question

tbird avatar image
tbird asked

Low voltage but Shunt reading high charge.

Hi there people,

So I’ve been away for a few weeks and my friend has been staying in my place.

I have a Smart Shunt, a 24/800 inverter and a 100/50 MPPT.

2 x 230ah 12v lead acid batteries wired to 24v.

Despite having a discharge floor of 60% - which hasn’t been reached - the 24v batteries have been as low as 18v which tells me they are wrecked.

I woke up this morning and I’ve got a low voltage alarm and no power despite supposedly having 82% in the tank.


I don’t understand how the deepest discharge can be 77ah and take the voltage down to 18.56. Calibration error?


Any ideas what I’m doing wrong?

How can the voltage get so low without hitting a charge cycle / the discharge floor?


Many thanks…

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2 Answers
pwfarnell avatar image
pwfarnell answered ·

Show us your SmartShunt settings, you may have it set wrong so it resets to 100% charged before it is actually fully charged. Also, batteries lead acid or lithium and what the is your absorption voltage set at.

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tbird avatar image tbird commented ·

Many thanks for your reply…

I’ve got lead acid batteries…

I just reset the settings to default, but I think absorption was over 29v and equalisation was 33v.

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chrigu avatar image
chrigu answered ·

Your charged voltage is at 24.0V, so anytime the battery goes over 24.0V and the charge current goes below 4% for 3min, the shunt resets the SOC to 100%


Set it to 27.4V, float voltage -0.2V

https://www.victronenergy.com/media/pg/Lynx_Shunt_VE.Can/en/battery-monitor-settings.html

And although the discharge floor is at 69%, i assume theres nothing actually stopping a discharge. Personally i would have set battery capacity to 115Ah, so the SOC will say 0% but actually its only 50%, but you cant really ask more of lead acid. Just a psychological thing, seeing 0% is more meaningful than seeing 50%

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tbird avatar image tbird commented ·
Much appreciated.


I’ve changed the settings although for these batteries I’m sure it’s already too late. I’m getting voltages of 30v…. I’m assuming that’s because they’re damaged?


And yeah, there’s nothing stopping a discharge from happening. In my little head the shunt would prevent the batteries getting damaged but I suppose I need another bit of kit to prevent draw on the system when the voltage gets too low…


Thanks again for your answer and the link

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chrigu avatar image chrigu tbird commented ·
Id say check the charger, 30V does not sound like 24V lead acid could produce that on its own. If they got damaged by the deep discharge, i would expect them to take a charge current but only get hot by it.


If it was only once down to 18V i would not be too worried. Sure they did not like it, maybe lost some life, but id be impressed if that already would have been it. Lead acids are more resilient than we nowadays give them credit. They've been used in hot engine bays, cranking in the cold winters, getting jump started, and done all that for decades
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pwfarnell avatar image pwfarnell commented ·
@TBird If the main charge is by solar it is better to set the SmartShunt charged voltage to 0.2V below absorption voltage i.e. 28.6V not the float voltage. This is because if you use float voltage then shading on your panels can reduce solar power so your charging voltage and current falls and the SmartShunt takes this as charged.

@Chrigu

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chrigu avatar image chrigu pwfarnell commented ·

Thanks for the correction, but your own manual says float, in the link above :-)

I totally linked the manual for the wrong shunt, but luckily for me it says the same in the smartShunt manual
https://www.victronenergy.com/media/pg/SmartShunt/en/all-features-and-settings.html#UUID-a7b9385f-6d53-49e0-2ccf-261ee8a434ad

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pwfarnell avatar image pwfarnell chrigu commented ·
@Chrigu It is not my manual, I am not a Victron employee and I and others have let Victron know that it should be changed to cover solar only installs which bring issues not present if using grid powered chargers. There are many instances on this sites of solar charged systems syncing to 100% early if they use the float voltage as reference.
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tbird avatar image tbird pwfarnell commented ·
Thanks for you input PW and Chrigu,


Recommended CV for 24v according to Victron is 26.4v, so if I use that as a starting point


So with my 1400w solar setup:

Charged Voltage: 26.4v?

Absorption: 26.6v?

Float: 26.2?

Equalisation:?


Does this seem about right?

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pwfarnell avatar image pwfarnell tbird commented ·
@TBird No, your proposed settings are completely wrong, your batteries will not charge. Your existing charge settings are correct


Absorption 28.8

Float 27.6

Equalisation 32.6, seems a bit high but check your battery manufacturer.

Charged voltage is then set to 28.6V.

Absorption is the critical setting, charged voltage is worked out from thiis and 28.8V is what your battery manufacturer will be recommending.

Please use this. @Chrigu unluckily gave some advice which was not the best and has confused the issue. Had I answered after seeing your settings I would have given you the 28.6V figure straight away.

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