MNBCM Question

[Frank S]

I had a 300 watt load on my batteries at night.  Checked late at night, no charging, and meter says 30% power remaining.  check controller and say batteries are at 23.7 volts.  I turn off load as I do not want batteries go below 50%.  Within two minutes meter now says 60% and controller 24.3 volts.  So question is did batteries really go down to 30% or 60%?

[Westbranch]

What you saw is a process called  "coup de fouet" or 'crack of the whip' where the voltage rebounds after the load is removed.

I would say it looks like your load is too much for the batteries or they are failing.  tell us more about your batteries...

[Vic]

Recall that Frank's system is similar to this:   6 Trina 295 watt panels, Classic 200 , 4 Trojan L16RE-B batteries - 24 V and a Samlex SA-2000K inverter.

Frank,  first,  Deep-Cycle batteries do on have a high degree of voltage regulation.  The 300  watt load -- about 12 -13 Amps -- that you mentioned is about 3% of the capacity of the battery bank,  not a trivial load.   The voltage drop of a  battery for a given load current does depend upon the State Of Charge of the battery.  A fully charged and a nearly fully discharged battery shows a greater drop for a given load current than does that same battery in the center 60 or so percent of the capacity curve.

Generally,  reading terminal voltage is not a very accurate indicator of the SOC of a battery.  A battery that has been removed from ANY charge or discharge for 3 - 6 hours does have an accurate voltage vs SOC indication.  This "Rested" voltage reading should be temperature compensated,  however.

The MN Battery  Capacity Meter  is a good general purpose tool,  but will not be an absolute indicator.

On your new system,  please find the recommended charge voltage recommendations from Trojan.  And,  be certain that you have a Hydrometer and know the exact recommended Specific Gravity for your batteries.  Unfortunately,  Trojan has changed the SG of the electrolyte in RE-B batteries.  Believe older batteries from 2012 and before were 1.260 (from memory),  and the new RE-B,  from March 2013 and later months used 1.280 SG.  There is a rumor that this March '13 Date Code is not an absolute.

Unless you have information from Trojan,  stating the SG used in YOUR batteries,  you may want to find the date code on your batteries and contact Trojan for the correct target SG.  IMHO,  Trojan should have changed the part number of the battery to help the customer know exactly which product they have.

It is very good that you are asking these questions here,  now when your system is young.  More later.  Vic

[Vic]

Frank,  Just noticed that I was one year off regarding the change from 1.265 SG to 1.280.  The change was reported to have happened in March 2012,  and batteries before this approximate date were 1.265 NOT 1.260.

Sorry for the confusion on my part.  Should have looked up the info.

If you have not seen it,  this is some data on L-16 RE-B batteries:
http://www.trojanbatteryre.com/PDF/datasheets/L16REB_TrojanRE_Data_Sheets.pdf

Trojan should be able to help you figure out what is the target SG for your batteries.   Good Luck,  Vic

[Photowhit]

I've never been a fan of voltage based battery capacity meters. Some say it's better than nothing, but my feeling is that if you don't understand what's going on, they may be worse.

Likely your voltage recovered even more after another half hour or so. Since you have a Classic, be sure to buy the battery monitor module for it when it comes out (soon?, later this year). I think MidNite has a target price of $59 or $69. This battery monitor will measure the amperage passing through the shunt so it will be a much more accurate measure of your batteries capacity.

Trust that voltage drops with a load, I can get my largish traction battery to do something similar by running a large load while there is no charging, like a microwave.

[TomW]

For what it is worth;

I often see what I think of as voltage sag and rebound when a substantial load is cycled like the well pump, water heater or microwave / toaster oven, etc.

I very seldom see any no load times here so there is always some load on the system and I mentally adjust for that watching voltage. I doubt specific gravity reacts so fast but never checked.

Just from here.

Tom