Running microwave shutting down BMS

[Tomthumb62]

Ok, @Mia , you're in luck as the ever knowledgable @Hedges has spoken in a post above. It's over my head most of what he said, but the phase sort of stuff I was mentioning before he goes into. In any case, what he's saying is basically that this is what can be expected for many microwaves running from inverter. His suggestion is to try the microwave two or three times and you might get lucky that it will start. I also wonder if operating the MW at half-power might be the trick (one I have read others have tried...I don't own a microwave).

 

[HRTKD]

Keep the posts on topic. I don't want to have to clean up the thread and hand out warnings.

 

[Mia]

No, you have twice what the BMS is rated for. If each BMS is rated for 125 amps then the total is 250 amps. That assumes that each battery can provide 50% of the requested amps.

Can you provide links to your microwave, inverter and battery/BMS? Let's see how much juice the microwave is like pulling and how much your system is capable of providing.

A picture of how your inverter and battery are wired, along with the specs of the wires, will help.

A 3000 watt inverter should be enough to run a microwave in an RV. However, if you had other loads that were competing for amps then I could see where the inverter and maybe the BMS would shut down.

The inverter is the original Renogy 3000 watt PSW inverter vs the newer version .

 

[Mia]

UPDATE:

Ok, so after troubleshooting this more throughout the day, we will see if it is reproducible tomorrow, but as of tonight, the microwave seems to now be working. So crossing my fingers!

(I do think that the one battery was actually locked out for discharging but it’s now unlocked.)

We also found that the battery voltage was dropping to 9.8 volts when it was running, so possibly a low voltage cutoff issue?

I then noted that the BMS was showing the cutoff set to 11.6v on both batteries, but the manual said 10.8v was appropriate for the low voltage cutoff, so then tweaked the BMS to 10.8v for both batteries.

We then stopped for a while to do other things and returned back tonight to continue with more troubleshooting… We left the SCC on to charge up the batteries as we were down 96% and came back and it was back to 99.8%

So we’ll have to retest again tomorrow after discharging the battery back down to the 96% or so and see what happens.

But so far it’s appearing to be related to the BMS and not necessarily the inverter. Only time will tell I guess! Will see how it’s functioning tomorrow!

Thanks for your input and suggestions and support! Greatly appreciate it!!!

 

[Mia]

I’ve also read that certain Renogy inverters don’t play nice with certain BMS’s. I don’t recall what brand of lifepo4 batteries that OP is using. Iirc, the story I read was with a Renogy 1000W inverter and an SOK (or was it Chins?) battery. Others had reported a similar quirk.

I’m not sure what brand of bms is being used. I’ll need to inquire with the battery company.

 

[Tomthumb62]

(I do think that the one battery was actually locked out for discharging but it’s now unlocked.)

We also found that the battery voltage was dropping to 9.8 volts when it was running, so possibly a low voltage cutoff issue?

Either of these issues could cause your microwave to not work.

If only one battery in parallel was discharging, then you would be limited to 100A discharge, which is not enough to start the microwave.

How did you measure that battery voltage was dropping to 9.8v? But however you determined this, that is the type of "voltage drop" that can cause all sorts of issues, including devices not powering on. Your inverter was not happy with only 9.8v and thus likely simply could not produce the amount of power that the microwave wanted. It may be a 3000W inverter, but it's with caveats..."this is a 3000W inverter if you provide ample and consistent voltage (not too much voltage drop) and enough amps for the job, otherwise it may only be able to produce 400W if conditions are poor enough."

My guess is that since one battery was locked out for discharge, the other battery sagged to 9.8v when the microwave tried to start up and the inverter tried to meet the power demands but there simply was not enough amps to do the job and this caused the voltage to sag so bad. This all happens within a second or less.

 

[Mia]

If only one battery in parallel was discharging, then you would be limited to 100A discharge, which is not enough to start the microwave.

How did you measure that battery voltage was dropping to 9.8v?

DH measured the voltage with a meter

 

[Mia]

DH measured the voltage with a meter

Definitely agree with what you noted Tomthumb62 as it does seem to explain the behavior we were experiencing! I will need to test again and see if it’s still behaving better today!

 

[Mia]

So the MW still seems to be working as one would expect today! Fingers crossed this issue has been successfully resolved now! I greatly appreciated all of the feedback, ideas and advice!!!