I'm trying to dial in my various Victron components on my DiY solar installation (still a work in progress) but I'd like to make sure I'm on the right track here. All advice would be appreciated.

I've got a BlueNova 26V 2.8kWh LiFePO4 battery, a Victron SmartSolar MPPT and a Victron MultiPlus inverter/charger. The battery capacity is a bit on the puny side due to budgetary restrictions (sadly). This means that during the day everything pretty much runs straight off the solar panels but at night it all depends on the battery. In order to make sure the battery isn't drained too far (causing the inverter to shut down and wearing out the battery prematurely at the same time) I'm setting up the Multiplus to kick in (powering several appliances from the mains and recharging the battery) when the battery reaches about 70% depth of discharge. Since the Multiplus is not the only inverter in the mix that drains the battery nor the only charger that recharges it, its internal algorithm to determine SOC is not usable. This leaves the battery's clamping voltage as the only means to trigger a recharge from the mains.

Unfortunately BLueNova doesn't publish discharge curves for their products. I can't afford a battery monitor at present so I'll have to make do with what I have. I've done some measurements using the VictronConnect app (which has limited options to export trends so all I can show here is a screenshot) and combined it with monitoring appliance consumption using a Kill-a-watt type plug-in monitor. An example of the graph produced by VIctronConnect is included with this post. The battery clamping voltage in the graph goes up and down because the inverter is currently powering only two freezers, so the load varies when the thermostat on a freezer switches on the compressor.

My question: how reliable is the battery's clamping voltage as an indication for Depth of Discharge?

Are all LiFePO4 discharge curves more or less the same (i.e. I can just use a graph for a different brand of battery since BlueNova doesn't provide one) or are there differences between brands? In the graph I'm attaching the voltage drops to a minimum of 25.62V until sunlight starts to provide additional energy again. I realize this is dependent on temperature and the amount of current drawn from the battery.

Any advice would be greatly appreciated!!

// FvW

Edited June 18, 20241 yr by frankvw

OK. Just in case this will be useful to someone somewhere: I've done a lot of experimentation and I've pretty much worked it out.

Determining the SoC on a LiFePO4 battery based on battery voltage is inherently somewhat inaccurate, since the battery voltage doesn't vary much between 10% and 90% or so SoC, while variations in load do cause voltage fluctuations that can easily exceed the voltage variation between 20% and 80% or so SoC. So this will always be a rather limited way of estimating the SoC.

That said, sometimes it's better than nothing. I have found through trial and error that when the battery voltage on my 26V 108Ah BlueNova battery drops to about 25.3V under a load of 0.2-0.35C, that means the SoC has dropped to about 30% (give or take). Recharging it to a battery voltage of about 27V restores the SoC to about 70% (once again give or take).

It's not perfect, but it helps. 😃