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AC Charger, Automatic Transfer Switch and Battery Life

TEDthebed

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May 18, 2021
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Batteries: Two (2) AGM 100 AH 12 volt in series (24 volts)
Solar Panels: Two (2) 100 Watt in series
PWM Solar Controller
Primary Use: 25 cu ft Fridge and a couple of lights

I'd like to get a 24 volt AC charger and keep it on the batteries at all times so that when there is insufficient sun and at night, the batteries will stay charged. I can't seem to find the maximum charge level, float level and boost level for any of the cheaper 24 volt chargers that I have found on Amazon. Does anyone know what these might be? 24 volts for all three? 24 volts is probably too low(?)

I am also considering an automatic transfer switch (ATS) to switch back to the grid if the batteries' DOD drop below 50-60%. I think I understand how to install and configure the ATS, but am wondering about any problems that I may run into with the AC charger also installed. I don't think it'll be a problem, but I have been wrong in the past … lol.

Also, my understanding is to occasionally (monthly?) discharge AGM batteries some for longer life instead of keeping them fully charged at all times. Is this also the consensus?

Thanks for any help.
 
25 cu ft Fridge and a couple of lights
Without real power usage numbers, everything is a guess.

So, I am guessing your fridge is like mine, a 27cuft energy star model.
Mine uses about 125w for about 20 min per hour. It idles at around 25w for 40 min per hour.

125w x .33h = 41wh per hour ... x24h = 990wh per day
25w x .66h = 16.5wh per hour ... x24 = 396wh per day
990wh + 396wh = 1386wh per day for your fridge

(2) AGM 100 AH 12 volt
2x 12.8v x 100ah x .5 dischargeable = 1280wh

Fully charged this will run your fridge for about 1280wh/1386wh = .92 days (22 hours)

1280wh battery / (2x 100w panels) = 6.4h to recharge (with no load)
6.4h / 5h per day = 1.28 days to recharge batteries with no load.


Looks like you are mostly very under paneled.
 
Thank you. I know I am under paneled that's why I want to add the AC charger.
 
Consider using a 30V 10A bench power supply similar to this - DC Variable Power Supply - connected full time to your batteries.

Set it at your lower discharge voltage, say 24.4 volts. Your solar will charge above that level and the power supply will kick in only when your batteries are at or below that point.

It would replace the functions of the charger and ATS you mention.
 
Consider using a 30V 10A bench power supply
How much standby power do you suspect that would consume?
I don't own a bench top charger or i would check. Just thinking that it would consume quite a bit of the power produced by 200w of panels in a 24h period.
 
Batteries: Two (2) AGM 100 AH 12 volt in series (24 volts)
Solar Panels: Two (2) 100 Watt in series
PWM Solar Controller
Primary Use: 25 cu ft Fridge and a couple of lights

I'd like to get a 24 volt AC charger and keep it on the batteries at all times so that when there is insufficient sun and at night, the batteries will stay charged. I can't seem to find the maximum charge level, float level and boost level for any of the cheaper 24 volt chargers that I have found on Amazon. Does anyone know what these might be? 24 volts for all three? 24 volts is probably too low(?)

Typical 24V AGM float values are 27.2-27.6V.

Also, my understanding is to occasionally (monthly?) discharge AGM batteries some for longer life instead of keeping them fully charged at all times. Is this also the consensus?

No. Lead-acid batteries last longest when kept at 100% SoC.
 
How much standby power do you suspect that would consume?
Good question, I can check and report back when I get home in a few days, but it shouldn't be much. Maybe someone else has a good number.

I'm thinking it would be plugged in to grid power so it would not consume anything from the batteries.

I never had a variable DC charger before this year and I find that I use it all of the time for different things.
 
Consider using a 30V 10A bench power supply similar to this - DC Variable Power Supply - connected full time to your batteries.

Set it at your lower discharge voltage, say 24.4 volts. Your solar will charge above that level and the power supply will kick in only when your batteries are at or below that point.

It would replace the functions of the charger and ATS you mention.
Hey JeepDaddy, Thanks for the idea. My understanding of this device is that if my batteries were to reach a certain preset minimum, this power supply would provide up to 10 amps of power to the batteries from the grid to supply to my Inverter until the batteries are charged by the power supply to a preset maximum, hence no need for the ATS. Right?
PS: There were several complaints about reliability on Amazon. Do you have any experience with it?
Thanks again.
 
Hey JeepDaddy, Thanks for the idea. My understanding of this device is that if my batteries were to reach a certain preset minimum, this power supply would provide up to 10 amps of power to the batteries from the grid to supply to my Inverter until the batteries are charged by the power supply to a preset maximum, hence no need for the ATS. Right?
Exactly. It would keep your batteries from going below whatever voltage you specify. It would not charge your batteries, per se. Your solar would take care of that.

My power supply (the one in the link) has worked fine for me, but you never know.

I would also recommend using upgraded leads to minimize voltage drop. A lot of people use 10 AWG wire with ring terminals instead of the banana/alligator leads that come with the power supply.
 
Exactly. It would keep your batteries from going below whatever voltage you specify. It would not charge your batteries, per se. Your solar would take care of that.

My power supply (the one in the link) has worked fine for me, but you never know.

I would also recommend using upgraded leads to minimize voltage drop. A lot of people use 10 AWG wire with ring terminals instead of the banana/alligator leads that come with the power supply.
So, if it does not charge the batteries, would it still be connected to the same battery terminals that connect to the Inverter?

Also, when the batteries are recharged to a certain level, would it automatically shut off?

I use 6 AWG wire connecting the batteries together and from the batteries to the Inverter which is 18" away.

Thanks again. This is a great idea.
 
So, if it does not charge the batteries, would it still be connected to the same battery terminals that connect to the Inverter?

Also, when the batteries are recharged to a certain level, would it automatically shut off?
Yes, and yes. It would just keep your batteries from going below whatever voltage level you select.
 
You could use a lighting controller which could be used to turn on the charger at night. This would not take care of the low light conditions but will solve the night time for about $15.00.
 
You could use a lighting controller which could be used to turn on the charger at night. This would not take care of the low light conditions but will solve the night time for about $15.00.
Problem I see with this approach is cloudy weather which could be mitigated I know by manually turning on the charger. Also, I'm not sure a $30
4 or 5 amp charger could keep up with the load over a couple of cloudy days. The power supply idea seems to make the most sense. I'm going to give it a shot and update. Thanks
 
Hi
I am in a similar situation with being under-paneled;).
Built a 10 amp charger which I plan to use at night while panels are hibernating, pushing 10hrsx10amps back into the system and in the morning, let the Solar controller take over.
 

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