Yearly battery math and a surprise

[Bruce Palmer]

I actually have a 24V system, but the below is converted to 12V equivalents as that is how many folks and charts think.

Every year I load test my AGM house batteries to see how they are doing.
The resting voltage starts about 13V. This year I put a 13 amp load on for 21 hours, or total draw of about 273 amp/hours. The meter said it was 282 ah, so lets round to 280 ah. At the end of the 21st hour, under load, the voltage was 11.85V (seems a bit low?). After removing the load and letting the batteries rest for 15 minutes voltage revived to 12.15V

There are many State of Charge charts out there so your mileage*may differ, but after much research I've settled on a middle of the road chart that says 12.15V = 60% SOC.* But I also have one that calls out 40% SOC.* It I pick the middle and say I drained my batteries to 50% SOC that would imply that the fully charged battery bank is worth 280/50% or 560AH.* The actual rating is 500AH of batteries.

Anyone see anything wrong with this?

For comparison, here are my notes from last year: Ran 12.5 amps 20 hours, meter says 246 a/hr. After 15 min rest 12.27V = 50% SOC by most charts. This indicates batteries (500 ah rated) in perfect condition. Amazing.

And now for the surprise. These are the original batteries and are 15 years old. Use averages 10 trips per year usually 2 nights on the hook each.

Comments welcomed. Note: voltage has been checked against several meters.

Thanks - Bruce

 

[tpbrady]

Bruce,

The amp hour rating varies with the rate of discharge. Take a T105 Trojan rated at 225AH if discharged down to 0 after 100hrs. That equates to a load of 2.5 amps per hour. Based on the data you presented you were discharging the batteries somewhere between the 40 and 50 hour rate. While watts consumed is the same whether it’s a 12v or 24v system halve the voltage and the amps double. I would say a key part of a battery test would be to determine the hour rate of discharge apportioned to each battery in the bank. I have 6 Firefly batteries rated at 116 AH each at 20 hr discharge rate of 5.8 amps. With a typical load of 10 amps when the boat is powered down at night that equates to about 1.7 amps per battery putting me at a rate between 60 and 70 hours. My readings on my shunt indicate my amp hour rating is somewhat higher because of the lower average rate of discharge.

It seems the way you are using your bank portends longer life. I got 9 years out of an AGM bank, only 6 of those were mine. They appeared to be toast after 4 years, but I revived them by removing wire that had the port and starboard banks paralleled 3 ways. I couldn’t make it overnight. After that exercise I got another five years out of an abused bank.

Tom

 

[AlanT]

They appeared to be toast after 4 years, but I revived them by removing wire that had the port and starboard banks paralleled 3 ways. I couldn’t make it overnight. After that exercise I got another five years out of an abused bank.

Tom

Can you clarify what you mean and what you feel helped the batteries recover, trying to understand (I do understand paralleling batteries). Thanks ~A

 

[tpbrady]

Alan,

The three parallel relationships seemed to have created multiple possible discharge paths that caused different discharge rates on each battery depending on the cable length from the load. I would have a battery when isolated show 11.5v after resting, another show12.2, and then a bank of two show 12.4. That would change if we used the inverter. After getting both port and starboard banks only parallel when the battery switch was in the both position, and getting the charger set to 14.4v from 14.2v for bulk and absorption, things improved immediately. I had earlier replaced the alternator as it was only outputting 13.6v while underway.

After all that I had about 90% of the bank capacity for the next 5 years. Then one dropped to 11.6v at rest and wouldn’t hold a charge so time for a battery replacement.

Tom

 

[ranger58sb]

15 years times 10 trips times 2 nights is only 300 cycles... and many decent deep cycle batteries are aimed at 1000 cycles to 50% depth of discharge...

-Chris

 

[sunchaser]

15 years times 10 trips times 2 nights is only 300 cycles... and many decent deep cycle batteries are aimed at 1000 cycles to 50% depth of discharge...

-Chris

Chris
A question with a tough answer is how many cycles can be achieved if one discharges to only about 75%. The only answer I have is more than my available cruising days.

TP
Given your experience with the Fireflys, how many years will they last with your use? Longer than T105s? Is the Firefly "advantage" working out as they advertise?

And Bruce, if you were to do a 100% unwired resting test over time on your batteries you may well find some weak links. One bad apple can spoil the batch so to speak thus suggesting replacing the entire bank sooner rather than encountering a bank failure under less than ideal conditions. I just throw my T105s away after 8 years as we often find ourselves far from cell phone coverage and cheap battery stores.

 

[tpbrady]

Tom

Since I am only going down to about 70% SOC on most overnighters, I suspect I would have more years available than I will probably own the boat. The best source of data on Firefly’s is Jay on Westerly. I think he is 5 years into his installation.

See you this summer?

Tom

 

[ranger58sb]

I think I've seen a chart that portrays that reduced % cycling... Maybe in the Lifeline technical manual...

-Chris