I’m trying to determine the optimal of the smallest MP2 models to match my DaHSolar 48V 100Ah LFP battery which has a specified maximum output of 100A. The max battery discharge current specified by Victron is at odds with the stated peak AC outputs, see my notes below, even ignoring the low end of the DC voltage range or the unknown efficiency during peak demand.
Surely the DC current must rise significantly higher than the published numbers to support that AC output? I would like to use the fully-allowable battery 100A for peak demand, which will be under 500 ms duration.
I think that the “peak power” rating is for <10s, during this time the Battery discharge current will exceed 110A, as the internal reservoir capacitors cannot store that amount of energy. So taking a nominal supply voltage of 50V, 9kVA output will produce a current spike of ~188A. So I suspect that the max discharge current is averaged over a ~1 min period - as the inverter power needs to be significantly reduced following a peak output event, to let the internal heat dissipate. With lead batteries (for which this spec was drawn up for), this current peak was never an issue, apart from the inverter fuse rating, but for Lithium batteries with internal BMS system, it can be.
How fast the internal BMS will react to a current surge over 100A depends on the internal design, but is likely to be <1s.
Your peak power output is therefore limited to <4800 kVA, However, this will depend on the loads of the installation, rather than the inverter. Certainly with the 100A limited current, you would probably be better off with the smaller inverter.
Thanks for that background and advice. It has dawned on me this week that the battery must match or exceed the load while inverter simply has to not get in the way.
My only load is a submerged water pump (induction motor) which draws 23A for 250ms then 5.2A ongoing. It only runs intermittently.
I have no information as to whether the 5kWh battery BMS could tolerate being over 100A, and if so for what length of time. I’d need at least around 115A for that 250 ms.
As such, the battery may not have the margin needed. But a new battery (10kWh) would cost me about 4 times that of a slightly smaller pump, which is thankfully an option I have. The energy requirements already are fine with the 5kWh.
So, I’d prefer to use the 3kVA MP2 provided it will fully exploit the battery 100A. Assuming that’s the case, I’ll buy one and test it. If it can’t start the pump I’ll purchase and install a new pump, the next smaller size will comfortably fit the power envelope.
Victron do have a “minumum” battery capacity indication for their inverters, Sorry, I don’t have the link to hand. But yes, the inverter passes the “peak” demand to the battery, modified by the inverter efficiency. The best way to look at this is via the max current demand, then if you have peak problems, particularly with motor starting, then electronic soft starters or VSD for the motors is an option. direct on line start of a motor can result in 6 - 7 times normal current for a few seconds.
