Adding panels to existing system

experimenter · December 2014

I have a 48V system now with only three 250W panels. I believe these are connected in series with each other (but am not sure). I have another 6 panels on the way, the new 6 are 275W panels because the original panels are no longer available. I'm wondering how best to connect these -- I won't actually be doing the connecting, an electrician will, but I want to make sure they are connecting them in the most efficient way. I assume that the electrician will be using the existing set as a pattern, and just repeat the pattern twice more so that I end up with 3 groups of 3 panels each. But maybe there is a better way?

The rest of my system is as follows:

1. Morningstar TriStar MPPT-60 charge controller
2. Magnum MS4448PAE Inverter
3. Bogart battery meter
4. 4 104AH 12V batteries (SunExtender PVX-1040T batteries) -- I will also be adding another 4 of the same batteries for a total of 8. These are/will be connected to give me 208 AH of 48V electricity.

Cariboocoot · December 2014

Re: Adding panels to existing system

Usually a 250 Watt or 275 Watt panel has a Vmp around 30, so on a 48 Volt system you would have three in series in order to have sufficient Voltage to charge the batteries: array Vmp of about 90, minimum needed for 48 Volt system is 70.

Your existing array is 750 Watts, which would put out about 12 Amps on a 48 Volt system. This is in fact sufficient for a battery bank of 104 Amp hours. Doubling the Amp hours means you need to increase the array, just as you are planning.

However, six 275 Watt panels is 1650 Watts and should produce 26 Amps on its own. With the previous 750 Watts you'd be at 38 Amps total. On 208 Amp hours that would be 18% charge rate. Fortunately these are SunXtender AGM's and they can take that sort of charge rate.

You should indeed end up with an array of three parallel strings of three in series. Note that the Vmp should be close on the panels (preferably within 5%) and that each string should have a fuse or breaker on it, best done with a combiner box.

If that is inconvenient/too expensive you don't really need that much panel for 208 Amp hours. The addition of a single string of the 275 Watt panels would suffice for that much battery and would not require the per string over-current protection. You would in either case need to make sure the wiring from the array to the charge controller can take the increased current, and the same for the output from the controller to the batteries.

experimenter · December 2014

Re: Adding panels to existing system

Thanks Cariboocoot, you've confirmed what I was thinking.

Now, about adding 6 additional panels instead of just 3 additional -- I was thinking having 6 more panels would allow me to maintain more battery charge during daylight hours when there is a load on the system. (Hoping that is clear -- using your number for example I have current 12 amps of charging on the 48 V system, plus 13 amps more from an additional 3 panels gives me 25A charging (when the panels are able to put out full power generation). At times when I am loading the system with say 20 amps of draw, now I will still have a +5A charge capability (whereas today with only 12A from the panels I am drawing down the battery charge by -8A in this condition.) And (I think) that in twilight hours when the panels are not putting out peak power generation having 3 times of something is better than 0.)

In other words I hope adding 6 panels instead of just 3 more is not a waste, and that the final 3 will still be adding capabilities to the overall system.

Cariboocoot wrote: »

Usually a 250 Watt or 275 Watt panel has a Vmp around 30, so on a 48 Volt system you would have three in series in order to have sufficient Voltage to charge the batteries: array Vmp of about 90, minimum needed for 48 Volt system is 70.

Your existing array is 750 Watts, which would put out about 12 Amps on a 48 Volt system. This is in fact sufficient for a battery bank of 104 Amp hours. Doubling the Amp hours means you need to increase the array, just as you are planning.

However, six 275 Watt panels is 1650 Watts and should produce 26 Amps on its own. With the previous 750 Watts you'd be at 38 Amps total. On 208 Amp hours that would be 18% charge rate. Fortunately these are SunXtender AGM's and they can take that sort of charge rate.

You should indeed end up with an array of three parallel strings of three in series. Note that the Vmp should be close on the panels (preferably within 5%) and that each string should have a fuse or breaker on it, best done with a combiner box.

If that is inconvenient/too expensive you don't really need that much panel for 208 Amp hours. The addition of a single string of the 275 Watt panels would suffice for that much battery and would not require the per string over-current protection. You would in either case need to make sure the wiring from the array to the charge controller can take the increased current, and the same for the output from the controller to the batteries.

Cariboocoot · December 2014

Re: Adding panels to existing system

The practicality of it depends on whether or not you are likely to have such draws. 20 Amps @ 48 Volts is 960 Watts, somewhat less on the AC side. Is this what your consistent loading would be?

I mention this because most systems do not have such high load demands and can manage to charge batteries and run average loads simultaneously with just the 10% charge rate. But if you have some exceptional power needs, by all means go for the extra panels. Otherwise you're buying 825 Watts of PV that won't really be doing much for you most of the time.

Adding panels to existing system

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