or all charge into them daily. I tend to find spots in the shade, so my solar does not bring my batteries back fully quite often, but they are totally recharged when I return home with my PD 4645 converter/charger. Even in late fall weather with furnace use and more lighting and inside occupation, I have never seen my Tri Metric meter show them at less than 40%. In the summer, like now, they seldom go below 80%. My last battery set lasted over 10 years in the Argosy. I am only on my second year in the Airstream’s set as it is new.
So, ask a simple question, get a load of too much information. But there are few simple battery question answers in truth. They all are filled with if’s and’s and but’s.
I hope this helps some.