monitoring battery state of charge

[Shadow Catcher]

This comes under the heading of what is sufficient to do the job. Deep Cycle batteries lose capacity over time under ideal conditions let alone if they have been discharged too deeply or allowed to lose too much water...
Reading state of charge as voltage is not accurate unless the battery is open circuit, not connected and charging or discharging for four hours.
Therefore monitoring voltage alone is less than accurate and the only really viable alternative is to monitor what current goes in and what current comes out of your battery with something like a Bogart Trimetric meter which does both, sort of. From what I can tell this is like trying to hit a moving target,. battery aging, and who knows what else.
The final question does it ultimately matter all that much that you do go below the 50% magic number perhaps as deep as 20% SOC as long as you fully recharge. The thinking is that you have a limited number of cycles, full to empty say 500, how likely are you to exceed that during the life say 10 years of your trailer out camping/boondocking not on shore power.
I have been contemplating buying an Trimetric ~$200 but I am reconsidering.

[parnold]

I use a Doc Wattson to monitor my usage. Tells me how many watts, amp, amp hours etc. It has been interesting to see what the various appliances/lights actually draw.

My typical use in hot weather (say fantastic fan) is three days to 50%. In moderate weather I can probably go 8 days to 50%. Knowing that, I will not bother to install a meter in trailer #2, but will add an 80 watt solar panel.

The deeper you draw down your battery, the less "cycles" you'll get out of it. will that matter in a limited use trailer? I dunno.

[fromeo]

This comes under the heading of what is sufficient to do the job. Deep Cycle batteries lose capacity over time under ideal conditions let alone if they have been discharged too deeply or allowed to lose too much water...
Reading state of charge as voltage is not accurate unless the battery is open circuit, not connected and charging or discharging for four hours.
Therefore monitoring voltage alone is less than accurate and the only really viable alternative is to monitor what current goes in and what current comes out of your battery with something like a Bogart Trimetric meter which does both, sort of. From what I can tell this is like trying to hit a moving target,. battery aging, and who knows what else.
The final question does it ultimately matter all that much that you do go below the 50% magic number perhaps as deep as 20% SOC as long as you fully recharge. The thinking is that you have a limited number of cycles, full to empty say 500, how likely are you to exceed that during the life say 10 years of your trailer out camping/boondocking not on shore power.
I have been contemplating buying an Trimetric ~$200 but I am reconsidering.

Another option for accurately measuring state of charge is a battery hydrometer. Mucking with acid is not on my list of fun things to do, but I have a considerable investment in batteries and I do dip mine on occasion to keep track of how they are doing.

As far as depth of discharge vs. life expectancy, you can find plenty of data online illustrating this. There is generally a pretty steep decrease in life expectancy when you start routinely dipping below that 50% mark, which is why it has become a bit of a rule of thumb. The most important thing to do is never leave the battery partially discharged for any longer than absolutely necessary.

I don't have first hand experience with the Trimetric, but I have heard good things.

I do own a monitor from Victron Energy, which operates very much like the Trimetric. I have an older version of these here: http://www.victronenergy.com/battery-monitors/bmv-600s%20and%20bmv-602s/

Does it work? Yes. Does it have it's quirks? Yes. Discharge tracking is great (and very accurate). The State of Charge reading in percent is great, because anyone can make some sense of it. Where I see the quirk is on recharge, it lacks in this department and is too quick to jump to 100% SOC (very prematurely!) if the charge voltage fluctuates a bit.

The Doc Wattson/Watts Up style of meters are cool and will provide a rough interpretation of what you have used. The catch is that battery capacity is NOT linear and depends greatly on discharge current (read up on the Puekert effect for more info), and those two products don't do any Puekert correction. If all your loads are exactly at C/20, the usage reading will be dead on. If the load is less than C/20 the meter will say you pulled more power out of the battery than you actually did (not really a bad thing in our cases). Where you could run into a "serious" issue is if your loads tend to run over C/20, in which case the meter will say you pulled less out of the battery than you did, thereby making it easy to discharge a lot further than you thought.

Another catch with those style meters is that they are not bidirectional, so they have to be swapped around to track charging. Since you have to swap them around, they wont keep a running total, you have to do that yourself. Battery charging is also non-linear, so you have to be concious of the fact that you will have to put more in than you took out in order to wind up at the same place. Finally, they have to be wired in-line, while i prefer a remote shunt.

Please dont take this as me knocking the Doc Wattson/Watts Up meters. I think they are neat devices at fairly reasonable prices. They just dont fit my definition of the "perfect" battery monitor. To be fair, the high dollar ones don't fit it either (as per my complaints above)!

So, anyway, one of my (too many) hobbies is electronics. A few months back I started putting together a design for my own battery monitor, one that does the things I feel are important. Prototype hardware has been built (pictures here: http://gallery.shadetree.org/main.php?g2_itemId=8081), and I've been working on the software when I have time. If anyone is interested in this and has some (atmel) microcontroller experience, feel free to chime in. I'm happy to "open source" the project if people have interest. FWIW, parts cost is under $40.

- Frank

[GuitarPhotog]

A battery hydrometer won't work with sealed or AGM batteries.

But as Paul says, is there any real possibility we'll exceed the charge/discharge cycle limits on our occasional-use teardrop trailers?

As an electrical geek of the 2nd order , I looked at installing a digital ammeter in my electrical panel when I built it, but decided that it would be showing me information I have no real use for. One of the maxims of my work was "don't collect data you aren't going to use" so I decided I could do without knowing my rate of charge/discharge.

<Chas>

[Shadow Catcher]

Frank I am interested!

[eamarquardt]

The Trimetric is kinda cool. It has only one shunt (500 amp which is pretty hefty IMHO) but is able to differentiate between current flowing into the battery and current flowing out of the battery. It does this, I assume, by sensing the polarity of voltage of the shunt.

It can be installed in a double gang electrical box so it's compact.

http://www.bogartengineering.com/products/TriMetric%20

Cheers,

Gus

[Shadow Catcher]

I understand what the Trimetric does and how, but back to the question is it in any way justified in additional expense. I think you know I am a not shy in using the bright, shiny and neat but short of abusing the battery.
I have one of these.


and one of these


which can be plugged into the fuse line to monitor amps, useful in spotting faults and finding consumption.

[eamarquardt]

I understand what the Trimetric does and how, but back to the question is it in any way justified in additional expense?

Probably not, but sometimes it's fun just to have something cool that you can play with. Upon seeing my shop (which is my biggest hobby that does contribute to keeping my expenses down but I don't do "jobs" for money) a friend asked if it pays for itself. My reply: "No but neither does a Winnebago!" He then understood.

Cheers,

Gus

[smatthew]

also check out the Victron BMV-600S. They just came out with the new "S" version - i believe it's more precise. It's a very nice battery monitor that does voltage, amperage flow (charge/discharge), state of charge, time to go at current rate of discharge, settable alarms, etc. Can be hooked up to a computer to download data as well.

Around $150.

[bdosborn]

I understand what the Trimetric does and how, but back to the question is it in any way justified in additional expense. I think you know I am a not shy in using the bright, shiny and neat but short of abusing the battery.

Is it a justified expense? Depends on how you look at it. Even though a battery is just a 70# hunk of lead, its surprisingly delicate and expense. I tend to be a worrier and the our Trimetric gives me instant peace of mind. I also have a higher electrical load than most because of our compressor fridge and I can run the batteries down quickly if I'm not careful. OTOH, I never ran the battery below 50% using just a voltmeter in teardrop but it was a much smaller electrical system than we have now.

For me its like the difference between determining how much gas I have in the motorcycle and the truck. I have a gas gauge in my truck that tells me to the gallon when I'm going to run out, which is pretty handy. I watch the odometer to tell when I'm low on gas on the motorcycle and that works good enough to keep me from walking.

I will say that I love the Trimetric and will have one in every camper I own from now on.

Bruce

P.S. A Doc Watson is a fairly inexpensive alternative to the Trimetric. It doesn't keep track of total battery amp-hrs in and out as well but it will keep track of the amp-hrs you've used.

[PcHistorian]

I checked out the Victron BMV-600S on-line.
quick search for lowest new price was $145. (add shipping and handling... (see bottom))

Another cool feature, besides accuracies in the mA range, is that there is a second input for monitoring a separate voltage, like a start-up battery.

http://www.globalmarinesource.com/victr ... p-934.html

I'm heading back to the charging by tow vehicle topic. (let me know when a used on of these comes around, otherwise I'll have to make one for myself... (sure, I'll put the schematic and parts list here, if I do. :-)

[Shadow Catcher]

Bruce I am also a worrier and our set up is very nearly identical as far as loads go to yours.
One of the replies I got from the Wind & Sun solar forum however struck a thoughtful note is changing my thinking about this.

"If this is a weekend/few months a year worth of use trailer--I would be tempted to let the battery go well below 50% stage of charge (as long as it is "quickly" recharged the next day or two). Yes, you may only get 500 "deep cycles" out of the battery bank--But if that is 50 cycles per year (i.e., camping every weekend a year), that is still 10 years worth of cycling--And the battery will probably "age out" much faster than that (3-7 years???). Just don't take the battery below ~11.5 volts (or below 20% stage of charge)--That can reverse a "weak" cell and kill it immediately (and trashing the entire battery)."

It is kind of hard to argue with the math.

[bdosborn]

From what I've read, a golf cart battery is the only type you can expect to get 500 cycles at 80% discharge. Still, it's a god point regarding aging versus cycles. The Trimetric keeps track of total battery amp-hrs, I'll check and see where I'm at. I doubt I'll work my batteries that hard, having them crap out unexpectedly would piss me off.
Bruce

[dh]

From what I've read, a golf cart battery is the only type you can expect to get 500 cycles at 80% discharge.
Bruce

I got a new Trojan golf cart type battery from a guy who was starting up a solar panel installation business. I think its 120AH, I'll have to check. Bartered about a $90 welding job for it. We used the same type batteries when we converted an S-10 to electric back in college.

[Shadow Catcher]

Bruce, my thinking is that I do not really plan on taking the level down to 20% but it makes a point that the battery is likely to age out before any damage from not being totally being obsessed at keeping it above 50% will affect its usability.
One of my concerns in relying too heavily on a Trimetric is that it can not account for battery age in its estimation of DOD.
I am mindful of such things as say binoculars and their quality of optics beyond a certain point human eyes are not as good as what less expensive optics can resolve. It is also like a audiophile sinking many thousands of dollars into a stereo system, I spent way too many years in industrial environments and despite my use of ear plugs my hearing is not what it was.
So where is the divide between "good enough" and "obsessive" particularly in view that our trailers are used rather sparingly?