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Battery Monitors

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As IT says. Wait 10hrs at minimum, preferably 24hrs preferable before measuring Voltage.

A possible problem you have is called "Surface Charge". More common with Deep cycle, but still possible with a start Batt. This is when the surface of the plate has a charge, but the depths of the plate does not. It can give you a false reading and the reading can even be higher than normal, suggesting the battery is really good, when in fact it is not. There are several reasons why surface charge occurs. One is normal charging coupled with Time. A Chemical reaction is what produces current and current fed back at charging creates the opposite chemical reaction. Plates have a thickness. Very thick for Deep cycle, not so thick for starting and the chemical reaction is thus much slower to take place in the depth of the plate. This is why a good battery monitor that does the calculations for charge replacement is essential. A simple monitor that tallied the current out and summed it against current back in is next to useless. A Voltmeter is eve more useless.
By allowing some time, as IT has said, allows the charge to settle deeper into the plate. The resting Voltage will decrease as this happens. 10hrs will give you OK accuracy. If you are wanting high accuracy, you need to allow 24hrs.

Another way to wipe Surface charge is to turn on a load or wind over the starter for 15seconds or so.
For a failing battery, you may very well get  good voltage reading just after charging has finished. But the battery is flat the next day or does not last long when turning over the starter or what have you.
There are some chargers on the market that have an automatic feature when on shore power, that if the Bank has been in float mode for some weeks, the charger stops charging and applies a load to start a discharge cycle for a short time. Then the charger will go back into a bulk charge cycle again and ck to float. This is because another form of Surface charge can take place. The internal depth of the plate loses charge while the surface of the plate remains at a full float Voltage. By discharging, the surface charge is removed and then the battery can undergo a proper full charge again. If this is not done, then you could go to the boat at the end of winter and find the battery has little to no capacity, even though a Solar panel or shore charger may have kept the Battery all tickity boo all winter.

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For check on SOC if I haven't waited at least 18hrs I use the hydrometer for check.

Apparently not affected by SC.


While we are drifting.......

Update on the SG200 after a couple of  15% discharges. (not really deep enough DOD to be fair)

Fully charged battery bank ie  Tail amps 5.2A  for 464Ahr bank;        Tail amps indicator 



Left 2 weeks with no charger or parasitic loads. (Except SG200 & other BM say 100mA.

V=12.7        SOC 74% :wtf:                 SOH = 90% (4yro FLA battery bank.)

Hydrometer reading  90 to 95% SOC. (Eyesight not great)


This current cycle I'm going to disconnect field on Alt  and wait to a DOD of 50% before repeating.


 Manual recommends tail amps setting to be  0.02C-0.04C. I have set it at 0.15C and will ratchet up incrementally if SOC doesn't fall into line. For my bank I am certain 0.04C isn't fully charged.

I wonder if this, recommended setting, is required to "massage" gauge  SOC. 

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Measuring SG (specific gravity) is a very accurate way to measure the state of charge and as Lateral has said, can be done without having to wait.  However, do be aware that there are many cheap Hydrometers on the market and it is hard to find a "real" one. If you do, they can be expensive. They can also be a tad tricky to use giving a bad result if you do not know what you are doing. There are also temperature compensation sums to do if you are wanting highly accurate readings.

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You mean the BMV712, right?

Good choice compared with Balmar SG-200.

* Cheaper

* More features, BT included onboard, relay for for control of charging at a specific SOC.

* Multiple buttons for input.

IMO, at this point anyway.


I bought the SG-200 because it was dumbed down for  family using the boat.

(Who were disinclined to want to obsess on battery state.)

Set and forget.

Jury's out on that one.



It would be nice to have that SOC relay to kill the field of the alt for that all day motor.

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To check state of charge (SOC)
Disconnect, wait, take voltage and compare with chart voltage versus SOC.

Health is a measure of capacity versus original factory capacity. To do this you need to do a load test.

Semantics I know, but otherwise it all makes no sense..


SOC chart     https://www.canalworld.net/forums/index.php?/gallery/image/8379-battery-resting-voltage-vs-soc/


SOH     https://batteryuniversity.com/learn/article/how_to_measure_capacity



Voltage with surface charge or after load is irrelevant  as it varies.

This is not a precise science,. There are lot of variables so numbers are 


Battery chemistry a major of course.

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