Off Grid Power

I know those battery cables look big, but they are creating voltage drop. You can measure it with your multimeter - one end at the battery lug and the other at the inverter. When the pump runs, you will see at least several tenths of a volt I guess. Rated for 400 amps does not mean 400 amps with no voltage drop; probably more like 10% voltage drop. Maybe you can live with it, but that would explain some of what you are seeing.

 

That's exactly the problem.......... I am not worried about when it runs its just the inverter idle which is sucking it away..... I was under the impression the bigger cables would stop the voltage drop.....Its not possible to get much bigger cables ? I guess I could double up but the hole in the floor to get them in/out is alarmingly big.

Smaller cables would be less of a drop ? I never had any heat from them...

FYI on the other post its not under panelled, it rarely gets used... we have a house to live in so the hut gets a shower (well once in the last 10 months) and that was just for testing.

One point to note the "lug" on the cable appear rated at 120 amps which worried me.. However I cant get any bigger from the stockist.... These came with the inverter so should be fit for purpose.... I was just thinking perhaps by cables are fit for purpose but the lugs are causing a bottleneck? I guess my 100amp fuse as a safety feature doesn't help either.

 

the large black cable in your last photo - that passes in front of the mains switch, is around the size that should connect your batteries to the inverter. The resistance of cables is proportional to the area for DC currents, so if you have to double the cables, it is not two of only half the diameter - area is proportional to the square of diameter. So two cables of half the size only carry 1/4 of the current each, or 1/2 as much in total. Resistance in cables is also proportional to length, so keeping the cables short is beneficial.

Your inverter is 3kW and the batteries are a 12V bank as I recall. To fully power the inverter, the batteries must deliver 3000W/12V = 250A. With #2 cable (6.5 mm) that will produce a voltage drop of almost 1V each 10 feet. The inverter will work but you will definitely notice the voltage drop. Of course your cable size is different, but there are various online calculators for this - I am just offering an example. Connections are the big unknown because the quality of the contact between the copper in the cable and the terminal is determined by the the workman. Likewise, the contact between the terminal and the battery is subject to problems. These places can produce significant voltage drops too and because they are small, you are more likely to sense heat there.

All through the system, you can check voltage drops with a good multimeter. Typically these resistances will be pretty constant, so if you see a 0.1 V difference while the system is drawing 10A from the batteries, that V difference will rise to 1.0 V when the current increases to 100A.

 

Thanks Chaps this is all making more sense now and exactly what I wanted... FYI its not an alt power sales droid its an industrial cable seller.... My cable is 3 times thicker than welding cable., it may be I am beyond the limits..... I don't understand why I have the drop with no usage and just the inverter on... but I guess the drop and loss is a drop and loss whenever it is used.


"With anything less, especially on 12Vdc Systems, that are drawing up to, or over 125 Amps, you NEVER get full use of the energy in the Battery Bank"

I am drawing standby on the inverter only and still seeing this statement ringing true. So a 1.5kw inverter would be EXACTLY the same... as would a 500w inverter on standby ? Or is this not the case... I assume a 3kw inverter doing nothing is.doing nothing..

 

well, the cable cannot be too thick - that is certain. But the voltage drop in the battery when the inverter is on with no load is simply due to the internal resistance of the battery when it is discharging. As soon as a battery is under load and has to produce current - even a small standby current to the inverter, it begins to discharge (if charging source has been removed) and there is a small voltage drop. And, the voltage on a freshly charged battery is often higher than "full" for a time - this is sometimes called "surface charge" but it quickly dissipates as soon as a load is present - again a voltage drop due to load. The numbers you gave earlier showed a voltage drop of about 0.1V from "off" to "on, no load" which seems OK - just the draw of the inverter in standby. The actual voltages were quite low - the battery was in need of charging.

 

I will get back with the multimeter today... I may run power up there... its charged back up to 39 percent which is good but I cant run 1000 watt pump without it complaining it cant get enough power from the batteries. There must be a problem as I not being able to use 74 percent to 100 percent due to straight drop and not being able to use the lower ends doesn't leave much left.... I can only think I botched the wiring somewhere...

 

consider how much power the batteries can hold. You appear to have two units; 6 or 12 volt? either way, say you have 200 amp-hours at 12 volts? that is 2400 W-hr; but you only want to use the capacity between 50% and about 80% - 100% would be great, but you will seldom achieve it. So you really only have about 30% x 2400 W-hr = 720 W-hr or 45 minutes of run time under ideal conditions with the 1000W pump. Misc. inefficiencies might reduce that to 30 minutes. If you ever get behind on solar charging, then it might take a couple of sunny days to really catch up. If the batteries have been at low voltage a while, they may have some permanent sulfation and thus some reduced capacity to store. . .

 

Welcome, Eric, from the misty, tall trees of the Pacific NW USA

 

Thanks Maxflax, KG and all... I have 500amps hours at 12 volts..The pump is VERY rarely used....

I think I have had my question answered in that the solar controller is just badly mis-interpretting the percentages.

I can put this to bed by just leaving the inverter off until its needed, Its just a little heart breaking seeing the battery percentage at 74 percent as soon as its turned on. I had hoped to have a little 4amp light on in the hut the whole time but that's never going to work due to the loss from the inverter. I will probably rig a 12v circuit to make it easier for constant light.

The long term plan is to have water storage 10 metres up to get the pressure and get rid of the pump....

I assume the only way to have always on 240v standby systems that aren't used too much is to start going to 24 or 48 v?

 

LED lights are widely available for 12V automotive applications . . . very low power consumption

 

I'm also seeing stand alone LED light arrays with separate light and battery/panel/mounts, allowing the solar panel to be mounted in a good light gathering spot, separate from the light (Lowes here in the states has them, a home improvement chain. Have one on my back deck, works great) and the d cell battery lantern I posted up here recently can last a long time with NMH batteries, and solar chargers are out there to recharge them

 

LED wont generate the heat I need.. Its just to stop the pipes freezing...

 

Missed that part..

 

electric heat tape on the pipes will be more energy efficient than an incandescent bulb

 

Heat is a byproduct of many things, such as radios and computers. Passive heat off these items, could also make it harder for your pipes to freeze. Make a list of what items you use, that generate heat normally, and keep them near the pipes when in use (if convenient)

 

HmM I have a hell of a lot of pipes to insulate.. there must be perhaps 15 metres in an area the size of a wardrobe.... Is it 12v ?