Deep cycle battery charge times

landyacht318 wrote:
Determining when an AGM battery is full, requires an ammeter, or a amp hour counting battery monitor.

HOld long absorption stage is to be held, is highly dependent of state of charge on the battery, how long since its last full recharge, the overall health of the battery, and temperature.

LifeLine AGM says 20 amps per 100AH minimum when deeply cycled. Hold 14.4v until amps taper to 0.5amps for a 100AH battery, and then the battery can be considered fully charged

I've seen my 90AH Northstar AGM taper to 0.42 amps at 14.46v in 3 hours, I've also seen it take 11 hours. 6 of those last ones at 1 amp or less.

Far too many people think that once a charging source drops to float voltage, that this means the battery is fully charged. IT DOES NOT mean this, only that absorption voltage was held as long as it was programmed to do so.

6 more hours might be required at absorption voltage before amps taper to 0.5% of capacity at absorption voltage.

The less healthy the battery, the longer it takes to reach this threshold where the AGM battery can be considered fully charged.

Get an ammeter and see what the battery is accepting at absorption voltage, otherwise you are basically completely blind and simply guessing as to AGM state of charge.

I would agree with most of this with a couple of additions.

An ammeter will tell you when the batteries are full, but a monitor that counts amp hours and goes by them to determine state of charge will not be accurate on the recharge side of the cycle because they can't accurately know the charge efficiency. Charge efficiency varies by battery, depth of discharge, and even where you are in the charge cycle, so it is nearly impossible to guess or calculate it precisely.

That said, most good monitors calculate the charged "indicator" separately from the amp hour count or state of charge percent. For instance, on a Trimetric you program in the voltages for charging and the amps you need to reach to be fully charged. When you get there it lights the fully charged light. The amp hours returned can be either over or under the amount you actually used depending on how the charge efficiency setting matched that particular charge cycle. Same with state of charge display, could be either way. Once you start a discharge cycle without charge voltage, the Trimetric will reset the amp hour counter to zero and the state of charge to 100%, so will be accurate on the discharge cycle.

The .5% that Lifeline states as full is pretty generic to cover new and old batteries, and you can go lower on new ones very easily. Our new ones will go to .2%. It is also voltage influenced so it will be higher if you go to high end of the charge range at 14.6v. It takes a long time to get there.

If you do stop at .5%, or any amount above the actual minimum, the charge will finish off on float of 13.3v on Lifelines, if you have the time. After going to our .2% at absorption we show .1 amp 13.3v float. After a week of float it is under .01 amps. This is on 440ah of battery bank. This would say that even if you take your batteries to the lowest amp in reading, they probably also can benefit from continued float rather than taking them off charge.

All of this gets pretty complex to accurately control when you have 3 charging systems (shore, solar, engine alternator), if you are trying to always get your batteries full when actually camping, driving, on and off shore power, etc, especially since you don't overcharge either. The good thing is that you can be fine if you get the batteries totally full every 5-10 charge cycles.

Determining when an AGM battery is full, requires an ammeter, or a amp hour counting battery monitor.

HOld long absorption stage is to be held, is highly dependent of state of charge on the battery, how long since its last full recharge, the overall health of the battery, and temperature.

LifeLine AGM says 20 amps per 100AH minimum when deeply cycled. Hold 14.4v until amps taper to 0.5amps for a 100AH battery, and then the battery can be considered fully charged

I've seen my 90AH Northstar AGM taper to 0.42 amps at 14.46v in 3 hours, I've also seen it take 11 hours. 6 of those last ones at 1 amp or less.

Far too many people think that once a charging source drops to float voltage, that this means the battery is fully charged. IT DOES NOT mean this, only that absorption voltage was held as long as it was programmed to do so.

6 more hours might be required at absorption voltage before amps taper to 0.5% of capacity at absorption voltage.

The less healthy the battery, the longer it takes to reach this threshold where the AGM battery can be considered fully charged.

Get an ammeter and see what the battery is accepting at absorption voltage, otherwise you are basically completely blind and simply guessing as to AGM state of charge.

Thanks Booster , I'm going to try the battery monitor

I think that basically, you don't really have any idea how full your batteries are at this point. The normal way to tell is to check specific gravity in wet cells, or rested open circuit voltage in wet or AGM batteries after letting them rest.

If your wet cells are not getting to their maximum specific gravity, and relatively even between the cells, they are either getting old and tired or need equalizing. Wet cells in good condition will be in the 1.270+ specific gravity range and not vary more than about .005 cell to cell. Most will show a rested voltage of 12.7+ volts. You need to charge them until the specific gravity doesn't increase any more, and if they are still uneven or low (which is highly likely) they will need to be equalized (15.5v for 1-4 hours our until the specific gravity quits increasing done very carefully).

The only way that I know of to accurately tell if AGM batteries are full is by measuring the amps going to them at the end of charge. It will be something in the range of .3 to 2% of the 20 hour battery capacity in amp-hours only applied to amps. 1% would be 1 amp per 100ah of capacity for instance. You just charge until the amps quit dropping and you are assured of being full. In the real world, it is usually best to stop a bit early to allow for variations due to temperature, etc, especially if you don't have a temperature compensated charger. Rested voltage of above 12.8 volts is an indication of full charge, but in reality you can get that voltage and still be considerably undercharged. Our Lifelines will hold 13.1 volts for weeks with no load on them if full, but 12.8v is considered full, for instance.

Chronic undercharging will "walk down" the capacity of the batteries quite quickly. They need to get totally full something like once every 5-10 charge cycles for long life.

Since the charge cycle will be different for every battery bank size and manufacture, and also vary by depth of discharge, any charger that tries to run on a timer is doomed to either over or under charge you batteries almost all the time. Most are setup to undercharge from what I have seen. I know they all say the extra time is in the bulk stage and the timers don't start until absorption, so they are accurate, but this is not true as absorption time also changes with different depth of discharge. Some very high end chargers try to estimate the depth of discharge by measuring the bulk time, which helps, but is still an approximation. The best charging systems simply measure the amps going into the batteries with a shunt to determine when the batteries are fully charged. Very simple, very consistent, and very accurate.

Getting the batteries totally full will take a long time, as the final few % are very slow to complete. If we go to minimum amps on our 440ah bank of Lifelines from 50% down, it takes close to 13 hours with a 100 amp charger. Only about 2 hours are at bulk full output, the rest at absorption, and the drop from 4 amps to ending at 1 amp takes nearly 1/2 the time. It is not a fast process. If we stop at 2 amps and let float finish the charge, which it will do, the time will drop by about 25% to get to float, but it will take about 12+ hours of float to finish the charge. The charge times that have been suggested in earlier posts are highly unlikely to come close to full charge.

Wet cells will charge even slower than the above, based on what we have seen in our old setup.

Based on the above, I would guess the OP is not getting anywhere near full and is running the batteries in the 20-80% state of charge range essentially all the time, as that is the part of the charge that will come back quickly. Many boats do this same thing to minimize generator run time, and just accept that it will shorten the battery life by something like 50%. With the system he has, it may be his only real alternative. I would, however, suggest that he put on a battery monitor so he actually knows his state of charge, voltage and amperage. They aren't very expensive, pretty easy to install, and give you all the information you need to know what is happening without guessing.

Set your adjustable Powermax to 14.4 to 14.8 volts.
Charge one hour after voltage hits the set point.
My WAG is for that use is two hours every other day.

Ive always tried not to discharge my AGM more then 20%,,takes about 45 min - 1 hr to fully charge up..

I go by what PROGRESSIVE DYNAMICS says in their CONVERTER brochures...

This is what PROGRESSIVE Dynamics states in their operating manual on how long it takes to charge a battery using the DC VOLTAGEs listed below: "Progressive Dynamics ran this test on the amount of time it took a PD9155 (55-amp) converter/charger set to three different output voltages to recharge a 125 AH (Amp Hour) battery after it was fully discharged to 10.5-volts.

14.4-VOLTS (Boost Mode) – Returned the battery to 90% of full charge in approximately 3-hours. The battery reached full charge in approximately 11 hours.

13.6-VOLTS (Normal Mode) – Required 40-hours to return the battery to 90% of full charge and 78-hours to reach full charge.

13.2-VOLTS (Storage Mode) – Required 60-hours to return the battery to 90% of full charge and 100-hours to reach full charge."

This is based on having 17-20AMPS DC current available for each battery in your battery bank... NOTE That Progressive Dynamics doesn't even list using DC Charge Voltages around the 12.0VC range as this would take alot more than 100 hours to achieve a 90% or 100% charge state.

Of course if you don't run the batteries down to the 50% charge state these times will different. I get better performance from my batteries by running them down to the 50% charge state before recharging. Topping off the batteries all the the eventually affects my performance in that the batteries will drop to the 50% charge state much sooner.

For me it better to discharge the batteries to their 50% charge state before re-charging... I can do the 50% charge state and get a good 95% performance out the batteries but if I do this 50% to 90% charge cycle more than 12-14 cycle times without doing a full 100% charge state than my batteries really start dropping off on their performance cycle. i.e. they will not operate as long as before producing high DC current output.

Roy Ken

Hard to say, depends on how much you discharge them. For us with 12 volt Exides 3 hours from 80% and 6 hours from 50%. When we had the on board converter its was days. That is why I use either solar or a smart charger.
Average absorb times 2 to 4 hours depending on battery manufacture.
Typical charge rate for GC-2's for bulk is c/10 where c = battery capacity in Ah at the 20 hour rate.