Question about solar for 48 volt battery bank

[phmccartney]

I have an electric sailboat currently powered by a 48 volt bank of traditional golf cart batteries. I am planning to upgrade my battery bank to LifePo4, either 2 or 3 banks of 12 volt 100ah batteries. I want to add solar to charge them, but un-shaded space on a 35 foot sailboat is limited. Due to the position of the backstay and the extent of the boom, my optimal configuration would be two 60 cell panels (typically 66" x 40"). Here is my question. Most of the 60 cell panels i see, have a Voc of around 40 volts and Vmp of around 33 volts, and are rated by some as a "20 volt panel" (eg Trina Solar TSM-DD06M.05-325) rather than a "24 volt panel". It not clear to me if two of these in series would generate enough volts to charge a 48 volt LiFePO4 battery back. I know that a 600 watt system wont generate many amps and may take several days to recharge the system if deeply discharged, but my question is whether they can generate enough volts to charge at all or if i should try hold out for panels that are capable of a higher Vmp, even if it means having to ship them rather than find a local pick up? I plan to go with a Victron 48 volt solar charger which will not have boost capability, so i think I need to be sure the panels will have enough voltage even in hot weather. Ive looked at options for using more panels in series, but all the configurations I've tried seem to increase weight aloft and result in lower total watts due to the constraints of placement on my boat. Thanks for any advice.

Peter

 

[DThames]

If they are in series, both need to be "not shaded" in order to produce any usable power. 60 cell will charge 24v battery, so two in series would charge 48v. Because of shading issues, if you can find a panel that would not require a series string, that would be better. You could put some in parallel where you would have sun or shade.....whatever was in the sun would then be producing. 96 cell panel might work.

 

[jameshowison]

Very unlikely to give you the needed voltage reliably. You’ll need something like 60v for reliable startup. You also need to adjust for the heat, Voc is lower in hot temps (and the temp that matters is the temp of the panel not the air).

Use the victron MPPT calculator and enter the panel details. You usually have to call for the temp coefficients.

For example I have the Rich solar 160w cigs panels. With three I thought it would charge reliably, but the heat meant I really needed four in series. Perhaps the cigs handles shading in series better?

If shading is a giant issue perhaps charge a 12v pack then use a 12-48v bridge. I don’t know much about them but wakespeed has the ws3000 and there are others. Speedy though.

 

[DThames]

Very unlikely to give you the needed voltage reliably. You’ll need something like 60v for reliable startup. You also need to adjust for the heat, Voc is lower in hot temps (and the temp that matters is the temp of the panel not the air).

Use the victron MPPT calculator and enter the panel details. You usually have to call for the temp coefficients.

For example I have the Rich solar 160w cigs panels. With three I thought it would charge reliably, but the heat meant I really needed four in series. Perhaps the cigs handles shading in series better?

If shading is a giant issue perhaps charge a 12v pack then use a 12-48v bridge. I don’t know much about them but wakespeed has the ws3000 and there are others. Speedy though.

I missed the "use Victron...." statement....please disregard my specific panel suggestion. I have seen some odd voltage panels up around 90v but they are some old solar farm oddballs.

 

[jameshowison]

I missed the "use Victron...." statement....please disregard my specific panel suggestion. I have seen some odd voltage panels up around 90v but they are some old solar farm oddballs.

Why would the 96 cell panels you linked not work with victron MPPT? They might be a bit big for OPs sailboat, but two of those in parallel seems feasible? Voc around 70v, could even work with the 100v max victron MPPTs (especially if low temps never encountered).

 

[rmaddy]

You usually have to call for the temp coefficients.

Or get the numbers off of the spec sheet online.