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Getting the most out of AGM batteries
This post was prompted by the recent thread about LiIon batteries and is aimed at the many new coach owners on the PC Forum.
We had new LifeLine AGMs house batteries installed in our coach in May of 2014 when we bought our coach. We (OK … “I”) was a naive coach owner. I relied on the WatchDog to start the generator and recharge the house batteries. Upon buying the coach we started building a race shop with FHUs including 50amps, for the coach, but didn’t have access to 50amps during construction. So for a few months I parked the coach and relied on the WatchDog and generator to keep the house AGMs charged. I also naively left the aux air compressor ON thinking that was necessary which exacerbated the problem (coach had air leaks, compressor ran often, drained the house batteries). I could have turned the main disconnects off on the house batteries, but I didn’t … the reasons for which are lost in the ether.
Then twice (maybe thrice) through negligence I allowed the house batteries to go to zero, complete 100% discharge. Down so low neither the generator nor shore power nor the alternator on the Detroit Diesel would charge them. I had to hook up a 12 volt car charger to the batteries to get a base charge in them. (Apparently the alternator on the Detroit and the inverter/chargers must see a minimal charge in the house AGM batteries before they will charge the house batteries.)
We also learned that the alternator on the Detroit Diesel did not bring the house batteries to full charge … not enough volts. Hence after driving all day the house batteries were not to full charge. So after parking in an Interstate rest area, making dinner, and watching TV for an hour or so … the generator would auto-start in the middle of the night; another event of allowing the house battery voltage to drop below the safe threshold.
The WatchDog on our Liberty Coach starts the generator at a voltage that is too low for the long term health of the house batteries. When the house batteries were about 4 1/2 years old, they’d last about two hours with minimal load before the generator was started by the WatchDog. At 5 1/2 years of age they lasted only 90 minutes before the generator would kick in.
During New Years week we had new Lifeline AGMs purchased from Tom Chilcote installed at East Tennessee Luxury Coach. Immediately while dry camping the house batteries lasted nine hours without getting close to 24 volts (we have a 24 volt coach) … big improvement!!!!
This is what we learned:
1. Minimize the amount of time you rely on the Watchdog (if you have one) to auto-start the generator. This will minimize the number of events that your house batteries drop below 12 volts or 24 volts (as appropriate for your coach). If dry camping without shore power, either leave the generator on and running or manually start the generator before your house batteries reach 12/24.
2. After driving all day and stopping to dry camp without shore power, run the generator to bring the house batteries up to full charge before retiring for the night.
3. DO NOT under any circumstances allow the house batteries discharge below 12/24 or WORSE YET fully discharge.
Learn from the lessons I learned the hard way.
(Tom and Jon: I’ve arbitrarily used the minimum voltage on an AGM of 12 and 24 volts. If there are better numbers, post them and I’ll edit as appropriate.)
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15 Replies
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Good summary.
I never use the gen autostart feature. It kicks in way too low. I could adjust the cut-in voltage but I like to manually start the generator.
When we dry-camp I run the generator 2 hours in the morning and two hours in the evening. We wake up with close to 25V in the morning. After a week of dry-camping, it is important to fully charge the house batteries to avoid sulfation. Two hours of generator time will only charge the batteries to 85% or so.
2000 Featherlite H3-45
2024 Jeep Grand Cherokee
Leesburg, FL -
One issue with auto gen start is that voltage is NOT a good measure of state of charge unless the batteries have been at rest – neither charging or discharging – for several hours. Lead acid batteries whether called flooded, sealed, AGM or gel all drop voltage the minute you start using them and drop voltage much more rapidly the harder you use them. For example, turn on the microwave and watch how quickly measured voltage drops. It is not uncommon to see far less than 12vdc. Run another high draw appliance (like an induction cooktop) at the same time and the measured voltage may drop below 11vdc.
A so called 12vdc battery fully charged at rest reads 12.6vdc. When it reads 12vdc at rest it is already 50% discharged. So, an auto gen start set to 10.5 volts May be correct if the battery bank is used hard all the time, it is way too low if the battery bank is lightly used like the case reported above.
A far more accurate way to measure battery state of charge is to install a shunt based battery monitor that directly measures amps going in and amps going out. If the battery manufacturer properly rated the battery capacity (total number of amp hours available), then you can directly read amp hours remaining IF the amp draw averages the fixed rate of draw the manufacturer assumed when rating the battery in the first place. Rating is usually done at an assumed amp draw of 20 amps per hour.
Confused yet? Given the amp draw in a Prevost is almost never 20 amps per hour, there is no surprise the manufacturers had to do a lot of head scratching to come up with these seemingly low auto gen start voltages that are usually set around 10 to 11.5 volts.
LiFePO4 batteries read about 13.6vdc when fully charged and stay reading more than 12.6vdc through 85% of their capacity. So, while called drop in replacements for lead acid batteries, that does not apply to auto gen start settings.
jerry
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Also, for AGM batteries, it is best to keep them completely topped up. If you are only alternator charging (and not using a smart regulator like the balmar 614), then you need/want to top off the AGM charge as often as you can – whehter thats with a smart 3 stage charger, inverter charger, or MPPT solar charger. AGM batteries do not like to sit below their full charge – for lack of a better term, think of them as NiMH type of batteries that develop a “memory” when not fully charged.
Fully charged AGM is usually 12.8. . You will not want to use a auto-gen start that low. as that is past the point of shortening your number of lifecycles. . . . .
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The Balmar regulator is the way to go for the house batteries.
It has a specific charging protocol for Lifeline batteries.
I would recommend one to anyone.
Put one on my 99 CC and it worked great.
Love my new avatar Tom
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I second that as well. The balmar (and also I think the wakespeed, which was developed by the balmar 614 engineer before he passed away), are the best regulators out there. They are programmable (although a pain to program) for any kind of battery. The issue is that they are “externally regulated” and many converter setups use internally (read : dumb) regulated alternators. So you either have to convert to an external regulator (not too bad, but you do need to understand how to wire/re-wire etc) or get a new p-type alternator that can handle external regulation. Which may not be a bad option to upgrade to a balmar or leece-neville constant duty rated high output alternator.
External regulation also gives you the ability to safely stop the alternator when you are done charging for say LiFePO4 batteries rather than abruptly cutting them off
Also the Balmar does not like to be hot or exposed to elements – eventhough it is a marine unit, you dont want to put it in the back engine bay and exposed to water and heat. . .
But the balmar mc614 is the one of the best altenrator regulators out there. and its user base in the marine world is wide and manufacturer support is extensive.
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This is nothing more than a guess, but Prevost used to set alternator voltage at 27.7, which is not a full charge for house batteries, but may be a full charge for chassis batteries due to the temperatures they can be exposed to.
Way back when Tom was promising me dues money I was testing Interstate Gel cell batteries (around 1990/91) and when they were failing (they were mounted above the chassis batteries on that coach) we measured temps at the batteries and they were as high as 170/180 degrees at the end closest to the engine.
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My alternator for house batteries is a leece neville with internal regulator set for 13.8V charge. I dont know what the 24v one is set for as I havent reached the one behind the engine personally to measure it.
Jon – ironically the 13.8 volt alternator is almost a safe charge voltage for LiFePO4 batteries without getting you in too much trouble.. . . . . But it isnt great for AGM, as it never tops them up.
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I did a little test on my bus Friday-Saturday which when not in use is usually plugged in at the shop. 97 Liberty Classic with 12V house, 3 month old Lifeline 8D AGMs (x3), 2 Heart inverter chargers (assume original). Had it parked at the house with no plug overnight as I was doing some cleaning on it. Original Amana fridge left on along with a small 110 VAC fan. Ran the genset to get back to full charge before going to bed. 12 hours later re-entered the coach, showing 35-40A 12VDC load on inverter #1, 0 on #2. Inverter #1 was indicating 11.5V, Inverter #2 indicated 12-12.5 if I recall correctly. Fridge still working great, fan still running, 12V lighting great, etc.
I think it took about 90 minutes to get back to full charge on the batteries. I will eventually upgrade the Hearts to Magnums and I’d like to add some maintenance solar for dry camping, particularly as the Mach V pump does put a hurt on power (flushing toilet overnight comes to mind).
I also put a meter on the B+ of my 12VDC Leece Neville alternator, showed 14VDC output, good to know. Like Anthem’s bus, it does not fully top off the batteries going down the road as I will bulk/absorption charge for 20-30 mins sometimes after plugging in/running genset.
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Chip: That’s a very good test. A test that every new coach owner should perform to better understand battery management of their coach. As the batteries age, they probably won’t last as long.
You saw 11.5 volts with a pretty good load. Next time check the voltage with that strong load and then turn the load off and see what happens to the voltage.
Based on my experience with our AGMs and what Jon, Tom, and others have written and told me … taking your batteries to 11.5volts has “hurt” them (notice I didn’t say “damage”). From what I’ve come to understand the impact of each 11.5volt instance with a load is small … but if it occurs often, small things add up. I suspect as you go lower than 11.5volts the impact on the batteries goes from “hurt” to “damage” probably geometrically. No empirical evidence, just my experience in shortening the lives of our new Lifeline AGMs from when we first about the coach. The worst thing we can do to our AGMs is to take them to “zero”.
IIRC the minimum voltage you should allow the AGMs to go to is 12.4 volts. At that point you need shorepower, the generator, or the alternator on the Detroit to recharge. If you don’t need inverter power at that point or can’t recharge, power down the house and disconnect the batteries with the switches.
Reminder to Liberty Coach owners with the WatchDog generator auto-start system … the WatchDog (while good in its day) will start the generator at TOO LOW of voltage to protect the batteries. Relying on the WatchDog to auto-start the generator will damage your AGM batteries over their lives, shortening their lives. BTDT.
As a result of learning this, we’ve changed how we rely on the WatchDog. Its on all the time, but we run the generator when we pull into a rest area for the night after running all day to top off the batteries and usually in the rest area we will start the generator in the morning as soon as we get up. This holds for any dry camping like at the race track. Generator in the morning, maybe generator mid-day, generator at dinner time, and generator before quiet time. Or … if one follows the Jon Wehrenberg’s Ten Commandments of Operating a Prevost, run the generator during the entire time you’re dry camping.
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The battery manufacturers have a wealth of information available about managing batteries. For our purposes we only need to know a couple of things to get the most from our batteries.
The first is to try to recharge them at or above a 50% state of charge. The charts vary a little but an AGM battery is at 50% SOC at 12.1 volts (no load).
As we know from experience allowing voltages to drop below that level won’t ruin the battery, but allowing batteries to drop below that point routinely such as some auto-starts will do impacts the number of battery charging cycles. It might be the difference between 5000 cycles or 1000 cycles.
The second thing important to understand is when batteries are charged the specified charge voltages and protocols need to be understood. Most AGM require a 14.2 to 14.4 volt bulk and absorption charge until float is reached, and then float needs to be maintained at around 13.6. Different manufacturers and batteries may require small variations from the above listed voltages.
But when we drive the bus and our batteries are being used and discharged and then recharged via the bus alternator if the voltage for charging is 13.6 or 27.2 or thereabouts the batteries are not fully charged. That is a big deal because unless batteries are fully charged the plates become sulfated, meaning the surface of the plates get coated, impeding electrical current flow and shortening the life of the batteries and the charge they can hold. So to avoid those problems make sure the batteries get a full charge after use. As Dick mentions, at the end of the day of driving I run my generator which provides power to the inverters to fully charge my batteries.
If those two practices are observed batteries are going to last a long time in service. While it might not be as convenient as always using auto-start with low voltage set points, or running a generator at the end of every day of driving just observing those two practices can significantly lengthen the times between battery replacements.
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All good stuff, just remember to not short charge your AGM’s. A good 3hrs on absorption for Lifelines before going to float when discharging to 50%. There are a bunch of variables, but 3 hrs is a good number.
Good post Dick, for you Liberty Watchdog guys who rely on auto start, keep me on speed dial.
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What voltage is factory voltage set on Marathons Invertres I have always used auto start my Lifeline battery’s lasted 9 years 5 were still good others weak after driving all day they would last 5 hr before gen would start
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Good post Dick
AL
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Dan,
My Marathon autostart was 21.8, easily changed with the Trace 4024’s to 24v. Never used it dry camping, but did arm it a few times when leaving the bus for an extended period.
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Thanks Tom
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