Battery Voltage

[Jriddle]

I have been able to use the bus this year after three years of sitting in the driveway being converted. Last weekend we took it out and had a nice two days of dry camping. I have a SW4024 and only have a few loads hooked to it at this time. I have not gotten all the TV and STUFF installed yet. I have four Cat 153-5720 batteries hooked as my battery bank at this time. I haven't hooked the generator auto start up yet but plan in the future to do so. One thing at a time I guess. The factory settings for the SW4024 seem to be working so far. I have had to adjust the  generator input amps down to keep it from tripping the generator circuit breaker. My question that started this thread is how far should one let the battery voltage drop before recharging? The factory default for the SW4024 is 22 volts. I have been reading the book and have been looking at each setting but haven't made many changes from factory default yet What should I look out for? Any advise would be welcome.

We got home from camping to find the power out and used the bus to get the local senior center refrigerator back down to temperature. (The wife is the director there.) Then went home and enjoyed power for our refrigerator, lights and TV. We had the power out for almost 24 hours.

Thanks John

[JackConrad]

John,
   We were told with our 12 volt setup to never let the batteries discharge more than 50% and that with 12 volt batteries 50 % was 12.0 or 12.5 volts (I think it is 12.5). Anyway we try to nnever let our batteries drop below 12.5.  On a 24 volt system, I guess that would be 25 volts.  Hopefully Sean will add some more precise facts & figures.  Jack

[Jriddle]

Thanks Jack

I was hoping that Sean would chime in as I have looked around and have not found what I thought was a good answer.

John

[KozyCade]

John

As a rule of thumb the Battery State of Charge is as follows:
Charge   Voltage    Voltage    Specific
Level    (12v)       (6v)      Gravity
------   -------    -------    --------
  100%     12.7        6.3      1.265
   75%     12.4        6.2      1.225
   50%     12.2        6.1      1.190
   25%     12.0        6.0      1.155
    0%     11.9        6.0      1.120

The life expectancy of a deep-cycle battery, like all lead-acid batteries, is directly dependent upon how heavily the battery is routinely discharged before being recharged.  Batteries that are regularly discharged until only 10 percent of their rated capacity
remains have a much short life expectancy than identical batteries that are rarely discharged below 50 percent. Therefore, you should install a 100 amp-hour battery if you plan on routinely using all 100 amp-hours between recharges.

A good rule of thumb is that a deep-cycle battery should not be depleted beyond 80 percent of capacity, with 50 percent being the threshold. A 50 percent discharge represents a good compromise between battery life and reasonable battery-bank size. Therefore, one would do well to plan on
least 200 amp-hours worth.

[Sean]

... My question that started this thread is how far should one let the battery voltage drop before recharging? The factory default for the SW4024 is 22 volts. I have been reading the book and have been looking at each setting but haven't made many changes from factory default yet What should I look out for?

The answer is (drumroll, please): It depends.

Don't forget that the voltage being measured by the SW4024 (except in charge mode) is the voltage under load.  At any given state-of-charge (SOC) of the batteries, you might see a full two volts or more difference in the measured voltage depending on load.  And while the inverter manufacturer could, conceivably, account for its own load and give you a "load-compensated" reading (but, alas, the SW4024 does not), most coaches will also have load on the batteries that does not flow through the inverter, namely any 24- or 12-volt DC loads, and there would be no way for the inverter to account for those.

Moreover, every battery manufacturer specifies different lower limits for voltage and SOC before charge is required.  Importantly, battery type will have a big impact -- AGMs will have different requirements from flooded batteries, and there is an even greater difference with gel cells.

Getting this setting right is a matter of trial and error.  It helps to know what your battery manufacturer recommends and what the "normal" amount of load is for your coach while using the batteries.  That said, I strongly recommend you get a decent SOC meter, such as the Tri-Metric from Bogart Engineering or the Link-10 from Xantrex.  This will allow you to keep an eye on actual SOC, and you can start to get a feel for what the voltage is, as read by the inverter, when you start to approach the depth-of-discharge (DoD) at which you want to start charging (generally 50% for most flooded batteries, 70-80% for AGMs and gels).

It's a safe bet that 22 is too low.  That recommendation is based on an average inverter load in the neighborhood of 50% of inverter capacity, or around 2kW.  FWIW, my LBCO is set at 22.6, and my 15-minute start voltage is set at 23.0.  And, yes, tenths of volts make big differences here.  I use that setting because more often than not, I have an air conditioner running when the auto-start kicks in.  If I did not run air conditioners from the batteries, my LBCO would be 23.0, and I sometimes use that setting in more temperate conditions when extended boondocking.  Also, I have AGMs and discharge to 20% SOC.

...
As a rule of thumb the Battery State of Charge is as follows:
Charge   Voltage    Voltage    Specific
Level    (12v)       (6v)      Gravity
------   -------    -------    --------
 100%     12.7        6.3      1.265
  75%     12.4        6.2      1.225
  50%     12.2        6.1      1.190
  25%     12.0        6.0      1.155
   0%     11.9        6.0      1.120

...

All true, but not useful.  That's because those SOC readings are only valid with the battery completely disconnected and after a "rest" period of several hours.  Since we generally can't turn everything off, disconnect the batteries, let them rest for hours, and then measure the voltage to determine SOC, we need to use numbers that more closely correspond to battery condition under load.

By this table, for example, a 24-volt system would be at 50% DoD when the voltage reads 24.0, yet I can assure you that my voltage regularly reads much less than that at 50% DoD.  This is why the Trace LBCO is defaulted to 22.0, which, by this chart, would be below completely dead.

It's not possible to make a handy chart like this for arbitrary load, which is why I recommend a good SOC meter -- it's the only way to know how much is left in your batteries under variable conditions.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

Forgot to mention:

When I said "trial and error", what I mean is that you will need to pick a setting and let the batteries discharge until the genny starts (or the inverter cuts out, if no auto-start), then look and see what the SOC really is.

It should go without saying that you should start high, and work your way lower, not the other way around.  Otherwise you may accidentally over-discharge the batteries, which could damage them.

I recommend an initial LBCO setting no lower than 23.0 and I would suggest even going to 23.2 or 23.4.  On subsequent cycles you can work your way down from there in 0.2 or 0.4 volt increments until you get to where you want to be.

Sorry not to have been more clear about this in my earlier post.

-Sean
http://OurOdyssey.BlogSpot.com

[Jriddle]

Thanks

I have not been below 24.3 on the trace meter and thought 22 volts would be very low. I will try Sean's recommendations and go from there hope others are learning as I am.

Thanks Again John

[Sam 4106]

Hi Sean,
While on the subject of SOC meters would an E-Meter be acceptable? A member of the Midwest Bus Nuts group has one for sale for $150 since he no longer has a bus. I have been thinking about getting an SOC meter but don't know enough about them to make a good decision.
Thanks, Sam MC8

[Sean]

... on the subject of SOC meters would an E-Meter be acceptable? ...

The E-Meter is identical to the Xantrex Link-10 that I recommended earlier.

Xantrex bought out the E-Meter folks (who designed the meter) years ago.  So your friend's item is pretty old.   Also, $150 is about the going rate for a used one (or one that old).

-Sean
http://OurOdyssey.BlogSpot.com