Battery C20 vs. "Minutes of discharge"?

[john61ct]

Note the protocol above is only for commissioning, not load testing for SoH / residual capacity.

I was just pointing out how similar they are, and refuting the idea that such maintenance routines are as harmful as the usual routine abuse from user ignorance.

For an outline of SoH testing protocols, CTRL-F search for

20 hour capacity

here https://marinehowto.com/programming-a-battery-monitor/

Better yet, read it all, and check out the rest of the site.

[Maine Sail]

Originally Posted by Unity

Or heck, take the 8 amp rating - in theory, 8 amps should give me more capacity, not less? Yet it still rates substantially below the C/20 @ 10.5a capacity.

I think what you found is simply a typo on the 8A reserve minute data..... Best to call Lifeline.

[Unity]

Originally Posted by Maine Sail

I think what you found is simply a typo on the 8A reserve minute data..... Best to call Lifeline.

I did contact Lifeline, and their head engineer got back to me confirming that the numbers on the site are inaccurate and that while the C20 rating is current/accurate, the minutes discharge are out of date.

Here is his email:

“Most of are ratings have been updated over the years. The 4D is one of the batteries that needs to be re-rated, specifically the 25 amp, 15 amp and 8 amp discharge rates. We rated those batteries back in 1994 and the amp hour at the 20 hour rate was not commonly used. As battery and manufacturing technology improved, so did the performance of the battery. Around 1998 the industry shifted more towards the 20 hour rating. When we ran the 20 hour rate it was and still is in fact a 210 amp hour battery @ 20hr rate. Sadly, the 25 amp, 15 amp and 8 amp tests were not redone to match this and they should have been. That was definitely an oversight on our part. We bench test these all the time and the published ratings are very low as the 4D is currently exceeding the 25 amp published discharge time by well over an hour. I will get with the technicians and have all these ratings redone so we can publish the true numbers. If anything, it is doing us a disservice by publishing very conservative numbers. As for right now the customer should use the 20 hour rating as a guide and we should have all the ratings published within the next 30 days.”

[Maine Sail]

Originally Posted by john61ct

Then discharge at .05C or 5A per 100AH, for about 20 hours. The battery voltage should then be resting around 10.5 Volts.

A resting voltage, after a 20 hour capacity test, of 10.5V means you went beyond 0% SOC or below 10.5V and this can be really bad for the battery. This is why the cut off voltage, regardless of the discharge test load, is consistently a cut off of 10.5V..

In other words 10.5V is the cut off voltage UNDER LOAD for a 120 hour discharge rate, 20 hour rate or a 1 hour discharge rate..

The rebound voltage for a 20 hour rate test will be well into the high 10's to low 11's pretty quickly. If left to sit long enough it would wind up at about 11.55V - 11.65V. You of course don't want to let them sit at 0% SOC for very long and if you see a rebound to 11.55V to 11.65V you've let them sit at 0% SOC for too long.

[Maine Sail]

Originally Posted by Unity

I did contact Lifeline, and their head engineer got back to me confirming that the numbers on the site are inaccurate and that while the C20 rating is current/accurate, the minutes discharge are out of date.

There was just no other explanation for the discrepancy. They are a great company but even small mistakes can happen. It's a lot of data to keep track of. Glad to hear they will be updating the specs.

For what it is worth it is extremely rare for me to test a new or barely broken in Lifeline AGM and not see it deliver the 20 hour capacity or within a gnats hair of it.. I wish I could say the same for all AGM and flooded battery manufacturers, but I can't..

[Maine Sail]

Originally Posted by CaptainSteven

I found using the C20 value gives misleading data. I found if I use peukerts formula with 72 hours instead of 20 I get data that more closely represents my true capacity with a constant of 1.2.

Here is the formula I use:

T = C/(I/(C/R))^n X (RXC)

The 20 hour test is used to arrive at your current SOH as compared to the factory capacity rating.

A 20 hour test is not going to be 100% representative of the actual usable capacity you have, at your own discharge rate (usually way below the 20 hour rate).It is used as a way to compare to the factory Ah rating. If your on-board discharge rate is lower you will have more usable capacity than the 20 hour rate indicates.

[john61ct]

Originally Posted by Maine Sail

In other words 10.5V is the cut off voltage UNDER LOAD for a 120 hour discharge rate, 20 hour rate or a 1 hour discharge rate.

Great point, thanks.

I believe got it from the Ample text cited?

Certainly did not mean to imply "resting" in that sense, always try to remember to say

Immediately start recharging as soon as 10.5V is reached.