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Pairing solar Panel Voltage w/ 24VDC System

I am designing my system around a 24 VDC inverter that I already have.  I also have panels that are 6VDC each.

What is the optimal voltage that I should wire my solar array up to?

Depends on the Charge controller you plan to use.

Panels > Charge controller > battery system > inverter
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(07-12-2020, 08:16 PM)daromer Wrote: Depends on the Charge controller you plan to use.

Panels > Charge controller > battery system > inverter

I have flexibility on the charge controller and BMS.  

I have 800 18650 cells rights now, and can have not picked out the charge controller nor BMS.

I am leaning towards the electrodacus, but may go with a Chinese BMS (don't know name, but the red one everyone is using and the 7510 MPPT controler.
Once you settle on the charge controller (ie an MPPT charge controller, different to a BMS), then you'll have an idea of the optimal Vmaxpower for your solar panels.
For a 24V system, people go with a 7S battery config.
So 7S x typical cell "full" of 4.1V = 7 x 4.1 = 28.7V
a) If you're going for a simple PWM charge controller, you want the solar input Vmp just above the battery full voltage eg 29 - 30V for best efficency.
b) If you're using a proper MPPT charge controller you would want the solar input voltage sitting at the controller's best efficiency point (typically 5-10V above the battery volts), ie more like 34 - 40V or so.
So (assuming "6V" is Vmp for your panels, not Vopen-circuit) you could go with either 5 x 6V panels in series for a) or maybe 6 in series for b)
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Can you share how many panels / total wattage you plan to deploy?   This will also affect what you choose.
(07-13-2020, 02:33 PM)OffGridInTheCity Wrote: Can you share how many panels / total wattage you plan to deploy?   This will also affect what you choose.

Thanks for the follow up mr offgridinthecity!

I have a total of 6 50 W panels @ 6 v each.  I can wire them up in either series or parallel (or sell them on craigslist and buy other panels).  They are just what I have on hand.

Below are some notes
Since the charge controller you had in your first post is a boost controller MPT-7210a unlike most MPPT controllers
The voltage of your array needs to be below the voltage of your battery. OC voltage
The amps of your array needs to be below 10 amps. Short Circuit amps
3 panels in series none in parallel would give you 20.4v x 7.36A = 150.144 watts you might be able to have 4 in series not sure
Another controller might work better
Afaik the 7210 is boost only.
later floyd
6 x 50 = 300watts of PV.  300w/24v = 12.5a.   6s = 36v = OK for 24v battery. 

For the MPT 7210a the 18v (3s2p) panel config should work.  If you already own the MPT 7210a, then definitely try it.   The output is limited to 10a (not the input) so you would potentially be leaving 20% of charging capability on the floor - but on the other hand, static panels are rarely more than 80% efficient for any significant portion of the day - so 6 x 50w panels would match up very well.

Next - you could move up  to a dedicated PWM charge controller.... 

The really cheap charge controllers ($15) don't do lithium / are not settable - so unless you find a specific one for lithium (which might be out there somewhere) - its not going to work for you. 

Next would be lithium friendly PWM such as these:
I would look for a lithium compatible charge controller such as this:     ...   but you probably need the $39.11 Bluetooth module as well - so $60 range.

This one has a screen - maybe enough and more like $52

Next - you could move up to a true MPPT charge controller....   such as this:

I started with one of these -  and withe the external unit, they fully settable for your lithium-ion battery bank.

Next would be large power charge controllers - but that's beyond this discussion.

The ones I link to above are just suggestions that are representative of the price range for the power range.  This also depends on $ you're willing to spend, long term goals, etc.   As you clarify your longer term goals and can say how large you want your PV input to be - I'm sure you can get several specific recommendations.

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