Saturday at home, installing an inverter

[Dave A]

The number of batteries is immaterial as long as the inverter has sufficient to supply it with the correct input voltage. A 24v pure sine inverter will always produce a pure sine wave output regardless of the battery configuration as long as it's getting 24 volts from them. The inverter doesn't 'see' the individual batteries as such it just sees a pool of energy at a certain voltage so a 24volt inverter needs a minimum of 2x 12v batteries in series or 4x 6v batteries in series, it makes no difference which.

Just a thought - One thing that might warrant consideration is voltage drop due to internal resistance in the battery, which would double (or more) at double the operating current. The simple solution to counter this would be to double up on the batteries in parallel.

[AndyD]

A lead acid battery is made up internally of multiple cells connected together. Each cell is basically 2 plates of dissimilar metals immersed in electrolyte (battery acid) and each cell produces about 2 volts nominally.


2V Cell

The size of the cell plates required to produce a high current is fairly large and wouldn't fit into a sensible sized battery casing so they cut the large plates into smaller ones and connect then together in groups like this.


2V Cells

These are still a single 2 Volt cell same as the top drawing but it's just a more size-friendly format.

For a 6 volt battery they'd take 3 of these cells and plonk them into a case and wire them in series.


6V Battery

For a 12V battery they'd take 6 cells and wire in series.


12V Battery

[AndyD]

So whether you use 4 x 6Volt batteries cabled in series or 2x 12Volt batteries cabled in series to supply your 24Volt inverter makes little if any significant difference. Whichever combination you go for you've still effectively got 12 basic cells in series so they're topographically the same when it boils down to it.

[Dave A]

A great explanation of the relationship between current and reactor plate surface area, Andy

I mentioned the issue more around the 48V vs 24V difference in the inverter systems. I was concerned someone might have thought halving the voltage of the battery system of the inverter would result in only half the number of batteries being required.

[Justloadit]

What is also important, when placing batteries in series and parallel, is that they must be of the same batch and same age. Rule of thumb is that when one battery of the series or parallel pack goes, the whole pack should be replaced.

The implications comes in the charging process, the faulty battery fools the charger and either under or over charges the full pack, because of incorrect feed back from the pack due to the faulty battery. Replacing the faulty battery with a brand new one, which will have discrepancies in the characteristics to the other batteries, will probably never work to its full potential, or may be over worked depending on how the battery pack is set up, series or parallel.

This situation is even more prevalent in Lithium battery packs.

[AndyD]

....I mentioned the issue more around the 48V vs 24V difference in the inverter systems......

Ahhh, I thought you were referring to the post you quoted where it says 'The inverter doesn't 'see' the individual batteries as such it just sees a pool of energy at a certain voltage so a 24volt inverter needs a minimum of 2x 12v batteries in series or 4x 6v batteries in series, it makes no difference which.'

[AndyD]

What is also important, when placing batteries in series and parallel, is that they must be of the same batch and same age. Rule of thumb is that when one battery of the series or parallel pack goes, the whole pack should be replaced.

The implications comes in the charging process, the faulty battery fools the charger and either under or over charges the full pack, because of incorrect feed back from the pack due to the faulty battery. Replacing the faulty battery with a brand new one, which will have discrepancies in the characteristics to the other batteries, will probably never work to its full potential, or may be over worked depending on how the battery pack is set up, series or parallel.

This situation is even more prevalent in Lithium battery packs.

Faulty batteries can present issues. We keep on-site and off site records of battery banks which is useful and with most of our installations we have a 6 month service contract where we check the individual batteries for failed or short circuit cells, take hydrometer readings, top up electrolyte levels and check the output under load etc.

The best strategy is to try to avoid single batteries going faulty in the first place. There's some key proceedures to be followed when installing battery banks and all too often they get overlooked which causes premature failures of a single cell or battery. A few key points are;

• Arrange the batteres in such a way that they all have sufficient space between them for cooling.
• Terminations are critical especially when there's more than one string of batteries in parallel. A poor termination upsets the balance between the batteries, they won't be loaded evenly, they'll run at different temperatures and they won't charge evenly. Poor or corroded terminations are very common especially when contact paste isn't used on the lugs and terminals.
• Never let the electrolyte levels drop below the top of the plates.
• Set the minimum battery voltage correctly on the inverter. We come across inverters where the min batt voltage is still on it's manufacturer default setting and the batteries are being over-cycled.

[IMHO]

I heard or read somewhere that the higher your battery voltage, the better. More efficient conversion or something. The downside is the higher risk, and that you need thicker cabling. Any truth in that?

I was still waiting for your answer on my laundry room, Andy, but subsequently learned that it will have to be such a big installation, that it is not really worthwhile the effort to do the calculations. So do not waste your time on that. Thanks.

[Dave A]

I heard or read somewhere that the higher your battery voltage, the better. More efficient conversion or something. The downside is the higher risk, and that you need thicker cabling. Any truth in that?

You certainly wouldn't need thicker cabling.

The thickness of the conductive cable required is determined by the current, not the voltage. To produce the same power output off a higher battery voltage base would require less current, therefor thinner cables.

Spot on the efficiency issue though. When it comes to line resistance losses, the critical formula to bear in mind is E = I²Rt
Assuming the same line resistance, half the voltage would require double the current to produce the same power. But in doubling the current your line resistance loss in energy quadruples (2²).

[IMHO]

Would this be true as well?

Assuming the same line resistance, double the voltage would require half the current to produce the same power. But in halving the current your line resistance gain in energy quadruples (2²).

hehe, what do I know?

[smitty]

Disclaimer I'm a layman.

Longer cables of a given conductor material and thickness result in greater resistance.

Thicker conductors of a given conductor material and length result in lower resistance.

So keep your cables short and thick to reduce resistance. Oh and stick to copper for your conductor.

[SilverNodashi]

Simple rule of thumb with DC voltages: keep the cables as short as possible, and thick enough to pass through all the current you need.
Generally, a 48V system would perform better than a 24V system, for a couple of reasons: V * A = Watt, so 24V * 100A = 2400W. 48V * 100A = 4800W, OR 48V * 50A = 2400W. Since you should only use 50% of your battery capacity, it would mean that in a 24V system you would need 200A battery bank, and 100A in a 48V system. The amount of batteries being used would be the same though, but your cables could be half the size. This shouldn't matter much in a short run, but could play a big role if your batteries can't be close to your inverter.

The Axpert 24V inverter can also only use 600W (i.e. 2x 300W) solar panels, whereas the Axpert MKS 48V PLUS one could take 3000W (i.e. 10x 300w panels).

[blackknight72]

SoftDux, that was very informative Indeed. My impression was that the impact of having a low voltage system would be on cabling alone.I just bought an Axpert 24 volt 3 kw system. I am just wondering how many batteries I will need to sustain 2000 watt output for 2 hrs and at the recommended battery drainage of 50 percent. I have purchased four 225Ah 6 volt batteries assuming this will be okay. However my installer says the same inverter sustained power only for 50 minutes on 100Ah high cycle battery at 1000 watts output at a recent installation. Any inputs in this regard by any members will be appreciated.

[Justloadit]

SoftDux, that was very informative Indeed. My impression was that the impact of having a low voltage system would be on cabling alone.I just bought an Axpert 24 volt 3 kw system. I am just wondering how many batteries I will need to sustain 2000 watt output for 2 hrs and at the recommended battery drainage of 50 percent. I have purchased four 225Ah 6 volt batteries assuming this will be okay. However my installer says the same inverter sustained power only for 50 minutes on 100Ah high cycle battery at 1000 watts output at a recent installation. Any inputs in this regard by any members will be appreciated.

Energy of battery = V x A, 225A x 6 = 1350Watts
4 batteries in series = 24V, 4 x 1350 = 5400Watts total
Using up to 50% = 5400 / 2 = 2700Watts
If your load is 2000Watts, then 2700/2000 = 1.35Hours or 1 hour and 21minutes

Your electrician says about a recent installation - 100Amps at 24V = 100 x 24 = 2400 watts
50% usage of energy = 2400/ 2 = 1200Watts
If load is 1000watts, then 1200/1000 = 1.2Hours or 1 hour and 12mins, if it only ran for 50 mins, then I assume that either the load was higher in power, or that there is losses within the inverter during the conversion process, at lower power loads on the inverter.

[blackknight72]

if I have to add additional batteries to prolong the back-up time should I stick with the same configuration of batteries as I have in the battery bank currently (4x6vx225 ah) or can I go for 12v cells (2x 12v x 200Ah)?