derBastler wrote: ↑
Sat, 02 Nov 2019, 21:56
We have VW E-Golf Batteries, VW Passat GTE, VW E-Up, BMW i3 and all other manufacturer batteries. A very typical batterie nominal voltage is 44,4V. Upper limit 50,2V, Lower Limit 38,4V
It would be great to change the PIP-5048MS to this values to bring a hugh mass of batteries into second use applications.
Do you think this is possible?
Weber and I have been considering this possibility. One thing that bothered us is how such a low battery voltage will generate enough bus voltage to generate a 230 V sine wave. In fact, we began to wonder how these inverter-chargers work down to as low as 40 V (the lowest possible value for setting 29, the Low DC cutoff voltage). We know from experience that the bus voltage is usually very close to 8.0 times the battery voltage, presumably due to the high frequency transformer turns ratio of 1:8. 40 V times 8 is only 320 V, which is not enough to generate the peaks of a 230 V sine wave (a bare minimum of 230 x √2 = 325 V is required).
Weber conjectured further that the 64 V models (which can charge the battery up to a maximum of 64.0 V) must have a lower turns ratio in their transformers, because 64 V times 8 is 512 V, and the inverter bus capacitors are rated for 500 V. Indeed, he found that his 64 V model usually had a bus voltage of very close to 7 times the battery voltage. Yet these models still allow the battery voltage to go as low as 40 V. With the battery voltage at 40.0 V, the bus voltage would only be 280 V... how does that produce a 230 V sine wave?
There is a circuit between the DC-DC converter and the inverter proper, called the buck converter in some service manuals. Its main purpose seems to be to buck a high bus voltage down to an appropriate voltage for utility charging the battery. In other words, it's a buck converter when power flows from the inverter output (connected to AC-in when utility charging) towards the battery. We wondered if that converter could act as a boost converter when power flows from the battery to the load (i.e. in battery mode). There seemed to be clues in the firmware that supported this idea. However, when we looked at the block diagram in this post
, it sure didn't look like the buck converter was bidirectional.
To sort out what really happens with a low battery voltage, Weber kindly reconfigured his battery from 16S of LFP (nominally 51.2 V) to 13S (nominally 41.6 V).His 64 V model inverter claimed it was generating 230 Vac on the LC Display. With the battery voltage at 44.4 V, the bus voltage (as revealed by the QPIGS command) was 311 V. (311 / 44.4 = 7.005). How was it generating 230 V? So we set up Weber's DSO to look at the generated waveform. This is what we found:
We measured the peak to peak voltage at 576 V, or 288 V peak to zero. The bus voltage would have been about 42.9 x 7.0 = 300 V, so there is an overhead of some 12 V.
Using the default output voltage of 230 V, to avoid clipping you'd need about 230 × √2 + 12 = 337 V. With a 58.4 V model (1:8 transformer), that means the lowest battery voltage to avoid clipping would be 337 / 8 = 42.1 V. That would be 3.51 Vpc for a 12S NMC or NCA battery, which seems to be about 12% SOC, per the table below.
With a 64 V model (1:7 transformer) the situation is much worse, even when you tell the inverter to only output 220 V, as shown in the following table. Using an output voltage of 220 V instead of the default 230 V helps, but only a little.
Onset of clipping
Model Out V | Bus V Bat V Cell V (12S) SOC (12S NMC)
64V 230 | 337 48.1 V 4.02 V 90%
64V 220 | 323 46.2 V 3.85 V 75%
58.4V 230 | 337 42.1 V 3.51 V 12%
In summary: Unless you don't care about the output clipping, using a battery whose nominal voltage is less than 48 V isn't going to work well with a 64 V inverter model.
[ Edit: many minor rewordings; thanks Weber. ]
Nissan Leaf 2012 with new battery May 2019.
5650 W solar, 2xPIP-4048MS inverters, 16 kWh battery.
1.4 kW solar with 1.2 kW Latronics inverter and FIT.
160 W solar, 2.5 kWh 24 V battery for lights.
Patching PIP-4048/5048 inverter-chargers.