12v battery guard4/6/2023 ![]() ![]() A 20 hour discharge rate is simply:įor this 105Ah rated battery the 20 hour discharge rate looks like this:ĭespite the slightly diminished capacity, 91.3% of the factory rating, I chose to keep the discharge rate at 5.25A at a controlled battery temp of about 76.4☏. I chose this slightly used battery to represent a more “ real world” scenario. It is factory rated at 105Ah, at the 20 hour discharge rate, but this particular battery, off my brothers boat, was used and only capable of delivering 95.69 Ah’s. The battery used in this test is a very popular AGM, a Lifeline GPL-31T. Sensing voltage anywhere else in the system can lead to erroneous voltage readings due to circuit voltage drops. The voltmeter should have both it’s negative lead and positive lead directly sensing/measuring the house banks physical battery terminals, or within a few inches of the bank on a distribution bus fed by large gauge wire. A voltmeter that can read 12.1 5V, or adds the hundredths of a volt position, will give you far better trend information. For example, a voltmeter that only reads 12.1 volts gives too little information. Always use a good quality, known accurate voltmeter that can display out to the hundredths position, not just the tenths position. On top of using an incorrect Voltage to SoC chart, using an inaccurate voltmeter or one connected or voltage sensed in the wrong location can also impact you negatively. Using the wrong chart can actually accelerate the murdering of your batteries. Using the wrong Voltage to SoC chart only exacerbates issues related to using voltage for SoC estimation. Cyclic On/Off loads, like we have on a boat, can also tweak this relationship. If you’re using voltage as an indicator of SoC, the rate of discharge, as well as battery temperature, can also affect your SoC to voltage points. ![]() Some charts you find on the internet do represent a “ resting voltage”, but you’ll also find some for a 20 hour discharge rate or an under-load voltage. Deep Cycle AGM will be slightly different than dual purpose AGM or TPPL AGM or GEL or flooded etc. Why? Because every battery manufacturer will have a slightly different voltage performance curve under load. You should avoid the use of use generic charts off the internet. I know this battery will rest at 12.93V once the surface charge has dissipated, but at 40-45☏ this can take weeks or as long as a month or more.Ī Voltage to SoC chart should only come directly from the manufacturer of your batteries, or through physical testing of your own batteries, as I have done here. Why? The battery temp is close to 40☏ right now and has not been much above 45☏ since I set the battery in storage. The “ at 77☏” is absolutely critical because at temps above 77☏ surface charge dissipates more rapidly and at temps below 77☏ the surface charge dissipates more slowly.Īs I type this I have a fully charged AGM battery in my storage barn that has been sitting 100% disconnected for well over a month, and the resting voltage is still 13.12V. A resting voltage is 12 – 24+ hours of no loads or charging at 77☏. Please be aware that some charts on the internet are for resting voltages. If we cross reference this chart to the 20 hour discharge rate, under-load voltages, in this test they line up pretty closely but if using a chart like this for a RESTING VOLTAGE you will essentially be murdering your batteries.Ĭhart Resting Voltage Actual Under-Load Voltage ![]() His battery bank was comprised of the identical battery being tested here. He was using it with resting voltages not under-load voltages as this chart was likely developed from. He located this voltage to SoC chart on the internet and I discovered it on his chart table. This image is an epic fail on our customers part, despite his very good intentions. ![]()
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