Battery...schmattery.... Voltage drop limit TRIGGER

[wade0000]

Hello there again,

I currently manually shut off the solar 24V battery banks running whatever I am running when my voltage gets down to 24.2 volts. (AGM) I have a wireless stream coming from the CC that I can see the instantaneous voltage from inside my house. The inverter and batteries are in a disattached garage outside.

However, I have noticed that when my fridge kicks in it drops the voltage way down and then once the load is reduced, the voltage rebounds at least 0.6 volts. So it is very hard to tell if I am really at my (self-imposed) 24.2V limit or it's just a surge on the demand side.

However, there is no way to tell from a distance if the load is big or small since the fridge is in my basement and so for now I err on the side of safety in that if I see the voltage is 24.2, I shut off the inverter and batteries and just go to the grid. But it could have just been bad luck in that the fridge was kicked in heavy.

So, my question is: If I run the voltage down to below my target 12.2 but it is only there because of the heavy draw at the time, is this really violating my rule of thumb and hurting my batteries? So, is the 12.2 real and affecting the life even if under no load it would be 12.8?

I know ideally I would want to disconnect both the panels and the load in order to see the steady state voltage but that is impractical.

Thanks.. I'll shut up and listen now.

 

[snoobler]

24.2V is approximately 50%, but that ONLY applies after being completely at rest for several hours or under extremely low current. A 24.2V under load is ABOVE 50% SoC.

If you insist on a fully automatic cut off, then you don't have a lot of choice, but if this is something you can tend to, then a lower cut-off below the "annoyance threshold" - where brief heavy loads may trigger LVD cut-off - may be more appropriate.

I personally have my LVD set to a very low cut-off, but my battery monitor alarms at 50%. Deep cycle AGM typical have around 1200 50% cycles and maybe 400 full cycles, so a single big dip isn't going to destroy it, but you might lose 4 of those shallow cycles. As an example, last visit to the ranch, the propane furnace blower ran almost all night due to very low temps - 8°F when we woke up in the morning. This pulled the battery down to 30% when it is rarely pulled below 80%. It would have been catastrophic if I had to deal with an arbitrary cut-off of 50% that night.

I battery monitor like one listed on Will's page:

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

Can help you monitor your actual state of charge as voltage can be a very poor indicator as you've gleaned.

While notably more expensive, the Victron unit takes into account Peukert's law to account for the fact that heavier loads reduce actual capacity as well as low temperature effect on capacity (reduced). The "Budget" model is way better than nothing.

 

[wade0000]

24.2V is approximately 50%, but that ONLY applies after being completely at rest for several hours or under extremely low current. A 24.2V under load is ABOVE 50% SoC.

If you insist on a fully automatic cut off, then you don't have a lot of choice, but if this is something you can tend to, then a lower cut-off below the "annoyance threshold" - where brief heavy loads may trigger LVD cut-off - may be more appropriate.

I personally have my LVD set to a very low cut-off, but my battery monitor alarms at 50%. Deep cycle AGM typical have around 1200 50% cycles and maybe 400 full cycles, so a single big dip isn't going to destroy it, but you might lose 4 of those shallow cycles. As an example, last visit to the ranch, the propane furnace blower ran almost all night due to very low temps - 8°F when we woke up in the morning. This pulled the battery down to 30% when it is rarely pulled below 80%. It would have been catastrophic if I had to deal with an arbitrary cut-off of 50% that night.

I battery monitor like one listed on Will's page:

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

Can help you monitor your actual state of charge as voltage can be a very poor indicator as you've gleaned.

While notably more expensive, the Victron unit takes into account Peukert's law to account for the fact that heavier loads reduce actual capacity as well as low temperature effect on capacity (reduced). The "Budget" model is way better than nothing.

Your commentary is extremely helpful and spot on with the exact issue I am having.

I will consider a more expensive monitor since the annoyance factor is getting on my nerves since the charging KWH is so low due to winter solstice.

Thanks. let me read Will's post now...

 

[wade0000]

So, just to double confirm. A below 12.2 V showing up only because the load is high... DOES NOT damage your battery or reduce cycles, correct?

 

[snoobler]

So, just to double confirm. A below 12.2 V showing up only because the load is high... DOES NOT damage your battery or reduce cycles, correct?

Correct.