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Small Cabin Forum / Off-Grid Living / Solar charging question
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# Posted: 7 Jan 2019 12:14

I am setting up a solar system.
It will be comprised of four 300 watt panels wired as 2 panels in series as two strings then connected together in parallel. This will feed into a Midnight Kidd charge controller. The charge controller should charge my battery bank at the rate of 21 amps at 57.6 volts from this panel array setup.

Now I will be using eight Trojan T150 batteries with an AH rating of 225AH.
I have two choices as to how I wire this battery bank.

24 Volt or 48 volt

The 24 volt arrangement would be as follows:
Two strings of 4 batteries wired in series then connected parallel.
225AH x 2 = 450 AH @ 24 volts = 10800 watt hours.

The 48 volt arrangement would be:
One string of 8 batteries connected parallel.
225AH @48 volts =10800 watt hours

Would the charge rate of 21 amps at 57.6 volts be better suited for the 48 volt system @ 225AH rather than the 24 volt system recieving 21 amps at 57.6 volts for 450AH?

The other question is would two strings of 4 batteries charge more evenly than one long string of 8 batteries?

Because the panel wiring layout would have the Kidd operating at about half it's limit I prefer to keep it wired as it is designed above. This way later on I can double the panels and double the battery bank using the same charge controller.

Is my math correct? Also are my questions valid?
I think I'm better off with the 48 volt battery bank wiring but would like other's opinions on this.

Also once I expand, 4 strings of four batteries each in 24 volt sounds like too many strings whereas 2 strings of eight batteries as a 48 volt layout sounds more in line.

# Posted: 7 Jan 2019 13:51

I prefer 48v as the system efficiencies between solar controller and inverter add up. Albeit to around 3-4%. Lithium batteries add 20% so ....

Lead acid, from what I've learned, parallel strings are best kept to a minimum.

Your solar controller will change the 57.6vx21a to whatever the battery needs; therefor your amperage rises to 42a at 24v. If the kid handles 42a. Otherwise you're seriously over paneled.

I don't know the specs on the kid. 30a?

# Posted: 7 Jan 2019 19:05 - Edited by: LastOutlaw

Yup 30A... will be setting up the batteries at 48 volt.

I'm guessing that means a string of eight six volt batteries.

Also can not double the panels in the future without adding another controller or running a different one. This Kid is at 70% according to the sizing estimator on their site.

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