Super cheap battery monitor

[Bill Roosa]

Hey folks
I'm working on building a super cheap ($50) battery monitor and need some info as I'm thinking I could turn this into a business. My assumptons are:
You already have volts and amp meters.
You do not need AH displayed but would like to know the state of charge of your battery banks.
You would like to know if the bank is charging or discharging.

Info I need:
max current drawn from your battery bank?
Size in AH of your battery banks?
The distance from your battery bank to the nav station?
Number of banks you have. (1+start)(2+start)(etc)

I'm thinking that a set of LEDs that indicate 50%, 60%, 70%, 80%, 90%, and full charge for each bank and two LEDs that indicate charge or discharg would be sufficient for a bare bones system.

Thanks

 

[Stu Jackson]

Yeah, right... C'mon Bill the "Battery Mon" (or something like that) just came out with something like that, snake oil on toast. % charger? Based on what? Voltage? Sorry, I'm not convinced. Without a shunt measuring amperage on a declared size bank, I'm very hesitant to understand how "% charged" works. I WANT to know amp hours, amps in & out, and voltage, this is very, very important information.

If you already have a volt and amp meter, how could you NOT know whether your batteries are being charged already, other than being blind?

Good luck in your first production run, but I'd like to know how it works, first.

Info I need:
max current drawn from your battery bank? less than 30A ever, mostly around 10, 15 with the fridge running, the water pump on and the macerator running at night with the running lights on
Size in AH of your battery banks? 390 and 60
The distance from your battery bank to the nav station? what does this have to do with it? Nav station? My PDP is 4 feet away from my 1-2-B switch, but the wire run is more like 8 feet. Two house batteries are further away, but connected to the PDP.
Number of banks you have. (1+start)(2+start)(etc) one house bank and one reserve bank.

 

[walt]

I agree about % discharge.. I have the Linklite with some complex "percent" algorithm based on Peukert .. ect and I don't use the % reading at all - dont even know if that button still works. I found it especially useless and inaccurate when my batteries had aged and degraded somewhat. I do find voltage, current and amp hours very useful.

What I want is something which gives me volts, amps, accumulated amps (ie, amp hours) and also "battery impedance". I think battery impedance is way more useful in telling both the short term and long term state of a battery. There is a device called I believe a "battery bug" which gives battery impedance but it must be mounted right at the battery (don't want it measuring the wire resistance). Having to mount it right at the battery makes it not useful to me since its not practical to look at the display.

What I would find ideal might be a module which mounts very near the batteries that includes the shunt and the battery impedance measurement and some electronics. The display and control would need to possibly be mounted some distance from the battery with some simple cable interface to the display and control module - probably with just power and a serial com interface in the cable. If you want to get fancy, a temp monitor to the battery could be added as temp likely affects the impedance measurement. I don't think this device will happen because of patents owned by the different companies..

FYI, Ive got a trailer sailboat - have and need only one single battery (that I know a lot about its state). One single battery "bank" is two six volt golf cart batteries.

 

[sailingdog]

Actually, the majority of boats don't have volt meters or amp meters. And neither is really useful in terms of figuring out what the actual State Of Charge on a battery is. The only way to do that is to have a battery monitor that measures amps in and out of the battery. If I'm going to spend money on something, I'd rather it have some actually useful information. Volts, as a indicator of the SOC of the batteries, are not really reliable... see Maine Sail's post on it. Amps is really useless unless you're watching the ammeter with a stop watch in hand and are anal enough to record the rate and duration reported... a battery monitor, and you can get a good one for less than $200, does all of this and more.

 

[Stu Jackson]

I agree about % discharge.. I have the Linklite with some complex "percent" algorithm based on Peukert .. ect and I don't use the % reading at all - dont even know if that button still works.

Walt, the algorithm uses kWhrs to do all of its measurements. Whether it is reading Amp Hours up or down, hours to go, or % of charge, it is simply doing different arithmetic to show those values, based on the declared amp hour capacity of the bank being measured. If you have a 400 ah bank and it's down 40 amp hours, it'll show 10%. Not too hard. It'll show either the 10% or minus 40 AH. In some cases the algorithm uses 50% of the declared house bank, since you don't want to discharge more than than 50%. Read your unit's manual.

The Peuket's equation stuff has to do with the 20 hour rate of standard discharge, and rarely, if ever, needs to changed or bothered with, especially on our recreational boats with relatively modest electrical draws on a house bank. Not understanding or even bothering with Peuket's Equation is no excuse for not understanding the rest of the monitor's operating functions.

You may be interested in the "Gotcha Algorithm" topic developed by our own Rich Stidger on this board, and I captured it here: http://c34.org/bbs/index.php/topic,4922.0.html

 

[Snotter]

I've always thought that a shunt connected to some USB based hardware and custom computer software would make an awesome app.

 

[walt]

I often check volts, amps and amp hours.

I recently changed my batteries but when they were getting old, I noticed a few things happening. First was that anything trying to charge them would result in a relatively high voltage occurring too quickly either "telling something" to back off on charging or that the charging might be compete when it likely was not. This messes up % full reading based on voltage and from briefly reading the link posted (Rich Stidger), would have also messed up the Liink 20 algorthm since it is also partically based on voltage. Alternatively, I would also see the battery voltage dropping a lot for a given load when the batteries were getting old also messing up a percent reading based on voltage.

I also noticed for the old batteries that my three stage charger (independent of the battery monitor) would think a lot more amps were required to charge a battery than to just get the amps hours net back to zero. It looked like the battery "self discharge" had gone way up from when the batteries were new.

When batteries are new, the percent readings are probably useful - don't know, I got used to not trusting the reading from nursing along my warn out batteries. I would much rather have a "battery impedance" measurement as I believe it would have been useful over the life of the batteries.

 

[Stu Jackson]

This messes up % full reading based on voltage and from briefly reading the link posted (Rich Stidger), would have also messed up the Liink 20 algorthm since it is also partically based on voltage.

Walt, that's simply not true. The voltage is a trigger, not a measurement, and was one of the two things that reported full banks prematurely. You may want reread that Gotcha algorithm post.

I do agree with you that % charge based on voltage alone is relatively useless information, given the technology available today. As I mentioned in a topic about this a year or so ago, it's so "way 70s technology."

 

[sailingdog]

Some of the more advanced battery monitors allow you to do this, but it is rather overkill on most small sailcraft.

I've always thought that a shunt connected to some USB based hardware and custom computer software would make an awesome app.

 

[walt]

When charging, the Link 20 considers the battery to be fully charged when two conditions are met. First the charging voltage must be at or above the entered parameter and the charging current must be below the percentage of the bank capacity that is also an entered parameter. In my case those parameters were 13.2V and 2%. 2% is about 9.2A for my bank of 460Ah.

This is copied from the link and is why I said the % algo is partially based on voltage. I do like the idea of disabling the % full by increasing the set voltage to 15 volts and the current to 1%. Although it doesn't really matter in my case, I'm missing some info by the meter reading full - at which point it appears to stop keeping track of amp hours into the battery.

 

[Stu Jackson]

Walt, I understand. Maybe it's just me being an engineer. That's exactly why I said it is a trigger, not a measurement. The two triggers are the voltage and the % of stated bank AMP HOUR capacity in NET charging AMPS to declare a premature indication of full (other than the kWhr to zero). That % is not the same as the previously discussed % of full, which is simply calculated by the machine as (Full amp hours less remaining amp hours) divided by full amp hours) -- this is done using the kWhrs.

When I say trigger, it's not that the machine is not "measuring" the volts and amps, it's just that the algorithm is doing a "check" on the two and comparing. Maybe measurement was a poor choice of words, of course they are being "measured," but are not being used in the same way that amp hours are being "measured" - maybe "readings" would be a better term, since they are instantaneous, rather than being calculated, evaluated, compared and used for logic.

Whew, damn engineers...

Our Link 2000 does "collect" amp hours positive from charging sources. The Link LITE, OTH, may not do so, since the manual says that once it says FULL, the manual doesn't appear to state that it will show input beyond its reset to full and zero amp hours.

 

[walt]

I think you are correct that the Linklite does NOT read positive amp hours - or at least I dont know how to get it to do this. Im pretty sure the Linklite only displays 0 amp hours and stays there when it determines the battery is full. But if I switch to amp mode, it will still show current going into the battery. The current still going into the battery is through a solar charge controller and solar panel. The solar charge controller must have a different "idea" about when the battery is full vs. the linklite.. Not really a big deal since I just leave the solar charger on all the time anyhow but Id prefer it kept showing net amp hours - even if positive. Still very useful (the amp hour reading that is..).

 

[Bill Roosa]

fair questions

I beg your pardon. I didn't realize the depth of understanding that my audience possess.
I intend to mound a current sensor (hall effect type) on the main negative return and several volt and temp sensors on the individual battery banks. Using an algorithm of my on "cunning" (yea, I know) design I intend to remove all that AH to % charge and voltage measuring madness to a very simple set of 5 LEDs. I agree that a volts only type monitor is not going to cut it as many of the capacity/state of charge functions are depend ant on temp of the batts, past history of use, and the rate of charge/discharge.
I'm not trying to sell snake oil here, what I'm trying to do is give the neophite a really simple and accurate view into how charged the batteries are.
Ask yourself this: Do I really need to know AH or do I just want to know that I'm xx% charged on y bank and zz% charged on bank u.
I need to know the number of banks and size so I can designe DIP switchs that accomidate the vast majority of users. I need to know the max charge/discharge rate to size the current sensor(s)
Thanks for all the imput so far and insights in to what you want.

For the record: I noticed that I use high current devices (windlass, electric winch...) when the engine is running. That means that I'm charging (150 amps if the circuit will take it) the same time I'm discharging. That in my mind is a wash from the engineering standpoint. i.e. it does not realy effect the state of charge much. But a new or non-technical person does not really need to understand all that. They just want to know the bank is still charged (or not). What catches most (guessing here) is the 5 amp refeer and 2 amp anchor light running all night or the running lights during a all night sail.
I really feel that if I can see my current volts (existing volt meter) and the current amp draw (existing amp meter) all I really need is a smart device that can tell me simply what the state of charge is.

I'd be interested in knowing what knowing the AH in the battery is telling you besides the mental math that results in the % charge.

 

[Shell]

Who cares about all of this. A friend spent a little under 2K installed a fancy monitor for his batteries. Shore power off, refrigerator left on. Dead batteries. He should have bought a new starter battery and left it on the dock. At $90 a battery, even buying a new one every year, it would take 22 years before he'd spend as much a the fancy battery led crap.

 

[Benny]

I thought I saw a gadget like that in Walmart Automotive for $14.95. Maybe two could be used, one for each bank.

 

[kenn]

I don't think the cost-effective 100% dependable commercial battery charge monitor has yet been made, so the opportunity still exists. Go for it, Bill.

I do think it's a tough problem for a number of reasons. There's the lack of a single, linear, easily-interpreted parameter in lead-acid battery technology.There's many different sizes, technologies and designs of storage batteries, each with different optimum charge/discharge cycles, load curves, capacities and lifespans. But mainly because no two boaters have the same set-up!

It's a changing field, and I've heard it predicted that in 5 years a newer battery technology could replace lead-acid as the dominant choice for boaters... which means a new chemistry and a new set of curves. In the meantime, battery charge monitoring remains a bit of a black art.

 

[Stroh]

I'm sure you have something more intricate in mind but I thought of this:

http://wingstuff.com/pgroup_detail/...p_list/gl1500/335_goldwing_Battery_Items/des/

 

[Stu Jackson]

Who cares about all of this. A friend spent a little under 2K installed a fancy monitor for his batteries. Shore power off, refrigerator left on. Dead batteries. He should have bought a new starter battery and left it on the dock. At $90 a battery, even buying a new one every year, it would take 22 years before he'd spend as much a the fancy battery led crap.

Heck, one could do that kinda stupid move with or without a battery monitor.

In the meantime, battery charge monitoring remains a bit of a black art.

Not quite. The science simply says that measuring amperage is more effective than measuring only voltage. And what we've learned are two very important things:

1. Do an energy budget. What you'll find, if you can do simple math from grade school, is that any energy budget is usually right on if you know the loads (easy to find from reference sources) and the time used, you can almost estimate to within 10% the amount of energy consumed in a 24 hour period, or a daysail. For most of us, the refrigeration is the "killer app."

2. Battery acceptance severely limits the amount of energy that can be put BACK into batteries. It is NOT linear. like the draw (use) is. It is a completely different thing, and a battery monitor can show you that.

Yes, it still is an art for those who either wish to complicate things, or, like me, don't read the manuals that come with them until after they've installed them.

Really, there are a few variables in the technology of both monitors and lead acid-type batteries, but for the USE of current technology for energy storage on recreational boats, they are a significant improvement over guessing and finding yourself dead in the water..

 

[alexco38]

A good battery monitor $200; an extra battery (decent one) $150-200. Well... why wouldn't I just get an extra battery if space permits?

A decent digital volt meter $50. A battery discharge meter $80. It's hard to make a business case for a battery state monitor business.

 

[Stu Jackson]

Well... why wouldn't I just get an extra battery if space permits?

Because a battery monitor allows you to understand why and how you may be seriously undercharging your expensive batteries. You have the math backwards. A good battery monitor will allow you to maintain your investment in batteries, rather than having to continually replace them because of incomplete charging and letting undercharged batteries die because of it.

And sizing a battery bank is a completely separate issue. Sure, if you have the space add batteries. But if you haven't sized your house bank properly to begin with, and it is undersized, you are seriously over-depleting your existing bank, which kills batteries as much as undercharging.