How long should absorb cycle take?

2-8hours depending on depth of discharge and charging rate. If your average depth of discharge is between 15-25% then use no more than 2 hours absorb. 25-35% depth of discharge use up to 2-3 hours. 35-50% depth of discharge use 3-5hours. Over 50% depth of discharge use up to 8 hours.

Another factor in determining absorb time is the rate of charge. A high current charger will get your battery upto the absorb voltage much quicker, but the battery will be far from fully charged - perhaps only at about 80% or so. Absorb time will need to be longer. At lower charge currents the battery will spend more time in bulk and will slowly build up to the absorb voltage, by the time it gets there it will be pretty close to 100% - maybe even as high as 95%. Absorb will not need to be that long.

If in doubt monitor the specific gravity and charge acceptance. If a 100AH battery is sitting at the absorb voltage but accepting less than 1 amp it is probably as fully charged as it's going to get...

larrybc1 wrote: »

Hi..I have a 48 volt 900 AH battery bank..8 Trojan 6 volt industrial batteries..just wondering how long should the absorb cycle take..currently have it set for 3 hours..manufacturer doesnt give any info on how long absorption cycle is..using Outback FM-80 charge controllers...thanks..

I doubt you have a 48 volt 900 ah battery bank, or at least I am unaware of Trojan making a 6 volt 900 ah battery. This would be about 2200lbs of battery, perhaps you have 8 - 6 volt golf cart batteries? If that is the case when you string the batteries only the voltage adds, the amps do not. so you end up with a 48 volt 220 or 230 ah battery bank.(not sure what they rate the t105 these days)

Once you reach absorb the battery is roughly 80-90% full, the voltage with out a charge controller would runaway and your battery would out gas, the charge controller at this point holds the voltage at a preset point. At this point the battery slowly reduces the amount of amps that it will absorb. Many people will set and 'end amps' your FM80 is equipped to do this. Setting this to around 2% of the battery capacity. Here is a thread that talks about this. About half way down Cariboocoot describes how to do this. End amps will work well unless the system is in use, then the charge controller doesn't know where the amps are going. If you are away during the day with minimal loads this might be the best way to top off you battery.You could increase the % if you have a lot of ghost loads.

There is a lot of good information in the Battery FAQs.

on YouTube I saw a video for setting up an Outback FM 80, specifically refers to "setting End Amps".

larrybc1 wrote: »

Hi..thks for all the info...i have the trojan industrial 6 volt 890 ah batteries...and yes they are heavy.. 400 lbs each! So during the day i charge the batteries and also run 1 freezer and 2 refs..use about 12 amps..and at night i turn off the freezer and ref...so how can i set the end amps in this situation with my loads during the day? Thks

12 amps only represents @ 1% of your battery bank, You might try 2% and see how long the system stays in absorb or even bump it up to 3% if you find it doesn't kick out. The fridges won't run all the time and the freezer likely has a minimal duty cycle, we don't know looking at the limited info we have. So if I was suggesting that is the route I would go.

larrybc1 wrote: »

... So during the day i charge the batteries and also run 1 freezer and 2 refs..use about 12 amps..and at night i turn off the freezer and ref...so how can i set the end amps in this situation with my loads during the day? Thks

Using EA from the CC only, requires that large-ish loads cycle during the time that the CC wants to end Absorb. Even, if on occasion, there are several loads on at the same time, and the resulting Absorb stage end is delayed, this will average out OK over time. Large, fairly continuous loads like a well pump, or similar, could delay Absorb end for some hours. This may not be too good, but perhaps still tolerable.

Continuous loads on the Inverters can be accommodated by simply adding the amount of current to the initial estimate of the desired EA value. Usually these 24/7 loads on inverters are fairly small.

Being able to measure the current flowing into the battery, as charge current is helpful in setting CC EA. Have not found that Clamp On DC Ammeters to be too helpful, when trying to measure currents over extended time periods. A good Shunt, and a 4.5 digit DMM can be acceptable (to measure the small voltage drop that these fairly low current EA values represent.

Have only ever used EA to end Absorb. Until about 15 months ago, used only CC EA, and this has worked very well. With the advent of the MidNite WhizBang Jt, teamed with the Classic line of CCs, we are able to use EA based on the actual battery charge current, which simplifies things. And, the WBjr is included, free with the standard Classic CCs (but not with the Classic Lites).

FWIW, Vic

CALLD wrote: »

... If your system allows for you to measure the actual net charging current at the battery then use this method, otherwise it's just guesswork for even the best charge controller...

Not to argue too much, BUT, for CC EA (without the Shunt and other hardware to end Absorb), the process begins with an ESTIMATION of a good EA setting to begin with. For Flooded batteries it is simple to measure how well this estimate is working -- NO GUESSING.

If one's household is out of control, with random, large loads running frequently during the time of day when Absorb is about to end, then, this CC EA will probably work less well.

However, we ran CC only EA for eight years on two different battery banks in use here, and it has worked extremely well, and NO guessing needed! The past two or so years, have used the MidNite Classic's team-mate, the WhizzBang Jr to end Absorb from Shunt-measured current, and this allows more flexibility in using Opportunity Loads during the Absorb ending time. This feature alone makes the Classic worth the cost to replace a perfectly good CC without this function.

As I read it, the OB FNDC hardware seems to work relatively poorly. The FNDC appears to have this Shunt current Absorb end function, but believe it works poorly ... or not at all (?).

And the batteries here are Surrettes, but the ideal EA setting for these (ten year old batteries) is just under 1% of 20 hour Capacity.
Another member that used to be active here -- Northguy -- had Trojan Ind batteries, but needed very, very long Absorb times with very, very high Vabs. It is possible that this situation settled down, once the batteries were cycled over 3 - 6 months or so.

But, larry, please DO carefully monitor the SGs of this new battery bank, and record these readings, EQ notations, water consumption and other important information in the Battery Logbook. Just number battery and each cell with a felt-tip marker.

Be certain to use a BTS on each charge source. These things will help your battery bank have a long and productive life.

FWIW, Vic

larrybc1 wrote: »

Ok thks ..so if i have 12 amps load during the day what should i use for end amps

what is the capacity of your battery ... EA should be about 1% of the capacity... +/-