Economical Battery Monitor

Bill Roosa · Aug 15, 2011

Battery SOC

All you really need is a volt meter, an ammeter and a graph
The graph is at:
http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

your standard boat volt and ammeter are fine for this and once you get a feel for your consumption (4 amps in your case) you don't even need the graphs any more.

And it is cheap to boot

Maine Sail · Aug 15, 2011

The Victron BMV-600 is about $158.00 and comes complete with the wiring necessary to hook it up. They are about half the price of a Xantrex. These are good monitors and will do and tell you a lot more than a "battery bug" and can work on banks considerably larger than yours, and smaller too....

Here's how to install one:

Wiring & Installing A Battery Monitor

Tim R. · Aug 15, 2011

Just spend the extra $18 for the Victron rather than trying to reinvent the wheel. It is the best value for battery monitors out there.

I have an Ample Tech Energy Monitor II and I never even glance at either my volt or amp meters.

You can do it your way or you can do it the right way.

mitiempo · Aug 15, 2011

As I understand the Battery Bug, after looking at their website, you will need one for each battery. And it will not do as much as a Victron monitor nor as good a job. Buy a proper monitor like the Victron BMV-600 and you will always know the state of your battery bank.

Stu Jackson · Aug 15, 2011

ancientmariner said:
1. That's encouraging, Bill! My original 1979 voltmeter and ammeter are fairly useless, but replacing them suits my budget better than the monitor prices I've seen.

2. I'm still considering a Battery Bug, I've found that they make a 300AH model which I can get for $140. But an accurate voltmeter and ammeter might be a better investment.

1. Not really. If they're still working and you know what you're doing, they should work, BUT, and here's the BIG but: you don't know how hard your fridge is working or when. ONLY a monitor can keep track of that. As far as charging is concerned, what we've learned is that what goes back into the bank takes a lot longer than you think because of battery acceptance - the fuller it gets, the longer it takes to charge as the amperage from ANY charging source, regardless of how big, goes down. Only a monitor will tell you that.

2. Fuggetaboutit unless you wanna waste your $$. The battery bug is a crock. All it does is use a silly algorithm to guess what the amps are based on voltage, which is hogwash and voodoo: what a mix!:cussing:

Do yourself and your new batteries a favor, get a real monitor like the easy-to-install Victron that Maine Sail suggested.

You might enjoy wandering around here, too: http://c34.org/bbs/index.php/topic,5977.0.html

galynd · Aug 15, 2011

I've had the Victron BMV-600 for about 2 years. It works flawlessly. It's a great value and very usefull.

Stu Jackson · Aug 15, 2011

And once you get and install your Victron monitor, to keep your batteries healthy, we suggest you keep them topped off at 100%. The best way is to buy a small solar panel, and avoid keeping them plugged into shorepower on float for extended periods of time. There's lots of battery information in the link I provided earlier, which includes a lot of Maine Sail's contributions and links to other useful information. Good luck, glad you asked.

mitiempo · Aug 15, 2011

Good choice.

Ken Cross · Aug 15, 2011

I built my own battery monitor

I purchased a digital 4.5 digit panel meter from eBay and installed it with a switch to select which battery to monitor. It works great and I think I have about $15 invested. It compares perfectly with my Fluke 77 for calibration

mitiempo · Aug 15, 2011

And does it tell you if you're batteries are at 40% or 60% state of charge like a monitor will?

Stu Jackson · Aug 15, 2011

Ken Cross said:
I purchased a digital 4.5 digit panel meter from eBay and installed it with a switch to select which battery to monitor. It works great and I think I have about $15 invested. It compares perfectly with my Fluke 77 for calibration

That's great. I have an ammeter on our boat, too. It shows when the fridge is on (5A draw) and when it's not (0A). What I do NOT know is how long the fridge has been on, you know, for like times I'm sailing, anchoring, in the cockpit, snoozing... So unless you have a crystal ball, or another ammeter in the cockpit, and are recording every minute of every hour that your ammeter is showing a draw, just how do you know how much is being taken out? And then, given battery acceptance, how do you know how much is coming back in. And also, I'd bet your ammeter isn't wired into your charging sources, only into your distribution panel, like most basic ammeters are, so you don't even know the rate of charging, putting aside battery acceptance. Unless you wired it so it would show, but I don't know how much you do or don't know about wiring ammeters. Don't kid yourself, there's a reason those of us who do know about boat electrical systems recommend these monitors to others. We're not in it for the $$

but we do enjoy helping folks out on this and other forums.

Bill Roosa · Aug 16, 2011

Hey Stu
Did I miss something? You don't know how long the load has been on? Well if it has a switch I'd say that is a pretty silly statement and if it doesn't have a switch then it has been on since installed. Not sure if that is what you mean though.
If you know that the reefer draws 5 amps and take the time to note the duty cycle then you know the average amp draw. If it cycles 20 minutes on and 10 minutes off the you have a 2/3 (on/(on+off)) duty cycle and you draw 3.33 amps effective. The fact that your calculations are "incorrect" over a 30 minute time frame would seem to be irrelevant.
In any case you can go to the graph from the paper I posted and using the C rating read off the SOC directly. Course you have look at the ammeter and use the actual amp draw.

I've been doing this for a number of years now and have never had any problem telling the SOC of the batteries. I guess I'm not that concerned with knowing if it is 76% or 77% charged as that info is pretty much useless for practical applications. What I’ve found is I’m not going to wait till I get to 76.54793% discharged to fire up the motors but if I only get to 76.54794% I will wait. I just wait till dinner time so I can have hot water, draw down the holding plates in the freezer/reefer, make some fresh water and all the other things that accompany running the engine.

Bill Roosa · Aug 16, 2011

And yes my charging devices are not read by my ammeter. This is only a problem if you are trying to determine the amp output of the device. Just wait till your battery voltage reaches the "I'm charged up value” from the chart or more practically wait till after you finish charging to check. My observation is you are not going to charge the batts for extended periods with the engine while at anchor, so just charge them for a while and then make a decision to motor for some additional time while leaving the anchorage the next morning.

I guess my observation is the SOC is handy to know but it does not change how I use the boat unless it gets pretty low (which is easily discernable from the volt meter)

I'd rather have the beer.

monkeybars · Aug 16, 2011

a simple 7 dollar voltmeter from harbor frieght will give you a reasonably accurate idea of the state of charge of your batteries once you learn what your readings should be for different levels of charge.why the concern for how long the fridge has been running?? it must be nice to be able to waste 300 dollars on a toy so i know my cooler ran for an hour and a half yesterday.all you really need to know is whats in your batteries now!!

Tim R. · Aug 16, 2011

Ok, so now we know Bill likes to do things the manual way which is fine but when you have a boat that has many 12v systems that cycle on/off either automatically or manually and also multiple charging sources(solar, wind, engine), having a proper battery monitor is the only way to go. In fact, I keep mine set by default to show the Percent remaining. I find this to be the most useful figure most of the time. I will check amp draw and SOC just so I know how efficiently my panels are working.

If you have AGMs then the SOC is very important in maintaining your batteries for maximum life.

Now my biggest problem is drawing down 520ah to 50% which I like to do monthly for the health of my AGMs. Yesterday was rainy all day and the only draw was my frige, AIS, antenna splitter and VHF. When I got home at 5:15, I also turned on virtually every lighting fixture, instruments, radar, fan, etc and by 9:30pm I had drawn the bank down to 80%. Turned off everything except frige, AIS and splitter at 9:30 and work up to 76%. Today is also cloudy and tonight I will power everything up again to draw down to 50% and then power up my battery charger.

FourPoints · Aug 16, 2011

Stu Jackson said:
avoid keeping them plugged into shorepower on float for extended periods of time.

Can anyone explain this? Why would you not want the batteries at (temperature compensated) float level for extend periods of time, and how do you define extended?

Also how does this correlate to those of us with 3 stage controllers on our solar panels that do run at float stage for most of the day, nearly every day?