This happens to be the set up screen for a Victron BMV-602 for the Charge Efficiency Factor or CEF. Here it is set for 90% which means it will calculate for 110% of the ampere hours that were removed, being returned.
Coulombic efficiency is the charge efficiency of your battery. In a perfect battery we would take 50 amp hours out and put 50 amp hours back in and this would be a Coulombic efficiency of 100%. With lead acid batteries it does not work like this and just like the Ah capacity changes with age, so does charge efficiency. Coulombic efficiency, like Ah capacity, is also a moving target.
Even the most charge efficient lead acid batteries often require 110% of removed capacity to be returned, and some batteries require 130% or more. As batteries age, and sulfate, charge efficiency can get worse. This means that if you use 100Ah’s you need to put back 110Ah’s to 130Ah’s plus in order to reach full charge.
Another issue with charge efficiency is that it’s different at different stages & different points in the SOC curve. Coulombic efficiency is also impacted by battery temperature as well. In bulk charging, when the battery is taking constant-current, the charge efficiency can be pretty close to 98-99% for lead acid batteries yet the last 5% of the SOC curve can see charge efficiencies at less than 50% efficient.
If you discharged to 50% SOC then replaced 20Ah’s, while never attaining a limiting voltage, you could take almost all of that 20Ah’s back out of the battery and wind up at the same exact SOC. BULK charging is a very efficient stage of charging, but absorption is not.
Sadly most Ah counters have no way to discern a difference between bulk and absorption or the gassing stages of charging where Coulombic efficiency is the worst.
"Charge Efficiency Factor: This is the traditional method used by the battery monitor, and the method used by some other similar monitors. This counts the amp-hours discharged exactly, at 100% rate, but when adding amp hours back (charging) they are counted at a lesser value, for example only 94% of actual charge. This requires that to make the “battery % full” display go back up to 100% charged, slightly more charge (6% more in this case) must be put back compared to what was discharged. This is the effect produced by the “efficiency factor” setting which is set at the factory to a recommended value of 94%, however you may program this to any value you wish from 60 to 100%."
Please let that quote from a battery monitor manual sink in. If you understand anything about charge efficiency you recognize that it's not the same throughout the SOC curve. By applying a fixed percentage as the Ah's are counted back up results in counting errors in calculation if you don't go all the way back to 100% SOC. On boats we also use different charge sources with different charge rates all of which impact the overall charge efficiency. What if you stop charging in bulk, which is darn near 100% efficient, but the monitor has already counted your SOC back up with a 6% or 10% handicap?
Sort of like Peukert, the rate of re-charge current can also cause changes in charge efficiency. A low charge rate of 4% of battery capacity, 4A for a 100Ah flooded battery, can have a better overall charge efficiency than 25% of capacity or 25A for a 100Ah flooded battery. Charge efficiency also changes with battery chemistry such as GEL, AGM or flooded and changes as the batteries age.
Unfortunately the typical Ah counter cannot track nor can it calculate for any of these variables in charge efficiencies. To further compound the issue many boats have multiple forms of charging, some with low rates such as solar and some with very high rates such as alternators.
With a Coulomb counter we simply need to set the charge efficiency the best we can. Some Ah counters can set CEF automatically and I strongly recommend using this feature but it relies on you getting back to 100% SOC regularly and can quickly get out of sync during PSOC cycling.
A charge efficiency factor is often set as 75%, 87%, 90% etc., if you choose to do it manually. This means a battery with a factory return ampere hour number of 110% gets set at 90%. Is this perfect? No, but it is the best we can do. All lead acid batteries require more ampere hours to be returned than were taken out.
Unfortunately Ah counters are not programmed to know the difference between BULK/constant-current and ABSORPTION/constant-voltage charging as related to the charge efficiency settings. The higher you go in the SOC range the less efficient the conversion of energy is. Charge efficiency is not simply 110%, 115%, 120% returned, at all stages of charging. The Ah counter only has one number to apply and that is the charge efficiency you set it to apply or that it has chosen to apply. This charge efficiency factor is applied regardless of where you are in the charge state, BULK or ABSORPTION, what the battery temp was, and regardless of rate of re-charge in current. It also ignores where you stopped charging in the SOC curve so PSOC cycling can throw a monkey wrench into the applied calculation...
Unless you do a full recharge cycle every time you discharge the Coulombic efficiency setting will lead to counting errors when calculated to reflect SOC. If you partial state of charge cycle (PSOC) in BULK, when off cruising, these errors can add up pretty quickly.
Testing for charge efficiency is considerably more complicated than Ah capacity so best to simply use the battery manufacturers stated charge efficiency factor or let the Ah counter calculate for you by discharging more than 10% of capacity and then doing a full 100% re-charge (Victron, Link's etc.). Many battery makers don’t publish a charge efficiency number so before buying batteries make sure you can get these numbers. Here’s a tip, don’t buy marine batteries from a company that can’t provide you a Peukert’s Constant, a general charge efficiency factor or the 20 hour Ah capacity figure.
Here’s an example of why a single number charge efficiency setting will lead to counting errors or drift in an Ah counter..
Lifeline Battery – “The amount of energy necessary for a complete recharge depends on the depth of discharge, rate of recharge, and temperature. Typically, between 102% and 110% of the discharged ampere-hours must be returned for full recharge.”
A single number for charge efficiency, such as 85% or 90%, is just not realistic in terms of overall accuracy. However that is what the battery monitor is set up for, and what we have to work with, so do your best to get it as close to accurate as you can. Finding this number may involve a call to your battery manufacturer. The Charge efficiency setting will cause out of sync events with an aging battery, no way around this, just do the best you can to ascertain the best charge efficiency suggestion for your batteries. Again if the Ah counter offers automatic CEF settings, I suggest using it.
WARNING: Not all battery monitors allow you to program for charge efficiency and they may try to internally apply a preset efficiency to all batteries. Be careful with monitors that leave this important programming option out or ones that do not offer an automatic CEF calculation.
I will leave charge efficiency with this quote from a US Government study of flooded lead acid batteries. In this test it was a Trojan G-31 flooded battery. Think about how an Ah counter would be calculating for CEF if you cycled your battery in the top 30% of the batteries capacity meaning 100% SOC to 70% SOC or say 60% to 90% SOC...
"Preliminary results agree well with established general understanding that the charge
efficiency of flooded lead-antimony batteries declines with increasing state-of-charge, and that charge efficiency is a non-linear function of battery state-of-charge. These tests indicate that from zero SOC to 84% SOC the average overall battery charging efficiency is 91%, and that the incremental battery charging efficiency from 79% to 84% is only 55%."
Think about that statement and how a single number CEF could impact your bank if you shallow cycled it in the top 30% of the SOC range.