Lead Acid LVD question/idea

[RJuelfs]

I have been looking for a LVD that I can set to 12.3V. Apparently, that is difficult to find. I’ve searched forums, YouTube, and google to no avail.
So, here is my thought. A $15 LVD and a$15 SSR from Amazon to control the inverter on/off.

 

[RJuelfs]

Thoughts?

 

[svetz]

looks like it should work ... but most LA LVDs are set for 11.6V for 50% capacity...

Some MPPTs and inverters have adjustable low voltage cutoffs, might be easier to use one of them, might look like:

 

[RJuelfs]

The charts i've seen show 12.2 as 50%. the only inverters i've found cut off at 10.5! FAR too low.
I'll look into it, and the LVC is adjustable so I can change it as I learn.

Thank you

 

[chrisski]

Some MPPTs (e.g., Schneider/Morningstar) and inverters (e.g., Outback, Magnum) have adjustable low voltage cutoffs, why not just use one of them?

On all the controllers I looked at, that cutoff attached to the charge controller is intended for low amperage loads. Perhaps it could be used to control an on off switch to an inverter by putting a relay on it if the inverter was capable of remote on off. My SAMLEX inverter is.

On my Victron Equipment, it has an option for a low voltage relay in the software. Since I don’t see anything this relay would plug into, I assume that this would be powered by an accessory device operated off Bluetooth.

The charts i've seen show 12.2 as 50%. the only inverters i've found cut off at 10.5! FAR too low.
I'll look into it, and the LVC is adjustable so I can change it as I learn.

For measuring voltage to determine SOC, this may not be the best way to have a cutoff. I found that higher wattage devices that may be on for a few seconds or minutes will drag the battery voltage down temporarily, giving a false low state of charge.

THe reason the inverter has a 10.5 cutoff is when you’re maxing the 2000 watt inverter on a battery bank, you will experience voltage loss, which changes to heat, more so the thinner wires you have. I have 4/0 on my 2000 watt inverter and I set my alarm for a 60% SOC based off voltage, and the alarm would trip because the voltage would drop low when a 1000 watt device came on, but the SOC measured off my battery monitor that calculates AH used and the Puekert Effect said I still had 80%.

There’s no way around this voltage loss on higher wattage loads. You can get thicker wires, change to lithium batteries, and switch from a 12 volt system to a 24 volt system, and it will be less extreme, but the voltage loss is still there.

You can calculate that voltage loss here:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

Start looking at 155 amp loads at 12 volts, which is close to 2000 watts. Use 6 AWG wire for a 10 foot run which is a realistic inverter to battery run. I say 6 AWG because really shady inverter manufacturers will include this with their inverters when you purchase that, and then move up to 2 AWG, then 2/0, and 4/0 and you’ll see this loss get less.

 

[RJuelfs]

I believe that the LVC I sourced has an adjustable hysteresis, so I should be able to minimize the issues. If not, It is on me to size the system/load to suit my needs.
As an engineer, I understand that there are trade offs in every design and and willing to accept the ones that I cannot practically design out.

Thanks for the replies!

 

[Gazoo]

I bought one of those SSR's and at the time did not realize it needs a heatsink. I also did not realize there is voltage drop across the relay. Then I started researching these cheap relays and realized one gets what they pay for.

I have a JunTek meter connected to my two Valence RT batteries that does the same thing as the meter you posted. I tested it with a mechanical relay and it does work. But the relay was heated up to the point it was too hot to touch and was drawing more current than advertised so I scrapped that idea.

I am thinking of ordering another JunTek meter for my new build and using a Tyco EV200AAANA relay for primary LVD with my LFE battery. The BMS will be secondary. The inverter I have on order won't work because the LVD is too low.

 

[Hedges]

The charts i've seen show 12.2 as 50%. the only inverters i've found cut off at 10.5! FAR too low.
I'll look into it, and the LVC is adjustable so I can change it as I learn.

Thank you

I noticed that for several inverters where forum members ended up with a discharged battery.
I think it is to protect the inverter, so it won't draw too much current trying to deliver a particular amount of watts. (Switchers don't like brownout.)
But doesn't help LA battery much. I think it can work for lithium.

Better disconnect control would be implemented with an inverter, charge controller, or monitor that counts coulombs and computes SoC, rather than just using voltage. Midnight SCC have a relay programmable for SoC, as does Sunny Island. I think there are other stand-alone monitors as well, which use a shunt.