Small off grid solar system query !

Submitted: Monday, Mar 26, 2018 at 21:52
ThreadID: 136477 Views:1535 Replies:4 FollowUps:18
Hello all. I have a small problem that is doing my head in. I wish to setup a permanent solar system on a onsite caravan. At this point in time I wish to run a 40 ltr fridge and led strip lights for a max of 2 weeks at most ( xmas ) and intermittent use at other times.
I have got five three y/o 33 ah UPS deep cycle batteries previously used in data room duty that I bought cheap and have charged up well and are holding charge. These are my practice batteries that I intend to run in parallel. The first of my problems starts here. trying to obtain the correct way of running the positive power off the bank. Some examples show it just feeding in one end and out the other or a more centralised junction. Ummm. Wtf
I bought Collyn Rivers motorhome and solar book and IMHO was a waste of money. Piss poor diagrams for the few that were there.
Back Reply Expand Un-Read 0 Moderator

Reply By: Ron N - Monday, Mar 26, 2018 at 23:19

Monday, Mar 26, 2018 at 23:19
Greg, there's some useful information on deep cycle batteries, solar systems, wiring, and the other relevant solar system issues, on the following website link.

Enerdrive - connecting batteries in series or parallel

Cheers, Ron.
AnswerID: 617857

Reply By: Allan B (Sunshine Coast) - Tuesday, Mar 27, 2018 at 01:46

Tuesday, Mar 27, 2018 at 01:46
.
Hi Greg,

I assume that your batteries are 12 volt each and that you require a 12v system. In that case you will be connecting the three batteries in parallel.

All you need to do is jumper your positives together and same for the negatives, then your take-offs can be wherever convenient, at either end or in the centre, it matters not. All that is important it to use adequate sized cable so that there is no voltage difference at each battery. For your current needs I suggest no smaller than 6 sq.mm cross section (10 gauge).

You may see published advice of taking the positive from one end and the negative from the other, and even more fancy arrangements, accompanied with expressions of avoiding voltage differences at the battery terminals. But in practice, none of this is necessary if adequate cable size is used. It is an elaborate scheme with application only to very large currents but is sometimes touted to impress the reader.

Cheers
Allan

Member
My Profile  My Blog  Send Message

AnswerID: 617859

Follow Up By: RMD - Tuesday, Mar 27, 2018 at 09:26

Tuesday, Mar 27, 2018 at 09:26
Same for the whole 5 batteries too.
2
FollowupID: 889680

Follow Up By: Allan B (Sunshine Coast) - Monday, Apr 02, 2018 at 18:58

Monday, Apr 02, 2018 at 18:58
.
Yeah thanks RMD. I should have said five, not "three".
The "three" belong to their age in years. doh.
Cheers
Allan

Member
My Profile  My Blog  Send Message

0
FollowupID: 889827

Follow Up By: GREG T11 - Saturday, Apr 07, 2018 at 20:39

Saturday, Apr 07, 2018 at 20:39
Thanks people, it is starting to make sense.

O.K today I got a 285w panel and a Victron 100/20 controller and some other odds and ends ( have already got 4mm twin sheath tinned solar cable and some 10mm tricable for the battery connections along with a proper crimping tool ).

But... when connecting the solar controller to the bank do I follow the same theory. As in + and - charge cabling onto one battery only and load off the other end, or run the load directly from the controller.

Cheers Greg
0
FollowupID: 889964

Follow Up By: Frank P (NSW) - Saturday, Apr 07, 2018 at 20:47

Saturday, Apr 07, 2018 at 20:47
I would run the load directly off the controller. That way the controller will supply the load and any excess output will go to charging the battery.

If the load is not present, e.g. a fridge not actually running, then all the output will go toward charging the battery.

If there is not enough sun to run the load, the regulator will automatically power the load with what is available from the sun and make up the difference from the battery.

It is usually better to charge a battery with no load on it. The above promotes that as much as possible while still guaranteeing power to the load.
1
FollowupID: 889965

Reply By: HKB Electronics - Tuesday, Mar 27, 2018 at 09:42

Tuesday, Mar 27, 2018 at 09:42
This maybe of interest:

How to wire parallel batteries and why
HKB Electronics

Business Member
My Profile  Send Message

AnswerID: 617867

Follow Up By: Allan B (Sunshine Coast) - Tuesday, Mar 27, 2018 at 12:15

Tuesday, Mar 27, 2018 at 12:15
.
That is one of the fancy schemes that I referred to.

In fact the conclusions are grossly incorrect, but the author does not provide his mathematical method for scrutiny. Given the resistances quoted, the current distribution of the four batteries would be 25.44A, 25.1A, 24.8A and 24.5A for a total of 100A. The dominant resistance in each leg is that of the battery internal resistances so that the jumper links represent only 0.16% of each leg losses. An insignificant amount. Normal variations of individual battery internal resistances are likely to far exceed this.

In Greg's case, his average current will probably be no more than10A so any imbalance would be effectively immeasurable.



Cheers
Allan

Member
My Profile  My Blog  Send Message

0
FollowupID: 889689

Follow Up By: Member - Trouper (NSW) - Tuesday, Mar 27, 2018 at 12:31

Tuesday, Mar 27, 2018 at 12:31
Well well HBK that is interesting and makes sense.
I wonder how the battery banks in PV cell installations in homes are wired up?
0
FollowupID: 889690

Follow Up By: Allan B (Sunshine Coast) - Tuesday, Mar 27, 2018 at 13:01

Tuesday, Mar 27, 2018 at 13:01
.
Trouper,
Within each battery module the cells are in series so the issue does not arise.
In the only installation I have seen with two modules the cables, of approximate equal length, were run from each module to the common terminals of the isolation switch so current would be equalised.
Cheers
Allan

Member
My Profile  My Blog  Send Message

0
FollowupID: 889694

Follow Up By: HKB Electronics - Tuesday, Mar 27, 2018 at 13:30

Tuesday, Mar 27, 2018 at 13:30
Allan,

I think you need to check your calculations for the first example using his quoted figures for the internal resistance of the batteries plus the links and got similar figures to his.
HKB Electronics

Business Member
My Profile  Send Message

0
FollowupID: 889697

Follow Up By: Allan B (Sunshine Coast) - Tuesday, Mar 27, 2018 at 18:49

Tuesday, Mar 27, 2018 at 18:49
.
Nope, just re-performed the calculation using a different method and got essentially the same result.

One factor in your referenced article does not make arithmetric sense and may account for the difference in outcome. It states that......

"the 20cm length between each battery will have a resistance of 0.00012 Ohms. This, admittedly, is close to nothing. But add onto this the 0.0002 Ohms for each connection interface (i.e. cable to crimp, crimp to battery post etc) and we find that the resistance between each battery post is around 0.0015 Ohms."

I make it to be as follows:

Crimp to post.. 0.0002 Ohms
Crimp to cable 0.0002
Cable.............. 0.00012
Crimp to cable 0.0002
Crimp to post.. 0.0002
TOTAL.............0.00092 Ohms

0.00092 Ohms is very different to 0.0015 Ohms and probably accounts for the differing outcomes.

I am satisfied that my assessment is correct and will leave it at that.



Cheers
Allan

Member
My Profile  My Blog  Send Message

0
FollowupID: 889709

Follow Up By: HKB Electronics - Tuesday, Mar 27, 2018 at 19:59

Tuesday, Mar 27, 2018 at 19:59
Alan,

There are four terminals involved and two lengths of cable therefore

4 X .0002 = .0008 crimp to post
4 x .0002 = .0008 Crimp to cable
2x .00012 = .00024 cable

Total = 00184 or he has stated around .0015
HKB Electronics

Business Member
My Profile  Send Message

0
FollowupID: 889710

Follow Up By: greybeard - Tuesday, Mar 27, 2018 at 20:34

Tuesday, Mar 27, 2018 at 20:34
The OP's got 5x33AH batteries, a fridge and a led strip light. It's going to make bugger all difference no matter how pedantic you guys get.
6
FollowupID: 889712

Follow Up By: Allan B (Sunshine Coast) - Tuesday, Mar 27, 2018 at 23:08

Tuesday, Mar 27, 2018 at 23:08
.
@HKB,
He stated.....
".....the resistance between EACH battery post is around 0.0015 Ohms."

@Greybeard,
Which is what I said in my first response.
Cheers
Allan

Member
My Profile  My Blog  Send Message

0
FollowupID: 889717

Follow Up By: qldcamper - Wednesday, Mar 28, 2018 at 07:48

Wednesday, Mar 28, 2018 at 07:48
You forgot about ventilation. What if one battery stayed 0.00005 degree cooler than the rest? That should keep you two going for a while ??
1
FollowupID: 889720

Follow Up By: HKB Electronics - Wednesday, Mar 28, 2018 at 10:31

Wednesday, Mar 28, 2018 at 10:31
Allan,

I stand corrected, my brain was looking at it from a battery to battery perspective I did not process the "per link".

I only posted the document as it gave an insight as to why it is a good idea to use specific wiring methods when interconnecting batteries, yes it does have some calculation errors but the principals are sound.

Still as it appears to have offended some that believe there's is the only way to do things I have removed the offending document. For those of you that may be interested I'll leave the article up on my website.

Cheers
Leigh
HKB Electronics

Business Member
My Profile  Send Message

2
FollowupID: 889727

Follow Up By: Allan B (Sunshine Coast) - Thursday, Mar 29, 2018 at 11:12

Thursday, Mar 29, 2018 at 11:12
.
@qldcamper,

Re: Effect of battery temperature.

Sorry it took so long, it was quite a consuming calculation as the characterisation of specific criteria was further compounded when taking into account the evolution of common-mode data over a given time period and the total system rationale evaluated against the normalised base factors of each functional node. This then necessitated apportioning the divisions of the root-mean-square dividends across the entire outcome field. Consequently some quantum approximation needed to be applied before relative trinomial allocation.
The analysis conclusion was that the internal resistance, or rather the thermodynamic impedance, of the cooler battery would asymptotically eventuate to between 287 and 315 nano-ohms lower than the other cells. The outcome could therefore be considered somewhat nugatory as was your question.

For your benefit qldcamper, I have kept the explication to lay terms for your comprehension and I thank you for participating in the discourse. Please feel free to proscribe yourself at any time.
Cheers
Allan

Member
My Profile  My Blog  Send Message

6
FollowupID: 889747

Follow Up By: Member - McLaren3030 - Thursday, Mar 29, 2018 at 11:23

Thursday, Mar 29, 2018 at 11:23
Well said Sir Humphry :-)

Macca.
Macca.

Lifetime Member
My Profile  My Position  Send Message

4
FollowupID: 889749

Follow Up By: qldcamper - Thursday, Mar 29, 2018 at 20:25

Thursday, Mar 29, 2018 at 20:25
Lol, you should have been a pollitician.
1
FollowupID: 889755

Reply By: swampy - Wednesday, Mar 28, 2018 at 19:12

Wednesday, Mar 28, 2018 at 19:12
hi
Greg
Great starting point for solar wattage also is 160--200watts per 100Ah batt
So 300-400 watts prefer Mppt controller . Controller at 25--30amp . An EP solar [brand ] Tracer series Mppt 30amp 100-200$ ebay .
AnswerID: 617896

Follow Up By: GREG T11 - Monday, Apr 02, 2018 at 18:42

Monday, Apr 02, 2018 at 18:42
Thanks everyone for your replies. Just returned from there and haven't had a chance to read all of it yet and shall do so now.
Hope everyone had a great Easter.
Cheers Greg

0
FollowupID: 889826

Popular Content

ExplorOz Shop Suggests (9)