# Why not always connect cells in series to increase voltage in solar component setups? [closed]

I am designing a solar system which I can take to an Indio village deep into the Amazon forest, and thinking about some principles. I am talking about a 2, 3, or even 6 kW system. I see a big problem with 12V DC systems is the enormous cable thickness that's needed (which actually makes me wonder why even bother with cables and not just use solid metal rods insulated with heat shrink tubing, it's not like those husky cables are flexible much?), so I wonder why we would not just generally put more cells in series? Both battery and solar cells?

If I have a 100 m run from the place we install the solar panels (in the sun) to the shed where we have the equipment (in the shadow), then it would probably make sense to put most panels in series to get at least 120 V DC or even more to be able to use manageable cable. No?

Then on the battery side, would it nor make sense to at least put 4 × 12 V batteries in series to make at last 48 V if not 8 to make 96 V.

Essentially, I am wondering why we should not always avoid parallel setups?

I know there is this thing about bypassing partly covered cells, so I am thinking, starting from 60 cell solar panels of 30V and 300W, if I use 4 of them, I would hook them up in series to get to 120V. Then I would put bypass diodes for each one of them, correct? And with that I would have solved all issues that come with series setups? And perhaps I need to dimension the cable to be suitable for half the voltage still without too much resistance, right?

And obviously then I would put 4 12V batteries also in series to get 48 V. The batteries need to remain balanced. To measure that, I once had a universal battery charger board which had 4 wires you would put at the terminals of each battery. I suppose these wires allow the voltage over each battery to be measured individually to understand if they are balanced or not. But I haven't quite seen that in the DIY solar power presentations that I have seen.

Would such voltage measurement inputs not be helpful for the solar panels too? At least for monitoring, even if not much could be done about that?

Given these considerations, why would you ever use parallel setup? (Except, perhaps, for safety reasons, not wanting to run a 1000V DC line near people where someone might accidentally cut it.)

And no, this question doesn't need more focus, because it is a general question. What is the reason for going parallel with anything, other than limiting excessively dangerously high voltage? Obviously matching maximum input voltage of the inverter and the current requirements at that input voltage, but that's a question of choosing the inverter that also has that higher input voltage.

In other words, why would I ever use a 12 V setup if I have more than one 12 V battery? I would better go to a 24 V x 100 A setup already than stay with 12 V and 200 A the two batteries in parallel.

So what am I missing?

• The correct gear depends a lot on what kind of loads you want to power with it, especially at 6kW. So please give more details. Is it a lot of 12V DC LED bulbs? Or AC mains equipment? Water pump? (got to think about the starting current) etc? Commented Apr 19 at 15:55
• Why not use 24v or 48v ? there are inverters available that work on those voltages. Also inverters that take higher DC voltages. Commented Apr 19 at 16:44
• @SolarMike that is exactly my question. Commented Apr 19 at 16:50
• No, you seem to be asking about "parallel"... Commented Apr 19 at 16:51
• There is a language problem here. "... why anyone would put two 12 V batteries in parallel and not in series to achieve higher voltage" can be read two ways. It could be "why anyone would put two 12 V batteries in parallel ... to achieve higher voltage" or "why anyone would [not] put two 12 V batteries in series to achieve higher voltage". I've read it four times and I don't know which you mean. I'm a native English speaker but I suspect that both of you are working in a second language (rather well, apart from this hiccup). Be cool, guys! Commented Apr 19 at 19:10

Talking about a 2, 3, or even 6 kW system, I see a big problem with 12V DC systems is the enormous cable thickness

Well yes.

The advantage of 12V is safety versus electrocution and it also works well with car/boat equipment. But for 6kW it is inadequate. Also due to the ridiculously high currents, safety against fire is not necessarily obvious...

At 3-6kW you'd be looking at a 48V battery system, and 6kW is already on the high end of that since that's more than 100 Amps already. That's what I have at home, it works fine.

There are 24V systems in the 3-5kW range but they are not optimal because current is impractical at this level of power.

Once you pick a battery voltage, you're married with it. You can still add more capacity by adding more batteries in parallel. For example I have 4x 48V LiFePo4 rack batteries in parallel. It's possible to wire lead acid batteries in series to reach 48V, but 48V Lithium requires specific BMS for protection and these require the batteries to not be wired in series.

If one of the batteries die I can simply disconnect it. For your jungle application I would not recommend high voltage batteries (>100V) because while they are more efficient they are proprietary and usually consist of a BMS (single point of failure) along with batteries wired in series.

If I have a 100 m run from the place we install the solar panels (in the sun) to the shed where we have the equipment (in the shadow)

Indeed transporting low voltage/high current over 100m would be a problem so you need to wire solar panels in series to increase the voltage.

I know there is this thing about bypassing partly covered cells, so I am thinking, starting from 60 cell solar panels of 30V and 300W, if I use 4 of them, I would hook them up in series to get to 120V. Then I would put bypass diodes

Normally the bypass diodes are included in the panels. When wiring panels in series, it can reach a high DC voltage, so it's important to have good connectors and keep the wiring tidy. Panels without bypass diodes would require a lot more wiring with tees, that would be a mess.

How to combine them depends on:

• Allowed input voltage and current of your MPPT charger.

PV's open circuit voltage and short circuit current must be below maximum allowed voltage ; PV's MPPT voltage and current must be in the MPPT range. Typically you'd wire them in series up to the MPPT's nominal input voltage, then perhaps wire several series strings in parallel if the MPPT can accept the current.

• Illumination: ideally you want all strings connected to the same MPPT to receive the same illumination.

If some panels in a series string are shaded, voltage will drop, which is okay. If a string which is in the shade is in parallel with a string that is in full sunlight, either some of the power may backflow into the shaded panels, or the MPPT will reduce its input voltage. In either case, not optimal. If two strings receive different illumination they should be connected to different MPPT channels so the MPPT can adapt operating conditions on each string.

Paralleling strings requires protection. If a panel shorts out due to damage, all the other panels in parallel will dump power into it, which can start a fire. So you need properly rated DC breakers. Note AC breakers used with DC tend to spontaneously combust due to being unable to extinguish the DC arc.

Anyway, with your 100m distance I'd recommend picking a high but safe voltage for transport between the panels and the battery shed, something like 100-600V. For 6kWp the ideal arrangement would be two series strings of 3kWp which avoids headaches with paralleling. You will need double the wire, but if you double them you'd need thicker wire, which is not necessarily cheaper.

All this depends ont the MPPT and inverter hardware, which depends on what the loads will be.

If most of your loads are at AC mains voltage, I'd recommend an all in one hybrid PV inverter with a 48V battery. There are many brands. 48V is standard, so you can pretty much use any battery with any inverter. Do not use a grid-tied inverter as it will not work without grid, it needs to be hybrid. If it has an input for a generator, that's a plus.

You can also use separate MPPT chargers and a 48V inverter.

The former is called "single conversion" because PV power is converted to mains directly, whereas the latter is called "double conversion" because PV power is converted to 48V then to mains voltage. Single conversion is more efficient, but results in all-in-one systems where, if it breaks, then nothing works, and you have to find one that fits your use (shouldn't be too hard). Whereas separate boxes with double conversion are more flexible, since you can mix and match, and if one element dies that doesn't stop the rest from working.

In particular, if you use separates, you don't need to parallel PV strings, because you can add more MPPT chargers. But if you go with an all-in-one with 2 MPPT inputs and you need 3 series strings due to shading, then you'll need to parallel two of them.

• Thank you for this answer. That's very helpful. I am surprised you could even give your answer after all this hostile shooting down of my question. As a general rule I think I took from your answer that solar panels are indeed preferred in series up to some maximum voltage and batteries in series are problematic to keep them balanced, I suppose in parallel a battery management system could charge each separately and thereby balance them, in series that's not possible. Also charging voltage cannot be well maintained for each battery in series. So thanks again. Commented Apr 20 at 2:19
• Yes that's the idea! Commented Apr 20 at 7:50

You can follow the instructions of my project, see photo: