Determining useful capacity of house bank

[billdre]

So this is my situation and what I am thinking of doing. What I'd like to know is, although it's crude will it give me the information needed to know it my batteries are still in good enough shape for my needs.
I have a house bank of 2 Lifeline 30HT batteries, 300ah total. I've had them for 9 seasons. I charge them full in the fall, Nov. 1, when the boat goes on the hard and around May 1 charge them back up and they have always held the charge and been fine. They start winter around 13.2 and end up in May at 12.9. This appears to be the case this spring too, but I'm not at the boat right now to double check. The boat goes into a slip with shore power all summer. I day sail and go on the hook for two or three days regularly each season. This is a 28' boat with LED house lights, VHF radio, AM radio, and Garmin chart plotter. No other draw.
My thought is that if it holds the full charge this spring I will stress test it by additionally operating the boat off shore power for several days of sailing, over night stays, etc. Then take a look at SOC and current voltage to determine if all is well or if I might need a new set. The bank is equipped with a Victron monitor.
How does this sound to you guys. I think I've never really drawn the bank down by much over the years but 9 years is a really long time. So...?

[Cheechako]

I think you are on borrowed time, but a good approach. You need to see how they perform under load and if they drain down too fast.

My experience is the useful capacity is only equal to your ability to refill it. You can have 1000 amp hours worth of capacity, but if you draw it down to half, it may take you a week to refill it.
Realistically, you need only what you will use in a time period you define. Say a 24 hour period. (every day you will recharge via engine, solar panels or etc)
ie: how much will you use while bringing the bank to your personal acceptable maximum battery depletion before charging? (how far discharged are you willing to go on a daily basis? 12V? 11.5V?)

If you use say 120 amp hours a day, what the point of having 1000 amp hours stored?
There are some battery life and backup storage considerations.

PS: The last acid cell batteries I had new were 12.5 volts at full charge. (this is after sitting overnight unconnected to anything, not right after charging which may read 13+ volts)

[Pete7]

Why not drop test them? Charge them up, separate one and put a 50w car light bulb on it and time how long before the light goes dim. Then charge it back up again.

The discharge time will tell you how much capacity it actually has compared when new.

However, I think the results will be disappointing if they are 9 years old.

Pete

[pjShap]

I have Lifeline AGM's as the house bank. I replaced them a few years ago, after the previous owner family lived aboard and cruised full time for 5 years, almost never with shore power, and I cruised full time for several summers plus more occasional cruising spring and fall. Sounds like your batteries have been treated a lot better than mine were, and mine lasted 10-12 years. I think you could just continue until you see a problem. If you're mostly day sailing, replacing should be relatively easy whenever it becomes necessary.

[leecea]

It sounds like all you need is that they support your typical current draw for 3 days. If they do that, maybe with a bit in reserve, you should be OK. So your plan to test them that way sounds reasonable to me. Even if they are not in the best shape, they may still be able to do that.

Do you have a separate engine start battery? You don't mention that.

[wholybee]

Originally Posted by Pete7

Why not drop test them? Charge them up, separate one and put a 50w car light bulb on it and time how long before the light goes dim. Then charge it back up again.

The discharge time will tell you how much capacity it actually has compared when new.

However, I think the results will be disappointing if they are 9 years old.

Pete

Yes, this is the correct approach. But instead of waiting for the light to go dim, use a voltmeter and wait until the voltage reaches 10.5V

The number of hours that takes, times the wattage of the bulb you choose, is approximately the number of Watt-Hours of capacity the battery has. Divide that by 12 for the number of Ah of capacity the battery has.

[billdre]

Originally Posted by leecea

It sounds like all you need is that they support your typical current draw for 3 days. If they do that, maybe with a bit in reserve, you should be OK. So your plan to test them that way sounds reasonable to me. Even if they are not in the best shape, they may still be able to do that.

Do you have a separate engine start battery? You don't mention that.

Yes. I do have a completely separate start battery charged by my outboard.

[billdre]

Thanks, that's really helpful idea.

[sailingharry]

I did a capacity test on my battery bank a year ago. Here is my approach.

* Fully charge the batteries on shore power, and hold on float for a while. Since I am on shore power, that was really not a defined step, that's normal condition.

* Determine the 20-hour discharge rate. My battery bank is 800Ah so the discharge rate is 800 / 20, 40a.

* Determine the Watts that give that power. 40 times 12.5 average voltage is 500 Watts.

* Find an AC load of about 500 W. As I recall, that ended up being a space heater set on low. I believe that DC light bulbs are an exceptionally poor form of establishing the load, because the power consumed drops as the square of the voltage drop. Note that the exact value is not critical, I'm my case 400-600 would probably have been close enough.

* Set my inverter low voltage cut out to the desired stop point, it may well have been 10.5 but I don't remember for sure.

* Make sure my battery monitor, in my case a Link 2000 is set up and showing fully charged.

* Fire off the load on the inverter with short power disconnected and go away. Set the start time to allow for me to be present before the end time.

* When the inverter kicks off, read the Ah discharge number from the battery monitor. This is the new capacity for the battery in Ah.

* IMMEDIATELY BEGIN RECHARGE AT MAXIMUM AVAILABLE RATE!

* Reset inverter low voltage cut out to a more reasonable number.

* Change the ah Bank value in the battery monitor to the new battery capacity.

Unfortunately, my batteries are only about 70% of their original capacity!

A slight variation on this would be to separate your bank into individual batteries to determine relative health between them.

[smac999]

google c20 battery test. it's the only valid way to test a house bank.

9 years on agm's is a good life. I would expect to change them soon

[Searles]

Lead acid ,on borrowed time ,may totally fail with no warning ,up grade while you can .⛵️⚓️

[NBs]

Hi,

I have lifeline 2 x 12V and 4 x 6V baterys 14 yars old and baterys is fine.

I have victron phoneix 12V charger and charger have a program that runs a bigger V about 15,4 and the Amps are low, the purpose is to remove possible sulfate from the plate surfaces and thus improve the battery capacity.

I do this about 6 times a year and the charger program takes about 1 hour. Lifeline recommends this for their batteries, information can be found on their web.

Lifeline say:

Conditioning
Conditioning should only be done when the battery is showing symptoms of capacity loss due to extended time in a partial or low state of charge condition. This could be caused, for example, by low charging voltage for an extended number of charge cycles, or by repeatedly charging to only 90% state of charge.
NOTE: Some chargers use the term Equalizing Charge instead of Conditioning Charge. An Equalizing Charge is generally applied to flooded lead acid batteries that are susceptible to acid stratification. However, an Equalizing Charge may be used to provide a Conditioning Charge for Lifeline® batteries as described below.
To apply a conditioning charge, first go through the normal charge cycle to bring the battery to full charge. The conditioning charge should then be applied by charging for 8 hours. At 77°F (25°C), the conditioning voltage should be set at 2.58 VPC (15.5 volts for a 12-volt battery). The conditioning voltage at other temperatures is shown in Table 5-2. By using the temperaturecompensated conditioning voltage, batteries that are not in controlled temperature environments may be conditioned without bringing them to room temperature. If temperature compensation is not available, it is best to bring the battery as close to room temperature as possible before applying the conditioning charge.

In systems with limited charging output or long time periods between full charge, a routine conditioning charge may helpful
in preventing capacity loss due to sulfation. If a routine
conditioning charge is used, the time duration should be less than the 8 hours specified above. As a starting point, a routine conditioning charge may be applied every 3 weeks for 4 hours at the voltage levels given above. The frequency should be fine-tuned to assure the batteries are not being undercharged or over-charged. In most cases, the optimum frequency will be between 2 and 4 weeks.

NBs

[Davidasailor]

I do a c20 test like sailingharry describes every few winters. I test the batteries separately so I just use a lightbulb on a dimmer switch with a cheap inverter. Once in a while I need to tune the dimmer switch as voltage and load change, but it’s pretty stable.

The last time I tested my old bank was when they were 7 years old. Both batteries tested within 95% of their rated capacity. They both lasted another 18 months, then one battery went flat dead and the other dropped to 70% capacity.

For your type of use the less scientific testing approach you propose would probably also be good enough. It won’t tell you the exact capacity but it’ll give you a pretty good idea of whether the can still meet your needs.