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Old 07-02-2018, 05:35 PM   #1
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Default Discharge experiment

Last weekend I had to leave the RV parked while we went for an overnight side trip. I fully expected to find the fridge cold and the battery in good shape after a 36 hour absence. Unfortunately the Fantastic fan was on thermostat and it got hot...so hot that the fan probably ran continuously and the battery showed 8.5 volts and the fridge was hot when we returned. It was also 102F. The batteries are 232AH Interstate wet Golf Cart batteries and are a little over 4 years old so I have been anticipating battery replacement soon anyway. By the time we got home the batteries were showing 232 amp hours and appeared to be charged. So yesterday I ran a battery life experiment. Wanting a load close to C20. I used the fridge on DC for 14.9 amps...about C15. The plan was to run the batteries down to 50% (116AH). I started the test at 10:00 and ran to 17:30 for a total of 7.5 hours. During that time the voltage decreased in a linear fashion from 13.17V at the start to 11.85V at the end. In the first half hour the voltage dropped from 13.17 to 12.41, apparently the surface charge because the graph was quite linear from there until the end. The current load remained fairly steady in the 14.X amps throughout most of the test.

The good news was that the battery held up fairly well to the 50% discharge level. The most interesting result of the test was the difference in discharge percentage between that shown on the AH meter and that shown on the Trojan SOC Vs Voltage chart. After the surface voltage burn off after 30 minutes, my AH meter showed 97% VS 78% from the Trojan chart. 3 hours in I was showing 80% VS 60% from the chart, 7 hours in 52% AH meter (11.92V) VS 39% chart. At termination (7.5 hours) 48% AH meter (11.85V) VS 35% chart.

So my question to the group: while it is clear that I am OK drawing down to 50% what conclusion can be drawn about the overall condition of the batteries considering the differences in apparent discharge percentage between the actual Amp Hour readings compared to the SOC VS Voltage comparisons to the Trojan chart?

As an aside I found that the RM2354 fridge running on DC dropped from 85F to 19F during the 7 hour test period, actually performing surprisingly better than expected. Outside ambient was in the 85F range during the test.
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Old 07-03-2018, 02:34 PM   #2
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The Trojan chart is likely an at rest SOC chart whereas the AH meter is probably the type that counts coulombs.

Was the end 11.85V measured while a load was still being drawn from the batteries? If so, a few minutes of no load rest at the end of the test would like see the voltage rise to to a level more often associated with a 50% SOC reading.
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Old 07-03-2018, 03:08 PM   #3
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Marco Polo Bear me to it, Trojans chart is the at-rest voltage, and it is for their batteries, although I've never found a chart for my Interstate batteries either. Others know more about batteries than I do, but I speculate that you didn't damage any cells and I would keep using them while keeping an eye on whether you seem to have degraded life in them.

When I got my RV, I did a life test with my refrigerator like you did to get a baseline. I plan on repeating this every spring and fall so that I can spot trends, plus it gives me a procedure I can perform in the field if I feel the need to. I did mine after sunset to take the solar panel out of the experiment. I drew down a little faster than you did, but my refrigerator's heating element is 0.68 ohms so I nominally draw closer to 19-18 amps.
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Old 07-03-2018, 03:33 PM   #4
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Quote:
Originally Posted by JohnnyFry View Post
Wanting a load close to C20. I used the fridge on DC for 14.9 amps...about C15.

The good news was that the battery held up fairly well to the 50% discharge level.

So my question to the group: while it is clear that I am OK drawing down to 50% what conclusion can be drawn about the overall condition of the batteries considering the differences in apparent discharge percentage between the actual Amp Hour readings compared to the SOC VS Voltage comparisons to the Trojan chart?
We agree with markopolo that the ‘load condition’ (or recency of load connection) is hugely important to any measure of state of charge (done other than by ampere/coulomb counting). Let’s take this a little further.

You indicated that your battery “held up fairly well to 50% discharge level.” Did it? How do we know? How do we know that you still had 50% left? That 50% determination was a ‘calculation’ as much as a ‘measurement’ and it was a calculation that your SoC meter made based on your previous programming - - you told your SoC meter that you had 232 ah batteries. What would that meter have said if you switched batteries . . . connected, instead, a 116 ah battery? It would have said the same thing based on its programming. Although your 132 ah battery would have, in fact, been completely discharged, your SoC meter would have reported a successful discharge to 50%.

The same would be true for your 4 year old batteries if they had, over that period, deteriorated. All we know for certain is that they’re still good for at least 50%.

We decided after initial installation of our system to run (and record progressive data on) a full discharge cycle with the goal being to confirm the capacity of the battery and, secondly, to create a ‘starting spot’ record against which we will be able to compare future capacity tests.

Now it is true that we’re running lithium and we are informed that we can safely fully discharge these batteries. But for the purposes of a very occasional test, we’d argue that you, too, can fully discharge your lead-acid batteries. You could trust your SoC meter and only discharge to 50% as you did in your recent test. But then, as noted above, you really don’t know if you have 50% left - - particularly when your charts are telling you that you are well below the 50% discharge level. So we’d run a full discharge cycle.

We, too, needed a ‘load’ to run our test. We happened to have a spool of several thousand feet of #18 wire and discovered that something in the order of 130' represented a 10 amp load at 13 volts. We trimmed and cut to get a fairly accurate 10 amp load. (The actual resistance of our length of trimmed wire actually changes enough over temperature that we can see small current changes as the wire gets hotter. We use this as a ‘feature’ to tweak our load . . . unrolling, fully or partially, to cool [increase the current] and visa versa.)

We have a 500 ah pack, so we run 5 or 10 hour discharge cycles for corresponding 10% or 20% discharge increments. After each cycle, we disconnect all charging and discharging (including our BMS) and let the batteries rest 10 hours. We have found that it takes several hours for the batteries to reach a steady-state voltage so the 10 hour interval is longer than necessary but assures a real ‘resting’ measurement.

Our three tests to date have netted the following full capacity measurement: 495, 509, and 518 ahs (interestingly, the 518 is the most recent).

As an interesting aside for those dabbling in lithium, one of our tests revealed a resting battery voltage of 13.36 volts represented both an 80% and 90% SoC while a second test netted 13.36 volts for 80% SoC and 13.37 volts for 90%. Either way, you can see why they say lithiums have a very flat voltage vs SoC curve. This 13.36 volt measurement is what we have adopted as our target ‘go to’ charging voltage for our lithium system - - wanting to keep it charged to something in the range of 80-90%.
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Old 07-03-2018, 03:51 PM   #5
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The 11.85 was before turning off the load. After 3 or 4 minutes the voltage recovered to 12.00 Volts.
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