Forum Discussion

landyacht318's avatar
landyacht318
Explorer
Apr 27, 2014

Judging/comparing Battery Performance

We've all read about person X who does nothing more than plug in their converter and claims the batteries are just as good as new 6 years later, although this is an impossibility.

More accurate would be is that the batteries still meet the needs of the owner, 6 years later.

Some of us are more anal, use the batteries more, and have a bunch of tools to measure and count amps into and out of the batteries, Specific gravity, and notice the voltage held under load for a certain amount of amp hours removed. Mex's "trends and tendencies"

While Fulltiming, I've been judging several sets of dedicated house batteries on how much voltage they were able to hold under such and such a load, but not until recently have I been seeing what the batteries would rebound to when all loads are removed as this would have required moving a bunch of wires. Back and forth in cramped quarters.

Now I can do so just by moving a switch or 3.

Right now, I have both a Northstar group 27 AGM rated at 90 amp hours and 930CCA and a Flooded USbattery Group31 Deep Cycle rated at 130 amp hours and 620CCA. I can task either for house or engine starting duties, by the turn of a switch or 2 or 3, which allows for much more flexibility and ease of testing/ comparing.


This has thrown a wrench into my expectations, because the lesser capacity AGM holds higher voltages for the same amount of amp hours removed( Upto 70 a/h so far anyway) It also has a much higher fully charged resting voltage of nearly 13.1v compared to ~12.7 for the USbattery.

Now neither battery has gotten so low as to to threaten the food in my compressor fridge, and my engine starts easily enough even with 70 amp hours removed from either battery when used as the starting battery. I rarely parallel them without a charging source available keeping them over 13.1v.

I do have small dashboard voltmeters with sense wires right to the battery's (+), and once again the AGM holds higher voltages during engine cranking and noticeably turns the engine over faster even when 70 amp hours are removed from either.

I have not been cycling the AGM as much as the flooded, because I really like the quick starting aspect of the AGM, though it does not really matter. I also have not been testing to see how much the voltage rebounds on the AGM after a nightly cycle and then loads removed before sunup. Unfortunately I have not been keeping records as to the number of cycles on each, so the longevity, when one or the other fails is not going to mean much either.

NorthStar claims 12.2 rested OCV is about 54% SOC. I cannot find a USbattery SOC chart.

I am not trying to turn this into another AGM vs flooded war that get so many riled up on these forums. This is my First AGM, and so far I find it impressive, but I paid nearly 300$ for it, and I also expect a lot from it.

What is my issue is I do not know how to judge its performance, as I only have, and have had, flooded batteries to compare it to, and my traditional judgement methods of voltage held for capacity depleted might not be fair to the flooded.

After all it is not as if either battery has come close to letting my food go bad, or been too weak to start the engine on its own. If I treated them exactly the same, who knows which would outlive the other. If we consider the ultimate longevity of the battery while it still meets the needs of the owner, how important is it how fast a battery can turn over an engine or power a microwave, or if it holds 0.1v more for the same A/h removed compared to the other?

Is holding a higher voltage under load, just bragging rights and nothing more?

36 Replies

  • landyacht318 wrote:

    Not sure if the above product is a tweaked version of this product
    Looks like the same unit. That temp probe would be ideal.
  • "Is holding a higher voltage under load, just bragging rights and nothing more?"

    It is real. Scenario- you are doing 50-90s and are down near 50 on the morning of your next recharge, which can't begin until gen hours at 9am. Meanwhile you want toast for breakfast using your real toaster on the inverter.

    You do not want the inverter to alarm off at 11v and so not get your toast. AGMs will hold that voltage up better than Wets. Also means you can have a smaller battery bank of AGMs than a bank of Wets and still make your toast at 50% SOC

    A forum member posted some good test results showing that AGM advantage clearly.

    Personally, I would be worried with AGMs where you don't have the hydrometer to tell you where you really are at. Others, less worried about such things, do enjoy the AGM advantage.

    The AGM advantage is in performance, not necessarily in any cost benefit. Wets may be less money but when you have restricted battery space/weight in the RV, cost is not everything.

    As usual it is down to what your rig can carry and what kind of camping you do, etc.
  • Fred, my USbattery flooded 31 has much higher charging voltage requirements than you state.

    http://www.usbattery.com/usb_images/charging_instruction_2011_3.pdf

    On two of the 3 charger profiles listed, they recommend a 15.3v 'finishing' charge. I've been denying it this voltage, and need to do a 15.5+ EQ cycle every two weeks or so to get SG back up into the 1.270 range.
    Gonna go Dip my Hydrometer in a few minutes and update 'my screwy 31 thread'
  • I do find it interesting that the smaller physical size group 27 AGM has a lower AH rating of only 90 yet has a much higher CCA rating of 930. I guess that AGM batteries are going to give off more energy quickly while cranking over a engine.

    The group 31 battery is flooded and thus has a 13.5 volt charging at about 5 amps then it is full, and will rest at 12.8 volts. You can equalize charge a flooded battery at up to 14.5 volts once a month or so, and then add a bunch of water to replace what was boiled off, and it is good for a flooded battery. Not so good for a AGM battery, 14.5 volts for 5 hours can do un-repairable damage to a AGM battery.

    That is why flooded and AGM batteries should not be connected together. The higher voltage AGM will try to charge the lower voltage flooded battery when there is no charging current coming in.

    I have an "E-Meter" in my motorhome. The original design was built for a couple of years before being bought out by Xanterex and then called the Link 10. It will carefully measure the amp hours leaving the battery bank, as well as the amp hours going back in (at a discounted rate). I say discounted rate because the designers of the E-Meter knew that there is in-efficiency with the battery, so as you slowly put back 10 amps at a time, then the meter counts down about 9 amps per hour as going back in. If you quickly put back about 47 amps per hour, it might count down at 40 amps per hour.

    In the morning I would have a reading of something like -120 AH. I would know that my solar system can make up about 100 AH daily when it is sunny, and would run the generator for a hour or so early in the day (when it will put out more than 50 amps per hour into the battery) if I wanted full batteries that day, or if it would be cloudy. If less than -100 AH, I would not need to run the generator.

    My first set of 4 golf cart T-105 batteries lasted 13 years and I gave them away to a friend who still used them a little longer. I was diligent with keeping it full of water, had hydrocaps to keep them full, and took good care of them, recharging when they where still about 1/2 full of power.

    My second set of batteries only lasted 6 years. I was diligent with keeping them full of water for only about the first 3 years, then became lazy.

    Hydrocaps recycle the gas leaving the battery back into water, so you only need to add water about once every 6 months. They have been advertized in HomePower.com for many years, and I bought some in 1997 for my then new motorhome.

    Fred.
  • I've recently been considering the AGM full when it accepts less than 0.1 amps at 14.6v.

    Though when new it refused anything more at 14.6v but had a rested OCV of 12.84v. It wasn't until I did a 50% discharge then a 25 amp recharge that it seemed to "wake up" and hold nearly 13.1 rested OCV. It also seemed to crank the engine noticeable quicker after the 50% discharge and 25 amp recharge initial cycle.

    I just found that Northstar has rebranded a 25 amp Ctek charger that has a temperature probe and says it can be a power supply too. I know 14.4v would be inadequate for my flooded battery, but I'd like to replace my 25 amp Schumacher and do not think I require a 45+ amp Iota PD or Powermax converter.
    http://www.northstarsitetel.com/1.0.1.0/254/NSBC_25.pdfhttp://www.northstarsitetel.com/1.0.1.0/254/NSBC_25.pdf

    Not sure if the above product is a tweaked version of this product:
    http://www.amazon.com/CTEK-56-674-Battery-Charger/dp/B0045F3SGY/ref=sr_1_2?ie=UTF8&qid=1398628385&sr=8-2&keywords=ctek+battery+25+amp
  • Mex likes to use load tests and kWh removed/replenished to determine the health and performance of AGM's (kWh removed/replenished can used to determined whether they're full or not).