Bad factory setting on charger will damage Aux battery

[daveb]

Could be how you're cycling the battery.

(From Northern Arizona Wind & sun)

A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film.



The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.

[vlamgat]

Thank you for your thoughtful input but there are some realities we all have to face. No easy way exists to count the number of cycles or track the level of discharge. So as in all things "consumer" we have to muddle through with approximations. I reckon that on an average Texas summer day with a thermostat setting of 85, the a/c will cycle on 2 times an hour for 24 hours. That's about 48 cycles a day. I do this typically for 2 full days, 8 times a year or say 1000 times a year. My guess is that the voltage drop from a nominal 12.8 (13.6 under charge) is to 12.4 which is about a 20% (??) discharge. So based on your table I should not expect more than 2 years usage from each battery?

[daveb]

Don't know, there are so many variables. Do you really think you're dropping the DOD to 12.4? It actually sounded like the batteries are being subjected to several light discharges. The only way to determine what the resting voltage ends up as is to discharge the battery after a typical session and let it sit W/O a load for 10 hours and then check the voltage. I'm guessing the only time the battery supply is used is when you're plugged into shore power and the battery bank supplies a slight boost for A/C startup, or is this an on the road thing? Perhaps you're overcharging the battery bank. Can you monitor the amperage of the charge? There are other factors...the rate of the charge, the amperage during the discharge, the temp your batteries are during the charge and more. Mind you this is all speculation on my part. Without really getting down and testing it is very difficult to actually tell what is going on.



This probably has no bearing on your issue but I finally found my alternator was insufficient to deliver a positive charge to my batteries at night with the Starcool on. The total amperage draw from the engine plus the Starcool was draining my entire battery system. I switched to a heavy alternator and that solved the problem. But I still have an issue with running it on shore power and it's similar to your problem. I want to completely isolate the Starcool from the battery system and run it off a 12vDC power supply via shore power only when vehicle is parked. I doubt it will happen as the Starcool uses parts of the stock A/C when operating. The only reason I need A/C is because it's hot, and I do try to avoid charging the batteries when it's very hot outside. JMO They do get hit by the alternator charge all the time when they're hot. Who knows, my batteries might crash and burn tomorrow. They are 3 years+ an still like new...knock on wood.

I wish you luck.