Inverter / Battery load question (Microwave et al)

[krawall]

Hi Guys,

I thought posting here again since I got so good advice last time. This time, it's about batteries and inverters.

I have inside the house bank 3 x Bosch N120 (120AH, calcium-calcium) batteries. They are 12V and connected in parallel.

These are connected to a Victron Multiplus Compact 1600 (1600/70/16) inverter charger.

I want to run a microwave that takes around 1100 Watts (measured).

I did not choose the batteries, they are what came with the boat (they are fairly new, at around 1 year old). I did choose the Victron because I had a Victron on my old boat and was quite happy with it (also I like the footprint)

I do have a genset I can run the microwave off, but ideally it would be nice if I can heat up something quickly without putting cold hours on it.

However here is the result:

I turn on the microwave, and it would run for around 2-3 seconds, then the battery would drop to 11.3 on the panel and the inverter control panel would yield low battery & overload simultaneously (which the victron handbook says means that the 12V supplied are not stable - they talk about a ripple voltage which is too high)

I turn the system off immediately, the voltage on the panel returns to 12.6 (or whatever it has been before) and after a thinking phase (on part of the charger I guess) the system starts recharging the batteries which finishes within seconds.

I have a bank of 4320 w/h and if I use around 1100 * 5/60 (5 minutes warming up time) it should use around 10Ah of my bank (360Ah) which seems OK to me.

Also, since I have three batteries, I would think that each of them would contribute around 33.33A (1100 Watt load is 92A so let's say with inverter inefficiency that's 100Amps) According to what I can find on the internet they should last for 245 minutes on 25 Amps.

Shouldn't this work?

What's the most likely reason this doesn't work. Do you think it lies in the batteries? The Victron is rated for 1600W continuous load.

Voltage on the electric panel drops to 11.3 Volts while the magnetron is on, what do you make of this (other than, obviously, high load)

Am I expecting too much or is it just bad equipment for the job?

Cheers and thanks,

Tom

 

[Dan Hill]

What your describing appears to be a voltage drop due to a choke point someplace. Using a volt meter, I would perform some voltage drop tests while the circuit is loaded. Mainly between the batteries and inverter, both legs. Good luck with your issue! Dan

 

[Don S/V ILLusion]

What size are the cables to the batteries and inverter?

 

[mitiempo]

The microwave is too large a draw for the inverter's output. A 700 watt microwave should work.

 

[azguy]

1100 watts for the microwave is massive

 

[Ron20324]

Maybe dumb question, but does the microwave work on shore power?

 

[Johann]

The microwave is too large a draw for the inverter's output. A 700 watt microwave should work.

A 700w microwave generally consumes about 1000w. The 700w is energy delivered for cooking, and you must account for efficiency losses in the microwave.

Regarding the OPs issue, I too would look closely at the gauge and total run of the cables from the batteries to the inverter.

 

[Maine Sail]

The microwave is too large a draw for the inverter's output. A 700 watt microwave should work.

The MP 1600 should handle 1200-1300W... Still 1100W pushing the envelope for that inverter..

That said the issue is either:

*Voltage drop due to undersized wire

*High resistance/poor terminations leading to voltage drop

*High resistance, poor connections and poorly wired batteries leading to voltage drop

*Bad batteries

*Bad batteries leading to battery voltage sag ADDED on top of voltage drop in the cabling


The last one is the most likely cause....

Rx:

Measure battery terminal voltage under load. Is is 9.3V when the inverter cuts off or is it higher? If higher then you have additional drop in the cabling. If it is the same as the inverter cut off point then your batts are toast. The MP 1600 LVC / low voltage cut off is 9.3V and if that is what your inverter is seeing it will shut down..

Measure the inverter end and battery end of the NEG cable. Connect + of DVM to inverter end of NEG cable and and - of DVM the battery end. Do you see a reading under load? What is it?

What is the actual voltage across the + & - terminals of the MP at load?

Measure the inverter end and battery end of the POS cable. Connect + of DVM to inverter end of POS cable and connect the - of DVM the battery end. Do you see a reading under load? What is it?

If you got readings on both cables add them together. This is your voltage drop at load... Example: 0.49V on the neg cable and 0.56V on the positive cable is 1.05V drop.....

This voltage drop gets added to the voltage "sag" that LA batteries have when under load....

 

[Bill Roosa]

Totally agree with MS. An alternate method to locate the offending voltage drop is to do a voltage drop (catchy name DYT) test on each connection. Probe on each side of the connection while there is load to determine how much voltage is being converted into heat at each connection. You can also test cables by probing at both ends with a load on. You should not see more than 0.3 volts on any one connection.

 

[krawall]

Thanks a lot guys. I'll give it a shot later this week.

Cable size: The inverter was just installed, not by me, I had a look and the cables looked massive but I can only measure them when I get on the boat next. Cables looked same size as Beneteau uses or same size as I see for other starter battery cables, etc.

Two batteries are really close together and to the inverter (within 2 feet cable runs) but the third battery is under the bed on the other cabin. add around 5-6 feet to that cable length. Haven't really checked how long the cables are, but the distance is very close (and I doubt they'd have used excessive cable lengths for no reason)

The microwave is built-in and came with the boat, it's loading a bit less than 1100 Watts but that was measured using the genset, so it may not bey very exact (no reference to compare).

Yes the microwave works fine under shore power and genset.

I'll try to have a look at the connectors and do the voltage drop test as many of you have indicated. It's just a bit difficult probably to find out how much load do I have to apply as it fails within 3 seconds under MW load and of course I don't want to repeat that often because I'm afraid something's gonna burn eventually. Can I check resistance when not under load with an ohmmeter or do I have to have load for symptoms to appear (and if so, how much load)?

I'll report back here, thanks for all the valuable input !

Cheers,

Tom

 

[Dan Hill]

Can I check resistance when not under load with an ohmmeter or do I have to have load for symptoms to appear (and if so, how much)

The ohm side of your meter won't work for this kind of testing. Most DVOMs nowadays have a min/max feature that allows the user to monitor and "record" the values seen across the leads. They can sense and record within milliseconds so 3 seconds of load isn't an issue.

 

[Bazzer]

Voltage at the batteries assuming that they are fully charged would not drop so low due to invetrter cables that are to small. You only have a 1600 watt inverter which is not a lot for your house batteries as described. I would check the battery terminals are not getting warm when the load is applied and then check the batteries themselves as I suspect they are not so good

 

[Don12364]

Internal Impedance of your batteries?

With batts at a full, or at least reasonable state of charge - but not on charge - I suggest switching the inverter on and measuring the voltage at the actual battery terminals and measuring the current draw. Then switch on a significant load - say between 500 watts and 1 KW nominal and measure both volts and amps again. Then by applying simple ohms law to the difference in voltage and the difference in current will give you the internal impedance of the battery bank. Problem is the battery makers are coy about what their batteries should achieve.
Also you might like to repeat the test at the inverter's input terminals to see how much your wiring, ammeter and switches/breakers add to all of this when it actually comes to converting the DC supply to AC.

Whatever else you will learn a lot (as I did) because you will see the voltage dropping by the second, plus how inferior your set up really is.
Good luck,
Don.