Battery connection

I have 2x80w solar panels connected to my solar regulator (SolarTech MP3129) and
I have 2 12v deep cycle batteries. 1x75ah & 1x100ah.
Is there a difference to connecting the batteries in parallel and have o pair
running to the regulator as opposed to have the batteries connected by separate
cable to the same connectors on the regulator?
I am using 6m2 cables as I have about 4m distance to cover.
My guess is 'no difference' .
I was hoping to have the batteries in different locations due to weight balance and space
What are your thoughts?



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Reply By: Ross M - Thursday, Jan 10, 2013 at 20:26

Thursday, Jan 10, 2013 at 20:26
G'day Marcel
I'm not quite sure what you are meaning there.
The Panels are best in parallel so the voltage, is above the battery voltage and the paralleling will provide/create max amp flow.

If panels in series, voltage may be more than reg will accept, "lets the smoke out" and will limit current to whatever the smaller panel can pass in full sun. This option not good and only suitable for some unusual situations.

The regulator should be as close to the batteries as is possible.
The batteries must be in a parallel configuration ie +ves to +ves and negs to negs.
Can be fed from reg with cable run to each battery.

Ross M
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Follow Up By: Imanoone2u - Thursday, Jan 10, 2013 at 20:34

Thursday, Jan 10, 2013 at 20:34
Thanks Ross,
As the batteries are different values (75ah & 100ah) I was merely wondering If parralel connection was more OR less beneficial to full charging as connecting both batteries to the same point at the regulator would be.
In essence i would treat the regulator like a double adaptor..
Would it make a difference to;
A) charging the batteries
B) discharging the batteries if one gets the fridge load and the other lights and coffee maker?
I hope I explained it better this time
FollowupID: 778554

Follow Up By: Ross M - Thursday, Jan 10, 2013 at 23:20

Thursday, Jan 10, 2013 at 23:20
I use the vehicle battery to charge a 105ahAGM in the rear of my ute and that is also connected to a 120ahAGM in a camper.
Solar feeds a PWM regulator and the engine is the source of amps while driving and when stopped the solar (connected all the time) charges the batteries. No trouble but does have good sized cables to minimize VD.

If you are going to drag BIG amps (I read your post about a coffee maker) the cables between the two batteries would have to be a fair size.
I wouldn't be running a coffee maker, Microwave oven or do the ironing either.
FollowupID: 778571

Reply By: Member - Frank P (NSW) - Thursday, Jan 10, 2013 at 23:34

Thursday, Jan 10, 2013 at 23:34

Putting aside the difference between the batteries, electrically there is little difference between parallelling the batteries at the batteries then running a single cable to the reg, or running separate cables from each battery to the reg.

As Ross said, it is important that the reg should be as close as possible to the batteries, so that is something you need to consider as well. Might be difficult if the batteries are separated for weight and balance.

You CAN connect a 75Ah and a 100Ah in parallel. Electrically it's ok, but it's not good for either battery. Batteries in parallel should be the same capacity (ie, in your case 2 x 75Ah or 2 x 100Ah), same chemistry and same age.



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AnswerID: 502197

Follow Up By: Allan B (Member, SunCoast) - Thursday, Jan 10, 2013 at 23:56

Thursday, Jan 10, 2013 at 23:56
It really does not make a lot of difference having two batteries of different capacity in parallel. Consider the large number of dual battery systems with a flooded lead-acid cranker and a differently sized AGM auxiliary which are in parallel on the alternator when the charging isolator is closed. Each battery will absorb current based on its state of charge (voltage) and its internal resistance and ultimately both will charge to the same terminal voltage. Being of differing chemistry is however not ideal for charging.

Much the same with two auxiliary batteries permanently wired in parallel. And they will contribute energy to the load proportional to their internal resistance. It is appropriate for them to be of the same chemistry however to achieve proper charge.


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Reply By: Allan B (Member, SunCoast) - Thursday, Jan 10, 2013 at 23:40

Thursday, Jan 10, 2013 at 23:40
Hi Marcel,

Yes, quite OK to separate the battery locations and run cable from each battery to the same connector on the regulator. Be sure to place a fuse in the +ve close to EACH battery as protection against cable short circuit. 30 or 40a fuse should do.

The 6mm2 cables at 4m length will drop about 0.1v at 10a current which should be satisfactory unless you intend drawing current much higher than 10a.

But as said in your post above, forget the coffee maker!


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AnswerID: 502198

Follow Up By: Imanoone2u - Friday, Jan 11, 2013 at 11:04

Friday, Jan 11, 2013 at 11:04
Thanks Allan,
When you say 'add a fuse on the +', I'm a bit stuck. I'm running a 6mm2 cable but can not find a fuse holder that I can add inline....???
I understand the need/precaution, but....
FollowupID: 778605

Follow Up By: Nomadic Navara - Friday, Jan 11, 2013 at 11:29

Friday, Jan 11, 2013 at 11:29
get something like this. Cut the loop near the screw cap bit of the holder and splice it to the cable to the regulator (joining crimp splice will do.) Crimp an eye terminal to wire to the long bit of the holder and terminate on the battery positive. That will get the fuse close to the battery. No need for a fuse at the regulator end, the regulator will not supply enough current to cause problems.
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Follow Up By: Allan B (Member, SunCoast) - Friday, Jan 11, 2013 at 11:46

Friday, Jan 11, 2013 at 11:46
Marcel, here are some suitable fuses, and Sidewinder is a Business Member on ExplorOz. Similar products can be found at auto accessory shops, BCF and the like.

Personally I prefer the 40A or 60A 'circuit breakers/trip switch' that you will find further down the referenced page. They have electrical advantages over fuses and also are easier to install as the cable is inserted and secured by a grubscrew whereas the fuse has a tail which has to be joined to the cable which can be difficult if you do not have the appropriate equipment. Keep the wire from the breaker to the battery +ve short and unlikely to contact earth. Take the load cables from the side of the fuse or breaker furthermost from the battery and these will also be protected by the fuse/breaker.

Be sure to open both fuses or breakers if disconnecting a battery as otherwise the removed cable will be 'live' from the other battery.


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