Energy Monitors and true battery state question

[Zanshin]

Here's something that has been bothering me for a while regarding my Xantrex battery monitor (the same issue affects Victron and others). The state of charge is measured by counting Amperes over time, but this seems to inherently have discrepancies.

Amps, by themselves, don't tell you how much energy is being used/supplied without knowing what the voltage is - the classic Ohm's Law formula of "Amps * Volts = Watts" applies here. Since the voltage on a boat changes significantly between charging and consumption using just Amps seems to be quite inaccurate.

For example, if I charge my 28V bank using my generator at 60A for an hour (actual values), the voltage goes from 27.8V to 28.8V, so if I use an average of 28.3V then I get a total charge of 28.3V*60A = 1698Watts.

Once the generator is turned off, the voltage under load drops to 25V or so. Thus, if I now draw a load of 60A for an hour (I have an electric galley, so baking bread would do this) then I've actually used 25V*60A = 1500Watts.

So even though my Xantrex would show me gain 60A while charging and then losing 60A while baking break to get a net difference of 0A, in reality I have actually charged 13% more than I removed (ignoring any charging losses)

I'm curious as to why battery monitors don't use Watts (they all measure both Amps and Volts quite accurately) rather than Amp-hours.

[fallingeggs]

I think you spelled out the reason for Balmar's Smart Gauge. It measures the voltage of the bank and compares to known curves to determine SOC (at least that's the guess). Note it doesn't work with LiFePO4, but you don't seem to have them.

See Maine Sail's excellent write up (and store!) here:

http://www.pbase.com/mainecruising/smart_gauge

Enjoy!


Sent from my iPhone using Cruisers Sailing Forum

[pcmm]

Originally Posted by Zanshin

I'm curious as to why battery monitors don't use Watts (they all measure both Amps and Volts quite accurately) rather than Amp-hours.

The equations aren't as simplistic as you lay them out to be. putting energy back into the batteries isn't 100% efficient. there are losses putting energy back into the battery. it can be up to 30% ( thought usually not) And the monitors sort of try to compensate for this. Also when you setup your battery monitor you need to enter the bank size, and a number for puekert. all these parameters affect how the monitor works. I do think the new Balmar smart gauge though does a better job than traditional monitors.

[Zanshin]

pcmm - I simplified matters in the post and didn't include Peukert's values or internal resistance, since they aren't germane to the actual issue at the heart of my question: if gauges only measure Amps without taking the voltage in account (and thus the actual energy moving in and out of the battery bank), then they are inherently inaccurate; but they don't need to be [as inaccurate] if they showed WattHours instead.

[Tikka]

Voltage in case of battery capacity can be ignored. You put 100Amps, if your battery is good you might get 80 out . / in fact how much you get depends on many variables, temperature, current vs time, internal resistance / that no figure would be exact.
I use voltmeter, 0.2 volts is 10% capacity /10.6 - 12.6/ with last 30% not usable, works for me last 20 years.

[btrayfors]

Zanshin,

You can measure watt-hours and all kinds of things with a simple in-line power meter like the "Watts-Up" or "Doctor Watson". These are available for about $40-60 on eBay. I've used mine for all kinds of power measurements....cute, reasonably accurate, inexpensive.

It would be interesting to see how the readings on this type power meter compare to your amp-counter readings. No reason not to put one in line and have a look.

Might also give you an idea of the efficiency of charging.

BTW, I'm NOT a fan of amp counters. I believe them to be inherently inaccurate for all the reasons mentioned. MaineSail's writeup is excellent.

The SmartGuage seems like a much more accurate device if you really have to have one on board.

Bill

[poiu]

I think you make a good point. A corollary of this is that power in a discharged lead battery is lower than the amp indicator implies. Even worse when you consider that for a discharged battery the voltage drop under heavy discharge is huge and therefore has an even lower power capacity. Much lower still than the now under reading amp counter shows. LiPo4 doesn't have these issues to any significant degree.

[TonyNA]

I am testing one of these monitors from VOTRONIC and so far love it.

Dual Battery Monitor Gauge Meter 12V24V RV Marine Boat Solar LiFePO4 Lithium 400 | eBay

The price in USD is also a deal...

[barnakiel]

You do not need the monitor to be 100% bang on other than in some very rarefied applications (e.g. the Solar Impulse). An average cruising boat is not likely one of such cases. Most boat monitors are simple on the chip solutions with only minimum of programming available to the builder.

If you want a nearly 100% accurate monitor, build one with a small PC out of hobby shop components and some hobbyist software. Level: easy.

Another option: look at Victron BM series and link one to a PC or a smartphone based software analyzer.

On our boat with have a Bogart. An outstanding piece of kit. Does everything the manufacturer promises.

b.

[sailorboy1]

I feel that battery monitors tell you most of what you need to know and spending more in order to obsess about them is a false economy. I'm starting year 6 of my house bank that cost me $456 (460AH bank of batteries plus some cables) that I abused for the first 2 years. The batteries seem to be have the same capacity as the first year (close enough that I can not tell). So why would I need to spend $300+ for a better monitor (Smartgauge) to better keep track of them? I just feel I have better things to spend my boat time and money on.

[Dockhead]

Originally Posted by sailorboy1

I feel that battery monitors tell you most of what you need to know and spending more in order to obsess about them is a false economy. I'm starting year 6 of my house bank that cost me $456 (460AH bank of batteries plus some cables) that I abused for the first 2 years. The batteries seem to be have the same capacity as the first year (close enough that I can not tell). So why would I need to spend $300+ for a better monitor (Smartgauge) to better keep track of them? I just feel I have better things to spend my boat time and money on.

I've used various types of battery monitors, and I generally agree with you. After years of experience, I have discovered that your own brain plus a simple volt meter is more accurate than any amp-counting meter, and is equal to the SmartGauge.

That being said -- I did end up going with the SmartGauge . Its only advantage over a simple volt meter (as far as I can tell) is that it gives you some idea of how much charge has gone back into the batteries -- something you can't tell from voltage (although you can learn something about it by watching the charge voltage gradually rise and getting to know what voltage point corresponds to what state of charge). Ironically, that is the one thing which MaineSail says the SmartGauge doesn't do very well. That tells me that the SmartGauge is just doing what we do ourselves when we watch voltage with some intelligence.

BTW, you don't have to buy the SmartGauge from Balmar. It's made in the UK by Merlin, and is far cheaper there. It costs 99 pounds or about $150, ex VAT and shipping. Balmar as usual charges more to stencil there name on it, than the underlying design costs -- same with their Leece Neville alternators.

[btrayfors]

I'm a 'voltage guy', too. I don't have an "amp counter", AKA battery monitor aboard and don't plan to fit one.

However, I do have an analog DC amp meter with shunt which monitors alternator output as well as a digital DC voltmeter. Taken together, with a bit of brain interpolation and reference to hundreds of times recharging my house batteries, these give me a close enough approximation of things to suit my usage parameters.

Additionally, every few months I measure battery health with a sophisticated though imperfect battery analyzer.

None of these techniques is 100%, but I've found them to be close enough for my purposes.

Proof of concept? Well, I've never been surprised by running my batteries flat or by getting less power over time than I had expected. And, battery longevity has been pretty good....7-8 years for flooded LA house batteries before their capacity begins to drop significantly, i.e., by 10-20% or so and I replace them.

Bill

[Dockhead]

Originally Posted by btrayfors

I'm a 'voltage guy', too. I don't have an "amp counter", AKA battery monitor aboard and don't plan to fit one.

However, I do have an analog DC amp meter with shunt which monitors alternator output as well as a digital DC voltmeter. Taken together, with a bit of brain interpolation and reference to hundreds of times recharging my house batteries, these give me a close enough approximation of things to suit my usage parameters.

Additionally, every few months I measure battery health with a sophisticated though imperfect battery analyzer.

None of these techniques is 100%, but I've found them to be close enough for my purposes.

Bill

Yes, I use absolutely all these techniques also.

I have an Argus battery analyzer which I use periodically to check capacity decline.

I've even been known to take SG and compare to what I think the battery SOC is.

I have a digital voltmeter/ammeter which shows system voltage and current out of the battery charger (and into the inverter).

All these bits and pieces of information all checked against each other give you a much better picture of the real situation, in my opinion, then any single store-bought device.

[Maine Sail]

Originally Posted by Zanshin

So even though my Xantrex would show me gain 60A while charging and then losing 60A while baking break to get a net difference of 0A, in reality I have actually charged 13% more than I removed (ignoring any charging losses)

I'm curious as to why battery monitors don't use Watts (they all measure both Amps and Volts quite accurately) rather than Amp-hours.

Batteries are energy storage mediums but they are fraught with issues when trying to accurately determine SOC. Ah counters give lots of good info but SOC is not one of them until someone comes up with a significantly better software for determining SOC.

If you want SIMPLE:

Don't discharge your bank below 12.2V using your average house load as your "bottom voltage".

So if your average house load is 10A when you hit 12.2V at 10A it is time to recharge.

SIMPLE.....

If you want to geek out and ATTEMPT to keep an Ah counter accurate you can read this.

Keeping Your Battery Monitor More Accurate



If you don't understand that article ignore the Ah counter for SOC estimation go back to the above SIMPLE method for when to charge......

[Dockhead]

Originally Posted by Maine Sail

. . .
If you want SIMPLE:

Don't discharge your bank below 12.2V using your average house load as your "bottom voltage".

So if your average house load is 10A when you hit 12.2V at 10A it is time to recharge.

SIMPLE.....

That's pretty much the method I used for years. I have a table laminated and stuck on my nav table elec panel showing SOC vs open circuit voltage vs specific gravity. I just look at system voltage and consider the state of charge to be what it says for open circuit voltage.

That's the wrong way to do it of course -- because open circuit voltage is when the batteries are not under load and have rested for some hours.

I used the following logic -- the error from this will always be on the conservative side -- it means the actual SOC will always be slightly better than what my table tells me.

I got very good results from this system. My bank is large enough that voltage does not sag much from normal loads. I checked it a few times by specific gravity and it always corresponded.

Now I have my SmartGauge installed, and its readings also EXACTLY correspond to what my old method showed. Uncannily.

So do what Maine Sail says! "Simple", as he calls it, is not even less accurate!