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Strettonman

Measuring battery SoC

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I am considering fitting a solar panel system and just wondered how you monitor the state of charge of the batteries. I presume there is a monitor I can buy or it is integrated with the controller but how do they work ? Do they measure energy in and out and then estimate the SoC ? If so it guess some will be better than others so does anyone recommend a particular unit ? 

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The best way to guesstimate SoC is to have an accurate voltmeter and ammeter (preferably with amp hour counter). Rested voltage will tell you the state of charge (difficult to do when solar is working) and tail current down to 2% of battery capacity at 14.1+Volts charging will tell you when you have got to 100%. No gauge can be relied upon to tell you accurate SoC at all stages of charge - hence I use the word guesstimate. I would not use a volt or ammeter on the solar controller as it just tells you what the solar is doing.

Get yourself a victron BMV 712 (or one of the cheaper models) or a NASA one which do the volts, amps and amp hour counting....but then you need to learn how to use them. They are not as simple as read the numbers and certainly dont rely on the SoC number they give you. You need to learn how to interpret the V and I readings.

Edited by Dr Bob

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There are two reliable ways to measure SOC of a part-charged lead acid battery:

A hydrometer

The RESTED terminal voltage.

 

Amp hour counting meters are snake oil.

Smart Gauge is OK when discharging, but not good when charging, which will be most of the time you want to know, with solar.

N

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32 minutes ago, Strettonman said:

I am considering fitting a solar panel system and just wondered how you monitor the state of charge of the batteries. I presume there is a monitor I can buy or it is integrated with the controller but how do they work ? Do they measure energy in and out and then estimate the SoC ? If so it guess some will be better than others so does anyone recommend a particular unit ? 

Assuming lead acid batteries, lithinums are different.

 

Any state of charge monitor shoudl be treated with a high degree of scepticism, most tell lies, especially those on solar controllers.

 

The simplest way is to use the rested voltage to infer the state of charge or more practically when you really need to stop using electricity and start charging ASAP.

 

Knowing when the batteries are fully charged is all but impossible with solar charging because they drop to a float voltage so the way we do it with alternators does not work. A charge source going into float in no way means the batteries are fully charged, it just means the charge source computer has decided they might be but they are all too often not. The basic "rule" is that when the charging current at 14.2V plus stops dropping for about an hour the batteries are all but fully charged but the solar would have dropped to about 13.6V long before that point.

 

A properly factory calibrated Smartguage is good for giving fairly accurate states of charge DURING DISCHARGE but is not so accurate during charging but you can do similar with a bit of knowledge and a decent voltmeter.

 

An ammeter gives a very rough idea of how well charged the battries are during charging at 14.2V plus once you get to know typical readings for your own boat.

 

A battery monitor from the major marine electronic suppliers that do show states of charge have been shown over the years to be very inaccurate in the hands of most boaters. Unfortunately their mode of inaccuracy encourages the wrecking of batteries in short periods of time. however if you invest time in understanding lead acid batteries, the monitor, and ensuring it is very regularly resynchronised with the batteries they are better than nothing. If you don't spend the time and effort nothing is far better.

 

Anything that counts Amp hours in will always be incorrect because how many of those Amp hours are concerted to stored chemicals and how many are wasted in heating etc. varies according to battery design, aged and temperature. They also have no way of knowing how much battery capacity has been lost to sulphation so the makers build in fiddle factors that experience show produce errors that are cumulative and overstate the true state of charge so boaters who believe them run with partially charged batteries that quickly suffer sulphation.

 

Sorry, no easy answer. get a decent voltmeter and ammeter (the ones on the battery monitors are accurate). Read the rested voltage at least half an hour after charging stops and with no loads drawing current and again as you turn in for the night and/or first thing in the morning while its still dark. 12.7 to 12.8 volts  implies more or less fully charged, 12.2 to 12.3 volts means stop using all but absolutely essential electricity and recharge ASAP.  Then at least once a week run the engine at 1200rpm or more in late afternoon and watch the ammeter. keep going until it has topped dropping over the last hour and reads between 1 & 2% of battery capacity. then if you have fitted a battery monitor resynchronise it because the batteries will then be as fully charged as possible and given  a reasonably accurate battery capacity input the percentage of charge will be near enough for a week or so. Finding the capacity is another thing altogether because after a week or so it will not what in on the label.

 

The Amp hours out reading on such meters is accurate so fully charge the batteries as above and then zero the Amp hours out. Discharge until the rested voltage reads about 12.25V which is 50% charged but for optimum battery life boaters should consider it as fully discharged. read the Ah out and double it. That is the actual battery capacity near enough.

 

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Wow - thanks everyone for these very detailed responses. As I suspected, it is clearly not a case of buy a piece of kit and all will become clear. I’ll spend a little time today trying to get my head around your advice and will certainly come back with further questions. Thanks very much. 

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Most of the solar charging units will have a state of charge function on their displays.

 

They are completely useless and incorrect.

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25 minutes ago, Strettonman said:

Wow - thanks everyone for these very detailed responses. As I suspected, it is clearly not a case of buy a piece of kit and all will become clear. I’ll spend a little time today trying to get my head around your advice and will certainly come back with further questions. Thanks very much. 

 

Have you found WotEver's Battery Charging Primer that is pinned at the start of the Maintenance section? If not its a good place to start.

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On 02/08/2020 at 10:21, Tony Brooks said:

 

Have you found WotEver's Battery Charging Primer that is pinned at the start of the Maintenance section? If not its a good place to start.

Save you looking...

 

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