The eternal question of “Do AGM’s charge faster with high charge rates applied?” seems to still be open for debate. This means constant-current or BULK charging. In this case, as you see next, it is 1:16 minutes before your alterntor would even begin to catch a break. It has some particular condition such as temperature range. At 1 hour the battery voltage has only risen to 13.8V at a .2C charge rate. You should also notice that if the battery warms up too much, that indicates something wrong in the battery. Here are a couple of things to keep in mind when maintaining your Absorbent Glass Mat (AGM) battery, including an OPTIMA® battery. The voltage is climbing slower in bulk at .2C than it did at .4C. Would you like to see more articles like this? AGM batteries have a wide range of functions. This image is just for just for illustrative purposes.. Do this a few times…….. Ouch! For example 80% DOD is also 20% SOC or 80Ah removed from a 100Ah battery. BULK: If we start at the top left of the chart we can see that the current held rock steady at 18.75A for *1:42. This is just represents a percentage of Ah capacity in either charging or discharging current. Discharge to 50% = 47.98Ah (left in the battery after discharge), 1 Hour .4C charge then discharged and counted Ah’s delivered back to 50% SOC = 33.43Ah, 47.98Ah + 33.43Ah = 81.41Ah of stored energy, 81.41Ah is approx 85% of the baseline Ah capacity of 95.69Ah’s. At 2 minutes the voltage at a .4C charge rate had already risen to 13.6V. Once voltage is held steady, or it becomes voltage limited, current begins to decline and the charge efficiency worsens as SOC increases. So, AGM batteries charging procedure is not the same as laptop batteries. By 1 hour in, our 42A or .4C charge current has already declined to 19.5A.. By two hours of charging, at .4C from 50% SOC, we are down to just 7.3A in current flowing into the battery…. The steps described above may help you a little bit. On this 1 hour .2C recharge the battery never attained the absorption voltage of 14.4V and was still in bulk when the charger turned off at the one hour mark. When 14.4V and 0.525A were attained is when I deemed this battery at 100% SOC. In this image we have removed 48.54Ah from the battery which tested at 95.69Ah’s of actual capacity. Let’s do the math: Baseline Ah Capacity = 95.69Ah, Discharge to 50% = 47.84Ah (left in the battery after discharge), 2 Hour charge then discharged and counted Ah’s delivered back to 50% SOC = 35.28Ah, 47.84Ah + 35.28Ah = 83.12Ah of stored energy, 83.12 is 86.9% of the baseline Ah capacity of 95.69Ah’s, RESULTS: The battery achieved approx 87% SOC from 50% SOC in two hours at a .2C charge rate. Double the charge current to shave 12 minutes? ABSORPTION – Absorption, float and equalization are all examples of constant voltage charging stages. AGM batteries need a particular type of charging system. There is a fixed range for AGM battery charging procedure. This is why many a boater has burned up their alternators charging AGM batteries. After a full recharge at 14.4V battery then spent 32 hours at a float voltage of 13.4V. It helps, not hinders, with overall longevity to charge at high rates. Lifeline wanted 0.5% and for Firefly we stopped at 0.5% as well. Most boaters charging AGM batteries would likely benefit from a compromise charge source current of about .25C to .35C for 1.5 – 2 hours +/- per day. Take for example the .4C charge rate. It provides the best output for your battery. Is so feel free to donate, support the site and keep it growing. For the first part of this test, 50% SOC to 100% SOC, I compared a charge rate of .2C with a charge rate of .4C on the same Lifeline GPL-31T battery. Over many years of capacity testing hundreds and hundreds of batteries, flooded, GEL and AGM I have yet to see a lead acid battery that could delivers its full storage potential at anything less than 0.75% in tail current at absorption voltage. This kind of battery technology introduces in 1985 in military aircraft. My best research of antiquated data and materials suggests that 2% @ Absorption voltage originated from the Ah counting industry not from batteries actually being “full”. The lithium batteries can be charged which means partially do not require full charging. The myth goes something like this: By using a battery combiner, on AGM batteries, and feeding the alternator or battery chargers charging current directly to the house battery bank first, “it will leave your start battery under charged“. Please remember that bulk is not a voltage limited stage of charging it is constant current. PERSPECTIVE: It is pretty clear that a 1 hour charge at .2C is an inadequate charge rate for AGM batteries that are routinely discharged to 50% SOC, unless you really like hearing your motor or generator run. The .4C charge rate had attained 14.4V within 19 minutes. Absorption or constant-voltage is where the charge source holds voltage steady, hence the term “constant-voltage”. If the battery store about 10.5 volts charge on it, then you can leave your battery for more charging. For charge and discharge rates I kept them based on the “as new” capacity rating, just as many boaters would do, to keep the test a bit more “real world“.. 3- 50% SOC Charged at .2C For Exactly 1 Hour Many companies, including Balmar and some others, do not correctly understand using the word “BULK”. The charging devices we use on boats are all considered CC > CV charge sources, or constant-current (BULK) then to constant-voltage (ABSORPTION, FLOAT & EQ). After cool enough, reconnect the charger again. 0.4C = 40% Charge Rate of the Ah Capacity. This is from the conclusion section of Dave V’s study: The study then goes on to suggest that a balance needs to be met between equipment and optimal cycle life. Check Pricing on Amazon. If you’re paying attention this battery, when charged at .4C from 49.3% SOC, stayed in BULK/CC for 19 minutes and entered ABSORPTION/CV charging, where current begins declining, at 63.3% SOC. Charging – BK Precision 60A Variable Power Supply W/Dedicated Voltage Sensing 5- 50% SOC Charged at .2C For Exactly 2 Hours I have long known that a higher charge rate, with AGM batteries, does not necessarily translate to *significantly faster charge times from 50% SOC to 100% SOC and thought it would be good to test this and measure the actual differences. SCALE IT UP: If we scale this test up, and it should scale well, a .2C charge rate on a 450Ah fairly typical cruising boats house bank would be a continuous 90A for 1 hour before your batteries even hit the absorption voltage set point. Moreover, AGM battery and gel-based battery technology are differing from each other. This current, 18.75A is .15C or a 15% charging current of a 125Ah battery. Modified on: Mon, 23 Mar, 2020 at 2:46 PM, To maximize the life of your Rolls AGM battery, it is important that it is properly charged. The battery was then recharged for exactly 2 hours at .2C and then discharged back to 50% SOC and the stored energy for that cycle was measured. For example if we return 10Ah’s with a 90% charge efficiency setting the battery monitor will only show that as 9Ah’s returned. *For more information on the effects of PSOC cycling AGM batteries please read the May 2015 and August 2015 issues of Practical Sailor Magazine. Here the electronic load has turned on and has now begun discharging the battery back down to 50% SOC. During charging if the battery has become too hot suddenly immediately disconnects the battery from the charger. It is a pretty rare boat that can muster a .4C charge rate, but some do. Unless the boat has a massive Electrodyne alternator or multiple stacked “Combis” (inverter/chargers) it is quite rate to see even a 200A continuous charge rate. The best protection from improper charging is the use of a quality charger and routinely. The charger cabling should be insulated and free of breaks or cuts. NOTE: Lifeline Battery recommends a .2C charge rate as the bare minimum for these expensive AGM batteries. Using return amps, at absorption voltage, (not float voltage) is usually the easiest method to determine 100% SOC when out cruising. So it will be best if you find a charger that has a desulfation method of charging batteries. Bulk charging is not a voltage limited stage of charging despite many companies bastardizing the term bulk for apparent marketing purposes. For this procedure, you need a voltmeter to measure the AGM battery charging amps. The stored energy removed was 20.46Ah from a max charge rate of 21A. If you’ve listened to net lore or dock lore for years, as I have, we all know AGM’s “don’t come up to absorption until 80-85%” right? If you think about the physical charger size, then it was a mistake! The discharge current at 77F that will yield a 20 hour run time before hitting 10.5V. Here we are at 3 hours and the accepted charge current is now down to 2.8A.. That last few % takes the longest due to declining CAR. If the connection does not do properly, then the whole procedure will be failed. Here we are at 100% SOC at the .2C charge rate and it took 5:42 minutes. But when the battery has become too hot, then you should not use it. The battery has a rated Ah capacity, at 20 hours, of 105Ah. Sulfating is a common thing in lead-acid type batteries. Your donations help keep the content coming and also help keep it free. Blended with solar you can get the bank to 85% SOC or so and let the solar take over for the long slow crawl back to 100% SOC. That helps to collect the battery condition information and regulates the voltage according to their current situation.

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