SmartShunt configuration w/very low Victron MPPT tail current (due to 100W solar array)

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I was/am running into an issue with my Victron SmartShunt where it is resetting to 100% SOC when the BMS (Wattcycle 280Ah Smart Mini) is clearly below 100% and no charging has taken place.

I found this article https://community.victronenergy.com…high-soc-with-low-volts-the-bms-shuts-down/88), and I think the recommendation to set the Charged Voltage = (absorption voltage – 0.2V for 12V systems) is a good one that will help with my issue.

Additionally, there’s a discussion of Tail Current, and the value that should be used there. It suggests that tail current for LFP should start around 4% (which happens to be the default on my Orion XS) but I think I’m going to run into a problem and would like some guidance on where to set my tail current.

To start: a 4% tail current on my 280Ah battery would be something on the order of 11.2A

My problem is twofold:

  1. My Victron MPPT SmartSolar has only a single 100W foldable panel attached to it that generates 20V. As such, I’m never going to get 11.2A out of it, and usually I’m getting on the order of 4A, with a max of around 7A when the panel is maxed out and generating 75W.
  2. I’ve also got my Orion XS, which is capable of charging at 50A, set to charge at 10A due to a relatively small alternator in my 25 year old Tacoma. I figure I’ve got the same problem with it that I have with my MPPT

With these two very low power charging devices, what should I be setting my tail current settings to/how should I be configuring my chargers to best address the 280Ah battery with a SmartShunt in the loop?

Thanks!

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This is a good question because the supposition in the FAQ document is that you have enough solar or charging capacity which would be 100W or 8A for a 100Ah battery, 3 times higher.

In your case I would set the tail current very low, maybe as low as 0.5% which will be just under 1.5A then see what happens during a charge / discharge cycle. If you start at 100% with a full battery (mains charger to get it full perhaps) then watch as it climbs back up to 100% without resynchronising and when it gets to about 99% or 99.5% on its own, note the voltage and current and use those. With such low charging rates, the voltage may not get pushed up very high. It will be trial and error. Make sure Peukert is 1.05 and charge efficiency is 99%.

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Thanks, I’ll give this a try and report back. I generally don’t use solar much when I’m out exploring, but I happen to have a week-long trip coming up where I’ll be mostly in one spot with good sun through the day, and that should let me play with this a bit! :+1:

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