adding charge termination logic to an rv converter

[John Frum]

I came up with a way to add automatic and configurable charge initiation and termination
to this https://www.iotaengineering.com/pro...r-24v-battery-charging-in-240vac-applications
using this https://www.victronenergy.com/battery_protect/smart-battery-protect
and this https://dlidirect.com/products/iot-power-relay

I named my invention after 2 of my favorite things.
I call it the drinking bird.

The iota charger comes factory configured to deliver ~27.2 volts with no charge termination logic.
It also comes with a "boost pendant" that bumps the voltage +1.2 for a total of 28.4 volts while inserted.

Its gets the battery plenty full for my usage model without the converter ever going into absorption mode.
The smart battery protect and the iot relay use ~0.0017 amps combined.
My usage model is an online uninterruptible power supply.

ac {
    mains->iot_relay.normally_closed->iota_converter
}
dc {
    pos {
        |<->battery.pos
        |<-ac2dc_charger.pos
        |->smart_battery_protect->fuse->iot_relay.control.pos
    }
    neg {
        |<->battery.neg
        |->ac2dc_charger.neg
        |<-smart_battery_protect.ground
        |<-iot_relay.control.neg
    }
}
configuration {
    set smart_battery_protect.restart $terminal_voltage
    set smart_battery_protect.shutdown $initial_voltage
}

On the main system board we see the converter and iot relay side by side.
The little green wires coming out of the converter are the "boost pendant".

Ont the bms side of the battery we see that the smart battery protect is connected to my ridiculously oversized balance leads.
The smart battery protect is not in the high current path and therefore reads a 30mv higher than the bms regardless of charge or discharge current.
I would not use 10 awg wire for the balance leads if I were to do this again.

 

[John Frum]

Just because I can...
I swapped out the smart battery protect for a raspberry pi that talks to the bms and allows me to make smarter initiation and termination decisions.
The pi uses @melkier 's jbdutil to do the heavy lifting.
Also logs the battery status every 6 seconds all day every day.

Here is a sample

2021-08-06 16:37:38 Pack=26.863V Lo=3.354V Hi=3.361V Average=3.358V Delta=0.007V Cell=26.8C Ambient=26.1C SOC=180.5AH Rate=11.8A Balance=00000000
2021-08-06 16:37:44 Pack=26.863V Lo=3.354V Hi=3.361V Average=3.358V Delta=0.007V Cell=26.8C Ambient=26.1C SOC=180.6AH Rate=11.8A Balance=00000000
2021-08-06 16:37:50 Pack=26.868V Lo=3.355V Hi=3.362V Average=3.359V Delta=0.007V Cell=26.8C Ambient=26.1C SOC=180.6AH Rate=11.5A Balance=00000000
2021-08-06 16:37:56 Pack=26.864V Lo=3.355V Hi=3.361V Average=3.358V Delta=0.006V Cell=26.8C Ambient=26.1C SOC=180.6AH Rate=12.0A Balance=00000000
2021-08-06 16:38:02 Pack=26.867V Lo=3.355V Hi=3.361V Average=3.358V Delta=0.006V Cell=26.8C Ambient=26.2C SOC=180.6AH Rate=12.0A Balance=00000000
2021-08-06 16:38:08 Pack=26.863V Lo=3.354V Hi=3.362V Average=3.358V Delta=0.008V Cell=26.8C Ambient=26.2C SOC=180.6AH Rate=11.8A Balance=00000000
2021-08-06 16:38:14 Pack=26.865V Lo=3.355V Hi=3.361V Average=3.358V Delta=0.006V Cell=26.8C Ambient=26.2C SOC=180.7AH Rate=12.0A Balance=00000000

 

[John Frum]

Just for fun I modified things a bit.
I set the iota converter to output 26.4 volts float and 27.6 boost.
I made my own version of this https://www.iotaengineering.com/products/dls-power-converters-battery-chargers#t=ablProducts&sort=relevancy&layout=card&numberOfResults=50&fproducttype=[Smart Charge Controller]
Using a raspi and a solid state relay.
The charger enters boost mode each night at 22:00 and reverts to float when the tail current is < 4 amps as measured at the battery.

SSR and rj9 breakout box.



Iota charger and raspi


Log sample

2021-08-27 20:43:23 Lo=3.308V Average=3.310V Hi=3.311V Delta=0.003V Balance=00000000 Cell=26.6C Ambient=26.4C SOC=135.5AH Rate=-7.1A float 17:43:19