Testing AGM Battery capacity?

[T37SOLARE]

With the help of a good electrician it appears I've finally worked out the bugs on the PO's battery upgrade on SOLARE. (4 group 27 = 440 amp hours)

He and the Link 20 manual say to do a battery condition test, to run 5% or your capacity until you get a 10.5 voltage reading. Then you can determine what your real available capacity is.

But, somewhere I recall reading that AGM's don't like to be fully discharged, and by doing so I'm shorting both the life and capacity.

Is this a good thing to do, or is there a safer way to determine actual capacity?

[k1vsk]

If you are unsure how to accomplish this properly, it may be best to simply input the stated battery capacity of your bank as a good approximation.

Out of curiosity, how do you get 440 amps out of four Gr27 batteries? Do you really have them all paralleled?

[camaraderie]

k1vsk...that's a fairly normal house bank with all parallelled. Mine is 1000+ with 4x8d's.
group 27's are generally around 90-100ah's each and 110 is actually quite high. I would simply use 90-100 ah's as my amount per battery IF they are new. It doesn't hurt to err a bit on the low side as extended battery life will be the result of charging before you reach 50% discharge.

SOLARE...you are correct...do NOT do what Xantrex suggests. If you are not content with estimates then you can do the following.
1. EQ the batteries.
2. Fully charge the batteries.
3. You have approximately 400ah capacity at the 20 hour rate which means that at a 20 amp load you should get 20 hours before you reach 10.5 volts. 4. Get a 20 amp test load and run the batteries down to 12.2V. (50%). They SHOULD last 10 hours before reaching 12.2V. If they only last 8 hours...you have 80% of rated capacity...5 hours is 50% etc.
5. Again...if the batteries are new...no need to go through this.

[hellosailor]

Cheap test load: Automobile headlights, or similar 12-volt lamps or heaters that are rated as high as you can get them.

Batteries are nominally rated at a 20-hour discharge rate, so if they are rated 105AH each, that's based on 20 hours at a 5-amp load. (Ratings for higher and lower rates are usually also available from the maker.) One automotive high beam lamp (~65 watt rated) is a conveniently close load for one battery at a 20-hour rate.

And I'd agree with Cam--cycling them down to the 50% point should be enough.

[T37SOLARE]

Originally Posted by camaraderie

SOLARE...you are correct...do NOT do what Xantrex suggests. If you are not content with estimates then you can do the following.
1. EQ the batteries.
2. Fully charge the batteries.
3. You have approximately 400ah capacity at the 20 hour rate which means that at a 20 amp load you should get 20 hours before you reach 10.5 volts. 4. Get a 20 amp test load and run the batteries down to 12.2V. (50%). They SHOULD last 10 hours before reaching 12.2V. If they only last 8 hours...you have 80% of rated capacity...5 hours is 50% etc.
5. Again...if the batteries are new...no need to go through this.

Thanks Cam!

These batteries were installed by the PO age unknown, and the fact the boat was keep on a mooring, I had no clue of their condition.

Good news is, I ran a 20+/-amp load on the bank and got it down to 12.2V at 12 hours, in addition the Link told me that I had consumed 215 amps.

So it looks like these batteries will be around for a bit longer!