question

ste7of1 avatar image
ste7of1 asked

How to understand SOC

I have a Zamp solar controller feeding LiFePo batteries. Multiplius 3000 inverter charger and a bmv700 connected to display/bluetooth. While offgrid, i turn on multiplus when required (microwave, coffee, etc) and state of charge slowly drops with usage. While unit is off, solar charge recharges batteries to 90-100% (as reported by bmv700), however, when i next turn on multiplus for use its state of charge is still at previous lower value and continues to drop with each additional usage state. Should i unplug the bmv700 display monitor and instead connect bmv700 directly to multiplus to get valid state of charge between units (Perhaps upgrade bmv700 to 712 Or ungrade solar controller to victron mPPT smart contoller?) When shore power is available for charging every thing seems normal but when off grid i’m afraid multiplus will continue showing lower discharge state until it reaches cutoff value and shuts down even though state is reasonably charged from solar.

SOC
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2 Answers
ste7of1 avatar image
ste7of1 answered ·

Or maybe said differently, how do i get agreement between multiplus and BMV700 on SOC as reported using connect app

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wkirby avatar image
wkirby answered ·

Your problem is that you have two independent battery monitors. One monitors the whole system (BMV-700) and the other one only monitors a portion of the system (the one inside the MultiPlus).
The best for you option is to disable the MultiPlus battery monitor and rely on the BMV-700.

The battery monitor within the MultiPlus is intended for use when it is stand alone with no other external charging sources.

To ties all the components together a GX device is used and brings the information from all the devices and makes a more coherent system. I guess you don't have one of these which is just fine, you'll just need to disable the MultiPlus battery monitor and use the BMV-700.

A BMV-712 will not help you and none of the battery monitors connect directly to the MultiPlus.

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I understand the separate aspects, but, the i was confusing SOC with actual voltage which multiplus uses to disable on low voltage - i guess even though SOC is going down, voltage isn’t, therefore i can just ignore SOC. maybe its time to add a cerbo GX.

thanks

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The SOC reading is based on the current in and out of the battery not on the voltage.

The MultiPlus can only "see" the current that goes through it (and only if it is switched on).

The BMV can see all the current, so it is more accurate and you can ignore/disable the battery monitor of the MultiPlus.

If you want to use the SOC in the MultiPlus to switch it of or for relay assistants you need a GX device and connected all devices there. The GX device will synchronize the SOC of the BMV to the battery monitor of the MultiPlus.

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