Battery charge state from 12v Planet, the definitive!

[Beneteau381]

How to use a multimeter to estimate the SoC of your battery

Good confirmation of what is what and how to check. Interesting that 12.1 volts is 50% and we know that 50% is the max a LA puts out

 

[Minerva]

Whilst that's useful to know an all, this bit;

First, you want to make sure that there are no loads drawing current from the battery, and no charging sources putting current into the battery. Once you've checked this, wait at least an hour before taking a measurement, in order for the voltage to settle and the readings to be as accurate as possible.

Means that using the voltage is pretty impractical for anyone to actually know their state of charge whilst the boat's in use. My Solar MPPT will kick in charging to some degree around 4am 'til 10pm during the summer and there will always be an element of draw from the batteries i.e from the instruments, VHF or fridge anytime I'm on the boat.

 

[Boater Sam]

Whilst that's useful to know an all, this bit;



Means that using the voltage is pretty impractical for anyone to actually know their state of charge whilst the boat's in use. My Solar MPPT will kick in charging to some degree around 4am 'til 10pm during the summer and there will always be an element of draw from the batteries i.e from the instruments, VHF or fridge anytime I'm on the boat.

That is the whole point, you isolate the battery and let it stand to lose the surface charge before checking the voltage. With any charging or load it is totally pointless to use battery voltages to indicate SOC.

 

[wingcommander]

Still doesn't tell you the battery condition though, eg , a 100ah battery 2 years old showing 12.7 volts probably won't give you anywhere near what you'd expect before hitting 12.1 v . A load test is the only way. An old Skipper i used to sail with once commented on how good his charger was at bringing his low voltage bank upto fully charged in only a short period , they even kept the charge overnight ( 12.7v ) at 06 .30 am next morning . Unfortunately upon switching on the navigation lights and plotter, within 30 min after engine was turned off, up popped his head from the companion way stating the plotter had hit its low voltage point and turned off . . ...The battery bank was fubar.

 

[oldmanofthehills]

The predicted voltage is also only the general expected. I was surprised when my van main lead acid battery seemed to settle at 12.3v when fully charged and dropped to 12.1 what with burgalar alarm and crash cam on standby, so asked Battery Specialist to check. He said cranking current a bit lower than new but sadly for him, he couldnt see a reason to sell me a new one. Our liesure battery fully charge reads 12.5v on its associated digital meter, though i have not double checked with my free ranging meter.

 

[roaringgirl]

On most boats it's relatively easy to isolate the battery. Then leave for a bit, and check the voltage. This will tell you the %charge of the battery. If you then find that (say) a 100% charged lead-acid 100Ah battery is down to 50% (ie empty) after 60mins of 10A load, you know that the useable capacity is 10Ah.

 

[Stemar]

I work on the fairly unscientific but practical basis that 12.5v is a decent state of charge under a light load, 12v is time to start turning things off, and if it gets below about 11.8, it's time to start the engine or do something else to put some juice back in. On no load, add 0.2v. It's worked for the best part of 20 years for me.

 

[Sandy]

How to use a multimeter to estimate the SoC of your battery

Good confirmation of what is what and how to check. Interesting that 12.1 volts is 50% and we know that 50% is the max a LA puts out

I looked at that and laughed.

Only one bit of data trying to answer a quite a complex question. I've seen so many people publish all sorts of figures all based on each others view of waving a bit of wet seaweed over a battery.

 

[Refueler]

My test is to try start engine. If it starts the engine ... its load test is OK.

I have two CCA batterys and can use either pr both for domestic and or engine. No desire to split them and have switch this ... switch that .... etc.

 

[Rappey]

Still doesn't tell you the battery condition though

Ancel battery tester! Gives internal resistance which seems a great way to know how much life your battery has left.
3 milliohms when new, 10 milliohms when needs replacing.

 

[wingcommander]

Ancel battery tester! Gives internal resistance which seems a great way to know how much life your battery has left.
3 milliohms when new, 10 milliohms when needs replacing.

Not if your cells are shorted out

 

[Refueler]

Not if your cells are shorted out

People talk about increased iR .. generally as a result of physical reduction of electrolyte recombining ... plates remaining 'coated'. But usual failure of Lead Acid cells is one or more plates buckling due age ..... once this happens - its Doo Doo land !!

 

[PaulRainbow]

On most boats it's relatively easy to isolate the battery. Then leave for a bit, and check the voltage. This will tell you the %charge of the battery. If you then find that (say) a 100% charged lead-acid 100Ah battery is down to 50% (ie empty) after 60mins of 10A load, you know that the useable capacity is 10Ah.

Doesn't always work like that.

 

[PaulRainbow]

There is only one, 100% fool proof way to tell you how good your batteries are (in terms of capacity) and that is a drop test over a few hours.