You really should read the whole article.. http://www.pbase.com/mainecruising/smart_gauge

Counting Ah's is wrought with issues, and has been for many years. I have been trying to help folks use their Coulomb counters more accurately but still they rarely if ever line up to any accuracy that is even within 10% when trying to monitor a max safe discharge floor and cycle the bank within a safe range.

Yes current is a valuable measure to be aware of but Ah's are pretty much meaningless unless you know the rest of the story and how those Ah's directly relate to your banks current capacity, its current temperature, at the rate of discharge etc. etc. etc.. This is why battery monitor makers insert important parameters such as Peukert, temperature, charge efficiency and Ah capacity to give a more accurate relation to SOC..

No need to get rid of the Ah counter, if you already had one, but adding an SOC meter that has some accuracy would be a good investment.. Keep an Ah counter and use it for monitoring current, charging or loads. Ignore the % charged and Ah screens and use the Smart Gauge for the SOC...

Ah counters can count Ah's very, very accurately.

The counted Ah's rarely if ever line up accurately to your bank.

For an Ah counter to be accurate, as related to SOC, is much harder than most folks are willing to work for...

At a bare minimum they need to know:

Bank capacity = ACTUAL not THEORETICAL (CWA)(nl)Battery Temp = Or a known "ballpark" that remains consistent(nl)Charge Efficiency = An Ah counter can't even begin to be accurate without this. (CWA)(nl)Peukert = If this is wrong there is no way it can even get close. (CWA)

*CWA= Changes With Age

On most boats auto-synch should simply be disabled and "known" full re-sets used.

To properly use an Ah counter the bank should ideally be 20 hour capacity tested once per year. This can be done two ways:

Method #1 (at a battery temp of 75F - 80F):

#1 Apply a load of C/20 to a FULLY CHARGED battery and start a stop watch. C/20 is the 20 hour Ah rating divided by 20. So a 125Ah battery would get a load of 6.25A for 20 hours before hitting 10.5V.

#2 Connect a DVM to the battery terminals.

#3 Adjust the load/current as voltage falls to maintain the C/20 rate as precisely as possible.

#4 Monitor battery & load until terminal voltage hits 10.5V then hit the stop watch and record the hours & minutes.

#5 Determine what % of the 20 hours it was able to deliver the 20 hour rate.

Method #2

Replace the stop watch with an Ah counter such as the Victron BMV-600 and count the actual Ah's delivered before hitting 10.5V. What percentage of the rated Ah capacity did you get? Use this as your new Ah capacity and you will be much closer in accuracy. You can also use the Victron to drive a relay and cut the "load" at 10.5V.

*In both methods the "load" needs to remain as constant as possible and requires human intervention to keep it steady as the voltage decays.

*After any discharge capacity test you must immediately recharge the battery bank!

Or just use a Smart Gauge and let it do its thing... The best readings for a Smart Gauge are in the am when there is no charging and the bank has been discharging all night.. You really don't need to know SOC more than once every 24 hours.

Ah counters can be used well, I use one on LiFePO4, but only because I have to. The issues are they need proper programing, proper wiring, accurate capacity figures, temp sensors or accurate programmed avg temp (this can mean multiple programming changes per season for us Norther sailors), and an accurate Peukert.

Unfortunately I have yet to see one boater who has done all of the following to ensure the Ah counter is working as it should.

Why do most Ah's counters appear to work so well?

Most boat owners get very lucky in that they draw the bank at well below the 20 hour rate, due to typical house bank sizing, and get more capacity out of the bank than it has at the 20 hour rate. This means their banks are discharging less than they assume from watching Ah screen only. Conversely it also means they are charging more frequently than they need to. This can lead to efficiency issues if you are not in bulk as long as you ideally want to be. If I had a dime for every boater who thought he/she was at 50% SOC due to counted Ah's yet the battery bank hit absorption in ten minutes, I'd probably be retired.. This is all Peukert related, a subject most books and authors shy away from, but one that is very, very important if you are trying to Coulomb count with any sort of accuracy.

I offer capacity testing for my customers with expensive banks such as GEL, AGM, Rolls batteries, L16's etc.... I have yet to have even 5% of the batteries tested line up with the "20 hour rating" and I have tested well over 100 batteries in the last few years... Last week I capacity tested a pair of AGM batteries, for a new customer, that had 178Ah's out of the 220Ah they are rated for. His battery monitor was still programmed for 220Ah, a Peukert of 1.25 (incorrect) and an average battery temp of 80F... His shunt was also miswired and had at least three wires "sneaking" around it. He, like many others, who don't know the rest of the story, swears his Ah counter was accurate... (wink) His Ah counter will be much better programmed this year but due to wiring and programming issues, among some others, his bank is pretty toasted.

Is the Smart Gauge perfect? No, ideally I would love to see a Smart Gauge with a current screen, but then we are back into more complicated wiring. For the vast majority of boaters who want to know SOC, so as not to go below their safe threshold for the best cycle life, then the SG makes a lot of sense due to its amazing simplicity and rather remarkable accuracy.