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cargol66 avatar image
cargol66 asked

brand new Victron AGM battery capacity

Hi everyone,

I've just bought a Victron AGM 220 Ah 12V battery.

As it will be inserted in a 48V battery pack, I've measured the C20 capacity as well as for the three other existing batteries.

Before to check the C20 capacity, the battery was charged with a Victron charger.

4.7 Ah was added to fully charge the battery.

Then a 11A constant current was drawn from the battery during 17:42 hours, wich gives us a 195 Ah C20 capacity.

Ambiant temperature was found between 15 and 17 °C.

Then battery was fully charged again with 22 A bulk current :

so 183.8 Ah in Bulk, 55,1 Ah in absorption, so a total of 238.9 Ah loaded in the battery.

charge efficiency is 81.6 %.

Is it normal that C20 was found 11.3% lower than specified for a brand new battery and morover lower than 2 years old batteries?

Does this capacity increase after a few discharge/charge cycles ?

What are the manufacturer tolerances values about C20 ?

Thanks for your Help.

N.B.: the three other 2 years old batteries C20 capacity was measured with the same method:

B1 : 212 Ah

B3 : 211 Ah

B4 : 207 Ah

Bruno.



AGM Batterybattery capacity
2 comments
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Al avatar image Al commented ·
Did you leave the new battery to charge until the end of an extended float period for its first charge?


More importantly, mixing old and new batteries is a false economy.

Are you actually hoping to mix 12v lead acid batteries with 2 years difference in age, in a 48v system and expecting them to last? If it's for a temporary emergency fix, ok, but you'll be buying an all new battery bank within a year.

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melbournepark avatar image melbournepark commented ·
New AGM batteries do not have full capacity when first used.


The B4 battery is 207 AH. I think this battery will draw power from the other 4 batteries. The best battery will contribute the most. Adding a new battery to old ones will shorten the life of the new battery to similar to the old batteries.

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2 Answers
kevgermany avatar image
kevgermany answered ·
AGM capacity drops with temperature. From memory capacity is for 25C. You will probably find that capacity increases after the battery has been used for a while and has gone through some charge/discharge cycles.

If you keep discharging this far, you're going to shorten the life of your batteries, Normal rule of thumb is to discharge by a max of 50%.

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cargol66 avatar image
cargol66 answered ·

Thank you kevgermany.

Of course, this low SOC is only for measurement purposes once a year to check batteries capacity.

Daily battery use will be with battery balancers, BMV, MPPT, Multplus 48/3000, temperature sensor and Cerbo in ESS mode to add some external power when battery SOC reaches 50%

Regards.

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