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Battery State of Charge

Knowing the state of charge of a battery is important for proper charging and maintenance of the batteries. The problem in the real world is finding a way to accurately reveal this.

Most manufacturer’s instructions will tell you that the Specific Gravity (SG) is the most accurate way (for Flood Lead Acid Wet cells).

Most well designed systems will have battery capacity meters included, but these need to be “tuned” to the system. Depth of Discharge (DOD), charging and discharging voltage will all affect the accuracy of these meters though. Once these meters have been “tuned” to the system, they will generally be fairly accurate.

Establishing the actual state of charge is not always really easy for the following reasons:

Since the SG of the acid of a fully charged battery is higher than water, the acid will tend to settle more at the bottom of the battery. Battery manufacturers try and design their batteries to minimize this, but I have seen the SG lag far behind what is actually happening at the plate level. Some batteries seem to mix the electrolyte better than others, and, different ways of using the batteries i.e. DOD, charging rates and such also have an effect on the SG that is read by the hydrometer. Remember also that SG readings vary with temperature. The hydrometer and batteries should be at the same temperature and most hydrometers will require an adjustment to the reading based on the temperature of the electrolyte.

  • The electronic meters record the incoming and outgoing current to the batteries. Some items to consider here is that:
    • When a battery is being discharged, the voltage is lower than when it is being charged, thus you can have a fairly high charge efficiency in terms of Amp Hours (AH), but the actual energy efficiency is much lower. For example if you take out 100 AH at 48 volts, you will have removed 100 AH X 48 V = 4800 Watt hours. If you then put in 100 AH at Absorb voltage (58.6 Volts), you will have put in 100 AH X 58.6 Volts = 5860 Watt hours. Therefore, your efficiency would be about 78% in terms of actual energy, but your AH efficiency would be 100 %.
    • These meters have parameters that need to be programmed in order for them to be relatively accurate, mainly the absorb voltage, current at absorb voltage and AH charge efficiency.
    • If the absorb voltage is not coupled to the charge controller temperature compensation, it may not be able to accurately reset the AH, especially in the summer or when the batteries are warmer than room temperature.

The theory behind electronic meters

The way that electronic meters behave is based on the following set points.

  • The absorb voltage (less a few tenth of a volt). This will tell the monitor to look for at least this voltage for determining that the battery is full.
  • The current has dropped to this value at absorb voltage. This has been specified by different manufacturers at different rates and varies for different battery types. Age also has an effect on this current, so this setpoint may require adjusting over the life of the batteries. I have seen 2% used frequently for Flooded Lead Acid (FLA) batteries, but have generally found that to be too high. When you look at the graphs in the battery section, .5% to 1% seems to be more common. I have seen less than .5% with some batteries. It would be best to determine where the bottom of the charge curve is and then set the charge controller and battery monitor accordingly. If return amps is used for the absorb to float transition, then the battery monitor amps will need to be set a bit higher than the transition amps. Transitioning too soon to float will cause deficit cycling and sulfation of the batteries (bad)
  • Charge efficiency ….. well, this one varies depending on how the batteries are used. Typically a good starting point is about 94% for new batteries. If the batteries are shallow cycled, this can be quite a bit lower, if cycled deeper it may be higher. The system will need to be monitored and this setpoint adjusted until it matches (at least fairly closely) what is actually happening. The best efficiency setting will show the battery at 100% when it has just finished charging the batteries and the return amps are at the bottom of the curve as previously discussed. If 100 % is reached prior to the return amps having dropped to the value entered, then the charge efficiency will need to be dropped. If 100% is reached significantly after the return amps have been reached, then the programmed charge efficiency can be raised. The preferred setting will have the AH go slightly positive before the 100% is reached.

What if my meter shows 100% charged, but days since charged is XXX?

There are times when, based on keeping track of the AH used and returned would theoretically mean that the batteries are at 100%. What has happened though is that the “Full” charge parameters that have been programmed have not been met yet. This may be due to the charge efficiency being set too high, the temperature of the batteries is warmer than room temperature or the charge controller has transitioned to float just prior to the programmed return amps being reached. This also typically happens more when a generator is used, as the generator is usually shut off prior to reaching the bottom of the charge curve because of the length of time it takes to top up the batteries. If a generator is required or used, it is best to use it in the morning and then let the solar top up the batteries.

Keep good records!

With the Boyd Solar Systems, you will receive a battery maintenance record book. Here you will record SGs, system voltages, AH removed and other maintenance performed. In looking at the patterns, you will be able to determine the health of your system much easier.

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