Does Voltage = Capacity?

[lawrence_craig]

I am looking to understand how close the correlation is between voltage of a Wet Cell battery is with capacity?

I have a battery monitor (Victron SmartShunt) with all loads and chargers running through the shunt. When discharging at night, the battery bank discharges to around 12.2 or 12.1 yet the SOC is >75%.

Any thoughts? Many thanks

[cal40john]

Most charts that show voltage vs charge state are based on no load on the battery for some time period. Some people say as long as 12 hours previously. If there is load on the battery the voltage will be lower, dependent on how much load.

[StuM]

Yep, voltage is a factor of state of charge and load on battery.

And voltage response to decreased load is not instantaneous, it can take hours for it to drift back to a figure that is actually indicative of SOC.

[Warby12]

I know this is a controversial subject but I do use this measure. One can be tricked by surface voltage: a current or recent load/charge will effect the surface voltage, which is the voltage being measured by our voltmeters but does not reflect the charge right through the plates of the battery. It is "wafer thin". But having said that! if you get to know your own batteries, voltage will tell you as much as any other measure (eg amp hours). If you leave a battery alone for long enough (no loads or charges, say for an hour or longer), it will tell you the charge in the batteries)
Of course re-reading this maybe the question is "does voltage = charge level"? Voltage has no relationship with capacity. A fully charged, old, worn out battery can show a high voltage but have not enough capacity to start a model aeroplane.

[markcouz]

https://batteryuniversity.com/learn/...tate_of_charge


The Victron BMV is a great piece of kit but it is not infallible. It will reliably count amps_in/amps_out and deliver its opinion of SOC based on the maths, but it can quite easily be fooled.


1. Obviously it needs to be given the correct parameters, you tell it the Ah capacity of your bank, if you made an error here then so will the BMV.


2. Batteries lose capacity with time/abuse. So while you may have installed 200Ah 2 years ago, it may have deteriorated and now be 100Ah (the BMV will still provide SOC based on the 200Ah parameter provided).


3. When solar charging the current going into the battery can vary greatly, the BMV may sense end of charge tail current and reset to 100%, when you actually are at 50%.

[chemist]

http://www.scubaengineer.com/documen...ing_graphs.pdf Check out this document

[lawrence_craig]

The interesting thing I find is that according to the Victron my engine alternator can charge it to 100% SOC yet I can see from the solar it never went into float mode.

I guess it is just indicative.

[smac999]

Originally Posted by lawrence_craig

I am looking to understand how close the correlation is between voltage of a Wet Cell battery is with capacity?

I have a battery monitor (Victron SmartShunt) with all loads and chargers running through the shunt. When discharging at night, the battery bank discharges to around 12.2 or 12.1 yet the SOC is >75%.

Any thoughts? Many thanks

voltage = SOC. not capacity. 2 different things. you can have a fully charged battery at 12.8v but it's 15 years old and has no capacity left, and would drain very fast.

if your voltage is 12.1v with no load, and your SOC meter is reading 75%. either it's wired wrong, programed wrong (wrong ah)

or more likely the batteries are no longer at their full capacity. IE you have a 100ah bank. and you pull 25ah out of it. the meter will show 75%. but after many years that 100ah bank now only has the capacity of a 50ah bank. so you have really pulled 25ah out of a 50ah bank. the voltage will show 12.0, which is the voltage of a bank at 50%, But the meter will show 75%, in this case the meter needs to be reprogrammed for a 50ah bank instead of a 100ah bank. or in that case, the batteries need replacing.

doing a 20h discharge test annually will give you the current capacity and you can change the meter settings each year.

voltage doesn't lie (with no load)

[Wotname]

Originally Posted by smac999

voltage = SOC. not capacity. 2 different things. you can have a fully charged battery at 12.8v but it's 15 years old and has no capacity left, and would drain very fast.............

This ^^^^^^

[KP44]

Originally Posted by lawrence_craig

I am looking to understand how close the correlation is between voltage of a Wet Cell battery is with capacity?

I have a battery monitor (Victron SmartShunt) with all loads and chargers running through the shunt. When discharging at night, the battery bank discharges to around 12.2 or 12.1 yet the SOC is >75%.

Any thoughts? Many thanks

Any discharge more than 50% will damage your batteries. So I would read 75% state of charge to mean I am at 50%. And a 50% SOC could be read as Zero in terms of what is available for anything but emergency use.

[Mike Banks]

Volts times amps times time under load without battery losing more than ten percent of its fully charged EMF (volts) is a good indication of capacity.

Capacity is the power the system can deliver at a nominal voltage under load, times the time for which it can safely deliver it--and at a standard of 12 volts, or whatever battery type you have, it is expressed in Ampere Hours.

So--if your battery can draw ten amps power for ten hours without depleting the battery volts under load below eleven and a half volts, that is about one hundred ampere hours-capacity.

Most battery makers, in my opinion, overstate the capacity of their batteries, some grossly so. There is ALWAYS some extra power remaining in a battery discharged to a safe level --but depleting the terminal voltage below eleven volts will reduce the life of the battery system.

I never let mine go below twelve if I can help it, but using a pulsed intermittent higher voltage charger, (I use 24 volts) I have managed to recharge batteries that were almost flat, to the extent they will after use accept a normal 14 volts or thereabouts charging emf, but if you do this to seriously discharged batteries, I would suggest you replace them as soon as you can.

The longer a battery remains discharged, the quicker it becomes sulphated to the extent if becomes at best unreliable, at worst ruined.

[mitiempo]

For a capacity test first fully charge the battery. Use a load that is 5% of the stated capacity - eg 5 amps for a 100 AH battery. Time the battery until it reaches 10.5 volts. If it is full capacity it will take 20 hours. If it is at 70% capacity you will reach 10.5 volts in 14 hours.

Recharge to full immediately after.

[CountyChick]

Watch this video and you will understand:

https://youtu.be/M5NuG_cIFA8