Inverter

Bruce Mc · Veteran Member

Inverter

Radar · Guru

Bruce Mc wrote:
I need to do a test on my system in my caravan to check a voltage drop issue I have. I plan to temporarily hook up a 300w inverter to my batteries then connect the inverter via a 240 volt extension lead to the caravan power input plug. All high current devices such as battery charger, microwave and washing machine will be turned off. Fridge, TV and phone charges will be the only devices used during the temporary hook up.
I have read some people caution against the connection to the caravan input, but I dont understand what issues it may create. Has anyone connected an inverter in this way or is it not safe to do so.

Well I do but for being safe I really have no idea.

I was told it was ok to do it by electrician.

Whenarewethere · Guru

300 watt inverter, let's say 30 amps. What is the total length of the + & - wires (resistance is not just the length of the positive wire), then you (or we) can calculate the minimum gauge the wire should be.

kgarnett · Senior Member

Tell us more about the voltage drop issue.

Mike Harding · Guru

It is perfectly safe to connect the caravan to an inverter.

Just be sure not to overload the inverter or it may suffer damage especially if it is a cheap one.

I run my caravan this way permanently. I have a Victron inverter and MPPT controller in a rain proof box which sits outside with AGM batteries close by and a 240V extension lead to the caravan. This method gives me much greater flexibility with solar panel placement which is often critical in forests.

Dick0 · Guru

Yes, no problem connecting that way. I run several 240v appliances connected that way through a 3000/6000W inverter (one appliance at a time). Even run the aircon that way. No more gennie!

Bruce Mc · Veteran Member

Im using the cables supplied with the inverter, that are approx 400mm each. The watts will be approx 150 only and the cables will be direct onto the battery.

Bruce Mc · Veteran Member

I have 2x130ah agm(3months old), 620w of fixed solar and 50mppt solar controller. At the end of the day we are 100 SOC as per the Victron Smartshunt. 12.9-13v. With TV on at a draw of 3amps approx the fridge cycles on and draws 6 amps, total draw @9amps. Toward the end of the night the volts while the load is on drops down to 12.1-2 and when this occurs the TV fades out. The volts at the solar controller read 12.1 while the smart shunt reads 12.3-4. The SOC as per the smart shunt is still 90+%. the power feed to the fuse box is via the solar controller which in turns is fed from the battery. Location of the controller forces a cable length of approx 6mtrs to the controller then 3 to the fuse box. I think this cable length is creating excessive resistance in the cables explaining why the controller reads lower battery voltage than the smart shunt. Running the power via an inverter will bypass the cables and will confirm this is the issue.

Jaahn · Guru

Hi smile

If you have a good multi meter you can easily read the voltage drop between the battery terminals and the inverter inputs. All you need is a length of small gauge wire long enough to reach and possibly a couple of clips for the ends. Allmost any wire is big enough. Set it with one end clipped to an inverter terminal and the other clipped to a multimeter lead. Set the meter on a low DC voltage of several volts and then put the other meter lead on the battery terminal of the wire run. You can do that with the system running. Then repeat it on the other cable run. It will give a direct voltage drop of both + and - cable runs including any shunts and connections etc. Tell us what that is. It should be much less than a volt or so in total I would hope.

Jaahn

PS as the battery voltage falls at the end of the night the current drawn increases so the voltage loss increases too.

-- Edited by Jaahn on Thursday 14th of July 2022 01:19:30 PM

Mike Harding · Guru

Jaahn wrote:
PS as the battery voltage falls at the end of the night the current drawn increases so the voltage loss increases too.

Jaahn, as usual, is correct but I'll point out why because I made a song and dance about this a couple of weeks ago as it seems to work against Ohms Law - but doesn't.

A 12V (or any voltage) DC to AC inverter is essentially a variable load constant current device: this means that to satisfy its specification it *must* output 240V AC at its specified current ie. it must produce a constant output power - in order to do this it must have an input power which is slightly greater than it's output power. As the voltage of a battery drops according to its state of discharge then the inverter *must* draw more current to compensate for the lower voltage in order to keep the power budgets in balance it does this by devious electronic means which, effectively, alter the resistance it presents to the battery. Of course the battery is not an infinite source of energy so the inverter can only abuse it so much and that is why they typically have a minimum input voltage specification of around 10.5V.

Mr Ohm is happy and all is well in the world of physics :)

jegog · Senior Member

Jaahn wrote:
Hi
If you have a good multi meter you can easily read the voltage drop between the battery terminals and the inverter inputs. All you need is a length of small gauge wire long enough to reach and possibly a couple of clips for the ends. Allmost any wire is big enough. Set it with one end clipped to an inverter terminal and the other clipped to a multimeter lead. Set the meter on a low DC voltage of several volts and then put the other meter lead on the battery terminal of the wire run. You can do that with the system running. Then repeat it on the other cable run. It will give a direct voltage drop of both + and - cable runs including any shunts and connections etc. Tell us what that is. It should be much less than a volt or so in total I would hope.
Jaahn
PS as the battery voltage falls at the end of the night the current drawn increases so the voltage loss increases too.

-- Edited by Jaahn on Thursday 14th of July 2022 01:19:30 PM

Or, assuming that the inverter has an on/off switch, measure the voltage at the inverter input with the inverter off and then subtract the voltage reading with the inverter switched on.

dorian · Guru

jegog wrote:
Or, assuming that the inverter has an on/off switch, measure the voltage at the inverter input with the inverter off and then subtract the voltage reading with the inverter switched on.

Clever!

Mike Harding · Guru

dorian wrote:
jegog wrote:
Or, assuming that the inverter has an on/off switch, measure the voltage at the inverter input with the inverter off and then subtract the voltage reading with the inverter switched on.
Clever!

Except when a load is applied to the battery its terminal voltage will drop by x and that reduction will be indistinguishable from the cable loss using jegog's suggestion.

Whenarewethere · Guru

Bruce Mc wrote:
Im using the cables supplied with the inverter, that are approx 400mm each. The watts will be approx 150 only and the cables will be direct onto the battery.

It may seem trivial on the surface, but what is the cross sectional area (copper diameter) of the 400mm cables (800mm + & - total).

It is surprising the voltage drop of even a short cable if it is not sized correctly. No point, say 10% or worse going up in heat due to wire resistance, which is easily avoidable.

dorian · Guru

Mike Harding wrote:
dorian wrote:
jegog wrote:
Or, assuming that the inverter has an on/off switch, measure the voltage at the inverter input with the inverter off and then subtract the voltage reading with the inverter switched on.
Clever!
Except when a load is applied to the battery its terminal voltage will drop by x and that reduction will be indistinguishable from the cable loss using jegog's suggestion.

OK, clever, but with caveats.

It might be worth investing in cable runs that incorporate sense wires, ie separate wires in the same sheath that don't carry any current.

-- Edited by dorian on Monday 18th of July 2022 10:03:16 AM

Bruce Mc · Veteran Member

The cables are 10awg. The voltage drop issue is not with the inverter, its with the standing 12 volt feed to the fuse box.

jegog · Senior Member

Mike Harding wrote:
dorian wrote:
jegog wrote:
Or, assuming that the inverter has an on/off switch, measure the voltage at the inverter input with the inverter off and then subtract the voltage reading with the inverter switched on.
Clever!
Except when a load is applied to the battery its terminal voltage will drop by x and that reduction will be indistinguishable from the cable loss using jegog's suggestion.

The internal resistance of a 12V lead acid batteries is a few m. If the battery terminal voltage drops under the load of the inverter then there is something drastically wrong with the battery(or the connections). Their short circuit current is over a thousand amps.

Mike Harding · Guru

jegog wrote:
The internal resistance of a 12V lead acid batteries is a few m. If the battery terminal voltage drops under the load of the inverter then there is something drastically wrong with the battery(or the connections). Their short circuit current is over a thousand amps.

I don't think so:

Ritar 100Ah battery data sheet

Wire resistance calculator

----

Battery internal resistance = 7.5mR

5m of 6mm(csa) cable has a resistance of 14.25mR

----

Assume current at 20A, therefore as V = IR

Battery voltage delta between 0A and 20A = 20 x 7.5^-3 = 150mV

Cable voltage drop at 20A = 20 x 14.25^-3 = 285mV

----

Therefore failing to account for the battery internal resistance represents a 53% error of measurement.

TimTim · Guru

Bruce Mc wrote:
The cables are 10awg. The voltage drop issue is not with the inverter, its with the standing 12 volt feed to the fuse box.

Sometimes we miss the most simple of things but have you checked the connections? Loose connections or even a washer inserted in the wrong place can make a big difference.

-- Edited by TimTim on Tuesday 19th of July 2022 06:11:56 PM

Jaahn · Guru

Bruce Mc wrote:
The cables are 10awg. The voltage drop issue is not with the inverter, its with the standing 12 volt feed to the fuse box.

Hi Bruce smile

If the cables are 10AWG then you have a real problem. That is not big enough to run even a small inverter. Have a look at this table below.

But my method of measuring the actual voltage drop will tell you that too I suspect.

Jaahn

American-Wire-Gauge-AWG-Sizes-and-Proper

Whenarewethere · Guru

I pulled my fridge apart & replaced the 16awg with 12awg. All the wire resistance adds up.

Swapped my 6awg wire to 4awg wire for the compressors. 6awg with 90amps was getting a bit warm.

TimTim · Guru

Hi Jaahn, I think Bruce meant he has 10AWG for the 12 volt circuit from his battery to the fuse box. Well I hope he did biggrin .

Well from the battery to the solar controller and then from the load terminals on his solar controller to the fuse box.

-- Edited by TimTim on Wednesday 20th of July 2022 06:04:34 AM

Whenarewethere · Guru

If I understand correctly the MPPT controller is 6 metres plus another 3 metres away from the batteries.

Let's say the panels are in parallel (based on my setup) so that is 40 amps out of the panels & could be 58 amps out of the controller, but it has a limit of 50 amps.

So 9 metres & 18 metres chart attached whichever is the total length of + & -

There may not be as many amps going into the controller if the wire from the panels is a bit thin for their length.