Adjustment of sine voltage inverter: Upper input voltage limit

[Phil1977]

Hi,

has anyone experiences with modification of these cheap sine DC-AC-inverters?

I have a 12V-->230V unit with rated 800W and 1600W peak power. I just want to power around 150W an average, so there should be enough margin.

I have a battery pack of 5 LiFePo-Cells in series. This delivers between 14V and 18V. If it´s not fully charged, the mentioned inverter receives less than 16V and is flawlessly working. The inverter uses a feedback voltage regulation, so I measure around 230V AC over all allowed input voltages.
If 16V are exceeded, the unit beeps and switches off due to overvoltage.
Here is the question: Is the 16V upper voltage limit negotiable? The caps in the circuit are rated for 25V, the FETs for 40V. Does anyone know if it´s possible to increase the voltage limit to 18V without further modifications?

Thanks in advance, best regards...

[moffy]

You really need a schematic specific to your device to answer that, sometimes there are little gotchas lurking in circuits that are not obvious. What might be safer is a preregulator, a pain, to keep your voltage at or below the 16V threshold, especially since the power is limited. The other thing to be cautious about is transients, the 40V mosfets will have switching transients and it is unknown how close they are to their voltage limits.

[Phil1977]

I did some more tests on the device.

It´s a two-step conversion. First the 12V are transformed to >300V intermediate DC and then a H-bridge converts the intermediate voltage to regulated 230V AC like a class-D-amplifier with feedback.

The step 1 converter seems to be push-pull transformer converter with open loop. So the intermediate voltage rises to 450V if the upper input voltage limit is reached.

Conclusion: Changing the overvoltage protection seems a not so good idea :-(

[Vovk_Z]

4S lifepo is perfect for 12Vdc rate inverter, so I wouldn't do it with 5S lifepo. That is too much of excessive work with redesign. (Why?)
A bit different thing is 24 VDC rated system. For 24 VDC rated inverter you sometimes may happily use 9S instead of 8S.
It is much easier now to buy just larger batteries than redesign electronix. There are hudge tonnes of both batteries and inverters of different sizes on the market. So, why redesign?

[Phil1977]

I already own the 5S battery in a rugged case fitted to drive a boat motor with 18V. It just would have been a nice additional feature to have a 230V inverter available.

For a short time I even thought about connecting the inverter to 4 of the 5 cells and add an active balancing system. But honestly I´m chickening out at this idea because malfunctions at that amperage level easily go out of control