Battery draw down range?

rp3703

Can someone explain to me how far my batteries can be depleted before I will start to cause damage? I have eight 6V wired for 24V. I use a Midnite Classic 150 CC and I am pretty consistently drawing them down to around 70% by morning. When I first got to our cabin, I forgot to turn off skip days and ran them down to 55%. I just want to make sure I’m not killing them. 

Photowhit

I'll give it a shot, after 20 year living on batteries, I have a clue....

Best to start here!
You bought lead acid batteries and they had already started dying! If you use them or don't use them, they have a finite life. 
Figuring out how to best use that life to serve your comfort and maximize their service is the key.

Things that affect that life are depth of discharge and length of time at a low level of charge.

Quality batteries will typically have a manual with a chart or graph showing a rough expectation of number of cycles to a certain SOC.

I actually did not find a chart with a quick search for Trojan T105! Perhaps companies are tired of trying to explain these...
Northern Arizona Wind and Sun offers this chart with no explanation of what type of battery it is;

Link to NAWS Battery FAQs;
Deep Cycle Battery FAQ | Northern Arizona Wind & Sun (solar-electric.com)

I would suspect this is for a tall case battery, I think golf cart size 6 volt batteries usually show around 2-300 cycles to 20% State of Charge (SOC). 

So, you get fewer cycles to a higher state of discharge. This chart shows what is often quoted as 2X as many discharges to 20% SOC as 50% SOC. This DOES NOT MEAN you shouldn't discharge to 20% SOC!!!

To answer your question, you are killing your battery every day!

I guess you could write an analysis of how much life you lose each day. For golf cart batteries figure you are losing 1 day every day you use it off of a potential 7-10 year max life. Add a loss of 1 day for every day you take it down to 50%, add 2 days for every day you take them down to 20%. Double the days if not recharged the next 24 hour period, square it for the next day, yep thats 4 days for everyday left at 20% and 16 days if left at 20% SOC.

It's certainly a touchy-feely thing.

I use 4 golf cart batteries to run a window air conditioner for 4 summers. I'd run them 4-5 hours a day along with my other minimal loads bringing them down to 50% SOC every day. They were a year old when I started using them in this manner and lasted 4 summers. I was happy to get a 5 year life from them. Would I have swapped 90 degree heat for a 7 year life...lol.

I recently bought some friends 5-year-old Trojan 8 volt golf cart batteries. They spoke as they had kept them above 70% SOC only a couple times dropping to 50%. My 12 year old forklift battery was rapidly dying. It had a dead cell but had managed 4 days of no charging running a 20 year old fridge so it had a lot of capacity in the remaining cells. I was hopeful it would survive the summer air condition season, but my inverter's DC to DC converter was over heating in the evening when the air conditioner was switching on and the system voltage dropped from 28-29 volts charging to 22 volts when the compressor kicked on in the evening with minimal solar assist.

A couple days ago I setup the 6 - 8volt batteries. Ran the air conditioner until midnight, last night and went to check on electrolyte levels this morning. Battery bank was at 24.8 with slight discharge above charging. I'll try to kill them a little more tonight as we start our 1st of 3 100 degree days. Looks like these will make it through the next year. Now I'm gambling on competition for the only UL1741 LiFePo4 battery prices coming down with competition VS inflation...

Hope this helps.

Photowhit

I should add the storage temps affect battery life as well. Mine live outside. May reach the median temp of 90 lose an extra day of battery life for days 10 degrees above 78-80 degrees. Less than 60 degrees will increase your expected battery life!

I don't auto equalize over the summer, rather choose cooler days to do it. I won't do it on days above 80.

rp3703

Thanks for that explanation. We are using cheap Deka 2nds golf cart size batteries, that we paid $80 a piece for back in 2017. If I make it to 7 years, I’ll consider it a success.

Our setup is only used 30 days each year at best and I have had it set for 10 skip days when we are not here but I have read that it’s better to only go 3. Our place is in Northern Ontario, so these batteries don’t get real warm unless we are here using them. 

Since you’ve been going 20 years now, how can you tell when the batteries are at the end of their life span?

littleharbor2

Lead acid batteries will appear to go from a depleted state of charge to full quite rapidly and appear to discharge just as rapidly. The problem is they have diminished capacity due to heavy sulphation.  If they are FLA, wet batteries with removable caps you can verify the state of charge and health with a hydrometer

I'm trying out a new "away" charging scheme this summer. All loads are off.  I have turned down the Bulk/Absorb voltage setting on the charge controller to 13.6 volts. Actually 27.2 volts as it is a 24 volt bank. I'm doing this on my neighbors batteries this summer while they're away on the cooler side of Baja. After 60 days now I had to add a total of 8 Oz. of distilled water to their 16 battery bank. As long as the voltage remains above 12.8/ 25.6 There should be no sulphation taking place. At least that's how I see it. Would there be any reason to worry about stratification with this plan?
 I would welcome any comments regarding this plan.