Smartgauge and ammeter

[GHA]

Well this is fun!!!
What a great piece of kit the victron smartshunt is, didn't take too long to get it talking to the trusty raspberry pi zero, data getting recorded now so will be interesting to see how closely it will be possible to get it and the smartgauge to agree, right now the SG says 95%. About 22:23 you can see the current draw drop a little when I turned the laptop brightness down a little
Would dearly love to do a proper capacity test but not easy, constant current not so hard with a variable load controlled from the Pi but constant temperature might be a step too far on a boat.
But so far having a smartshunt and a smartguage looks to be a huge step forward in knowing just how tired the batteries are getting on a cruising boat, the 2 agree pretty closely in the mornings with the smartshunt set to 209AH for T105's which suggests still over 90% capacity, over 5 years old so doing OK so far Such fun!!!

 

[Hoolie]

I also use SmartGauge to control the battery ventilation fan. The fan also runs when the fridge is running as on our boat the two spaces are next to each other.

 

[GHA]

And another thing to bear in mind, very good chance your bms is showing 100% too early, the smartshunt with default efficiency settings thinks they're back to 100% at just before 12.30, but still they acceptance rate hasn't leveled off. Not so obvious here but earlier in the charge cycle it show just how much of a difference a few hundred millivolts can make to the amps going into the battery as the fridge cycles and pulls the volts down a little bit. .

 

[sailinglegend420]

Charge efficiency also varies greatly at different SOC for many of the same reasons

Charge efficiency has been mentioned twice now and it is a very good reason why battery monitors cannot be 100% accurate - no mater what the age of the batteries.

Battery Charge Efficiency is not to be confused with battery Charge Acceptance which is the amount of current that a battery will accept at a given voltage and given state of charge. As the SoC gets closer to 100% the current into the battery gets lower and lower - and the time to reach 100% gets longer and longer.

Battery Charge Efficiency is how much of that current going into the battery is actually converted to stored Ah. A typical default value for Battery Monitors is 85%. Wet Lead Acids can be anywhere from 70-85% and AGMs can be as high as 98%. Battery efficiency is very non-linear, so at 50% State of Charge it may be 95% for most batteries, but above 85% SoC it may be less than 50%. This means with every 10Ah input only 5Ah get stored. This gets much worse the closer to 100% SoC you get, which is another reason why it takes so long to charge the last 2-3%. So if you want to store 100Ah in an FLA bank with an 80% Charge Efficiency then 125Ah has to be input. (80% of 125 is 100).

But this value is used as the average value in your battery monitor when discharging to 50% and then back to 100%. If your battery bank is large and a large solar array maintains you SoC, then you could be regularly cycling your batteries from 100% down to only 80% each 24 hrs so the Battery Charge Efficiency should maybe set as low as 50%. If your SoC changes due to weather or seasonal variations then the programming of the BM may need to be changed often. With my 1050 Ah bank that was the final solution to getting my BM to synchronise with my SmartGauge.

Another very good reason why SmartGauge is always much more accurate than a shunt-based BM.

Graphic showing research done by Sandia National Laboratories, Photovoltaic System Applications Department: