jodusno1 avatar image
jodusno1 asked

Battery voltage dropping with no load

Hello all,

Would be grateful for any help.

Off grid system for my tiny house. I have 2x 380W solar panels in series, 60A Victron MPPT charge controller, and 2x 140ah Sterling AGM deep cycle batteries in parallel.

Its been horrible and misty for weeks now. I moved out a couple of days ago because the charge controller hadn't gotten past Bulk charge for about 5 days. Today I went back at about noon to disconnect the fusebox incase there was any slight power draw. Victron app at this point reported a bulk charge and a battery voltage of about 13.8. I went back just now about about 4 hrs later and now the app reports battery voltage of about 12.8V. Having never left Bulk charge.

Why has the battery voltage dropped when there has been no power draw? It has been a horrible rainy day but I thought it would still manage to put something in the batteries?

Second question. I would like to know how to tell the DOD of my batteries. I came across images like this:1668531257357.pngWhich I thought might be useful. But then I noticed that when my batteries are being charged to Float state by my panels, the Victron app reports the max voltage as being about 14.5V, and min as 12.7/12.8. Does this mean min voltage is the more useful data point? This makes no sense to me because this is what the victron app is reporting now even though my batteries haven't made it beyond bulk charge for days.

Some screenshots incase this is helpful.


Less miserable weather: 1668532067401.png

Thank you all for any help offered,


AGM Batterybattery system voltagedischarge level
1668531257357.png (188.3 KiB)
1668532034001.png (125.0 KiB)
1668532067401.png (122.8 KiB)
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2 Answers
kevgermany avatar image
kevgermany answered ·

Bit of background first

Voltages in the table are for a battery the has no load and is not being charged and has rested for a while since charge or load.

Voltages in the rest of the answer are general values for AGM and my differ from you setup.

Normal fully charged voltage of AGM is around 12.7V, as shown in the table. In float, this means charger has identified fully charged battery and will maintain it at 12.7V.

Charger will not switch out of bulk until battery reaches absorption voltage, usually 14.4V.

Every day the MPPT starts a new charge cycle. During this it will try to charge the battery up to 14.4V, then switch through absorption down to float, and the battery will remain at the float voltage unless/until there's enough load to trigger moving back to bulk. If there isn't enough light, the battery voltage will never reach 14.4V.

Your MPPT screen shots show that there isn't enough light to move the charge cycle out of bulk, and that the panels are not generating enough power to raise the battery voltage by strong charging. If you look at the panel Vmax line, it's around 70V on the bad days, but around 80V on the better days. They're also showing that there is enough ligh to keep the batteries charged at float voltage.

So nothing wrong, system working properly. But you need to give the sun a kick.

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jodusno1 avatar image
jodusno1 answered ·

Thank you Kevgermany - good to know that things aren't as bad as I expected. I will keep an eye on the min battery voltage and compare to the DOD chart, and not worry so much about the max voltage and the charge states. Probably still not the moment to turn my 12v fridge on again though! Still raining.

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