How charge controllers stop battery discharge

[ButchDeal]

Clearly I read the OPs first post and the title of this thread differently than you.

check it again:

It seems to me that in order to stop the battery from discharging at a certain point, there would have to be something to cut off the electricity between the battery and the inverter.

So how can a CC prevent over-discharge? Do I have my system wired completely wrong?

This instructable says connect the inverter to the charge controller, but that is a heavier load than a light, especially if I have my devices running on the inverter. So that doesn't seem right.

[AzRoute66]

Could be, but it's not. The OP was asking about how to prevent the inverter from over discharging the batteries.

Clearly I read the OPs first post and the title of this thread differently than you.

[Sunking]

Charge controllers do two things and two things only:

Prevent back feed at night
Tries it best to charge batteries and does a poor job of it.

[Mike90250]

Way back in the day, didn't they used to say that the charge controller prevented battery discharge back through the panels at night? Could that be what the OP read?

Could be, but it's not. The OP was asking about how to prevent the inverter from over discharging the batteries.

then there is nothing between the inverter and the cc to stop stop discharge.

Charge controllers still prevent back-feed into PV panels at night.

[Sunking]

My inverter will set off an alarm when voltage hits about 10.6, and will cut off when voltage hits 10.5.
The only problem with that is that although you can go as low as 10.5 (50%), everthing I ever read says you should not go below 11.9 (80%) for better battery life.

11.9 volts is Open Circuit Voltage on a rested battery. That has nothing to do with a loaded battery.

I can take a new fully charged 12 volt batter, and make it read 7.2 volts. In fact that is a Industry Test to determine Cranking Amps. Hit a starting battery with 500 amp load and the voltage will crash. So when you see a 12 volt battery with a CA of 500 amps, that 500 amp sis delivered at 7.2 volts on a 12 volt battery. Take the load off, and the battery recovers to 12.6 volts fful charged OCV.

10.5 volts on a 12 volt battery under load is 100% discharged. Disconnect the battery, let it rest, and it will recover to 11.9 volts. 10.5 is industry standard to keep your Inverter from destroying the battery. You do not want to raise that voltage. If you do and hit the battery with a heavy load, your Inverter is going to Falsely Trip Off-Line from under voltage when there is nothing wrong except your battery is not sized correctly.

[Wy_White_Wolf]

My inverter will set off an alarm when voltage hits about 10.6, and will cut off when voltage hits 10.5.
The only problem with that is that although you can go as low as 10.5 (50%), everthing I ever read says you should not go below 11.9 (80%) for better battery life.

That's because they are intended to cut power based on voltage sag instead of theoretical state of charge voltage.

WWW

[PNjunction]

You are on the right track - the inverter's own low voltage disconnect is a last-ditch effort to prevent immediate damage. So an external low-voltage disconnect is needed prior to that to be more conservative. Probably the easiest thing to do is re-engineer your system so you never pull more than 50% capacity regularly. A larger battery might be the easiest way to control this.

And yes, disconnect the inverter when not in use as many will draw some amount of idle current.

[AzRoute66]

Way back in the day, didn't they used to say that the charge controller prevented battery discharge back through the panels at night? Could that be what the OP read?

[ckuttruff]

So you could either add an external Low Voltage Disconnect (LVD) relay or program the LOAD terminal for a reasonable lower limit voltage and use it to drive a relay to cut off the real load connected to the battery through the relay.

Thanks so much for this info. Do you know where one can acquire one of these devices that suppirts 100A and ~24V shutoff? Had a difficult time tracking anything down...

[wpmasterdesign]

Well like I said, due to what I have been told by so many, I never let my batterry drop under 11.9. But I suspect you are correct.

[littleharbor]

When your batteries are less than full and you hit them with a heavy surge load from the inverter the voltage will sag , then recover. If the LVD on the inverter were say, 11.5 volts then you would be tripping it all the time at anything above idle.

[wpmasterdesign]

My inverter will set off an alarm when voltage hits about 10.6, and will cut off when voltage hits 10.5.
The only problem with that is that although you can go as low as 10.5 (50%), everthing I ever read says you should not go below 11.9 (80%) for better battery life.

[NorthRick]

Doesn't your inverter have a low voltage cutoff? Mine do. If the voltage drops too low the inverter will shutoff stopping the discharge.

[wpmasterdesign]

Thats kinda what I was thinking guys. Since I use small modular "as needed" units, I think I am better off doing it how I do now and that is to simply disconnect the battery from the controller when I am not using the power. Thanks for clarifyin!

[inetdog]

So you could either add an external Low Voltage Disconnect (LVD) relay or program the LOAD terminal for a reasonable lower limit voltage and use it to drive a relay to cut off the real load connected to the battery through the relay.