question

lawrence-craig avatar image

Absorption Vs State Of Charge

Hi there.

A few disclaimers firstly:

I appreciate that SoC is not an exact science, depends a lot on chemistry and settings.

I have also ensured that the settings are such that the SoC doesn't switch to 100% too early

I have a 560aH flooded lead acid batteries (less than 6 months old) with 3 X MPPTs, SmartShunt and a Multiplus.


So heres my question:

I have read a number of Victron litterateur that absorption should start when the batteries are about 85-90% full. Where as on my SmartShunt, the SoC is often at 97-99% when Absorption starts. To put it into context, my usage is such that when my solar chargers start in the morning, I have used about 75aH overnight which is approximately 15% of capacity (again, I know there's no way of know that without testing). My SoC reads 87%. Perfect, about what you'd expect. The batteries are then charged during the day at about 7amps per hour for 10 hours. They then switch to absorption for the last 3 hours when SoC is is about 98% and go to float after 3 hours absorption. (3 hours absorption based on adaptive charging on my MPPTs).


So based on the above, I would assume that the SoC is working as expected, and my interpretation of "SoC=90% when absorption starts" is incorrect? Any ideas?


Thanks

MPPT - Solar Charge Controllerchargingsmartshuntabsorption
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3 Answers
Alexandra avatar image
Alexandra answered ·

@Lawrence Craig

Absorption charge usually starts when the target voltage, Set on whatever charger you have. The percentage of SOC for lead acid based systems will usually be around 80% as it needs longer to fully reverse chemistry. If you dont mind some indepth reading this book is helpful...

https://www.google.com/url?q=https://www.victronenergy.com/upload/documents/Book-Energy-Unlimited-EN.pdf&sa=U&ved=2ahUKEwij-9P0ubfxAhXKTcAKHWZDDWMQFnoECAkQAQ&usg=AOvVaw2qujpOYl5gxqZqZl6ExjiX

The length of absorption again varies depending on the start voltage or DOD and if it really was fully charged previously on last charge.

Whereas lithium has shorter absorption times so 90 to 95%. That is why they are described as being more efficient.

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lawrence-craig avatar image
lawrence-craig answered ·

Thank you for your explanation. However, based on that logic, my batteries should be in absorption when my SoC is 85-90% where as in my original post, that is not the case.

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Soc as estimated by a multiplus, will be different to how a BMV calculates Soc.

Setting up an BMV properly >>

(48) How to set up BMV Battery Monitor for lead and lithium batteries - YouTube


pwfarnell avatar image
pwfarnell answered ·

The figure of 85 to 90% SOC is based on having a charging source that will provide around 10 to 20% of the battery capacity in Amps, for your 560Ah that would be 56 to 112 Amps. In your case if you are only charging at 7 Amps so you will be in bulk for much longer as it will take a long time for the voltage to increase to the absorption value. The 85% is a rule of thumb for typically sized charging sources. Larger charge sources reach absorption more quickly and smaller one more slowly.

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ah ok, that makes a bit more sense. So with an absorption cycle on a slower charger (i.e. I have a 300W solar system delivering about 7-10 amps per hour for 12 hours), what should I expect the SoC to be roughly when absorption starts?
One rough rule of thumb is absorption current as a percentage of battery capacity is equal to the depth of discharge (DOD). So for your 560 Ah, at 80% SOC = 20% DOD, then absorption would start at approx. 0.2*560 = 112 Amps. For your actual case, with say 10 Amps charging current, this is about 2% of battery capacity, so absorption would start at about 98% SOC. Remember, this is a crude rule of thumb not scientific theory.