battery discharge monitor question

[artist]

I'm heading off shore and want to be able to monitor the discharge of my 225AH marine gel 8D house battery. Here's an idea and I was wondering if anyone thinks it might work. I am trying to avoid installing a bat discharge gauge and was hoping to be able to tell the depth of discharge by voltage. I was thinking of hooking up a pre-determined DC load to the fully charged battery and timing it to remove 110 amps (50ish%), then recording the voltage. Then recharging and doing the same removing 45 amps (leaving 80%) and recording the voltage. If I operate between those two voltages (50%-80%), I should be Ok right?
Any feedback would be appreciated... Thanks!

[sailorboy1]

Unless you can compare the voltages at a given load that is the same each time and know the previous loads were the same then the voltage isn't really telling you much.

with battery monitors being fairly inexpensive if you really want to know what your batteries are doing you should get one

[jeepbluetj]

Or, just look up voltage as relates to SOC in one of the tables available for gel or flooded batteries. (And is it gel or AGM -- as that's different too)

But voltage is a pretty poor indicator - it's generally taken "at rest" - and a battery with any load on it will always measure a lower voltage than one that's been resting for 12 hours.

So use a battery monitor (that monitors current out/in)

Or a swag. Take a good estimate on your loads and recharge when you're around 50%. If your running lights take 2a, and you leave em on for 12H, thats -24ah right there. Tougher to average things like autopilots that the current draw will change with sea state. -- Voltage coupled with some decent guesstimates can leave you with a decent enough idea of SOC to know when you need to recharge.

(If it was a flooded battery, a hydrometer is quite accurate, but won't work at all on a Gel )

[noelex 77]

What you are suggesting is possible, in fact it is similar to how I used to monitor the batteries on my boats before battery monitors became available, or at least cheap enough to be practical.
However it is not without its problems. The biggest one is that the voltage measurement needs to be taken with zero charge/discharge, preferably for 24 hours, or more. This is not practical if you are using the boat.
In practice is you have a consistent load the measurements can still be useful.

I used to measure the battery voltage in the morning before the solar panels had any output. The load overnight was constant and the same from day to day (the anchor light). A rising battery voltage, compared to previous days, indicated my solar output was overall greater than discharge and vice versa. There are tables that make some compensation for the discharge current, but It was difficult to quantify the exact SOC. Still better than nothing.

There are other problems, the voltage reading needs to be adjusted for temperature and also needs to measured with high accuracy and precision.

These days I use a battery monitor. They have become readily available and provide more accurate and easy information on the SOC. The better information leads to better battery management and a longer battery life, offsetting the cost. The AHr counting feature is the most valuable, but the current display helps show you where the power is going and the effects of turning down screen brightness on the laptop etc. A quick glance before going to bed will warn if you if you have inadvertently left something on.

For most cruising boats a battery monitor justifies its cost.

[artist]

Originally Posted by sailorboy1

Unless you can compare the voltages at a given load that is the same each time and know the previous loads were the same then the voltage isn't really telling you much.

with battery monitors being fairly inexpensive if you really want to know what your batteries are doing you should get one

I see what you mean, Everything would have to be shut down to get an even comparison. Looks like a Bat level gauge is in my future ... sigh, more money.

[smac999]

every boat should have a battery monitor. they are so nice

[tuomas]

Definitively install a battery monitor. Easy to install, accurate, most can account for the effect of current drawn to remaining capacity (Peukert's law) etc. They themselves consume only some milliamps so you can leave them on. Prices range only between 100-200 euro.

I installed mine last summer and immediately started to wonder how I could have managed without one for so long.

btw, this is a good opportunity to update charging systems too, as some of these alternator to battery chargers (like sterling) have a monitor included.

[Dockhead]

I'll take a slightly contrary position and say that amp-counting battery monitors are somewhat useful but by no means any panacea.

The state of charge displayed as a percentage is very hard to calibrate so that it says anything useful -- the practical capacity of your batteries is always changing, a boat which is not often on shore power will rarely reach a true 100% allowing for a reset, Peukert is not perfectly considered, etc., etc., etc.

I eventually stopped looking much at the % discharge screen of my Victron battery monitor, and started looking more at volts, applying some interpretation. I think if you have a reasonably big battery bank and you look at volts where there has not been a heavy load for some hours, then you will have a better indication of state of charge than your battery monitor can calculate. Looking at the amp/hours used screen can also be somewhat useful, but without applying a Peukert factor, that is also not necessarily telling you all that much about your state of charge.

For really accurate state of charge data, you need to do a specific gravity test. But for day to day use, voltage is actually ok, I think, if you measure it properly and interpret it carefully.

One good thing about using voltage, is that errors will be on the conservative side. So when I get up in the morning and see 24.5 volts, I know I need to charge. That would be a 60% charge if it were a true open circuit measurement. Since there's been a little load on it, I know that it can't be less than 60%. But it's not going to much more, either, since the load will have been a tiny fraction of C. That is more than close enough for our purposes, I think. If I don't really want to charge right at that moment, I can wait and watch for the voltage to hit 24.2 -- which would correspond to 50% if it were measured in true open circuit state. But what do I care if it's 53% or 55%? Why do I need to measure it so precisely, if I'm sure at least that it's not less than 50% SOC? I feel pretty sure that this is at least as accurate and probably more accurately than the % SOC calculated by amp-counting battery monitors.

Have a look at the Smart Gauge -- which does not count amps, but uses voltage and analysis of voltage patterns to come up with % SOC. I am betting that this actually is more accurate than amp-counting battery gauges. My Victron battery gauge has been broken for more than a year, and rather than repairing it again, I am thinking about installing a Smart Gauge, although the voltage measurement I have been doing from the system voltage function on my chart plotter has actually been quite satisfactory for the last year.

[Oceanride007]

There is another method that I was taught in Marine Electrical Practice, it required Charge discharge curves, so If I remember right vertical axis is volts, Horizontal is % charge. There were several curves representing Amps 5, 10, 15 etc one set for charging another set discharging (difference is Peukert) anyway, these curves are a hysterisis shape and you can create your own by doing discharges/charge at different rate an ordeal I know. Of course there should be allowance for temperature.
The point is these and SG were the bees knees before amp hr meters, now, I reckon that the manufacturer has them and were once publicly available. Now, no doubt commercially confidential.
Anyway just an idea, personally I would want this info as a backup to the Amphr Meter. If you interested, I could dig up a old text book for a image.

[sailorboy1]

I use my BM more for the amp-hrs/out more than the SOC. If the battery voltage is way lower than expected for the amp-hrs out I start to question things.

Though I will say that since I installed solar the SOC and the amp-hrs track pretty well.

[btrayfors]

I agree completely with Dockhead: a battery monitor is no panacea. I do things aboard Born Free much as he does, and have no plans to install a battery monitor.

For many years, the constant monitoring of house battery voltage via a digital voltmeter mounted for easy visibility anywhere in the cabin has been all I need to ascertain approximate State of Charge (SOC) and to support decisions on when and how much/how long to charge my 675AH house battery bank.

HOWEVER, I'm a working marine professional dealing constantly with marine power systems, batteries, chargers, inverters, and the like -- and have done considerable research on battery behavior over the years. This ain't for everyone.

Monitoring via the "voltage" route requires some knowledge, thinking, mental calculation, and an intimate knowledge of your boat's electrical setup and workings. Dockhead's long explanation is right on, but many cruisers will not want to "do the math", preferring to simply glance at a battery monitor giving them a calculated SOC reading. These CAN be fairly accurate, but they can also be wildly and increasingly misleading, too, as your batteries age and after repeated "resets" when the batteries are not brought to a truly full charge condition.

Either way -- voltage monitoring or SOC monitoring via a battery monitor -- you'll still have to learn something about your boat's electrical system in order to realize satisfactory results.

Bill

[sailinglegend]

Originally Posted by Dockhead

I'll take a slightly contrary position and say that amp-counting battery monitors are somewhat useful but by no means any panacea......
Have a look at the Smart Gauge -- which does not count amps, but uses voltage and analysis of voltage patterns to come up with % SOC. I am betting that this actually is more accurate than amp-counting battery gauges....

I have a SmartGauge and a BEP battery monitor which has become highly inaccurate now that my batteries are 10 years old. The SmartGuage is proving to be very accurate. It learns the battery status and becomes more accurate the more it "learns" - all just by measuring the voltage, which it does over one thousand times a second.

It was the UK military who finally demanded more accurate battery monitors - so enter the SmartGuage. Because it doesn’t use a shunt, it is cheaper and very much easier to install as only two wires need to be connected. It also has some sophisticated alarm functions with a relay that can control a Genset. If you get a SmartGauge do take the trouble to instal a relatively cheap ammeter/Ah counter to keep an eye on loads and charging currents.

[btrayfors]

SailingLegend,

Yep! Exactly what I -- and I assume Dockhead also -- would expect.

Smart voltage monitoring, either by humans or by a device such as SmartGuage -- is likely to be more accurate than dumb (or casual) use of a SOC battery monitor.

A better battery monitor with a very simple hookup...what's not to like? Unfortunately, the price. This side of the Atlantic it's marketed by Balmar and the street price is about $300.....almost twice the price of the Victron BM600 SOC battery monitor. Oh, well :-)

Thanks for posting your experience with these two types of battery monitors.

Bill

[Dockhead]

Originally Posted by btrayfors

A better battery monitor with a very simple hookup...what's not to like? Unfortunately, the price. This side of the Atlantic it's marketed by Balmar and the street price is about $300.....almost twice the price of the Victron BM600 SOC battery monitor. Oh, well :-)

Wow, Balmar have a lot of nerve, don't they? The full list price from the maker in the UK, including 20% VAT, is 150 pounds or about $240, so $200 or so without the VAT. I think you can order it direct from the manufacturer, and VAT won't be charged on an international shipment.

Even at $300, it won't be, I think, much if any more expensive than the Victron, if you consider the much more complication installation with installation of a shunt into your main ground cable, running wires, etc.

I am thinking more and more to order one and give it a try.

[Dockhead]

By the way, I would think that a perfect battery monitor could be made by combining the Smartgauge approach with data about loads from a shunt -- surely voltage will give you a perfect picture of battery state if you know precisely what loads have been on them for the last few hours.