Calculating maximum panel watts for 48v AGM bank? (4x 12v 65ah)

AutisticMorty1111

New Member
Joined
Nov 8, 2021
Messages
4
I've been looking at building a 48v emergency back-up power system, with hopes to add panels and batteries in the future.

My intention was to build a temporary emergency back-up, and then in a year or so I'll be moving the system to power an off-grid tiny house that I'll be building, where I'll be adding more panels and batteries.

I wanted to start with:
  • 4x 340W panels (40v/10a each, in formation of 2S2P)
  • 48v 2000W inverter (peak 4000W)
  • 48v 30amp charge controller
  • 4x 12v 65ah AGM batteries
But I learned that the 12v 65ah batteries should only be charged at 6.5a (maximum 19.5amps, but will cause degradation).

So that means I should only connect 312 Watts of solar panels (48v x 6.5a), correct?

So having 1360 watt of panels is pointless...?

I wanted to start with a 48v system, so when I upgrade in the future, I can continue using the inverter and charge controller.

I'm in Europe and LifePO4 is too expensive right now, so I don't want to buy 4 lithium batteries needed for the 48v system, I wanted to wait another year or two for prices to fall before getting lithium.

I was hoping I could oversize the system, so I could use the electricity generated during the day to ie. run laundry, using the energy before it was fed into the battery. But I guess that won't be possible unless I get an all-in-one system that will let me set a max. charge current, while being able to consume the higher generated current as needed...?

Thoughts? I think I'm back to square-one on the drawing board. Not sure what to do now.
 

sunshine_eggo

Happy Breffast!
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Oct 26, 2021
Messages
1,886
Location
USA
I've been looking at building a 48v emergency back-up power system, with hopes to add panels and batteries in the future.

My intention was to build a temporary emergency back-up, and then in a year or so I'll be moving the system to power an off-grid tiny house that I'll be building, where I'll be adding more panels and batteries.

I wanted to start with:
  • 4x 340W panels (40v/10a each, in formation of 2S2P)
  • 48v 2000W inverter (peak 4000W)
  • 48v 30amp charge controller
  • 4x 12v 65ah AGM batteries

65Ah AGM for the other components is like putting a Prius engine in a Bugatti. You should likely have about 300Ah AGM for that setup.

But I learned that the 12v 65ah batteries should only be charged at 6.5a (maximum 19.5amps, but will cause degradation).

Established by YOUR battery specification. General rule is 0.1C. Many are tolerant up to 0.15 to 0.20C. Most are tolerant to 0.3C under 55.2V.

So that means I should only connect 312 Watts of solar panels (48v x 6.5a), correct?

48V is nominal. Using the above 55.2V, 55.2 * 6.5 = 358.8W.

However, if you keep the absorption at 55.2 and float at 54.4, you could easily accept 3 * 358.8 = 1076.4W Given that you'll likely never be in a situation where you'll get the full 1360W from your array (you'll likely be in absorption well before peak solar), it's not an unreasonable configuration, albeit suboptimal.

So having 1360 watt of panels is pointless...?

I wanted to start with a 48v system, so when I upgrade in the future, I can continue using the inverter and charge controller.

You should probably take a step back and design the 48V system you want to end up with based on your energy needs and available solar - then decide how you can piece it together over time.

I'm in Europe and LifePO4 is too expensive right now, so I don't want to buy 4 lithium batteries needed for the 48v system, I wanted to wait another year or two for prices to fall before getting lithium.

Good luck with this. LFP prices have increased BEFORE the global supply crunch. Increased demand for lithium from the EV and powergen market are going to put UPWARDS pressure on LFP prices. The last "halving" of LFP costs took a little over 4 years, the industry says that the next "halving" will take 10-12 years assuming demand doesn't change.

Now is as good a time as any to get LFP, especially if you can do DIY and save boatloads.

I was hoping I could oversize the system, so I could use the electricity generated during the day to ie. run laundry, using the energy before it was fed into the battery. But I guess that won't be possible unless I get an all-in-one system that will let me set a max. charge current, while being able to consume the higher generated current as needed...?

Your 65Ah setup would work fine for that. The key is to keep the absorption voltage to 55.2 and to take the system offline when solar is no longer available.

In fact, using loads WHILE charging takes stress off the battery diverting charge current from the battery to the inverter, so it's something of a solution.

You'll just need to pay attention.

Some MPPT like Victron allow you to set a max charge current. You could default it to 13A if you're not planning to use the system and bump it to 30A if you plan to use loads.

Thoughts? I think I'm back to square-one on the drawing board. Not sure what to do now.

Plenty. See above. :)
 

wattmatters

Photon Sorcerer
Joined
Apr 16, 2021
Messages
1,062
But I learned that the 12v 65ah batteries should only be charged at 6.5a (maximum 19.5amps, but will cause degradation).

So that means I should only connect 312 Watts of solar panels (48v x 6.5a), correct?
Use a charge controller for which you can set the maximum charge current limit.

Having the extra panels will be useful in low light conditions, early and late in the day, cloudy and rainy days. Else the charge controller will just pull what it needs from the array and no more.

65Ah * 48V nominal = 3.12kWh of nominal storage, however to get any sort of useful life from your AGM batteries you do not want to be discharging them below 50%, so in effect you have about 1.5kWh of useable storage. Whether that's enough for your backup needs really depends on what you need to power and for how long. That will keep a modern efficient medium sized fridge running for about a day.

I was hoping I could oversize the system, so I could use the electricity generated during the day to ie. run laundry, using the energy before it was fed into the battery. But I guess that won't be possible unless I get an all-in-one system that will let me set a max. charge current, while being able to consume the higher generated current as needed...?

You'll need to choose your all-in-one inverter wisely. While most will supply power to loads over and above any energy used to charge (or float) the battery (assuming the PV array can supply it), some of these inverters limit the PV output to a power no more than the maximal battery charge current limit. e.g. if the battery is 48V and the charge current limit is set to 10A, then the inverter will draw no more than 480W from the PV array no matter how much load there is.

Here's an example of that in action when I tested my system by changing the battery charge current limit between 20A and 40A. At all times during the test the load is in excess of what the PV array can supply and hence battery was always discharging.

As you can see, when the charge limit is 20A the PV array output is limited to ~1kW, when I up the charge limit to 40A the PV array's output increases to the max it can supply at that time (which was less than 2kW).
 
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