Need help getting smart shunt to synchronise

DIY
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1

Hi all, I recently put together a MPPT 100/20 with a 300 A smart shunt for a hobby project. For completeness, I have a 370 W Trina panel, and intend to add a second identical in series at some point as my learning progresses. More details at the bottom, I’m here to ask about the somewhat traditional DIY “how to synchronise”? question.

The manual states 0.2 V below float, the forums recommend 0.2 V below absorption. I’ve got it at 0.3 V below absorption while I’ve been testing, but I believe that should make it even easier to synchronise? I wasn’t sure about tail current, but I’ve read the shunt should be a higher tail current than the battery. As you can see, I have 2 A tail current on shunt, and 1.5 A on battery. Finally, the charged detection time is 3 minutes, which I think was the default. If I’m trying to trigger a synchronisation, this should be brief, and I can tune it longer if needed.

I’m in Perth, Australia, so we’ve had plenty of uninterrupted sun the last few weeks.

Screenshots of settings

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Battery

Bosch AGM datasheet: BAC 12 75FR

  • Absorption and float within spec sheet ranges
Setting Value
Capacity 80 Ah @ 20C
Absorption voltage 14.6 V
Float voltage 13.7 V
Tail current 1.5 A (1.875%)
Temp compensation -30 mV/°C

Shunt

Setting Value
Peukert exponent 1.19 (20C / 3C)
Charged voltage 14.3 V
Tail current 2 A (2.5%)

Load

  • 20 W active, 5 W idle (small board computer + system fan)

Logs
This is my system monitoring from home assistant for today (currently mid afternoon). Hopefully you can tap it to see full size, but:

  • Goes into float around 10:30 am
  • Drops from 14.4 V to 13.5 V
  • Shunt current in float is initially 1.7 A, drops down (currently 0.7 A)

Screenshot_20260126_164240_Home Assistant
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System details

I’m happy with my cabling, PV rated circuit breakers, and load fuse. The entire system is built inside a ex-army steel carry case, with some ip65 grommets to pass PV in and load out. The load needs a bit more work, another anderson plug, and a proper PSU on the end - I currently have an automotive plug with a USB-C PD module with trigger board to request specific voltages.

I use GitHub - Fabian-Schmidt/esphome-victron_ble: Use official Victron BLE endpoint for fetching data from Victron devices via Bluetooth LE via ESPHome. to sniff the BLE packets into Home Assistant for long term remote monitoring.

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TL:DR

  • While writing this post I noticed that because I have dramatically oversized panels for my load, I reach float very easily every day. This means for SoC sync, I drop below absorption voltage before my tail current is below the setpoint, which could cause a failure to automatically synchronise.
    • I could revert to 0.2 V below float?
    • Is absorption reference charged voltage recommended for solar systems because they aren’t likely to hit float each day, so SoC drifts too much?
  • But is there something else I should consider?
2

Id leave the charged detection voltage at absorption -0.2V, but adjust the tail current up to 4% on the shunt. It looks like the charge current was at about 3A before switching into float. Adjusting the tail current up will lead to an earlier synchronisation.

The MPPTs adaptive absorption duration also plays into this, since it will only stay in absorption for a short period of time if the battery voltage was already high before the sun got up. Since your battery was over 12.5V in the morning, the max absorption time was divided by six due to the adaptive duration, so in theory it stayed in absorption for an hour. Hard to tell on the coarse time grid on the graphs, but could be correct. More info an the adaptive absorption in the manual

Id leave adaptive absorption enabled, but you can give it a try with it disabled. Then the MPPT will stay in absorption until its own tail current setting is reached.

3

Looks like you have the mppt on the battery side of the shunt.
Battery side of shunt should only have battery connected, all other connections go on load side of shunt.

4

Thanks for checking - The cables are a little hard to see there - I can confirm the Shunt is wired first in series from the battery negative, with a thicker gauge cable. Not even a breaker or fuse before it.

1 Like
5

Thanks for directing me to the Adaptive absorption reading material, I’ve overlooked that. I’ll see what happens in the sun tomorrow and give those a try, as well as bumping the tail current up. Sounds like I should leave adaptive absorption enabled for light discharge duties:

Disabled - … this option can potentially result in overcharging your batteries, especially for lead batteries and if only shallow daily discharges take place

6

I asked created this post a few months ago and got some good answers.