lead crystal 150 ah 4 years old cables etc all fine sunliner install ..    everything else works fine 

Hi
Yes I agree that your 150Ah battery is very marginal to run a microwave through an inverter even when new and 100% charged. It would struggle to sustain its voltage enough at the high current draw. Add to that any voltage drop in the 2 cables and 4 DC connections, which all have a small resistance at large currents, and it is easy to see what is happening to the voltage at the inverter input sensing.
The inverter is telling you that the voltage is dropping too low. That is why it it sounds the alarm. Then it cuts out ! Get more battery capacity is my first suggestion. Get another battery the same size. 
Also you can shorten the DC cables to the minimum required, clean all the connections and solder the lug crimps, and use a conductive paste on the surfaces, all to help a bit. You might get another minute or so.
Jaahn

-- Edited by Jaahn on Sunday 9th of January 2022 10:01:43 AM

Thanks mike appreciate the advice microwave is 800 watt draws about 3amp  even when vehicle was new same outcome .. we dont use microwave that often just for that morning coffee fix !! heat up the milk etc .. coffee machine works great no issues .

lead crystal batteries after lithium have a much longer life span than Agm from what ive read up to 18 years which i'm not too sure about !! dealer that supplied the battery says 4 years is still in its early days  .. i have a mate who's battery is 7 years old lead crystal still going strong but he doesn't use a microwave .

-- Edited by goldenyears on Sunday 9th of January 2022 10:34:54 AM

What is a Lead Crystal battery?

The unique Lead Crystal® battery consists of a number of special features including: a micro porous high absorbent mat (AGM), thick plates cast from high purity lead calcium selenium alloy (which ensures an extended life), and a SiO2 based electrolyte solution. During the charge / discharge cycles the electrolyte solidifies and forms a white crystalline powder. This eventually results in a safer, high performing and environmentally friendlier battery. The Lead Crystal® battery can be used as a substitute for most battery technologies in the lead category, such as lead acid, lead gel and AGM.

What is the lifespan of a Lead Crystal battery?

Lead Crystal® has a design life of 18 years.

What is the storage capacity of a Lead Crystal battery?

The Lead Crystal® has an extremely low self-discharge and can be stored for more than two years without any top-up charging prior to use.

-- Edited by goldenyears on Sunday 9th of January 2022 10:47:58 AM

-- Edited by goldenyears on Sunday 9th of January 2022 10:51:22 AM

What the 3 previous posts said.

You said it was 800 watts and drew 3 amps. That is 3 amps at 240 volts which is 60 amps at 12 volts.

You also have to add the inverter inefficiencies of probably 10% or more. So the inverter would be sucking say 900 watts. Probably more than that if it's a cheapie inverter. And then you have the losses in the cabling. If it's say 8 gauge cable and 1.5M long (=3M return distance), you are probably losing more than half a volt in cable loss.

So, if the battery state of charge of 76% is anywhere near accurate it probably reads around 12.5 volts before you start. The instant you press the button, the inverter would only be seeing 12 volts or lower due to cable loss. As the battery quickly gets drawn down, and voltage drops below 12, then 11, etc, amps increase, further increasing the cable loss. So, to me it's no surprise that it trips the low voltage cutout in 2 minutes.

A quick test on the cabling is to feel how warm it gets. Hot means undersized cable and wasted energy. More battery capacity would handle the current draw better but if you get a new one and wire in parallel, the old one will drag down the new one. A workaround would be to run the microwave for a minute. Let the battery recover and then give another minute. Not ideal but it will get there in the end.

I would use the stove instead.

when using the 12 volt  invertor with microwave and nothing else drawing power the bms states its drawing 3.2 amps not 60amp am i reading this wrong ?

not connected to mains  240 ...                              but the invertor converts it to 240 volts  am i right ?  so maybe bms is correct !   confusing lol !!!

-- Edited by goldenyears on Sunday 9th of January 2022 11:28:01 AM

-- Edited by goldenyears on Sunday 9th of January 2022 12:42:05 PM

I'm not familiar with the BMS, but if the microwave is 800 watts, that is a little over 3 amps at 240 volts. The standard formula is

Watts divided by Volts = Amps.

There are slight modifications to that in practice but it is fine for now.

So your microwave is 800 watts and it runs on 240 volts. Using the formula:

800W / 240V = 3.3A

But when you need to supply that same 800 watts with your 12 volt battery you need to recalculate the amps.

800W / 12V = 66.7A

It's very common that people mix these up, so don't feel bad. If you can't see it, it's not as easy to understand. But for caravan life it's really helpful to know how it works.

I suggest doing a search on Youtube for "Volts Watts Amps explained". Watch a few and it will become clearer.

 

As I wrote this I thought of another solution that may help. Turn down the microwave power level. It will take longer to heat things, but in doing so it may be enough to avoid the inverter low voltage cutoff. Some microwaves just switch the magnetron on and off (you can hear when it does so) while others reduce the power. No need to go further into that.

 

-- Edited by Are We Lost on Sunday 9th of January 2022 12:53:00 PM

Yes. The inverter takes the 12v supplied by the battery and converts it to 240 volts which can then be used by your standard household appliances. The solar would provide power to charge the battery. So, it's not quite "or Solar".

 

-- Edited by Are We Lost on Sunday 9th of January 2022 12:58:26 PM

Good point kgarnett.

i'm definitely not a  technical person and find it all so confusing going by what everyones saying the bms read out is  perhaps wrong   arghhhhhhhhhhh lol ...  will contact redarc  to make sure .

maybe this old fella should just buy a small butane stove to heat up the milk lol !!!   have a diesel ceramic stove just takes a bit longer than gas to heat up .

 

so the 3.2amps is what ??    not the draw from the  battery !!!!   drawing 125amps from 150amp battery doesn't leave u much in reserve eh ! 

 

allot of different comments on here but all appreciated 

-- Edited by goldenyears on Sunday 9th of January 2022 01:41:23 PM

-- Edited by goldenyears on Sunday 9th of January 2022 01:43:25 PM

-- Edited by goldenyears on Sunday 9th of January 2022 01:45:19 PM

yes mike you could be right ! or maybe you could be wrong !  lol !!!   Im in no position to comment on that i guess .. but willing to learn .

 

There is a Redarc dealership and workshop not far from here so will see what they have to say ...   Redarc gear has great ratings from 4wd enthusiasts that swear by the set ups .. think that was what swayed me in what i wanted installed ... 

 

goldenyears wrote:

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i'm definitely not a  technical person and find it all so confusing going by what everyones saying the bms read out is  perhaps wrong   arghhhhhhhhhhh lol ...  will contact redarc  to make sure .

maybe this old fella should just buy a small butane stove to heat up the milk lol !!!  have a diesel ceramic stove just takes a bit longer than gas to heat up .

so the 3.2amps is what ??    not the draw from the  battery !!!!   drawing 125amps from 150amp battery doesn't leave u much in reserve eh ! 

allot of different comments on here but all appreciated 

 

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Hi goldenyears 

My best answer about the 3.2amps is this, based on looking at Redarcs information on line here; https://www.redarc.com.au/Images/uploaded/REDARC%20Manager30%20BMS%20Brochure.pdf

the BMS only monitors the charging inputs to the auxilary battery which it controls (and the car battery),  that's good so it will estimate the battery state of charge and the real time voltage and charging current.  It says "The BMS1230S3 is supplied with the standard monochrome remote monitor which provides information such as state of charge, current, voltage and temperature as well as a simplified battery percentage and charge rate"

But it does not monitor any output currents from the battery. It has no shunt to measure these outputs. So it can only estimate the battery capacity by the voltage changes under loads. That will not give any indication of the amps being drawn by the inverter. That small 3.2 amps is the charging current that was available to the BMS when you read it and probably the solar panel input.

My best answer for your information. Cheers Jaahn

 

Jaan it has a shunt with unit sometimes called battery sensor .. my negative on portable solar connected to chassis side ... so i can get a reading from bms .

goldenyears wrote:

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In short, the REDARC Inverter takes your caravan's 12V (Battery or Solar) power source and converts it into 240V in traditional power sockets, just like you have at home .

 

slowly getting there and good to know my bms {redarc battery management system } read out of 3.2 amps is correct ...and not 60amp  thanks to everyone that contributed .

 

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Hi Goldenyears,

You said earlier your inverter is Projecta, whereas the Redarc Manager 30 is a combined Mains charger, DC/DC charger, and solar MPPT controller.

The Redarc and Projecta cannot communicate with each other, they use proprietary communications protocols, so the Redarc has no idea what's happening with the inverter, all it can see is that there is a massive flow of current going out of the battery to - somewhere?

The Redarc's sees this because of it's shunt. Simply put, this device measures the DC flow of current in and out of the battery and from that calculates the battery state of charge (SOC). It does not measure AC current. To understand how this works I find it's useful to use an analogy of your water tank. Imagine it's opaque so you can't see the water level and its not fitted with a gauge, but you know it's overall capacity is 100 litres. So, you fill it up until it will accept no more and you can then assume it contains 100 litres of water. Then you measure every drop you take out, and put in, and keep a running tally on a piece of paper. So for example you use 30 litres for a shower, and 10 litres to cook dinner. But then you pour a 20 lire jerry can of water back in. So you now calculate the water in the tank as 100 - 30 - 10 + 20 = 80 litres, so it is 80% full. That's basically how the Redarc works but with electricity rather than water. But for this to be accurate four things are needed.

1. You need to tell it the capacity of the battery in Ah through the settings menu on the remote display. This is akin to the size of your water tank in our analogy above. 

2. You need to let it go through it's calibration process by charging it up to 100% so it knows whereto start it's calculations from. This is akin to filling up our water tank at the beginning.

3. You need to make sure everything that goes in and out of the battery goes through the shunt. If it doesn't go through the shunt, it won't get measured and your displayed SOC% is meaningless. This is akin to keeping tally of your water use on a piece of paper, if you don't write it down, when you calculate remaining water it won't be correct! The way to do this is to ensure all system negatives terminate on the load-side terminal of the shunt (marked GND), and only the one wire from the battery-side terminal of the shunt (marked B NEG) goes to the battery - nothing else!

So, you may be wondering what this rambling monalogue has got to do with your current predicament with the microwave?

Well, the fact that your Redarc display is saying only 3.3 Amps DC (remember the Redarc only measures DC, not AC) is going out of the battery while your microwave is running, whereas from the calculations of others above we know it's actually more like 100 Amps DC, suggests to me one of two things...

1. There is something wrong with your shunt/Manager and it is not measuring correctly, OR

2. Whoever wired in your Projecta inverter wired it directly to the battery negative instead of via the shunt, and the current the inverter is taking out of your battery isn't getting measured. This is not unusual because the needs of the inverter are different to the rest of the system in terms of size of fuse and wiring because of the large current draw, so usually is wired directly to the battery terminals, and whoever did it may not have understood the need to go via the shunt. 

Either way, the SOC reading on your Redarc display will be totally wrong, and bear no relation to the actual state of charge of your battery, so it may well be that when your Redarc display says 76%, the battery is actually very low and hence why it can't sustain the current draw from the inverter for very long.

I think at this stage you'd be wise to get a good sparky in to check your system over, as if what I'm suggesting above is true, you risk killing your battery altogether.

Jaahn wrote:

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goldenyears wrote:

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Jaahn it has a shunt with unit sometimes called battery sensor .. my negative on portable solar connected to chassis side ... so i can get a reading from bms .

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 Yes, that shunt is measuring the input from your portable solar panel for the benefit of the BMS estimations. That's good !

BUT it has no way of measuring the load output of the battery. The wiring diagram shows no method of doing this, and a very large shunt would be required for the inverter.  

I believe what I have said is what Redarc will tell you. The reading is correct but it is not what you thought it was reading. Sorry !

Jaahn

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 Jaahn,

The Redarc Manager 30 comes with a shunt rated at 500 Amps so is capable of reading the draw of quite a large inverter. All that is required is that the inverter negative goes via the shunt (as with every other load) rather than being wired direct to the battery negative as I suspect it is with Goldenyears system, and then it will be accurately including the inverter draw in it's measurements.