Excerpt from the article on making your Ah counter more accurate.
What is Ampere Hour Capacity?
The Ah capacity of deep cycle marine batteries is based on the BCI (Battery Council International) 20 hour discharge test. For a 100Ah battery this means it should deliver 5A at 77F for 20 hours before the loaded terminal voltage falls to 10.5V. This is your ideal factory rated Ah capacity, but you’ll notice two things.
The first thing that stands out is the 5A current. In order to meet the 20 hour capacity test figure, the current is held absolutely stable even as the voltage decays/falls during the test. As a battery discharges, the load it is discharged at will change the usable capacity of the bank, at that load. The only way you can get 20 hours of run time, at 77F, is at the 20 hour discharge rate. Any load greater than this and the battery will not deliver its full rated capacity. On the flip side if we draw the battery at less than the 20 hour rate we can get slightly more Ah capacity out of it.
I prefer to call this the Peukert effect. I hesitate to call it Peukert’s law because it is not a law, like Ohm’s Law is, it’s an effect that modifies usable capacity based on rate of discharge.
In order to test the battery at the 20 hour rate, and do so accurately, you ideally need to hold the discharge current steady as the voltage decays to the 10.5V cut off point. This is a tedious and imprecise process for the average DIY. There is test equipment available to conduct a proper 20 hour Ah capacity tests but they begin in the four figure price range, and take a lot of time to complete. A 20 hour test is just as it implies, 20 hours long, not including the time it took you to charge the battery and get it to a controlled temp of 75-80F.
“How do I determine my batteries 20 hour discharge rate?”
This part is easy, you divide your batteries 20 hour Ah capacity by 20, eg: 100Ah ÷ 20 = 5A. If you had a 210Ah battery the math is the same; 210 ÷ 20 = 10.5A. If you want to test for Ah capacity, which is the only test that matters for an Ah counter, then this is the formula for determining the constant-current discharge load you will use.
The second thing you will notice is the temperature. Just like the rate of discharge, battery temperature affects your usable capacity. If you do not maintain a battery temp of 75-80F, during testing, you will not arrive at or get the correct 20 hour capacity. When conducting a capacity test, in order to properly program a battery monitor, discharge current and battery temp ideally need to remain constant & stable while the battery is discharged to 10.5V. A few degrees F either way sill net a "close enough" approximation but not pin point. For most boat owners "close enough" is fine...
If you don’t start with a known & confirmed Ah capacity, your Ah counter may never give you reliable information. It simply can’t unless it is programmed well. At a bare minimum, for cooler climates, a once yearly 20 hour capacity test to 10.5V should be conducted. In warmer climates, defined as average battery temps above 80F, bi-yearly is a much better choice.
“How can I conduct an accurate 20 hour capacity test?”
20 Hour Capacity Test:
#1 Fully charge battery, equalize if possible, then allow it to rest disconnected for 24 hours
#2 Make sure battery temperature is between 75F & 80F
#3 Apply a DC load that = Ah Capacity ÷ 20 (small light bulbs and/or resistors can work)
#4 Connect an accurate digital volt meter to the positive and negative battery terminals
#5 Start DC load and a stop watch at the same time
#6 As battery voltage drops, during discharge, adjust the DC load to maintain as close to the C÷20 rate as is humanly possible
#7 Immediately stop the discharge test when battery terminal voltage hits 10.499V
#8 Note the hours and minutes of run time on the stop watch and figure your percentage of 20 hours that it ran. This is your batteries current Ah capacity or state of health as a percentage. For example if a 100Ah battery ran for 16 hours it is at 80% of its original rated capacity. Flooded lead acid batteries are considered “end of life” when they can no longer deliver 80% or more of their rating.
#9 Recharge the battery immediately at the 20 hour rate. Follow this up with equalization level voltage and measure specific gravity until all cells match. (not for non-Lifeline AGM or GEL). A long slow recharge can have a slight reforming effect on flooded batteries and can actually recover some lost capacity. It is not uncommon for a battery to have more capacity after a 20 hour test than it had going into it.
“But isn’t 10.5V bad for my batteries?”
A once yearly discharge test, done correctly, is arguably less damaging than taking your battery to 50% SOC and leaving it in that range for a day or two, or the continual PSOC cycling the average boater thinks nothing of when cruising. Regular PSOC cycling is more damaging than a once or twice yearly Ah capacity test done correctly. A proper capacity test simply counts as another deep cycle and can actually revive some lost capacity.
“Twenty hours is a long time, can I conduct an approximate 20 hour capacity test?”
Approximate Capacity Test:
#1 Fully charge battery, equalize if possible, then allow it to rest disconnected for 24 hours
#2 Make certain battery temperature is between 75F & 80F
#3 Apply a DC load for 2 hours that = Ah Capacity ÷ 20 (small light bulbs and/or resistors can work)
#4 Allow the battery to rest for at least 10 hours at 75-80F (24+ hours is significantly more accurate)
#5 *Check the specific gravity or resting open circuit voltage and compare to the manufacturers SG or SOC to OCV tables
#6 Use basic math to determine the approximate Ah capacity. For example, a 100Ah rated battery has been discharged at 5A for 2 hours. This means so you removed 10Ah’s of capacity. If the battery was in perfect health specific gravity readings or open circuit voltage readings should show the battery at 90% SOC. *If SG and OCV only show the battery at 60% SOC then the battery has lost approximately 30% of it’s Ah capacity.
*This is an approximation only and not an accurate Ah capacity test. Variances can be anywhere from 10-18% off an actual 20 hour capacity test depending upon your particular battery.
CAUTION: The only time your batteries should regularly be taken below *12.1V is during a capacity test. For regular house use, at your average house loads, your deepest loaded voltage should ideally not dip below 12.1V or better yet 12.2V. Unless you are running short duration high load device such as an inverter, windlass, electric winches, thruster, water maker etc. don’t let your bank voltage dip below 12.1V.
For certain situations, such as an off-shore passage or open ocean racing, discharging to 70-80% DOD is acceptable provided the batteries receive a proper 100% SOC re-charge as soon as you get to the destination. Discharging below 50% SOC on a regular basis, in a PSOC environment, drastically shortens battery life when compared to 50%.
*Firefly AGM, large format 2V or 6V batteries, & some GEL batteries would be an exception for regularly discharging below 50% SOC.
