Batteries SOC - State Of Charge

[goboatingnow]

To be honest determining SOC from terminal voltage is a very hit and miss affair. Even if you leave the batteries stand open circuit ( ie resting ) for several hours the results can mislead you where a battery is ageing and not holding charge.

Voltage metering can give you a “ feel” for how the battery is performing .

Overtime your boat loads are fairly consistent so each day you can build up a personal picture of the the relationship of the volt meter to the battery state , it’s not very scientific or accurate but it suffices.

[smac999]

Originally Posted by sailinglegend

To answer the original post (OP):

The DoD can be estimated by measuring the battery bank voltage,

The op question Was how to determine soc while charging.

But how to measure it when your wind generator and solar panels give you a constant 13,6V load into your network?

So using a volt meter Please tell me the Soc of a battery at 13.6v? 14v? 14.4v?

You can’t. You need an ah counter.

[h2ojunke]

One can NOT get any meaningful SOC data from a voltage reading while under load. As stated above, battery monitors measure amps in and amps out to display an approximate % SOC. This assumes it is set up correctly, all the loads in and out go through the properly installed shunt, and the system is periodically synchronized when the batteries are full. Agreed that marine how to is a great source.

[Dockhead]

Originally Posted by rslifkin

As long as the batteries get topped off periodically to allow the meter to re-calibrate, a meter that knows how many amp-hours have gone in and out, as well as the current amps flowing in/out and voltage, along with the capacity and Peukert exponent of the batteries can produce a pretty good estimate of how charged they are. Certainly one that's good enough for most purposes.

In my experience, amp-counting meters are basically useless, and besides being wildly inaccurate, they tend to err on the reckless side, so much less useful than simple voltage readings. I threw mine away and installed a Smart Gauge, which is supposed to do some kind of sophisticated analysis of voltage, but I see no evidence of that -- it always agrees with my own analysis of system voltage.


To the OP:


I think your state of charge when a charge source is connected is basically unknowable. Nor do you really care to know this accurately?


What you really want to know is when you need to charge. This is easy to know -- provided there is no charge source attached, no matter what the load, if the system voltage is above 12.10, you cannot have less than 50% SOC. If the bank is under much load, you will have more than 50%. But if you make a habit of charging any time you see system voltage under that (or better, under 12.24 for 60%), you can't go wrong. Only caveat here is you must be sure there is no surface charge -- so you cannot have been charging recently.



If you've got a charge source attached, then system voltage doesn't tell you the SOC of the batteries, but you STILL know whether you're OK or not. You can tell by system voltage whether the charging source is actually putting charge into the batteries besides just supporting loads, and you can tell whether the loads are too big for the source and your batteries are being drawn down.


So in my opinion system voltage will give you the information you need to make all critical decisions in operating a lead-acid battery bank.


For more convenience, so you don't have to use a table, you can use a Smart Gauge, but in five or six years of experience with mine, I never found it telling me anything I couldn't know with system voltage and a table.



If for some reason you really need to know how far you are getting, charging on solar, and if your solar charger doesn't give you this information, then you could use an amp-counting meter specifically for this purpose, but NOT relying on it to tell you when you need to charge.



Balmar has a new version of the Smart Gauge which I believe combines amp-counting with the original Smart Gauge principle. I haven't used it so can't comment, but maybe that's the best solution of all.

[Dockhead]

Originally Posted by goboatingnow

To be honest determining SOC from terminal voltage is a very hit and miss affair. Even if you leave the batteries stand open circuit ( ie resting ) for several hours the results can mislead you where a battery is ageing and not holding charge.

Voltage metering can give you a “ feel” for how the battery is performing .

Overtime your boat loads are fairly consistent so each day you can build up a personal picture of the the relationship of the volt meter to the battery state , it’s not very scientific or accurate but it suffices.

I beg to disagree. I would say on the contrary that system voltage IS the most scientific and accurate way to measure SOC.

You don't need to leave the batts standing open circuit -- small loads create small errors which are all on the conservative side.

If the battery is aging and has reduced capacity, this is EXACTLY where voltage shines -- when you calculate 50%, it's 50%, and if you have some loads on, it's "not less than 50%" -- which is EXACTLY what you need to know. It's 50% of a different number than with a new battery, so it's not telling you how much power you have left, but you care about the percentage, not how many amp/hours you have left, when making decisions about when to charge.

An amp-counting meter is what becomes completely useless when batteries age. It tells you how many amp-hours have gone out of the batt, but tells you NEITHER how many amp-hours you have left NOR what % SOC you are at. It will give you can increasingly erroneous % SOC which is an error all on the harmful side.

[goboatingnow]

Amp counting combined with a end of charge trigger ( properly metered , subtracting any loads ) with a proper prukert exponent set and a charge efficiency factor also set actually correlates well with SOC. it’s just too many meter install one shunt and don’t separate charge and load currents and hence don’t reset

Of course this assumes you can fully charge

As I said with experience you can roughly determine some data from voltage measurements

Smart gauge is in my opinion smoke and mirrors.

[Dockhead]

Originally Posted by goboatingnow

Amp counting combined with a end of charge trigger ( properly metered , subtracting any loads ) with a proper prukert exponent set and a charge efficiency factor also set actually correlates well with SOC. it’s just too many meter install one shunt and don’t separate charge and load currents and hence don’t reset

Of course this assumes you can fully charge

As I said with experience you can roughly determine some data from voltage measurements

Smart gauge is in my opinion smoke and mirrors.

Amp counting MIGHT be accurate IF you do it this way, but all the mass market systems use a single shunt.

But PLUS you must know peukert numbers AND real capacity of the bank, which are constantly changing as the batteries age. I believe charge efficiency ALSO changes with battery age.

Another huge disadvantage of amp-counting meters I didn't mention before is that the typical off-grid cruiser doesn't achieve a 100% charge very often. If you go several days of partial charge and discharge, then any amp-counting meter will lose the plot completely as it doesn't get to reset itself to 100%.

So in real life -- single shunt, no re-measuring and resetting battery capacity and peukert every weekend (or at least every month), all the more, frequent partial charging, then no, amp-counting can't be usefully accurate.

Smart Gauge probably is smoke and mirrors. As far as I can tell it just calculates SOC against a lookup table vs voltage, with damping built in to deal with short term loads. It is useless when charging.

However, despite that, it has been tested and proven to be much more accurate than amp-counting meters, during discharge, even with new batteries. The advantage becomes much greater as batteries age.

[rslifkin]

The amp counters aren't perfectly accurate, especially as batteries age. But if you reach full charge often enough for it to reset, they'll get you in the ballpark. Certainly better than trying to measure no load voltage in many systems (many don't ever have truly no load).

The big question is, how accurate do you need? I figure if it tells me the batteries are at 70% when I wake up in the morning, I don't care if they're really at 65% or 75%, as I know they aren't being over discharged or anything. And in my setup, the Victron monitor usually shows 99% (give or take a couple of tenths) when I hit the tail current limit to drop to float, then it slowly creeps up as the current drops off in float and then syncs up to full charge.

If you don't have solar, etc. and are regularly seeing PSOC cycles without hitting full charge, the meters become a lot less accurate. That was pretty much my biggest reason for adding solar: get to 100% almost every day when off shore power, mostly to be nicer to the batteries (but better meter accuracy is good too). On a decent to good day for solar conditions, I'm in float some time between 1 and 3 PM depending on power use and solar performance.

As far as the "12.1 volts can't be under 50%" thing, that varies by battery. I've seen some AGMs that run very hot in the voltage department compared to most lead acid types. As in, draw down to 50% SOC by amp hours and it's still resting at 12.5 volts. At 90% SOC, resting at 13+ volts. So there's no hard rule for estimating based on voltage, as different batteries behave differently.

[goboatingnow]

I would agree where you accuracy measure charge current into the battery , and have the ability to reset the amp counter , the resulting meter is more accurate then a voltage meter because voltage and SOC have a very complex relationship varying with discharge Rate , SOC , temp , age and battery type

In my case I wrote the software for the meter and the charge efficiency is modified as is the peukert exponent based on averaging the recharge amp count to estimate decreasing charge efficiency

No data yet as it’s just fitted so I’ll report back in the autumn when I have some data.

[Dockhead]

Originally Posted by goboatingnow

. . . .In my case I wrote the software for the meter and the charge efficiency is modified as is the peukert exponent based on averaging the recharge amp count to estimate decreasing charge efficiency

No data yet as it’s just fitted so I’ll report back in the autumn when I have some data.

Wow! That sounds like a breakthrough. I have been talking about off the shelf dumb amp-counting meters like the Victron one I threw in the trash, not something like this.


And how about bank capacity? Are you doing dynamic reevaluation of that as well?

[goboatingnow]

Originally Posted by Dockhead

Wow! That sounds like a breakthrough. I have been talking about off the shelf dumb amp-counting meters like the Victron one I threw in the trash, not something like this.


And how about bank capacity? Are you doing dynamic reevaluation of that as well?

Well yes. I have a lookup table

What I do is use the charge efficiency to evaluate battery life , as the charge efficiency drops ( ie less Ah goes back in from a known discharge point ) I can modify peukerts exponent , charge efficient factor and I could add overall AH capacity.

At present the lookup “ degradation “ factors are based on some published data and guesstimates

To really be useful I need to build some data from usage and see what’s exactly happening

I’ve a good battery logging system so I can collect usage data over time.

Whether this actually delivers an improvement will take time to evaluate

( and I’m close to going to Li anyway !!! Where cycle degradation is much less problematic )

[Dockhead]

Originally Posted by goboatingnow

Well yes. I have a lookup table

What I do is use the charge efficiency to evaluate battery life , as the charge efficiency drops ( ie less Ah goes back in from a known discharge point ) I can modify peukerts exponent , charge efficient factor and I could add overall AH capacity.

At present the lookup “ degradation “ factors are based on some published data and guesstimates

To really be useful I need to build some data from usage and see what’s exactly happening

I’ve a good battery logging system so I can collect usage data over time.

Whether this actually delivers an improvement will take time to evaluate

( and I’m close to going to Li anyway !!! Where cycle degradation is much less problematic )

Well, this is brilliant. Sounds like a totally different level of battery management than what we have.


As for me -- I will soon go to lithium myself. I was actually planning to do it last year but then just ran out of time when we unexpectedly qualified for a race. Pretty soon we will forget why we got so involved in this Dr. Frankenstein 19th century technology like lead-acid batteries.

[goboatingnow]

Originally Posted by Dockhead

Well, this is brilliant. Sounds like a totally different level of battery management than what we have.


As for me -- I will soon go to lithium myself. I was actually planning to do it last year but then just ran out of time when we unexpectedly qualified for a race. Pretty soon we will forget why we got so involved in this Dr. Frankenstein 19th century technology like lead-acid batteries.

Yes my boat has a complete custom monitoring system with multiple shunts so I can monitor all charge sources separately and load currents ( broken down by major consumers )

This is all fed to a custom Android touch computer mounted in the belowdecks chart table. ( I use android to remote view my chart plotter ) I have a custom android app that then collects all this data onto an SD card for evaluation by spreadsheet.

[Boatwright]

One advantage of a solar system that is matched to your energy budget comes with the features found in better quality solar controller / chargers.

Our Renogy MPPT controller also has an automatic equalizing mode. The MPPT can increase the charge voltage to 15 + volts for a timed period. This is done at the end of a normal charge for an hour or so once every month. This overcharge will balance the cells in your bank, slows down harmful sulphation, and will extend the life of your batteries.

In my experience boats untethered from shore power that experience short battery life have two problems:

1) Not enough battery capacity. (Sometimes due to using big-box store units that say "Marine Deep Cycle" on the case instead of true deep cycle batteries and confusing "cold cranking amps" with Amp/Hours.)

2) When cruising, the Skipper is burning through the electrons with the nav gear, the Cook has a regular suburban kitchen plugged in to the inverter, the kids are wired, etc..

Combined these two things kill batteries fast and installing an expensive over-sold monitor won't fix either. Understanding your system at a fundamental level takes a bit more effort than looking at a meter but like anything else it's worth doing.

There are big boats with big banks, gensets, radar, water heaters, etc., etc.. Big bucks for big boats means professional design and engineering. A properly engineered battery control, charging, and monitoring system with the right shunts, isolators, etc. is complicated and expensive. If you have to have it, get ready to pay for it. A do-it-yourselfer buying gear out of a catalog is likely to end up frustrated with money wasted.

I like to watch YouTube sailing videos too, but have to keep reminding myself that Cap't Bryan on Delos is an electrical engineer. For those of us on a budget: Stick to the fundamentals and Keep it Simple.

[goboatingnow]

equalizing is great for accessible cells in a wet acid battery , its not a good idea in anything sealed

like most question and answers , it depends is often the situation

DIY is fine if you understand what you are doing. its when you exceed your knowledge that things go wrong , sadly male humans in particular tend to be over confident in their own abilities