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solarricht avatar image
solarricht asked

How low should I set my low Battery voltage cutoff?

Hi folks, so I got a 840ah Trojan battery bank, about 2yrs old. I have a battery extra desulphating unit connected all the time. My Victron energy meter is set up correctly I believe. I have an auto start generator set to come on at 11.05volts. However, my %SOC usually states this is just 84% of my capacity. I know voltage is a tricky way to judge the discharge, as if I take a high draw,say 800w coffee machine, the capacity has to be over 95% for the voltage not to drop below 11v. We leave our inverter on 24/7 to run a germicidal lamp for our water system. So my question is: if I set the generator to come on at say 10.5v is that going to damage my batteries if it is still saying I’m above 50%capacity? Today the Generator came on at 84% or -156ah. I don’t seem to be making full use of the capacity of the batteries. I find looking after the batteries a minefield of difficult judgements!
Any help much appreciated!

battery capacity
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1 Answer
handy avatar image
handy answered ·

Hi, I also find that the SOC of batteries Is based on an estimated value and although calculations can be made it does seem a bit of a dark art and walking a minefield blindfolded, and application specific.

I have had to alter the voltage on my system over time to feel like I'm perfecting the settings.


Trojan batteries do however have really good estimated data available regarding discharge rate and temperature derating on there website and the values can often be found on the bottom of the data sheet specific for the batteries. From memory 1.75v per cell is the low level when deep cycling at 30 degrees centigrade and 1.2v per cell when Cranking at 30 degrees. Trojan also have really good info on there website regarding maintenance, Check out the section on Discharging.

https://www.trojanbattery.com/tech-support/battery-maintenance/


I'm on a marine off grid system and find we share the same thoughts, sadly batteries are sacrificial. By using them, they can fail, by not using them, they can also fail :) The art seems to be how long can we make them last.

6 years ago I could last 10 days off grid, now I'm down to only 4 and still dancing for Sunshine!

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solarricht avatar image solarricht commented ·

Thanks for the helpful reply! Being winter it’s hovering around 0-10 degrees at the moment so less capacity than usual. Interested to know what voltage you allow your bank to drop to when using a high powered appliance? Cheers.

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