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Depth of discharge settings


Johandup

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A question for the experts.

I need to know at what cutoff voltage must an inverter be set to for a 20% dod on its batteries. 

The batteries are 4 off 12v 200ah solar lead acid connected to give an output of 48v.

It is currently setup to switch off at 42v. But the batteries are not monitored in any way except by the inverter. Is this recommended?

Are there any practical way to set the batteries up to limit the kwhr output?

Please show the calculations if possible. 

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7 hours ago, Johandup said:

I need to know at what cutoff voltage must an inverter be set to for a 20% dod on its batteries. 

Morning Johan, the only way Voltage can be accurately used for SOC calculation is when the bank is disconnected from any load. While connected the load will influence the voltage and the SOC calculation will be different for every applied load, for Example, at 200 watt load the Voltage might be 51 volts, but as soon as you start a 1.8kwh Load the battery voltage will drop to 50 Volt or less instantly. If you used voltage for the SOC calculation it would mean that the batteries went from 98% to 75% instantly, and that is not correct. 

7 hours ago, Johandup said:

the batteries are not monitored in any way except by the inverter. Is this recommended?

Not at all, the best and only correct way is to add a Battery monitor to the setup, a basic BMV700 will be enough, the other models have additional functionality, but for SOC alone the 700 will work. 

7 hours ago, Johandup said:

Are there any practical way to set the batteries up to limit the kwhr output?

Not without something like a Pi and a couple of sonoffs to control the loads. You can only control the output by controlling the load. 

Wait you have the Super 4 if I remember correctly. They can limit the battery usage by way of setting, but only while the grid is available. It will then Blend the allowed power from the Battery with Grid. During a power failure, you will still need to manage loads manually or by the use of sonoff's 

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6 hours ago, Jaco de Jongh said:

the only way Voltage can be accurately used for SOC calculation is when the bank is disconnected from any load

Jaco, how is the dod then setup for disconnect if it is not as per voltage? 

Many inverters are set up for disconnect when the required depth of discharge Ir reached. 

I realise one is spoilt by running pylontechs with their bms.

Thanks for the reply though. 

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55 minutes ago, Johandup said:

, how is the dod then setup for disconnect if it is not as per voltage? 

Easy, it is setup according the voltage. A guesstimate, not accurate at all. Most inverters use that method, but install a battery monitor and compare the two and you will see there is no correlation at all. 

We have discussed this at length numourous times before on the forum. 

The voltage varies greatly with load and can not accurately be used to indicate SOC. Using voltage as SOC Indication can only be done accurately with no load connected to it. 

That is why we always advise newcomers to add a battery monitor as the most important addition to lead acid batteries. It's the only way to really know the SOC of the battery. 

 

EDIT: Here is some interesting reading that might explain it better than ME.

Voltage Method

Measuring state-of-charge by voltage is simple, but it can be inaccurate because cell materials and temperature affect the voltage. The most blatant error of the voltage-based SoC occurs when disturbing a battery with a charge or discharge. The resulting agitation distorts the voltage and it no longer represents a correct SoC reference. To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid. This makes the voltage-based SoC method impractical for a battery in active duty.

Each battery chemistry delivers its own unique discharge signature. While voltage-based SoC works reasonably well for a lead acid battery that has rested, the flat discharge curve of nickel- and lithium-based batteries renders the voltage method impracticable.

Info taken from https://batteryuniversity.com/learn/article/how_to_measure_state_of_charge

 

 

 

 

55 minutes ago, Johandup said:

Many inverters are set up for disconnect when the required depth of discharge Ir reached. 

Agree. but again it is not accurate at all... 

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