Connecting an electric 2 amp battery maintainer to a solar system

Tonytee · March 2023

Hi, I have a 12 volt backup mini solar system to provide 120 volt power to run my wood furnace fan in the event of a power failure while we are sleeping or away from the house and the fire is still burning in the furnace. It consists of a 20AH lead acid battery, a couple of 100 watt solar panels, an automatic transfer switch, and an inverter. The inverter must remain in the ON position 24/7 for the system to work as designed. My inverter draws about 1/2 amp at idle. The inverter remains at idle due to the transfer switch as long as there is city power available. I want to know if I can attach a 2 amp battery maintainer to the battery to keep it topped up overnight with the inverter is in the ON position. or will it fry the electronic components in the inverter ?

Cheers,
TT

BB. · March 2023

Welcome to the forum TT,

A good place to start with is your loads (amps@12 volts, Watts, and Watt*Hours) and your expectations (how many hours per day, etc.)... Then look at the load(s) and figure out if you can do any conservation (smaller, more efficient load, etc.). Next do a few paper designs for system to support those loads. Finally, go buy the hardware and build out the system...

To answer your specific question, Yes, you can use good quality float chargers to keep the battery topped off with AC power. HOWEVER, you need to purchase good a good quality float charger and not just a random inexpensive cube charger which may over voltage your lead acid battery and "boil" (i.e., hydrogen+oxygen gassing) it dry. Many "trickle chargers" will overcharge the battery.

What you are looking for is a charger that will "float" the battery around 13.4 to 13.8 volts or so... If it is (long term) charging at >~14.0 volts, it will over charge the battery. Most inverters have a "high voltage" warning of 15.0 to 16.0+ volts.

The bigger question is the design of your system... a 20 AH @ 12 volt battery is not very large and may not supply your load nearly as long as you would like... Measuring the DC load running from battery power (not just 1/2 amp when the inverter is idle, but when the fan is running) is a critical first step. A Kill-a-Watt meter is a good start... And/or an AC+DC current clamp DMM... Some quick links:

https://www.amazon.com/s?k=kill+a+watt+power+meter
https://www.amazon.com/UNI-T-UT210E-Capacitance-Multimeter-Resolution/dp/B075ZHDQFP (inexpensive, good enough clamp meter)
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (mid-range AC+DC clamp meter)

Say you have a 150 Watt AC fan on your stove... Some quick math (estimate):

150 Watts AC load * 1/0.85 AC inverter eff * 1/10.5 volts battery cutoff = 16.8 Amps worst case load current

That would drain your battery to dead (not good for Lead Acid) in, roughly, an hour...

If you wish, give us the DC current from your battery (under load) and we can give an example of the back of the envelope design for your system...

https://pvwatts.nrel.gov/pvwatts.php

Month	Solar Radiation ( kWh / m2 / day )	AC Energy ( kWh for 1,000 Watt solar array )
January	2.64	42
February	3.91	56
March	4.74	74
April	4.67	68
May	4.71	69
June	4.66	65
July	4.88	69
August	5.21	74
September	4.78	67
October	3.96	59
November	2.54	38
December	2.17	35
Annual	4.07	716

For example, if you are near Halifax Nova Scotia Canada, you may get as little as 2.17 hours of sun per day (long term average) in December... If you have a 150 Watt load, that would need a solar array of (December break even):

150 Watts * 24 hours per day * 0.52 off grid AC system eff * 1/2.17 hours of sun per day (Dec ave) = 3,190 Watt array (Dec "break even")

Say you want to power a 150 Watt fan for 24 hours to 50% planned battery discharge (for longer battery life) on a cloudy winter day (no sun)... The battery bank would need to be:

150 Watts * 24 hours per day * 1/0.85 AC inverter eff * 1 day of backup * 1/0.50 max discharge * 1/12 volt battery bank = 706 AH @ 12 volts

Of course, I am just guessing at your system (Watts of fan, hours per day from battery, your location, December worst case usage, etc.)... But it turns out a small/medium size load that runs 24 hours per day (fan, refrigerator, etc.) are "huge loads" for a smaller solar power system... The above example would run a full size refrigerator pretty nicely.

Anything that runs 24 hours per day and is induction motor based--Is difficult to do with "small" solar. Using a more efficient fan, having a thermostat that cycles on/off at 60F on 100% of the time (say 50% duty cycle saves 1/2 the electrical usage), all help.

Your thoughts, corrections, questions?

-Bill

Tonytee · March 2023

Thank you Bill for the feedback. I will dig deeper into this project and get it right.

JRHill · March 2023

I have used one of these for a long time to control the fan behind our wood burner, which is our only heat source:
https://www.amazon.com/gp/product/B01HXM5UAC/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1

Inkbird ITC-308 Digital Temperature Controller 2-Stage Outlet Thermostat Heating and Cooling Mode Carboy Homebrew Fermenter Greenhouse Terrarium 110V 10A 1100W

Is there a safety concern related to the wood burner if the fan isn't running?

Tonytee · March 2023

As far as the safety concern goes, My house is 115 years old and totally made of hand sewn timbers. There is no drywall in the basement to act as a fire break. Also, we load up the basement with around 6 cords of fire wood to last the winter. When the fire is burning, the temp can get quite hot if the fan stops running. If we are sleeping and the power goes out, there could be the possibility of a fire starting. I do have a 9 volt main power failure alarm that will sound (120 Db's) to wake us up. I also have a temp alarm in the main supply duct that will trigger if there is still power, but the fan motor decides to crap out. If a fire started in the basement the house would go up in a flash.

The battery/inverter backup system I have is to keep the fan running only when there is a city power failure while there is a fire still burning in the fire box. I have a backup generator system setup to provide power to the fan as well as other household items ( Fridge, freezers, lights, TV, Internet Etc.) I just use the battery/inverter backup to keep the fan running while I get things setup with the generator and turned ON, so maybe 15 minutes. Also, if during a time when the power goes out, and we are away from the house for whatever reason. The fan would need to keep running until we got back, or the fire burned out. So maybe 1 to 4 hours depending how much wood remained when we left the house. I have an automatic transfer switch that switches to battery/inverter power when there is a power failure. I wanted to know if I could hook up a city electrically supplied trickle charger to the battery while the inverter is connected to the battery and turned on. (The inverter has to be turned ON 24/7/365.) I wasn't sure if it would destroy the inverter. I don't want to setup solar panels to charge the battery. The backup system would only be using battery power when there was a power failure. It would be in standby mode while there was city power. Due to the inverter always in the ON position, it would deplete the battery over time. We don't have that many power failures throughout the year; maybe 1 to 2 on average. That is why I need the trickle charger to keep the battery fully charged, so when there is a power failure the battery would be fully charged, and I would get maximum run time from the fan. I could setup solar panels and a solar charger to charge the battery, but I don't want solar panels on the house. The trickle charger would not use that much power anyway.

Also, With regards to the temp controller you mention, how does that control your wood furnace ?

BB. · March 2023

No--A good quality float charger with good output voltage control will not damage the inverter... Many times, a "poor quality trickle charger" will first "boil the battery dry" from over charging (gassing the battery).

There are inverters with remote on/off and "search mode" (basically send a 120 VAC "pulse" every few seconds... When a load over ~(typically) 8 Watts, then the inverter "turns on".

Years ago, these types of features were relatively rare on smaller AC inverters... Today, there are quite a few smaller/high quality AC inverters that have these features.

Watching for "things" that draw power 24x7--Costs can add up pretty quickly. As an example 6 Watts * 24x7 for 30 days:

6 Watts * 1/0.85 charging eff * 24 hours per day * 30 days per month = 5,082 WH per day = 5.1 kWH per day
5.1 kWH per month * $0.20 per kWH (California is 2x that cost) = $1.02 per month utility costs (est.).

Not a big deal, but these things can add up (inverter tare losses, running a 100 Watt fan @ 50% duty cycle, etc,). Again, another example:

100 Watts * 0.50 duty cycle * 24 hours per day * 30 days per month * 1/1,000 WH per kWH * $0.20 per kWH = $7.20 per month fan

Addressing possible fire issues... Rocking (or concrete backer board on walls/ceiling, tile on floor if wooden) around the fire place, etc. may a good summer project. Would hate to have something bad happened if fireplace overheated or sparks got into the wood pile in the basement.

-Bill

JRHill · March 2023

Tonytee said:

Also, With regards to the temp controller you mention, how does that control your wood furnace ?

It's a wood burning stove stove in the main room and I have a box fan set on low behind it. The thermostatic AC control has a sensor on a cord that is on the rear heat shield up toward the chimney. I set the ON temp at 110F. I still have to control the fire manually.

Tonytee · March 2023

I have a separate oil furnace and a separate wood furnace that are tried together by the ductwork. The oil furnace has the fan ( but we don't burn oil anymore). I built a combustion air damper control using an Arduino board and a metal gear RC servo. I have a temp sensor in the supply duct. I programmed the Arduino with 4 temp & servo settings. I maintain 125Deg F supply air with it.

Connecting an electric 2 amp battery maintainer to a solar system

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Inkbird ITC-308 Digital Temperature Controller 2-Stage Outlet Thermostat Heating and Cooling Mode Carboy Homebrew Fermenter Greenhouse Terrarium 110V 10A 1100W

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