Fixing failing off-grid systems

lazza · July 2011

Dear Forum

I've been looking at exsiting off-grid installations here in my area (many of which are very badly installed- poor quality materials and poor installers) hoping to offer solutions for people who are ending up with no electricity in their homes.

Questions:

Do inverters lose power with increased temperature? The installer blamed the owner for a blown inverter, saying that although it was a 3kW inverter at 30ºC it only provides 2.4 kW ?? It sounds like porkies to me ("pork pies"=lies).
The inverter blew because the owner was using a 2.4 kW electric saw- but if the starting current was above the inverter's power rating, surely it should have over-current protection?? and therefore only cut out and not blow. Am i correct?
It's a strange set up of 10 Gel 12V batteries 200Ah, in five parallel series of two. I understand that for Gel batteries, when charging, you have to be very careful, and not to go above 29V (24V system), and that you shouldnt charge more than C/20 or 5% of the Ah rating. My question is, is this 5% of the Ah rating of the whole system or just one battery? i.e. should the maximum charge be 10 Amps or 50 Amps in this case?
And finally, measuring the current with a clamp meter (which although cheap has proved reliable and accurate) there is about 1 amp difference between the current on the +ve lead compared to the -ve lead at the PV charge controller. Does this imply a ground-leakage current of about 1 Amp? What would cause this?

Apologies for so many questions at once, but any assistance will be much appreciated

Larry

Cariboocoot · July 2011

Re: Fixing failing off-grid systems

lazza wrote: »

1). Do inverters lose power with increased temperature? The installer blamed the owner for a blown inverter, saying that although it was a 3kW inverter at 30ºC it only provides 2.4 kW ?? It sounds like porkies to me ("pork pies"=lies).

Yes, inverter power does fall off when the temperature goes way up. 30C (86F) shouldn't be that bad, though.

2). The inverter blew because the owner was using a 2.4 kW electric saw- but if the starting current was above the inverter's power rating, surely it should have over-current protection?? and therefore only cut out and not blow. Am i correct?

An over-loaded inverter should fault almost instantly and shut down. It should not burn up or fail.

3). It's a strange set up of 10 Gel 12V batteries 200Ah, in five parallel series of two. I understand that for Gel batteries, when charging, you have to be very careful, and not to go above 29V (24V system), and that you shouldnt charge more than C/20 or 5% of the Ah rating. My question is, is this 5% of the Ah rating of the whole system or just one battery? i.e. should the maximum charge be 10 Amps or 50 Amps in this case?

This is probably the real cause of the trouble: gel batteries. They're not suitable for RE because they can't deliver heavy current, such as running 2.4 kW on a 3 kW inverter. And they are very picky about charging. Five parallel sets of batteries is a likely disaster of maintaining current flow no matter what kind of batteries. The charge rating is for the whole battery bank: in this case five times 200 or 1000 Amp hours. You are correct that the 5% max would be 50 Amps peak current.

I suspect what really happened here is a bad set-up allowed a fairly high current draw for a long time in hot weather and the inverter (a cheap one, perhaps?) was cooked.

4). And finally, measuring the current with a clamp meter (which although cheap has proved reliable and accurate) there is about 1 amp difference between the current on the +ve lead compared to the -ve lead at the PV charge controller. Does this imply a ground-leakage current of about 1 Amp? What would cause this?

Looked at as a percentage, a 1 Amp difference could be nothing more than reading error of the clamp-on meter. Considering the wiring sounds a bit suspect on the system anyway, it may vary that much from when you read one side to when you read the other. If it was a difference of 10%, I'd be worried.

BB. · July 2011

Re: Fixing failing off-grid systems

Marc has pretty much addressed everything... A few details (where the Devil seems to reside):

Many inverters are over spec'ed... Even some of the well known good guys really do not meet data sheet spec's.
- Check DC voltage at inverter's input. Many wiring installations have just too much voltage drop.
- Batteries, as they age, loose capacity--both storage and surge current.
- We have seen 5kW 12 volt AC inverters. Working the math, the current requirement (worst case) design (including a 1.25 US National Electric Code safety factor for wiring/fuses/breakers):
  - 5,000 watts * 1/10.5 volt cutoff * 1/0.85 eff * 1.25 NEC = 700 Amp circuit rating
- A recommended starting point is ~1-1,200 watt inverter @ 12 volts; 1,200-2,400 watt inverter @ 24 volt; Over 2,400 watt, look at a 48 volt battery bank. This keeps the nominal current to around 100 amps maximum--a reasonable amount of copper and breakers/fuses/switches for a reasonable cost.
Use your clamp on current meter to measure the current in each parallel battery string (under heavy load and/or charging). Look for unequal current sharing (over ~2:1 difference in current, check for bad wiring, bad cells, etc.).
Also use a good volt meter to measure battery voltage (and even wiring voltage drop). Looking for differences (too high/too low battery voltage compared to rest of bank, cable/connection with too much voltage drop, etc.).
Note that Solar Charge controllers are not designed to give accurate current output readings. Many vendors consistently read 5-10% high. In some cases, a few models (again good vendors) are known to read 30% high. Controllers are still good performers, just not accurate (and do any owners complain when systems produce "more power" than the expected :roll:).
They are not perfect, but I like to recommend Battery Monitors (Victron apparently makes nice units too). Especially for Sealed batteries where owners cannot measure specific gravity. As close as you will get to having a fuel gauge on your car's fuel tank.
Measuring current/power in a Solar PV system--Even with a battery monitor, you are probably doing well to be within 2%-5% accuracy.
For poorly charging systems, check if they have desulfators attached. We have a few reports of desulfators and even inverters causing confusion for MPPT type solar charge controllers (reducing their output current on bright/sunny days due to electrical noise on battery bank).

-Bill

niel · July 2011

Re: Fixing failing off-grid systems

one has to watch how inverters are rated for that 3kw might have been it's peak surge power. add to that the heat and high powered load in conjunction with failing to fuse properly (my guess) and bang it blows.

i would say it was poorly designed at the very least. can't say on the equipment being good or not, but i would not recommend gel batteries unless bought with a real good deal as they are very picky.

lazza · July 2011

Re: Fixing failing off-grid systems

HI Guys

Thanks very much for your replies. Pretty much confirmed my thoughts on the installation. A cheap inverter, bad battery set up etc.

The only thing that still bothers me is the current differential between +ve and -ve on the solar charge controller. As the difference was 8A on +ve and -9A on -ve, this is more than a 10% difference. As mentioned, my clamp meter has proved pretty accurate up to now. If this were not a reading error, could it be that there is a current leakage to ground?

Cheers
Larry

BB. · July 2011

Re: Fixing failing off-grid systems

Put both + and - leads together in the DC clamp meter... You will then measure only the "common mode" current.

Do this on the battery output--if you see that 1 amp--you probably do have some leakage current.

Do the same measurement on the solar array input... If you see the same 1 amp current flow, then you have a short to ground on the solar panel side--possibly on the negative side (or somebody grounded the solar panel to earth ground directly and you have some ground current flow).

Also measure the current flow on any of the other wires leaving the controller (earth ground wire, any communications wire, etc.).

If you have significant difference between +/- leads--it is not right.

-Bill

lazza · July 2011

Re: Fixing failing off-grid systems

Ok, many thanks.

When we go back to revise his system- I know exactly how to verify.

Larry

BB. · July 2011

Re: Fixing failing off-grid systems

You may also have a ground loop between Solar Array - and earth ground, or Battery Out - and earth ground.

As far as I know--No solar charge controller should have the - lead tied to earth ground in more than one place--And some will not work correctly if the solar array - lead is earth grounded (for example, the Xantrex MPPT charge controller puts the array current monitor in the negative lead--this will confuse the MPPT calculations).

New systems may have a DC Ground Fault detection setup. Basically a fuse or circuit breaker between DC - and Earth Ground. If there is more than 1/2 to 1 amp of ground current, the fuse blows (and electronics shut down) or the breaker trips which trips a second breaker on the solar array + lead.

For DC Ground Fault systems, there should be no other DC - to earth ground connections for the circuitry to work correctly (no Battery - to earth ground either).

-Bill

Photowhit · July 2011

Re: Fixing failing off-grid systems

I know I'm late to the game, but perhaps have someinsite;

1. Yes

2. Some cheap inverters have fuses, some internal some external, nearly typical for a MSW cheap inverter.

3. Odd comfiguration unless your likely to want to do a large power draw like running a 2400 watt saw from a gel battery. Should allow more available draw.

4. While I only have a cheap clamp meter, I do know statistics and significate digits, and the difference between 8 and 9 amps when an Amp is the lowest digit can be well less than 1%.

Fixing failing off-grid systems

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