Battery voltage issues/questions on a DIY solar kit

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KennyM

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#116235 22-Apr-2013 21:38

Hey there

I have just designed a built a home made solar station.
The goal at this stage is to power the man cave, then onto the shed and whatever else as it expands.

I am having issues with the battery voltage at the moment.
I have a DC power monitor installed that records every 3 minutes.

Basically have a 130ah deep cycle battery - with a 1000w inverter to power everything.
The battery in charged state sits at around 12.7volts - which is pretty normal from what Ive researched.
As soon as there is power drain it drops straight away as per my screen shot - thats only drawing less than 250w for my computer

Spiky blue is the battery charging in the sun, the first spike is me using my computer, then i turn it off, i then turn it on again, and about 1/2 of that time is spent playing a game which is the high usage, then off for the night. then todays charging.

It shows the battery voltage drops as soon as the power is drawn - and then the voltage goes back up when the draw is off.....This is only about 250watts out of 1000w possible from inverter.

Now, my issue is that when the voltage drops, the controller notices the voltage drop, and it has an automatic shutoff to protect the battery from over discharging. This never happened last nights, but it did get me wondering what the drop would be like if I used more power.

Today the battery was back up to 12.3volts when I tried it out, but this time I plugged in the toaster (750w) nothing else (moved PC back to mains) As soon as I turned on the toaster the voltage dropped to 11.06, then over the course of the next 2 minutes it dropped to 11.00 - the toaster then finished....I left it to cool down - I was doing the graph above when all of a sudden the controller turned everything off - after about 2 minutes without any load - I guess its a bit delayed

I personaly think the issue is with the battery
The battery is brand new - and supposed to be just what I want. I see no way that this could work at all for me - I cant get anywhere near the 65h out of the battery i should be able to (getting battery to 50% discharged)
But how do I test/diagnose that it is the battery?

Bits that confuse me:
Battery says its a 'D3' - doesnt that make it an April 2003?
On the white sticker up the top - the 1 line with the black background says 'not suitable in cycling applications' ....I could be wrong but isnt a cycling battery exactly what I want?

Below is the design map, ive tried disconnecting everything except the controller and battery to make sure its not 'leaking' some where - but doesnt seem to be

Battery is 130ah NOT 135 as image shows.

jeffnz

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#803963 22-Apr-2013 21:49

D3 is April 2013 and it is a Deep cycle battery, American made so it isn't rubbish.

I know nothing of what you are using it for but work for the company that imports those batteries.

PM me your contact details and I'll talk to one of the guru's at work tomorrow. There should be another date code stamped on the terminals which is manufacturing date can you also include that.

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Niel

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#804043 23-Apr-2013 07:24

Every 100W will draw 10A off the battery. A resistance in any cable connection will cause a voltage drop. Deep cycle is not designed for high power use, so might not work well for 1kW applications. Normally you use a bank of batteries, not just one.

You can never have enough Volvos!

jeffnz

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#804114 23-Apr-2013 09:41

just having another look at this through not so tired eyes and a couple of things.

"I cant get anywhere near the 65h out of the battery i should be able to (getting battery to 50% discharged)"

you won't get 65 hours out of it unless you are drawing 1 amp or less. Looking at your draw you will be lucky to get 4-6 hours use with computer.

Secondly, 12.3 volts is basically 50% drained so not surprising you get hardly any use from it.

http://www.energymatters.com.au/renewable-energy/batteries/battery-voltage-discharge.php

Basically it I don't think its the battery its what you are trying to do with it.

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ubergeeknz

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Vocus

#804120 23-Apr-2013 09:51

I'd say you're going to need more batteries. With that kind of draw you're not going to get close to the rated capacity. If you put another 2-3 batteries in parallel (same type) then you might be in better shape. This is where the costs of "off grid" start to grow :/

edit: oh and if it's not getting to 12.7v with an hour or so of sunlight to spare, you will need more panels too ;)

KennyM

221 posts

Master Geek

#804527 23-Apr-2013 20:10

Hey guys, thanks for the replies.

It is an issue with 'the size of the bank' - too much current for the battery to handle.

Im going to get 2 bigger batteries, tho not perfect, it will do me for the next couple of years while I spend money on the panels.

Thanks for the help.

Niel

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#804530 23-Apr-2013 20:14

Drawing 1000W off a 130Ah 12V battery you might get 60-90 minutes of run time while drawing 80A. The battery is likely not rated to do this kind of current for more than 10 minutes. Battery capacity is also specified over a certain discharge time, often 10h not 1h.

The battery has a very low internal resistance, but the contact resistance at the terminal is often significant and can be in the order of 50m Ohm which at a 1000W load will waste about half the battery capacity. You can improve the terminals, but the most effective solution is multiple batteries.

I have not looked closely at Pb batteries, but this is an excellent reference: http://www.buchmann.ca/toc.asp

What I have worked with for a few years is LiFePO4 which is not the highest energy density battery but it is the safest Li chemistry available and ideal for cyclic use. Pb is not suitable for cyclic use, deep cycle is just better than normal but still not suitable because Pb batteries suffer damage if they are left discharged while there is a week of rain and you cannot fully charge them. With LiFePO4 you are looking at about $100 per 12V 12Ah, so about double the price of Pb but the performance and service life is so much greater that it pays for itself a few times over. The only issue with getting hold of them is the stupid IATA freight regulations do not recognise the safety of this technology. Sorry, I'm ranting, but so frustrating when you try and develop a product when freight companies are scared of perceived danger, we should warn them there is pure hydrogen and pure oxygen in water!

You can never have enough Volvos!

jeffnz

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#804531 23-Apr-2013 20:14

KennyM: Hey guys, thanks for the replies.

It is an issue with 'the size of the bank' - too much current for the battery to handle.

Im going to get 2 bigger batteries, tho not perfect, it will do me for the next couple of years while I spend money on the panels.

Thanks for the help.

sorry never called back but basically the guy looked at it and said same as everyone else it is about what you need not the battery then I just got busy and forgot, age age and bad attitude I put it down to.

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Niel

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#804542 23-Apr-2013 20:20

Just do not parallel the batteries straight away, get a resistor to connect them together and let them equalize to the same voltage before removing the resistor. Suppose you have 2V difference between 2 batteries, if you use a 1 Ohm 5W resistor to connect the batteries together (or prefer 10W to have headroom) then you will draw about 2A from one into the other. Leave it overnight, or until the voltage across the resistor is zero, then it is safe to remove the resistor and connect the batteries together.

You can never have enough Volvos!

KennyM

221 posts

Master Geek

#804545 23-Apr-2013 20:27

jeffnz:
KennyM: Hey guys, thanks for the replies.

It is an issue with 'the size of the bank' - too much current for the battery to handle.

Im going to get 2 bigger batteries, tho not perfect, it will do me for the next couple of years while I spend money on the panels.

Thanks for the help.

sorry never called back but basically the guy looked at it and said same as everyone else it is about what you need not the battery then I just got busy and forgot, age age and bad attitude I put it down to.

Not a problem.

Niel: Drawing 1000W off a 130Ah 12V battery you might get 60-90 minutes of run time while drawing 80A. The battery is likely not rated to do this kind of current for more than 10 minutes. Battery capacity is also specified over a certain discharge time, often 10h not 1h. !

I wanted to work my way up to being able to use that much power, but it turns out its just not going to be possible.
Which is fine - my understanding of batteries has changed...

As per what has been suggested to me, ill be taking back the 130ah battery and replacing it with 2 x 235 (i think or close too it) 6v batteries. And ill run my computer off them...
This is aparently best aH per $, and easy to add 2 more batteries latter...But I wont, this will do me, and the batteries should survive until I change too a grid tied system.....But I wont be able to power as much stuff from the batteries during the wait.