BP65 mode for Victron 12V AGM deep cycle battery

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electronman asked Jul 29, '22

BP65 mode for Victron 12V AGM deep cycle battery

I'm just working my way through the specs for the battery BAT212200080 (12V 22Ah) and it states 20h discharge time End Voltage = 10.8V but I'm not sure what the 100% refers to. Would it be best to use mode 6 on the BP65 which cuts off at 10.5V or should it ideally be mode 3 11.25V which is nearest 10.8V? I'm assuming, although it says "deep cycle" it still needs to be shutdown at 50% discharge as it's an SLA battery. Or can "deep cycle" batteries such as this be 100% discharged? Thanks for any advice.

Battery Protect AGM Battery

agm.png (120.0 KiB)

bp65.png (99.3 KiB)

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Answer 1 · 2022-07-29T13:40:51Z

kevgermany answered · Jul 29, 2022 at 01:40 PM

It's percentage of the rated capacity.

What this shows is that if you discharge over 20 hours at amps = capacity/20, you will get the rated capacity with a final voltage of 10.8V.

However, don't discharge by more than 50%. Using the total battery capacity has a very big impact on service life.

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Answer 2 · 2022-07-29T15:06:06Z

electronman answered · Jul 29, 2022 at 03:06 PM

Thanks for that, that makes it clearer.

I'm powering a 9W LED from the battery (10V 900mA):

battery = 132Wh (half of what's written on it as can only use 50%)
LED = 9Wh

so the LED can run for 14.6 hours before the battery is at 50% capacity and needs to be shutdown.

What that equates to in end voltage isn't clear from the table though. The BPA needs to know at what voltage to shutdown.

The spec says:

"The rated capacity of Victron AGM and Gel Deep Cycle batteries refers to 20 hour discharge, in other words: a discharge current of 0.05 C"

discharge current of 0.9A = 0.04C

14.6h @ 9W = 131.4 = 49.78%

Given the end voltage is 10.8V for 20 hours or 10 hours, I'm assuming it will be 10.8V for 14.6 hours too. However, the spec says between 42% and 55% usage has an end voltage 9.6V. That would suggest the BPA should shutdown at 9.6V, which it can't do. It can only do 9.5V or 10V.

Or am I completely off in the wrong direction?

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