Battery Bank Capacity

[KeithP]

I have been a Link-10 user for quite a number of years now and have found it a huge aid in maintaining my battery bank, in particular learning how much individual appliances draw. There is great comfort in knowing how much you are sucking out of the batteries, even down to sitting at anchor and seeing if I reduce the brightness on the TV it saves 0.5Ah. We have a genset on board and generally I never let the batteries discharge anywhere near 50%... well as far as I can guess. Guess being the important point, spending a lot of time away from shore power (home is a river mooring) I start to distrust the Link-10 and don't want to run the genset all day to get back to being confident of being close to full charge. So regular charging is the order of the day.

So recently I've been reading up on the SmartGauge which by all accounts gives an accurate state of charge. Whether this is by magic or not I don't care as long as its accurate, if the military are using it then my feeling is that it must work and that the science behind it must be sound.

Hence we have one device that gives us a pretty accurate measure of amps being drawn and another that gives a pretty accurate SOC. But what would really assist me in planning day to day battery charging is a good knowledge of what my battery bank capacity actually is, given it reduces over time and my batteries are years old now. Knowing my Ah draw is good, knowing SOC is good but neither on it's on tells me whether to recharge before I leave the boat for a trip ashore or wait till I get back.

So, the question that I would like to answer, and I'm sure the answer is far more complicated than anticipated, is this...

If I have both a SmartGauge and an Ah Counter, both correctly wired, assuming the SmartGauge has settled in and learnt the bank and the Ah counter is as well configured as the average boater can (Peukert etc), then would the following hold good.

If I have no current draw and all forms of charging are switched off, then I read the SOC from SmartGauge, say 95%. I then make a note of the Ah count (or zero it) and apply typical current draw (fridge etc) but no charging. Wait until SOC has reduced by 25%, say down to 70% note the Ah count over this period. Can I now say this is approx a quarter of my bank capacity.

Hence my bank capacity is 4 x the amps drawn.

Does that work or, given the complexity of batteries, just fantacy?

If it is a decent rule of thumb then keeping track of bank capacity year on year should be an easy thing to monitor with both devices fitted.

Thoughts?

 

[Bill Roosa]

Determining SOC

If you have an ammeter and voltmeter you can determine SOC directly
See below, C/x is the battery capacity in AH / amps into or out of the bank.

 

[KeithP]

If you have an ammeter and voltmeter you can determine SOC directly
See below, C/x is the battery capacity in AH / amps into or out of the bank.

That seems very simplistic, are those curves affected by temperature? Mr Peukert? etc.

I think Maine Sail did a write up on this somewhere (don't have a link to hand), but I seem to remember that making accurate SOC calculations required the battery to be at rest and the voltage settled. Depending on various conditions such as battery temperature. last load etc, it can take a battery many many hours to stabilise assuming no further load applied. Not sure how long I'll be allowed to switch off the fridge for to get an accurate SOC!!!

And of course SOC doesn't tell me the capacity of the bank.

 

[Stu Jackson]

Ken, I think you're over-thinking this.

Realistically, I have found that any good energy budget is pretty spot-on when it comes to how much energy is flowing OUT of a house bank.

Record of Daily Energy Use of 100 ah per day:

http://c34.org/bbs/index.php/topic,6353.msg41471.html#msg41471

What is much harder to estimate is the amount going INTO the banks when charging because of battery acceptance.

That's where a battery monitor really shines. And most folks with them don't know how to use them, reset them or change the defaults for their own system.

For everyone installing a battery monitor: The "Gotcha Algorithm" thread, a "MUST READ"

http://c34.org/bbs/index.php/topic,4922.0.html

DEFAULTS are factory settings that are made to be modified to suit your setup.

So, if you already know your bank's capacity and can reset your battery monitor by simply reducing the ahs after a few years, there's not much else to do.

The downside I see with battery monitors is that folks tend to get over impressed with double digit decimal places. While these are fairly sophisticated pieces of equipment, they are more of a "good indicator" of the information being presented, rather than a perfect measuring tool.

Treat them with respect, of course, but recognize that they are giving you very good but not precise information because of all the variables. Much better than no technology, but if you can estimate your energy use, then the Smart Gauge would be my choice if it was one or the other.

Now, that said, it comes from decades of boating and electrical system experience. For anyone just buying only a Smart Gauge, I'd encourage them to learn how to do an energy budget first. A guy with a fridge and single 100 ah battery CAN work just fine if he's a marina hopper, not so good if he likes to anchor out a lot.

Your boat, your choice.

 

[Maine Sail]

I have been a Link-10 user for quite a number of years now and have found it a huge aid in maintaining my battery bank, in particular learning how much individual appliances draw. There is great comfort in knowing how much you are sucking out of the batteries, even down to sitting at anchor and seeing if I reduce the brightness on the TV it saves 0.5Ah. We have a genset on board and generally I never let the batteries discharge anywhere near 50%... well as far as I can guess. Guess being the important point, spending a lot of time away from shore power (home is a river mooring) I start to distrust the Link-10 and don't want to run the genset all day to get back to being confident of being close to full charge. So regular charging is the order of the day.

So recently I've been reading up on the SmartGauge which by all accounts gives an accurate state of charge. Whether this is by magic or not I don't care as long as its accurate, if the military are using it then my feeling is that it must work and that the science behind it must be sound.

Hence we have one device that gives us a pretty accurate measure of amps being drawn and another that gives a pretty accurate SOC. But what would really assist me in planning day to day battery charging is a good knowledge of what my battery bank capacity actually is, given it reduces over time and my batteries are years old now. Knowing my Ah draw is good, knowing SOC is good but neither on it's on tells me whether to recharge before I leave the boat for a trip ashore or wait till I get back.

So, the question that I would like to answer, and I'm sure the answer is far more complicated than anticipated, is this...

If I have both a SmartGauge and an Ah Counter, both correctly wired, assuming the SmartGauge has settled in and learnt the bank and the Ah counter is as well configured as the average boater can (Peukert etc), then would the following hold good.

If I have no current draw and all forms of charging are switched off, then I read the SOC from SmartGauge, say 95%. I then make a note of the Ah count (or zero it) and apply typical current draw (fridge etc) but no charging. Wait until SOC has reduced by 25%, say down to 70% note the Ah count over this period. Can I now say this is approx a quarter of my bank capacity.

Hence my bank capacity is 4 x the amps drawn.

Does that work or, given the complexity of batteries, just fantacy?

If it is a decent rule of thumb then keeping track of bank capacity year on year should be an easy thing to monitor with both devices fitted.

Thoughts?

As for the Smart Gauge you simply get used to it. It is nothing any different than the fuel gauge on your car. After a while you know that with a half tank you can go another 150 miles or so. Same goes with the Smart Gauge at 75% you will learn that you have 1 more day or half a day etc....

Unless your Ah counter is very accurately programmed, and virtually none ever are, the -Ah's or time to go screens are misleading at best. (see linked article below)

You can play around trying to get the Ah counter to match the SG but don't bother. Use teh AH counter for informational uses and use the SG for a more accurate level of SOC.

If you want to use loaded voltage you will need to conduct tests and create spread sheets because the loaded voltage in relation to SOC varies with age just as Coulombic efficiency, Peukert & capacity do...

For those who don't really fully understand Ah counters, or how to properly program them..

Programming A Battery Monitor (LINK)

Note: This article is not yet 100% complete (I just started writing it yesterday) but enough to get you a better understanding of how an Ah counter works and what is actually required to keep it more accurate.

Warning that article is long and detailed....

 

[Ron20324]

SOC is just a number, and it "Is what it is". I get more use out of knowing amps used, but SOC knowledge is also nice to have sometimes. I know that on a summer night when we use fans, we us 50 amps. Other times, without fan, 40 amps. This tells me if i have to, i can survive a summer night with only one of the batteries. With a Victron 600 and the solar controller monitor panel, I have had alot more confidence in my power management- and I don't need to worry about using the power.

 

[Don Lucas]

If you want to use loaded voltage you will need to conduct tests and create spread sheets because the loaded voltage in relation to SOC varies with age just as Coulombic efficiency, Peukert & capacity do...
.

This is the reason I'm considering adding a SmartGage even though I have a regular battery monitor (and it is programed correctly except for the capacity and how really knows what that is). With a load on the batteries the voltage is always lower and it is hard to correlate that to the SOC.