Electrical question for the guru's

[strongmanmike]

Ok, so I have dispensed with the idea of using a generator to power my new observatory site, in fact I did buy a brand new (Chinese) generator from Outbaxcamping in Sydney and it was dead on arrival... and to cut a long story short...now some two months later it seems I may have finally secured a refund :rolleyes:

anywaaaay....:mad2:

So I have a 160W solar panel with a good quality 30amp regulator to do the charging and I will have 2 X 120ahr AGM batteries in parallel and a 1000W inverter (and a 12V outlet) to supply all my power :)

My questions are:

1) If I have the two batteries wired in parallel, does it make any difference if I charge them by connecting the charger (solar panel via regulator) to the terminals of only one of the batteries?

I understand the charging method, where I would place the +ve from the charger to the +ve on the first battery, and the -ve from the charger to the -ve on the second battery.

But does it matter? If the batteries are connected in parallel would the charge trickle over from one battery to the next anyway if the charger was connected to the terminals of only one of the batteries?

2) In the same vain does it matter how I connect the inverter across the batteries?

Thanks for any advice

Mike

[Astro_Bot]

The potential across both batteries should be as close to identical as you can make it, hence the "cross" wiring as you describe (+ve input to +ve of one battery and -ve to -ve of the other battery). That maximises battery life.

Not only should you charge in that configuration, but also draw power that way as well.

The wire resistance may be small but at 12V can have a measurable effect. What the "cross" wiring achieves is equalizing the wire length on the +ve and -ve sides and for each battery.

I can't really tell you much about inverters other than, as mentioned, connect the 12V input "cross" wired as for charging.

[GeoffW1]

Hi,

It does not matter if

(1) The charger is connected to only one of the parallel wired batteries, in the manner you describe. In this case you have effectively one battery (in 2 halves) of the same voltage but twice the Ah capacity of just one. Whatever happens in one battery will happen in the other as long as they are parallel connected.

(2) For the same reason the inverter connection can be made to either battery

The gauge of the parallel connection wiring should not be too light though, as it will bear at least half the peak current draw. Then, if one battery begins to die early, it becomes a shunt resistance unable to hold a charge, and the other battery begins to bear all the load of charging and running. It will not last long after.

Cheers

[strongmanmike]

Hmmm? well as I suspected :question:... two opposite answers :scared: :lol:

Maybe I should do a poll :shrug:

Anyone else..??

Mike

[GeoffW1]

Haha

They are not opposite answers, they are exactly the same advice. :thumbsup:

You are surrounded by genius here :eyepop: (geniuses? Genii? :lol:)

Well, you asked us :rofl:

Cheers

[strongmanmike]

Huh?...o-k, what am I missing here... :question: to me you are saying different things :shrug:

:help:

[GeoffW1]

No no,

Connect +ve to +ve and -ve to -ve on the batteries, for parallel wiring (you know that OK), and it gives you effectively a twice size battery of the same voltage. Then it does not matter which terminals you connect to to for the rest of it, provided you observe polarity of the load.

All just as you said originally.

Cheers

[acropolite]

Mike, as the others suggest, keep the leads between the batteries as large as possible, your 1Kw inverter running at full capacity will draw close to 100 amps, any resistance at that current will result in significant voltage drop and power loss. Think thick automotive type connecting leads of a gauge similar to the ones connecting your car batteries.

The real issue is the 1Kw inverter, you can expect from fully charged less than 2 hours of running at the full 1Kw load, less load and your run time will be longer, it's really mental arithmetic, to get a rough idea divide your power consumption in watts by 12 then add around 20% for losses in conversion and that will give a ball park figure for current draw.
I'd imagine with the equipment you're running the constant load will only be a couple of hundred watts.

Battery capacity varies with discharge. The higher the discharge the lower the amp hour rating, your 120 amp hour rating would be at the 10 hour rate, i.e. around 12 amps for 10 hours, draw higher currents and the amp hour capacity will decrease. If you're curious get the data sheets for your batteries, there should be graphs showing the rating for various discharge levels.

The killer will be recharging, your 160 watt panel on a good day in summer will probably return around 60 amp hours of charge per day, on a cloudy day could be as low as 5amp hours, your winter charge will be at best 1/3 of the summer values and bugger all if it's heavily overcast or raining.

If your combined 240 amp hour bank is totally flat and you get a fortnight of cloudy weather you might as well kiss your batteries goodbye, they will likely have sulphated irreversibly.

Make every thing as efficient as you can, understand the arithmetic and how to care for your setup and it will reward you, otherwise you're in for an expensive experiment.

[glend]

Early in my working life I installed telephone exchange battery backup systems, so I know something about this topic. I would agree that your system is too small for what your trying to do. I won't write a long thesis on the subject and there is plenty available on the Net concerning the design of PV 12V systems. I run an off-grid system for my shed and astro supply, which also feed to my house to provide some power (isolated from the Return to Grid solar system on the house).

Your battery bank wiring should look like this:

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcTBYTgOxk3sq-a4QIfnsfmRkV-pDstruTkbfhv3vPSJvNJa1MSNmA

As you design the system keep in mind you do not want the voltage of your battery bank to drop too low, as deep discharge will shorten the lifespan of the batteries. As a personal rule I don't let the bank voltage get below 12.3V and my MPPT controller tells me exactly how much capacity I have left. The bank floats at 13.1V, and input bulk charge early the next day is capped at 14.5V by the MPPT controller. Its worth investing in a good charge controller that can accept input charge from multiple sources and will manage the bank voltage.

Keep the cables between your batteries as short as possible and as thick as you can. Don't try to terminate battery cables yourself, buy them pre-swagged. You can get them at most Supercheap Auto stores but a boat supply house like Whitworths often has much better 12V cable and gear.

BTW check the nonimal charge current for your panel, it should be on the label on the rear of the panel. I would be surprised if you get more than 9 amps out of it at peak.

Find one of the online design tools it will help with working out how much panel size and battery amp/hrs you need for your load.

Good luck

[strongmanmike]

Thanks for the feedback guys, all good info :thumbsup:

So.. it appears that for both charging and powering from the inverter wiring the +ve and -ve across the two batteries (preferably with equal length wires) rather than from the one battery, is at least theoretically, the better way..?

Ok the battery I am looking at is the Kickass AGM (http://www.australiandirect.com.au/buy/kickass-120ah-deep-cycle-agm-battery/KA12120) from Australian Direct

My imaging system currently consists of: ASUS Lap top, SXH694 CCD, lodestar guider, USB filterwheel, FLI Atlas focuser, FS2 mount control, secondary mirror heater (Kendrick S2 pad), Orion tube heater (42W) and three rear cooling fans. All up the constant total current draw is about 10A. This will increase temporarily for a few moments when the hard drive activates or the FS2 slews which occur usually only once or twice a night for a few seconds only (I don't do mount modelling) or I turn on some low power LED lighting.... and sometimes less than 10A, when the laptop is idling and the heater pads are cycled off, so... about 10A should see my whole imaging system chugging away nicely.

So, if I am not mistaken, a 120ahr @ 20hr rate battery like the Kickass will provide 6amps for 20 hrs or say 10A for perhaps 12hrs? So having 2 X 120ahr batteries I would have thought adequate so that I don't discharge the individual batteries by more than about 50% even after an all nighter with everything running the whole time, even allowing for some efficiency losses in the inverter and any extra current draw here and there from some low power LED lighting and the occasional GOTO slew etc..??

Then, with the 160W panel it should only take a couple of sunny days to fully recharge the batteries again...?

Although more than I need to do astroimaging alone, I thought a 1000W inverter would be handy to have on site in case I need to briefly use a power tool or some other higher power appliance occasionally?

Am I on the right track here..?

Mike

[Astro_Bot]

I'm not a solar expert, but my gut feel is that the solar panel is a bit underpowered.

General rule of thumb: average power (averaged over a typical 24-hour period) is around 10-15% of the panel rating, so that's 16-24W. Let's pick 20W for argument's sake. Assuming 100% efficiency for charging (adjust downward as necessary), at 12V, that's 40Ah per day. In other words, that'll take 3 average days to recharge 120Ah (60Ah into each battery, or 50%). If you have better figures for average power output at your location, use those instead of the rule of thumb. Edit: I suppose you could go with the 160W panel initially, and add more at a later date if it is indeed underpowered. Who knows, you may never draw all the power you estimate.

As for the 1000W inverter, capacity vs current is non-linear, so the battery will be drained very much faster at high current. Assuming 100% efficiency for the inverter (again, adjust down as necessary), a 1000W (4A at 240V) power tool will draw 80A at 12V - that's 40A from each battery - you'll need to check the spec for your battery to see what the drain characteristic is at that current, but it'll be much shorter.

As mentioned above, that also calls for very thick cables.

If it's just for occasional power tool use, personally I'd get cordless tools - charge them up at home, and skip the inverter altogether.

[strongmanmike]

Fair enough an extra day it will ahve to be...I got the solar panel for free so I'll just have to make do as I'm on a strict construction budget :doh:



Yes I agree I would use cordless where possible but I envisage it would only be occasionally that a higher power tool or appliance might be used..?? and the way I read it the constant current discharge graph for this battery suggests it can supply 80A for nearly an hour :shrug: so two of them should power the tool for nearly an hour and neither battery drop below its 50% charge level..?

Mike

[MrB]

Yes, the rest is lost as heat due to the internal resistance.

You would expect that if one is drained 100% at 80A, two in parallel would be drained 50% at 80A, but it's even better!
Because the two are connected in parallel you have halved the current draw between them, 40A each, so there are less losses. Expect approx 40 or 45% DOD(depth of discharge) at 80A for an hour.
80A / 240A = 0.33C (C = capacity) Extrapolating from the graph in the datasheet, a 0.33C discharge (somewhere just to the left of the 0.25C curve) would give approx 2.5h run time, so 1h/2.5h = 0.4 or 40% DOD. :thumbsup: