KID & WBRj Questions about and "Depth of Discharge" and "Low Battery Disconnect"

[canuck webmaster]

I have an off-grid weather station powered by 4 x 140W panels in series, 8 x 6V/220Ah AGM battery bank wired in 24V (so I have 440Ah), and a MideNite Kid with WBJr module. The load, according to the WBJr, is 1.0A when outside temp is over +5C (40F). Below +5C there is a heating device inside the rain gauge (to avoid ice/snow) and load goes up to around 2.5 A.

I also have an Air 40 wind turbine (24V / 160W). I understand the Kid is not meant to control a wind turbine charge, so we wired it directly to the battery bank. The Air 40 has an internal charge controller, and the rotor has a built-in brake to stop the turbine to spin when batteries don't need to be charged. I noticed the turbine spins only during the night or on very cloudy days, which I think is quite helpful to keep the batteries happy. The Kid doesn't seem to notice any interference.

Some settings we used:
- Battery bank size: 440Ah
- Battery efficiency: 96%
- Battery temperature compensation reference: 25° C
- Percentage of change per degree C: 1%

Once, I disconnected the panels for 48 hours (but kept the turbine), and the state of charge (SoC) got down to 89% with 394Ah remaining, and battery voltage down to 26.2V. Load was 1.0A at this time, with temp around 20C (70F).

On clear sunny days, SoC is around 96% at 9:00AM in the morning when I first check the Kid. 'Float' state and 100% SoC is reached usually around 11:30AM. Sunrise (on my rooftop) is around 7:00AM these days.

On very rainy and cloudy days, it's 92% at 9:00 AM, and if I'm lucky I might get to 'Float' and 100% SoC at around 4:30 PM, sometimes the best I get is 'BulkMPPT' and 97%.

I could play with the weather station settings to avoid excessive heating, in case somebody here thinks this  2.5 A load is going to be too high in winter (much less daylight, snow on panels, temperature below freezing point for at least 3 months in a row, here in Canada at lat. 45). My weather device has some clever eco-modes, so that heating is switched on only in some very restrictive situations (when freezing rain is detected for example).

So I have 3 questions for forum visitors and moderators here:

1 - is this setup looks ok to you?

2 - we sized the battery bank for 4 days of autonomy, is this realistic? Do we have too much, or not enough Ah?

3  - I'd like to program 50% Depth of Discharge (DoD) to protect my batteries. How can I find the proper values to program the Low Battery Disconnect option? According to the Kid manual, for a 12V setup, recommended value for disconnect voltage set point (absolute discharge limit) is 11.8V. For the reconnect voltage set point, it's 12.5V (I don't like the 'reconnect on Float stage' option). So, for a 24V setup, should I use 23.6V and 25V ? Is it that simple?

Thank you

[ClassicCrazy]

I would lower the efficiency setting  to 92 or 94 .

Is your weather station running off of 24v or do you have a voltage converter ?  You might loose some efficiency in that .

Just from my own experience with an APRS weather station - with the amount of solar panels , wind , and batteries you have you will probably be okay.

If you could turn off  the heater if the batteries got too low - down to a certain voltage that would make your system run longer.

How does your weather station report ?

Larry

[Westbranch]

where is your bank situated? Inside?  Distance to anemometer from batteries?  wire guage?

[canuck webmaster]

Thanks guys for feedback!

where is your bank situated? Inside?  Distance to anemometer from batteries?  wire guage?

Weather station will be mounted on a 20 ft self-standing tower (design similar to a light-pole). So cables length is around 10~15 ft for solar (mid-mast mounted), 20~25 ft for wind (top), same for weather equipment.

Battery bank will be outside, but inside an isolated casing at the base of the tower (so temperature will be less cold than outside temp, but only around 5C to 10C warmer, so we can expect freezing temperature anyway).

Wire gauge: sorry can't tell. My solar panel reseller knows what he's doing, and he choose not too thin and not too big wires. With a 24V system, I think we can use smaller wire, so less loss. So I hope that's what he choose. Will check next time I have a chance.

Is your weather station running off of 24v or do you have a voltage converter ? How does your weather station report ?

All our weather equipment and cameras runs on 24V, except one device (datalogger), so we do have a small 24V to 12V converter, but only one.

The station sends data (weather and images) via a cell modem to nearby public cell towers, then to a distant server. On the server a software crunch the numbers and display web pages in almost real time (15 min delay).

I hope you'll help me setup Low Battery Disconnect option! I have no way of remotely switching on and off the heaters, sorry. What I can do is choose an eco-mode, and pray for the best.

[ClassicCrazy]

Kind of tough to decide on what low voltage disconnect you want . What is your low outside air temperature ? The batteries will be cold so you won't be able to  get as much power from them in the winter .  Make sure you have the Kid remote temperature sensor in the battery box.

[canuck webmaster]

Kind of tough to decide on what low voltage disconnect you want . What is your low outside air temperature ?

Coldest month is January, with mean temperature -10C (14F), mean minimum temperature -15C (5F) and extreme minimum temperature -30C (-22F). Freezing temperatures are expected from December to March.

I don't have (yet) the battery temperature sensor, but there is a sensor in the Kid casing, right? And since the kid is inside the battery enclosure, do we really need the remote sensor?

[Vic]

As far as I know,  there is NO temperature sensor inside the KID that is used in lieu of the real RTS/BTS.

You will want the RTS.

Believe that the new MkdNite Brat PWM CC uses an RTS inside its enclosure.

FWIW,    Vic

[ClassicCrazy]

Maybe  you can find a local ham radio operator or group that may also like to add an APRS modem and radio to also  send out the weather to aprs servers available on the interenet. With the Argent Data OT3M you would also be able to use it's capabilities to monitor voltages, additional temperatures, light levels, etc via telemetry and or scripting  and also have a power switch that could be remotely activated to turn things like your heater on and off.
https://www.argentdata.com/products/tracker3.html


I wouldn't want my batteries going below 11.8 volts ( 23.6 v for 24v system )  before they needed to recharge. You can take them down farther but it is not good for the longevity . But I am thinking of flooded lead acid batteries but AGM might be a bit different in how the voltage corresponds.