Electrical – One battery discharges faster than the other in the solar power system

There are really two questions here.

1. To calculate the run time you first need to estimate the amount of current being drawn by the inverter. If you assume an efficiency of 85% (your millage may vary) then you can calculate the current as
   ((120V / 24V) * 7A) / 0.85= 41A. This number is a bit on the high side since the voltage will initially start around 26V and drop down as you discharge the battery. The run time for 50% discharge is then calculate as a) (200 AH/41A) / 50% = 9.7 hrs. In case b) you have double the capacity so the runtime is increased by two.
2. You don't get any more charge discharge cycles by combing batteries in series. However, if you combine them in parallel then you would only be discharging the banks by half as much for a given a given amount of time. So even though your charge/discharge cycles wouldn't change, you would get twice as much work out of the batteries.

I believe that if all of the data presented was correct, then the observed behaviour wouldn't be happening. Therefore something is wrong in the data given - we just don't know which part(s) to disbelieve.

Obviously there is lots of data here. There are a few points which are unclear to me within that data, but I formed a hypothesis and I can't see any data which disproves it (there just seems to be a belief that my suspect component(s) are OK).

All indications are that the battery bank is full.

Yes, but full to what actual capacity?

My hypothesis is a battery-related problem - one or more of the batteries either:

• Don't have the capacity claimed; and/or
• Have (at least now) a higher internal resistance, which prevents the capacity being used to power a higher-current load (even though they might have a higher usable capacity with a lower discharge current) [† see below]; and/or
• Those batteries (despite any claims by the vendor to the contrary) may not have been designed to handle the specific load being applied.

Some of those possibilities overlap e.g. battery design could result in a higher internal resistance than required for supplying that load.

[ † See the update below for another possibility which would produce the same symptoms, and the test necessary to confirm or eliminate it. ]

Two examples of the behaviour which fits with that hypothesis includes:

• The batteries seem to become fully charged during the day, due to the low eventual charge current in daylight of 200 mA;

  Yet...

• Under the 260W (or 300W) load from the inverter, the overall battery voltage drops much more quickly than would be expected for the claimed battery capacity of 6149 Wh.

If one or more of the batteries has a lower-than-claimed capacity (or higher-than-acceptable internal resistance) now, then this is exactly the behaviour I would expect.

If I have missed some data which disproves this hypothesis then great - eliminating a hypothesis is a step towards finding the solution. However just believing the battery vendor's claims of the battery capacity, does not disprove this hypothesis.

I don't know the company "Johnson Marine" but are they really the battery manufacturer, as mentioned - or just the vendor (retailer)? Where is the datasheet for these "29DC" batteries? I couldn't find one online. But even if there was a datasheet, it could only be used to influence further testing; again, it would not disprove the hypothesis, since the batteries may not meet the specification in the datasheet.

You might find something useful from a quick check of the battery voltages even before disconnecting them. Obviously the total voltage of each series "pair" must be identical (since the two series pairs are connected in parallel). But what about the voltages of each battery within each series pair? Is there an indication of one battery with a significantly higher voltage that the other one, within a series pair?

Assuming that each battery has a very similar voltage when (they appear to be) fully charged, I would look to design a test similar to the following:

• Start with (what the existing charge controller believes is) a fully-charged set of batteries.
• Disconnect the series/parallel wiring from the batteries.
• Discharge the batteries individually, while measuring their individual capacity, using a load which simulates the load on each battery of the 300W AC load on the inverter; my back-of-an-envelope calculation suggests that may be around a 12 A load on each battery, but do check this.
• Charge them individually from a controlled, different (i.e. non-solar) source (thereby eliminating the "opinion" of the existing charge controller etc. as to when the overall "battery pack" is charged), while watching the voltage curve and measuring the charge current.
• Review the results.

My hypothesis is that the data collected during the discharge and/or recharge tests, will not show the expected behaviour from one or more batteries.

Note: I'm assuming that we can believe the voltage readings provided (I guess they are from the charge controller's own ADC). I'm also assuming that there isn't an additional (e.g. unintended) current drain on the batteries, in addition to the inverter. I would probably use a DC current clamp meter on relevant cables during charging & discharging, to make sure that the currents shown were in-line with expectations.

One concern is that although the kill-a-watt claims 300 W AC power from the inverter during the testing, I don't see anywhere that the DC current from the batteries has been measured at that point, to confirm that it fits with that. Is it possible that relying on one piece of equipment (the kill-a-watt) and its reading might be misleading the investigation, if that reported reading is incorrect? Again, appropriate use of a DC clamp meter, would help give some confidence.

Update:

† Another possibility which would have the same symptoms as those described, would be if there is an unexpected high-resistance path in the wiring between the batteries and the inverter. That might be the short wires between the batteries, or the longer wiring between the batteries and the inverter.

This would have the same effects as one or more batteries having a high internal resistance as described in my hypothesis above, and would cause the observed increased voltage drop at the inverter under load only.

The effect would be that the batteries were being discharged only from 100% to, say, 90% of their capacity before the inverter correctly detected an excessive voltage drop. However the voltage drop wasn't at the batteries (the inverter cannot measure that) but instead the voltage drop was at the terminals of the inverter at the end of the wiring.

This would also fit with the apparently quick battery recharge times since, indeed, they wouldn't need much recharging.

The test for this would be to measure the voltage drop between the ends of each high-current cable, when under load. Although it might seem that you could measure the resistance of each high-current cable instead, there are disadvantages to that method. Measure the voltage drop across the cable instead.

In summary, I believe that somewhere you've got some unexpected resistance. It may be inside the batteries (e.g. due to their design, or their life so far) or in the wiring between the batteries and the inverter. It may be helpful to review the calculation of the expected voltage drop, for the wiring you have used, at the likely currents.