I've read your treatise on installing battery monitors as well as the post on "Smart Guage," noting your comments re. deteriorating accuracy of traditional Ah / Coulomb meters over time. As I have such a one, I'll be keeping it.
So, if I fully charge and equalize my house bank, measure the specific gravity of the cells and calculate the available Ah remaining based on the SG, and use that to reprogram the total Ah on my battery monitor, will I have made my monitor more accurate for the coming boating season? Or, must I do a 20Ah test to determine the available Ah?
Thanks for your contributions to the boating community.
SG or ROCV (resting open circuit voltage) readings tell you the same thing, a batteries SOC at a given point in time after being fully rested. SOC is all you really need to know to not abuse the batteries but this can be very tough on-board while the batteries are in use as the readings will not be accurate..
An Ah counter needs to know the actual physical Ahcapacity of the bank to determine SOC because it does not use voltage and instead uses Coulomb or ampere counting... If it does not know the banks current as-is condition, and its physical Ah capacity, then it can not accurately tell you the correct SOC...
As batteries age and get used the physical capacity is also changing and the physical capacity is never really static for very long. I have done yearly 20 hour capacity tests to keep my Ah counters as accurate as possible. My last set of
Wal*Mart batteries are still going strong into their eighth year. This still does not make Ah counters 100% accurate but between that, proper programing and always using manual "known full" re-sets or synchronizations they can be pretty darn accurate.
A 20 hour test is easy but requires patience and, well, an Ah counter. You also need a way to control the load or adjust it as the voltage falls to keep it steady. Most boaters will never do this. I have found that deducting 5% year one then approx 3% each year there after results in
pretty good tracking but this is for banks I know are well cared for.
Cheater Pseudo Capacity Test:
#1 Conduct a manual reset when the bank is "known full". Known full is less than 1.5% of Ah capacity in current acceptance at 14.4V, NOT AT FLOAT VOLTAGE!! Note: This is not truly full but as full as you will usually get so base Ah's on this.
#2 Take out 50% of your assumed capacity then let the batteries sit and attain a resting voltage. At 80F this should be at least 24 hours rest with no loads or charge sources. At 40F this can take up to a week!! If the ROCV lines up pretty well with what the Ah counter is telling you then it is pretty well adjusted. Use an accurate DVM as the difference between 50% and 40% SOC is just a 0.1V difference..
TIP: You MUST know what your batteries full ROCV is. Many marine batteries are 12.72 - 12.73V, some AGM's are even higher. You need to know what
your batteries rest at when full!
100Ah Battery Capacity Programming (very rough guide)
Year #1 95Ah
Year #2 92Ah
Year #3 89Ah
Year #4 86Ah
Year #5 83Ah
Year #6 80Ah
Good Ah Counter Practices:
*Don't count on "auto synch" adjust the parameters to NOT auto synch
*Always rely on "know-full" manual re-sets (Less than 1.5% net acceptance at 14.4V)
*Program the proper Ah capacity
*Wire the shunt properly, all negatives must be on load side of shunt including start batt
*Program the batteries
factory Peukert exponent
*Adjust Ah capacity downward each season
*Do pseudo capacity checks occasionally - battery case temp must be considered
Best Ah Counter Practices:
All of the above plus:
*Conduct yearly 20 hour capacity tests and use this as your new Ah capacity
