Are my FLA batteries dead?

[RadicalDad]

In October 2021 I purchased a model year 2021 teardrop trailer that seems to never have been used. (The propane tank had never been filled, among other signs.) It came with two Interstate HD24-DP flooded lead acid batteries connected in parallel. Based on the build date of the trailer, the batteries can't be more than 18 months old at this point. Yes, I know these aren't great batteries. My question is whether the previous owner possibly killed them through poor maintenance. The details:

The batteries seemed to run down to 50% SOC very quickly (based on resting voltage) given the light loads this trailer presents. I've checked for vampires and all amp draws seem reasonable. I took the batteries to a local Interstate Battery store where they did a "tune up" for me - charged, equalized (I assume), plus checked the water and electrolyte levels which didn't need adjustment. After the "tune up" they went back into the trailer under the care of the factory-installed solar panel and Victron MPPT charger for a few days.

Then I tested for voltage drop at the battery terminals under light loads typical of my usage. So this doesn't become a discussion issue, I disconnected the MPPT controller from the battery before running the test. Here is the core of my question. Starting voltage with no load was 12.82 V, but that may be spurious because I didn't allow much time for possible surface charge to bleed off. I ran a 1.58 amp load for 8 minutes. By the end of that test, voltage had stabilized at 12.6 V. I then ran a 4.6 amp load for another 8 minutes. The voltage dropped fairly quickly and was stable at 12.40 volts for the remainder of the test. Then I let the batteries rest under no load for 10 minutes and the resting voltage was 12.75 V.

Using the final resting voltage of 12.75 V as the baseline, I saw an 0.35 V drop under a maximum 0.0359 C load (based on a nominal 128 Ah capacity for the parallel set of batteries). Are my batteries toast, or is this normal? I've searched for FLA-voltage-drop-under-load charts without success. I even wrote to Interstate Battery but didn't get a useable answer. Does anybody have any insight or knowledge about what might be normal voltage drop under light loads for a healthy battery? It could be that the "tune up" fixed the poor performance problem I saw in my earlier trips, but my next trip will be far longer and I don't want to end up without power by the end of the week.

Thanks for any help or insight.

 

[time2roll]

I would verify with a hydrometer if that concerned. Run them down slow to 50%, let them rest overnight and check the hydrometer reading and voltage. Post the results.

The initial tests don't seem out of line to me. Worst case use them hard, have some fun and plan for replacements of something better.

 

[Substrate]

If you want to stop going to Napa for tune-ups, invest in getting your own gear. Use a multimeter to measure the terminal voltages of each battery *individually*.

Invest in a charger, like 5-7A or so and charge each one *individually* before placing them back into parallel service. Know the difference between a charger and a maintainer - ie, don't get a 1A little maintainer for this job with your individual 64Ah batts.

Getting involved with your system will be much more satisfying than crossing fingers and relying on tune-ups from others when you can see it for yourself at your own leisure.

 

[TacomaJoe]

Sounds like you have the means for testing. I would charge them up fully then give a known load to pull 60A out (3A for 20 hours, 6A for 10 hours) and look at the battery level at that point. Pulling 60A from the pair should keep you above the 50% SOC level. A couple of tests and you will know your batteries. A shunt monitor is real handy, I love mine.

 

[STW]

Exercise them a few times. I have the same problem with my trolling motor. It gets better and better after being cycled a few time… as it sits most of the year.