Battery Virgin Needs Help!

[charliegolf]

Hi All,

Just had PV grid tie installed, and bought an extra panel 'to play with'! I currently have it attached to a fairly new 110 Ah leisure battery (donated), charge controller and 600W inverter (gift from daughter). The load, such as it is, are some very low watage outside lights.

The charge controller has a little display, cycling through 3 screens:
Battery Voltage (12.7V);
Current to the Battery Charhing current? (1 Amp) and the
Load (0.7 A).

Questions:

Why is there 0.7A dischage (correct word?) with no load as such?
How can I know the 'health' of the battery over time? (Does the voltage tell  me anything?)

All advice happily accepted.

Cheers,

CG

[billi]

Hi , perhaps the inverter on its own draws the 0.7 AMP ?

[johnrae]

Assuming your inverter is connected (with no load) then it is obviously drawing 0.7 amps, so it imposes a 9watt load just to keep its circuit oscillating.  At 600watts output it'll impose a 50amp+ load on the battery  -  probably nearer 60amps.

If that is the case, then you should not have the inverter connected unless you actually need a 240volt supply.

Battery voltage won't give you a reliable "health" status, only a general state (apart from when it reads less than 11, at which point the battery is flat). Taking an SG reading is your best bet.

[charliegolf]

Thanks both. Inverter drawing .7A makes sense. As for the 600W inverter limit- it's never been my intention to run it at anything more that 20 or 30W. My plan is to use 1W LEDs for outside lighting- not much else at this stage.

CG

[johnrae]

That being the case then use 12 volt leds.

[jonesy]

Yep.  Johnrae is spot on.  With the inverter wasting almost 10W just to give mains, it makes much more sense to use 12V LEDs.  Also, if you put a 10W load on an inverter it will take around 15% more out of the battery, so about 11.5W when it's well loaded, possibly 20-25% more at light load.
And as you're playing with the mains, don't forget to earth the inverter, and install proper RCD and fusing.  Mains off an inverter is more dangerous than mains in a modern house (in terms of its ability to kill you)  And whatever you do, never connect your inverter into a live house socket.

I'd also strongly recommend  a DC isolator, appropriate fusing and good quality thick cable on the DC side.  Any fault on the AC side eg a short circuit will result in extremely high currents on the DC side - hundreds of amps - which can cause very quick heating in thin, unfused DC cables.  We all tend to think of 12V DC as safe, but it ain't!

[charliegolf]

All noted, thanks. As for 12v LEDs, I have long cable runs- does that change the advice? Ta.

CG

[jonesy]

Long cable runs.  A 12W lamp will draw 1A at 12V.  A 0.5mm2 cable, 10m long, would lose about 0.25V or 2%.  5% is considered the limit for volt drop, so you could double the distance or half the cable size and get a 4% loss.
A thicker cable can be used to a point of distribution and then thin cable to the individual lamps.

[billi]

Surely , if your consumption is mainly at night , without the sun , than  direct 12 volt is the way to go ,  cannot tell  about cable size ,cause depends on watt and length  , but a 4-6 mm2 solar cable (under 1£ per metre) can supply a lot  of watt for LED lights over a longer distance

Personally , i  like more PV , instead of DC products , that are generally more expensive , but my heavy consumers in the house are run during the day ..... and  sun helps direct

Its a bit schizophrenic  situation  , for example , i should get an more efficient fridge/freezer , but why kick the existing one out ? And not use the 500£ to buy 800 Watt more PV  

Billi

Oh.... Jonesy was faster ..... so my cable diameters where a bit over the top .....

[Justme]

4% losses might be ok for mains AC but for 12v DC its far to much.

A full 12v bat is at about 12.8v. Loose 4% & your down to 12.28v at the device.

Depending on the min voltage your equipment will run at you could soon get the the point when it wont run yet the battery is still at a decent state of charge.

Its more of an issue with larger stuff like 12v fridges as boat dwellers soon find out.

[jonesy]

4% losses might be ok for mains AC but for 12v DC its far to much.
Depending on the min voltage your equipment will run at you could soon get the the point when it wont run yet the battery is still at a decent state of charge.
Its more of an issue with larger stuff like 12v fridges as boat dwellers soon find out.

I hadnt thought about discharge down to say 11.8V with SLA, so another 0.5V off that is no good.  So would 4% drop be ok for LEDs and the like, justme?

[johnrae]

1 watt of LED is a fair amount of light, so 12 watts is LOTS.  Rather than just stating "long cable runs" if you can give some idea of your set up with distances , wattage required at specific points, etc you will get better guidance to your enquiry.
Forget about potential problems with % loss, 12 volt LEDS are quite happy running at lower voltages - they are current sensitive devices.

[Justme]

I hadnt thought about discharge down to say 11.8V with SLA, so another 0.5V off that is no good.  So would 4% drop be ok for LEDs and the like, justme?

I would never run my bank down to 11.8v.

Even under high loads I would want it higher than that.

You would have to check the LED's you want to use. They all have differing voltage ranges they are happy with.

Lower voltages will give lower levels of light.

[biff]

Be kind to your batteries .
                         Do not discharge below 12.4volts.If you want the happy life and a happy battery bank,keep your voltage above this.
                                                                          Biff