Increase the capacity of a 2x12v battery setup

Hey all,

We have two 12v marine batteries connected in parallel - they power some LED lights and a small inverter used to charge a laptop and the odd camera battery. The marine starter batteries are about 2 months old and are charged from a couple of solar panels.

If we purchase a new deep cycle battery (or two new ones) what is the recommended way of connecting these to the existing batteries? Is there a name of a device that handles the different batteries (ie: charge / discharge separately)?

My understanding is connecting new different batteries in parallel would not be efficient (crappy ones drain from better ones).

Coming up to Mt Isa and looking to buy something there - need to know what I'm asking for :)

Thanks heaps!
Dave & Hayley
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Reply By: Member - Michael A (ACT) - Sunday, Aug 14, 2011 at 12:07

Sunday, Aug 14, 2011 at 12:07
look at Redarc or ctec products for multi battery options as a cheaper option 1 of the prianha products may work. Look up their sites via your search engine and phone the companies I have found them all to be extremely helpful when discussing your needs and requirements.
on a good point Redarc talked me out of my needs and talked me into a cheaper option requirements which suited me better technically and money wise.

Redarc and Piranah products are both Australian I am not sure about Ctek.


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AnswerID: 462557

Follow Up By: blueteddy - Sunday, Aug 14, 2011 at 14:06

Sunday, Aug 14, 2011 at 14:06
Thanks Michael!

I'll pop into a redarc / piranha dealer in Mt Isa and have a chat to someone.

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Reply By: Sigmund - Sunday, Aug 14, 2011 at 12:31

Sunday, Aug 14, 2011 at 12:31
What's the aH capacity of each battery?

What's the voltage of your panels and the effective charge of the batteries?
AnswerID: 462559

Follow Up By: blueteddy - Sunday, Aug 14, 2011 at 13:55

Sunday, Aug 14, 2011 at 13:55
Hi Sigmund,

The solar panels are two old BP 280F (75W?) ones. The voltmeter shows 12-14v at 5-9amps coming out of them when they are charging the batteries. Is that the effective charge?

I'm not sure how to work out the aH capacity of each battery (they are not "deep cycle" as such - no aH label on them). However they do have: 900A(MCA) / 720A (CCA) XV27MF Marine written on them if that helps? (I don't have other info as they were in the van when it was purchased).

We get 30-90mins out of our little 380W Powertech inverter when charging/running two laptops at the same time. Time varies if we've got solar / what else batteries have been used for / what we're doing on the laptops.

If an extra solar panel fits in the budget we'll be getting one as well in Mt Isa (there's an extra plug for a 3rd one). However at this stage our problems seem to be at night when we are running out of juice hence the idea for extra/better batteries (and hopefully able to use the existing rather new ones in the van).

FollowupID: 736368

Reply By: Puma1 - Sunday, Aug 14, 2011 at 13:53

Sunday, Aug 14, 2011 at 13:53
All the amph have to be exactly the same, if you mix them you will only have the equivalent of the lowest one, so this means the highest amph battery will never charge to 100% so it will fail (not - might - WILL) and that in turn will kill the other one too!

Same can be said for mixing old ones with new ones it will work but the new one is going to have a short life.
Only real long term solution is the expensive one.

As with solar panels if you mix different watt panels you will never have higher than the lowest one in the string, ultimately means you can screw up your solar panels - Doh!
AnswerID: 462564

Follow Up By: blueteddy - Sunday, Aug 14, 2011 at 14:19

Sunday, Aug 14, 2011 at 14:19
Thanks! Yep I was tempted to just wire a new deep cycle battery into the mix, before reading about the problems. Oh what a waste of money that would have been. That's why I'm now on the hunt for some battery management system that will let me use these two existing batteries along with new different ones - all together somehow.
FollowupID: 736371

Follow Up By: Member Al (Sunshine Coast) - Sunday, Aug 14, 2011 at 14:32

Sunday, Aug 14, 2011 at 14:32
Sorry Nigel, You are incorrect on both counts.

It is quite acceptable to connect two or more batteries of the same chemistry but differing Ampere-hours capacity in parallel when they are being charged from a simple voltage source such as a vehicle alternator or a solar panel. It is not so good if the source is a multi-stage charger as the charger will be unable to clearly detect the conclusion of the various charge cycles. The problem with paralleled batteries arises when one of the batteries is failing and self-discharging which then affects both the charge and storage of the good battery. Further difficulty is parallel connecting batteries of different chemistry requiring differing charging regime.

As for the solar panels, provided that they are the same nominal voltage there is no problem parallel-connecting panels of differing wattages. They will each contribute to the total charge in accordance with their ratings although you may get a little less than their simple arithmetic sum. Such connection will certainly not "screw up" your panels.


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Follow Up By: blueteddy - Sunday, Aug 14, 2011 at 14:40

Sunday, Aug 14, 2011 at 14:40
Interesting! Thanks Allan.

So if we find a battery in Mt Isa that is the same chemisty as our "900A(MCA) / 720A (CCA) XV27MF Marine" battery, it would be acceptable to wire it in as parallel with the existing two? So adding a better "deep cycle" battery to our 2 month old "XV27MF" bank would be ok (not perfect, but not horrible). And maybe check the health of each battery individually if we notice any performance drops to see if one of the batteries is on the kicks and needs replacing?
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Reply By: Sigmund - Sunday, Aug 14, 2011 at 14:01

Sunday, Aug 14, 2011 at 14:01
Starter batteries are not deep cycle; they provide short-term high power output.

That'd be your prob IMO but I'm no expert.
AnswerID: 462565

Reply By: blueteddy - Sunday, Aug 14, 2011 at 14:56

Sunday, Aug 14, 2011 at 14:56
Well I've done some more googling, and a few websites list the "XV27MF" battery as a Deep Cycle 95aH battery. Maybe these are deep cycle batteries? And it is possible to add a new/different one in parallel with minimal fuss?
AnswerID: 462570

Reply By: Member Al (Sunshine Coast) - Sunday, Aug 14, 2011 at 15:29

Sunday, Aug 14, 2011 at 15:29

The XV27MF is a combination Cranking/ Deep-cycle battery of about 100 Ampere Hours capacity. Batteries of this type are a compromise and never as good as the correct type for each duty, cranking or deep-cycle. Nevertheless, they are not too bad if maintained properly.

You say that these batteries are only 2 months old, so if that is true (did you buy them?) and they are being correctly charged then they should be capable of supplying your fairly modest needs as you do not mention a fridge load.

If the batteries are OK then I would be suspecting your solar charging system. The 5-9 amps you quote represents 20-36 ah contribution to the battery if available for say 4 hours, providing perhaps 16-29 ah battery output. This is possibly not enough for your needs.

If the bottleneck is your solar system then adding further batteries will not solve the problem and in fact would possibly make it worse. I would suggest having the existing batteries tested by a battery supply retailer, although unless he is of repute he may condemn them in order to secure a sale. The best way by far is to separately charge these batteries fully with a quality multi-stage charger then connect a known load such as a 50w 12v lamp (auto or domestic halogen) and observe the discharge time to reach 12.2 volts which is considered 50% "state-of-charge" and as low as you should be discharging. 50w draws about 4 amps so a perfect 100ah battery should be providing about 12 hours to consume 50% of its 100ah. In reality, the battery will not be perfect and so will last for more like 10 hours.

If the batteries are reasonable then you will need to consider the solar charging system. Bear in mind that batteries are like your bank can only draw as much as you have put in. Getting further bank accounts will not help if your deposits are inadequate for your withdrawals! LOL


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AnswerID: 462572

Follow Up By: blueteddy - Sunday, Aug 14, 2011 at 16:26

Sunday, Aug 14, 2011 at 16:26
Champion! Thanks :)
FollowupID: 736379

Reply By: Battery Value Pty Ltd - Sunday, Aug 14, 2011 at 17:54

Sunday, Aug 14, 2011 at 17:54
Hi Dave,

looking at your loads of two lappies, the two batteries are able to power for 90 minutes, that's about 15~20Ah all up.
Add 20% for inverter and p/s inefficiencies and you're looking at a measly 25Ah juice from two batteries which should provide 190Ah (if they're specced 95Ah each).

Three possibilities for this low Ah output:
batteries not fully charged (very likely), battery capacity permanently reduced by chronic undercharging (quite likely), or a combination of 1 and 2.

What you can do:
Use a quality solar regulator, or a mains powered 3 stage charger to try and bring the batteries up to their maximum remaining capacity.
It may take several days of 'cycling' the batteries.

As Al has already mentioned, the amount of solar mightn't be enough to do this for you, so you have to either invest in a proper 3 stage charger, or have someone baby the batteries for you.
A load test with a known load is also a good way of finding out about the remaining battery capacity.

If the capacity can't be recovered to higher than say 90% (85Ah per battery) don't wire them in parallel with a new deep cycle battery (you mightn't need a new one if you get to this stage anyway).
Reason for this is the open circuit voltages of the weak batteries are lower due to their more dilute electrolyte which in turn can partially discharge the new deep cycle battery, leading to accelerated ageing of the new one.

For a good quality/best value for money deep cycle battery, ideally suited for your loads, and a good quality true MPPT solar regulator with expert battery/solar advice thrown in for free, look in the profile of Battery Value Pty Ltd. We ship all over Australia most economically.

cheers, Peter
AnswerID: 462586

Follow Up By: drivesafe - Sunday, Aug 14, 2011 at 18:29

Sunday, Aug 14, 2011 at 18:29
Hi Dave and as Peter has posted, it sounds like your batteries are not being charged properly so they are loosing capacity.

Because they are only two months old, you should be able to get them back to full or near full capacity.

Those batteries are Calcium/Calcium ( Ca/Ca ) batteries and as such, they need a high voltage boost charge about once a month to keep them at about full capacity.

I’d guess that you are not getting them anywhere near fully charged at any time with the solar panels and you are loosing a bit more capacity every day.

Again as Peter has stated, you will probably need to cycle them with a boost charge quite a few times to get them back up to a good condition.

If you do nothing, they will continue to loose capacity and will be useless in a very short period of time.
FollowupID: 736390

Reply By: Member - Joe n Mel n kids (FNQ - Sunday, Aug 14, 2011 at 19:17

Sunday, Aug 14, 2011 at 19:17
Hi guys, i looked at all the options and the associated prices and it really gets expensive and i would add "complicated" so i looked at some of the "systems" that have been around for years in trucks and busses and have stood the test of time and went out and brought 2 N70Z's, Centrys at a good price, made a secure mount in the tray, set them up with the "charge" link to the main battery with a 200amp soliniod that simply clicks in with ign on and out with ign off and it works really well, i run the engle and other stuff off it and it all is isolated from the main starting batt when ign is off so dont matter if it goes flat and charges with ign on and motor running, if i have it parked up for more than a few weeks i connect a chatger to them and that is as technical as it gets ........... in 2 years time yes they will be stuffed but for under $300 i can simply buy 2 new batts ...
Works for me..
Oh and i got it all from Isa :O)
AnswerID: 462593

Reply By: Bob Y. - Qld - Sunday, Aug 14, 2011 at 21:57

Sunday, Aug 14, 2011 at 21:57

If you're set on buying more gear in the Isa, there won't be a lot of places that will have what you might need.

TJM & Atlas Superstore(they got Batts, panels & chargers) are in Simpson St, at each end of the block facing Maccas.

Autobarn has some deep cycle batteries, and 3 way chargers(I got a 16 amp Projecta there, that does Calcium as well)

Supercheap is just up Marian St about 150 metres from A'barn. Isa Auto Supplies are also in Marian St, next to BP, & Muffler Shop. There's also Dave Clancy Electrical, in Cook Crescent, who I think has some panels, if you want to go that way.

Blackwoods are over in Traders Way. on north side. They've ususlly got everything, except that one part you really want. Sorry, Mr Balckwood.

Good thing with the Isa, being a mining town, if they haven't got something, it can be up overnight, depending on what it is.

And go & see George at Bearing Service, as he sells lots of other stuff, like chargers & tools, oh, & batteries too.

No affiliation etc etc

Seen it all, Done it all.
Can't remember most of it.

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AnswerID: 462620

Reply By: blueteddy - Monday, Aug 15, 2011 at 19:38

Monday, Aug 15, 2011 at 19:38

Hopped into Autobarn, Supercheap, TJM and finally Atlas. The battery fella at Atlas is amazing :) highly recommend going there if you need anything camping/electrical.

He jumped in the van with a multimeter and found the charge coming out of the "solar controller" was a bit lower than what it should have been. He wanted me to remove all the dials and make sure none of them were causing issues, so back to the caravan park I went and got in with the screwdriver.

2 minutes in I found the problem (and started to smell it as well) - melty!

back to atlas and left with a new solar controller (7amp as opposed to the previous 20amp - not sure if that will slow charge down?)

It's wired in now and I'm going to leave it charging the batteries tomorrow - I'll test the batteries tomorrow night and see how long it'll run the inverter for.

Next purchase will be a 3 phase charger to make sure the batteries are up to scratch between powered caravan site visits.

Thanks for all the recommendations and advice, I learnt a lot :)
Looks like we'll be skipping the new solar panel and battery purchase if this fixes the problem.

AnswerID: 462678

Follow Up By: blueteddy - Monday, Aug 15, 2011 at 19:40

Monday, Aug 15, 2011 at 19:40
FollowupID: 736497

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