question

motorambler avatar image
motorambler asked

Charge controller settings for idle FLA batteries (gone for 6 months)?

Current system is made up of:

  • 12 x 2V 960AH FLA batteries
  • 3 x 350W panels connected to a Xantrex 150V/60A charge controller
  • 3 x 350W panels connected to a Victron 150V/60A charge controller
  • Trace SW4024 24V inverter

I have plans to be out of the country for 6 months but am concerned about leaving the battery bank unattended.

I am planning on turning off the Xantrex charge controller and only using the Victron being fed by 3 x 350W panels.

My thought is to set the Victron charger to 27.2 volts across all charging parameters (bulk, absorption, & float). Would this be enough to prevent unnecessary gassing of the batteries? Should I use an even lower voltage and go further and limit the charger output to say something around 10A?

There will be zero loads and the Trace SW4024 inverter will be turned off while I'm away. I will also put a full charge into the batteries (via generator) just before I leave.

Thoughts?


Info on my battery bank (from manufacturer):


Table 1: Recommended Voltage and Current settings for recharging Davidson Batteries

table-1.jpeg

Specific Gravity

The specific gravity of your Davidson battery is somewhat different from other brands of battery. They are actually fully charged at a lower specific gravity than most batteries and as a result they will actually be charged before most hydrometers will indicate full charge. The most accurate way to use a hydrometer to measure state of charge is to take note of when the specific gravity stops rising. Take readings every 1/2 hour during the last part of Absorption Stage, when the hydrometer does not rise from your last reading the batteries are fully charged.

The correct fully charged specific gravity of your Davidson battery is:

1.265 - 1.275 ( this will vary dependent on battery age, condition and ambient temperature)

Safe Operating Voltage

Another result of the special formulation and design of these long lasting deep cycle batteries is that they tend to operate at lower voltages than most higher specific gravity batteries, thus the safe operating voltage range of Davidson batteries is lower than most other batteries.

It is quite normal and safe to operate your batteries as low as 2 Vdc per cell5. They should always be recharged when they reach a standing voltage of less than 1.975 per cell6. Table 2 shows the conversion of Volts per cell settings to 12, 24 and 48 Vdc values.

Table 2: Converting Volts per cell to battery bank voltage.

table-2.jpeg


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3 Answers
kevgermany avatar image
kevgermany answered ·

Might be a better idea to ask the battery maker/supplier for this advice.

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regman54 avatar image
regman54 answered ·

Normally Flooded Lead Acid batteries do best over an extended period of time when left on a trickle charger, which is what a float cycle is/does. What makes their batteries special I don't know.

But note in the top chart it ecommends float Time continuous is a charge rate of 2.3V DC per cell (2.3V x 6 cells = 13.8Vdc). ??

But I agree with Kev, contact the battery manufacturer and discuss it with them. Good luck.

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kevgermany avatar image kevgermany ♦♦ commented ·
Guessing these are tubular or spiral cell batteries, but no idea. Some makers specify a storage voltage, maybe the OP gets lucky. But as he points out, there's the issue of water loss.
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regman54 avatar image regman54 kevgermany ♦♦ commented ·
Yeah, that was my guess as well. But I find it interesting the battery manufacturer's information provided in the Table 1 shows an indefinite float charge of 2.3V/cell. I can see some water loss happening if the controller goes into an equalizing charge but those are usually done manually or at a specified time like maybe once a month. I'd be looking at the charge controller settings and turning equalization off if possible. If not possible I'd be using a small trickle/maintenance charger instead.
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motorambler avatar image
motorambler answered ·

Thanks for offering to help! :)

Yes, these are tubular cell batteries (model 85T-19) and were made by Global/Davidson but they went out of business some time ago.

Interestingly, on another forum, someone advised leaving the voltage alone and setting the 'Tail Current' setting to something like 6A because the Victron will try to bulk/absorb charge and the battery will already be full, it will reach the 6A tail current setting, then go into float. Or, to try Victron's 'adaptive setting'. I have never used the Tail or Adaptive settings (both are disabled) but am open to enabling them if that's the best option.

Man, this is getting more confusing... :(

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