Foretravel Owners' Forum

Your battery manufacturer may differ, but I have Lifeline 8D batteries in my coach and Lifeline says 11.8V is the minimum recommended voltage.  At that point, the batteries are 30% depleted and should be charged.  My generator auto start is set to fire the gen for charging at 11.85V.  I'd let it charge up and get them in the mid 12's, then let it hold there instead of at this lower voltage if it were my rig.

I would suggest you either run the Generator to keep the batteries up, or plug in to power if available.
You don't want the batteries too low.
Is coach winterized? if not, how are you keeping it warm?

Your battery manufacturer may differ, but I have Lifeline 8D batteries in my coach and Lifeline says 11.8V is the minimum recommended voltage.  At that point, the batteries are 30% depleted and should be charged.  My generator auto start is set to fire the gen for charging at 11.85V.  I'd let it charge up and get them in the mid 12's, then let it hold there instead of at this lower voltage if it were my rig.

For lifelines or other AGM batteries, I am pretty sure at 11.8 volts your batteries are 70% depleted.  50% (12.2 volts) is generally the minimum you should get to before a 100% recharge.  Discharging to 50% and recharging to 100% will get you the nominal minimum battery life. Discharging to 70% is much worse.  Discharging to 75% remaining and recharging to 100% every time will more than double your battery life (measured in cycles (discharge and recharge).  Recharging to less than 100% will reduce capacity over time.  If you recharge to 90% every time pretty soon that is as much as you will be able to charge it to. And that last 10-15% can take as much time or more than 50% to 85%.  Treat your batteries with great care.

A fully charged 12v AGM battery should have a resting voltage of 12.7-12.8 volts.  This is with no charger hooked up and some load applied for a few minutes.  Chargers are going to be about 13.8v.

Lithium batteries are different.

Your batteries aren't really charged to 14V.  The charger is applying 14V but the batteries are much lower.  If you start at 11.9V, it will take many hours to charge the batteries to 12.8V.  If you can get them to 12.6 after they have been disconnected from the charger for 30 minutes, you will be in good shape.  That should take several hours at least, probably more.

Rich

For lifelines or other AGM batteries, I am pretty sure at 11.8 volts your batteries are 70% depleted.

I'm no expert for sure, but 11.8V is the voltage Lifeline telephone technical support recommended as the set point for the auto-gen starting point to maximize battery life.

Thank you. I will continue to charge, possibly switching to daily until I can move inside on Monday. My batteries always charge to 14.0 v during a one-hour charge. The batteries are one year-old Trojan T-105 6v batteries, in case that makes any difference in characteristics or charging strategies.

That you are using Trojan T-105 batteries does make a difference in your charging strategy.  I can't lay my hands on the information but if my memory serves me the T-105 is a true deep cycle battery with solid lead plates.  As such it isn't as easily damaged by deep discharge as batteries more suited to starting, lighting, and ignition service.

That being said, plan on a 4-hour generator run* "today" and check the voltage tomorrow.  Resting battery voltage really should be measured after 3 hours of rest, preferably 24 hours.

*Once the charging voltage reaches 14.4V.  You can adjust the charging time once you gain some experience with your battery bank and smart charger combination.

http://www.trojanbattery.com/pdf/datasheets/T105_Trojan_Data_Sheets.pdf

Art

Thanks again for your help!

You're Welcome.  Those are nice batteries you got there.  Congratulations.

FWIW, the charging voltage rises as the batteries require fewer electrons.  Or to put it another way, your smart charger "rolls off" the voltage to limit current flow to what the smart charger can deliver.  It's a plus because charging current should be limited to 1/10th the battery bank's amp-hour capacity to prevent overheating.

The on-line "Battery University" is published by a company that makes commercial battery chargers, and the information provided for your reading pleasure is pretty much middle of the road.  And tilted toward their constant current charging solutions.

Home Page: Battery University Homepage (https://batteryuniversity.com/)

Battery Charging:  BU-401: How do Battery Chargers Work? - Battery University (https://batteryuniversity.com/article/bu-401-how-do-battery-chargers-work)

Thank you very much sir.

Today, 24 hours after charging to 14.5v yesterday, my batteries are back down to 11.9v It is still bitter cold here, but only one day to go before warmer weather and moving inside, with 30a shore power.

It's down to 71 outside on the way to a predicted 68, and we're using the air conditioner to dry the air.  The daytime high was 83.  Should see high 30s Monday morning.

What are you powering with the batteries?

Art, nothing is currently being powered by the batteries, as the coach is winterized and all DC power draws are off. Typically, the battery banks are kept topped, either by solar, shore power or both, but in this unique situation, I have no shore power, and the solar panels are covered with snow. I know there is probably some DC draw, but I've got everything I can think of, such as the easy-to-overlook propane detector switched OFF. The DC master switch, too, is OFF.

Oh this isn't good.  That is a lot of electricity to just go up into the ether.  One pair of those in series will store 225 amp hours of 12V electricity.  Just how old are your Trojans?  Ten years is just about their upper age limit.

Art

The batteries are two years old.

That is not a good thing either.  The next step is to charge them up, move your coach into storage is disconnect the ground lead from those batteries.  I use plural because I'm assuming that you have two or four T 105 batteries powering your house.  It's really only one ground  wire.  If, after 24 hours the batteries have lost their charge you need to have a talk with Trojan.

Randall, there are two main types of lead acid battery construction.  The more common battery is a Starting Lighting and Ignition battery, one meant to deliver electricity on big torrents for a short period of time.  The less common Deep Discharge battery can deliver lower amounts of current to a deeper state of discharge without sustaining damage.  Your Trojan T-105 batteries of true deep discharge batteries with solid lead plates and a space underneath the plates for the material shed during normal operation to accumulate without reaching the bottom of the plates.

If after 24 hours you find your disconnected batteries in a low state of charge, well, sorry?

Understood. I plan to move inside tomorrow, whereupon I can charge the batteries by shore power. I will then disconnect ground and see what happens. I have six Trojan T-105s for house and two Interstate for chassis.

Again, many thanks for the help and insights.

I had been so focused on replacing my current pair of 4D batteries with a pair of Interstate 8D I never considered doing the math for 6 T-105s. 
The latest change in plans has been the addition of an oxygen concentrator and Bi-Pap machine.  So time to re-think power storage needs.

With wet cell or AGM's, you can use about 30% of the Ah capacity before needing to recharge, to maximize battery life.  With Lithiums, you can use about 80% before needing to recharge.  So a 100Ah lithium has 80Ah of usable juice, where you'd need a 260Ah lead acid battery to have the same 80Ah usable.  Lithiums are more expensive as well.

AGM vs wet cell - you don't have to worry about AGM's cooking off the electrolyte like you do with wet cells on a constant charge.

That's what I've gathered so far anyway.