battery string connection in parallel

Davo000

Hi everyone Im new to this forum I live in australia and I have just got my off grid system up and running.
I Have a plasmatronics 60amp controller 6x37ah agm batteries and 100 wats solar at 12 volts. I plan to add another 2 x 200 watt 12v panels in the near future and then I just need a decent inverter.

I have a fairly straightforward question,
I have connected 4 of the batteries in the configuration shown in the attachment
I need to connect the last 2 batteries but I dont know how, I have tried to figure it out on paper but it just made my brain hurt.
Could someone show me how to add 2 batteries to this arangement If it is possible?
In hindsight I probably should have used a couple of bus bars to bank them up but its kinda late now!
I can guess the proper way would be to add the last set onto the 2 terminals where the load goes but thers not alot of room left.
Thanks :cool:
Attachment not found.

waynefromnscanada

Re: battery string connection in parallel

Welcome to the forum Davo.
Well, to be honest, we never recommend more than 3 batteries in parallel. Why? Because it's just too difficult to ensure all batteries beyond three will be equally charged / discharged, leading to early battery failure. Even three can be tricky. Far better to have one or two larger capacity batteries, or lower voltage batteries wired in series, even two parallel strings of series wired batteries rather than what you already have, or worse, what you propose. You can do it, but expect early battery failures no matter how you wire them.

NorthGuy

Re: battery string connection in parallel

You said you need a new inverter. I would suggest getting a 24V inverter instead of 12V. This way, you can arrange your batteries into 3 strings of 2 batteries each. This is much better configuration. Your controller may or may not be able to work with 24V bank, so you may need a new controller. If you do, this gives you an opportunity to get an MPPT controller which will get 20% more energy from your panels.

Davo000

Re: battery string connection in parallel

NorthGuy wrote: »

You said you need a new inverter. I would suggest getting a 24V inverter instead of 12V. This way, you can arrange your batteries into 3 strings of 2 batteries each. This is much better configuration. Your controller may or may not be able to work with 24V bank, so you may need a new controller. If you do, this gives you an opportunity to get an MPPT controller which will get 20% more energy from your panels.

I hear ya and that is the plan when I can afford a new inverter I will move to 24 volts.
They are second hand batteries that I got fairly cheap so Im not expectin much of them and there wont be any tears if they start dying on me. The controller is not mppt but it was 700 dolars worth and goes up to 48 volts.
What should I do In the mean time would the batteries be better off leaving as is and discharging the 4 of them a bit more or wire the other 2 in and have more capacity but also more diferences in the bank? they are all within .06 volts diference.
thanks for your replies .

waynefromnscanada

Re: battery string connection in parallel

$700 for a non MPPT controller? Sounds pretty steep. Do you have a make and model # ?

Davo000

Re: battery string connection in parallel

aussie made plasmatronics PL 60 ps I think mppt is overrated :cool:

niel

Re: battery string connection in parallel

Davo000 wrote: »

aussie made plasmatronics PL 60 ps I think mppt is overrated :cool:

your prices out there are a bit higher than here, but you still have mppt ccs that are available at a cheaper cost than what you are getting with that particular cc. that is highway robbery for a pwm. you could have gotten pwm ccs imported at a far cheaper price.

now pwm does have 1 advantage in that it has a low operational loss, but until you have seen or used an mppt for yourself you can't really judge them as "overrated". it is true mppt generally has a high price tag, but your pwm takes the cake. care to buy a bridge?:p

vtmaps

Re: battery string connection in parallel

Davo000 wrote: »

What should I do In the mean time would the batteries be better off leaving as is and discharging the 4 of them a bit more or wire the other 2

One way to deal with this is use a battery selector switch. With a switch you could charge/discharge a couple of strings at a time.

To understand some of the problems with parallel batteries, read: http://forum.solar-electric.com/showthread.php?14674

One of the problems with parallel batteries is that when one cell ages or fails, it may be difficult to notice until the whole bank fails. This is because when one cell in one parallel string fails, it is compensated by the other parallel strings. When you turn off your charger you may see one string discharging into another parallel string. (another reason to put a fuse in each string)

Think of it like a single engine airplane... There is a certain probability that the engine will fail on the next flight. Now suppose you have a two-engine airplane with two of the same engines. The probability of an engine failing is now TWICE the probability of engine failure on the single engine plane.

So it is with batteries... if you have four parallel strings you have four times the probability of a cell failing, and when one goes down they all go down. On average, parallel strings of batteries do not last as long as single strings.

--vtMaps

waynefromnscanada

Re: battery string connection in parallel

niel wrote: »

your prices out there are a bit higher than here, but you still have mppt ccs that are available at a cheaper cost than what you are getting with that particular cc. that is highway robbery for a pwm. you could have gotten pwm ccs imported at a far cheaper price.

now pwm does have 1 advantage in that it has a low operational loss, but until you have seen or used an mppt for yourself you can't really judge them as "overrated". it is true mppt generally has a high price tag, but your pwm takes the cake. care to buy a bridge?:p

My thoughts exactly. I nearly fell off my chair when i saw $700 for a PWM unit. But what's done is done. Just a shame this site hadn't been found before the purchase.
Hopefully we can help out from this point on.
BTW, I've had both PWM and MPPT. I'll say this - - I gave away my last PWM after switching to MPPT, and I won't be going back to PWM.

Davo000

Re: battery string connection in parallel

I didnt start this thread to talk about controllers but il say I'm happy with my controller, I looked at all the options and decided mppt was too epensive for the gain plus I couldnt find any other controller that could do what this one can do at the same price. at least if something goes wrong with it i dont have to ship it across the world to get it fixed. a mppt controller with the same features as the pl 60 would have been around twice the price so I could not relly justify it. I was planning to put in a small wind turbine also and use the 30amp load terminals as a dump but im still mulling that one over.

So I hear that with a series string you can see that one battery may be getting weak because the voltage will come down but not so with parallel?
As i understand also the higher the charge rate or discharge rate the more the battery alignment will become an issue so when I add another 400w of panels the bateries might not last a week. ATM im only charging at 6 amps max so i think the batteries would be charging fairly evenly and completely.

Without fusing between the parallel batteries is there a possability of fire or explosion ? if this rig has a melt down Il be living in a cave
considering what I have now learnt im glad i started with second hand batteries as I was going to start with 2 new 100ah batteries and add more on later ( bad idea)
Also Im not that keen to go above 12 volts right away because I have a plan to wire up all the led downlights in the house to the load terminals on the controller, It can give 30 amps.

is a 24 v solar panel more efficient/ better than a 12volt panel asuming that same price? I can get a 200w 12v panel for $200 or 24v for a few dolars less.

NorthGuy

Re: battery string connection in parallel

Davo000 wrote: »

is a 24 v solar panel more efficient/ better than a 12volt panel asuming that same price? I can get a 200w 12v panel for $200 or 24v for a few dolars less.

With PWM controller, you have to match the volatage of the panel to the voltage of the battery bank. So, for 12V system you'll need 12V panels all in parallel. For 24V system, you'll need 24V panels in parallel or 12V panels wired into strings, each string consisting of two panels.

Davo000

Re: battery string connection in parallel

NorthGuy wrote: »

With PWM controller, you have to match the volatage of the panel to the voltage of the battery bank. So, for 12V system you'll need 12V panels all in parallel. For 24V system, you'll need 24V panels in parallel or 12V panels wired into strings, each string consisting of two panels.

I know thats why im buying 12v panels in pairs.

BB.

Re: battery string connection in parallel

It is hard to say...

The problem is that manufacturers are chasing cost sensitive customers/installers. So, they tend to do whatever it takes to reduce price and increase reliability (hopefully both at the same time).

And for "us" that is both good and bad. The good, obvious falling prices... From $5-$10 per watt not more than 1-2 decades ago, to near or under $1 per watt today.

The down side is that the volume driver are large Grid Connected systems. They all use MPPT type Grid Tied inverters that can take a wide range of panel string input voltages (typically from ~200 to 400 VDC or so)... That means that the installer does not care too much the exact Vmp of the panels they use. As long as the string and currents add up to what the GT inverter needs to operate.

So, the panel mfg. tend to make higher voltage panels (~30 volts Vmp seems to be popular at the moment). But that voltage does not work well for any of the standard 12/24/48 volt battery banks (we would be recommending ~17.5 to 18.6 Volt Vmp per 12 volt increment) to match well with a PWM type charge controller.

Higher voltage panels means lower current--Which means less copper inside the panel. And sealed junction boxes with pre-attached cables gives us lower costs and (hopefully) more reliable electrical connections--But it also denies access to where panels could be reconfigured for other voltages--and repair failed bypass diodes).

You can, sometimes, still find ~18 or 36 volt Vmp panels out there in the >100 watt range... But they can be rare. And with the churn in the solar panel market, what is available today at a nice price point may not be available in 12-18 months.

So--There is no simple answer about 12 or 24 volt panels... You probably need to do a couple paper designs (such as a PWM system with the "12 or 24 volt available panels" and a second design looking for lower cost >>100 watt panels and an MPPT controller that will meet your needs.

It may not be obvious... The costs of the two systems may be fairly close (expensive panels+cheap PWM controller vs cheap panels+expensive MPPT controller). In general, for smaller systems (say under ~400 watts) a PWM system may be less expensive. For larger systems (say >800 watts) a MPPT system may be more cost effective.

The real driver for Vmp-array voltage design is usually the distance from the array to the charge controller/battery shed... Trying to have a "large" array on a 12 volt battery system with PWM and any distance over a few tens of feet will usually force one to use very heavy gauge of copper cables from the array to the controller. If you go "high voltage (upwards of 100 volts Vmp for a typical high end MPPT controller), the wire gauge can be many times smaller and less expensive for the run.

-Bill