Baterries set up in parellel

Submitted: Monday, Mar 06, 2006 at 12:52
ThreadID: 31473 Views:2266 Replies:7 FollowUps:7
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Have a question, if i run 2 x 55 amp optimuim (Gel) batteries in parellel, do i get 110 amps of usauage current to run my fridge. I have a Piranaha 180s duel battery management system with 1 battery already, was planning to add another in the back cab (of course wirred correctly with thick cable etc.). Was also wondering how this would charge up. Would think that the batteries (pair) would charge at the same rate etc. but would take twice as long. Am i on the right track? Also had another thought, can i ad the bigger 80 amp gel battery ( in parellel), in other words can they be charged at the same time in the same circuit?
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Reply By: Member - Jay Gee (WA) - Monday, Mar 06, 2006 at 14:01

Monday, Mar 06, 2006 at 14:01
You don't actually have a "55 amp" battery. You have a "55 amp-hour" battery.
Which means you can consume:
1 amp every hour for 55 hours
2 amps every hour for 27.5 hours
5 amps every hour for 11 hours

If you run 2 x 55 Amp-hour batteries in parralel - you actually have 1 x 110 amp-hour battery - which would double the above duration figures

What you need to know is how much current does your fridge draws. Then you can work out how long the battery will last/
AnswerID: 158881

Follow Up By: Mike DiD - Monday, Mar 06, 2006 at 16:12

Monday, Mar 06, 2006 at 16:12
Instantaneous current flow is AMPS (like Litres per hour) and that total current flow over a time period is AMPHOURS (like litres).

Battery capacity is specified in Amphours, but only at the 20 hour rate. If you discharge your 55 amphour battery at 55 amps it wil not last for 1 hour - closer to 35 minutes.

"5 amps every hour for 11 hours " - actually.
- you can draw 5 amps for 11 hours
- you can draw 5amphours per hour for 11 hours.

FollowupID: 413415

Follow Up By: Wok - Monday, Mar 06, 2006 at 20:58

Monday, Mar 06, 2006 at 20:58
There is a hidden gem in what Mike has posted,

[Quote]If you discharge your 55 amphour battery at 55 amps it wil not last for 1 hour - closer to 35 minutes. [Unquote]

The reverse is also true, you should expect more then double the running time [under the same load] with two 55Ah batteries in the paralleled batteries would perform as a single battery at C40 specification.

FollowupID: 413482

Reply By: Flash - Monday, Mar 06, 2006 at 15:14

Monday, Mar 06, 2006 at 15:14
Yes, You could get away with charging all at the same time (although it's not the perfect setup, it will work.)
However, I'd separate them during use- any time you parallel non identical batteries you'll get poor results. Even two supposedly identical batteries (type, age) being discharged in parallel can lose some efficiency as one can "drag" the other down.
If you parallel an old battery and a new one when off charge, the new one will simply discharge into the old one. ie: separate them when off charge.
AnswerID: 158888

Follow Up By: phil - Monday, Mar 06, 2006 at 15:43

Monday, Mar 06, 2006 at 15:43
Actually if you look at the charge/discharge voltage curves for lead acid batteries you will find that there is not all that much charge transferred from a full battery to a flat battery when in parallel. You certainly do not end up with 2 half charged batteries as some people believe.
If the 2 batteries are similar chemistry, condition and type, which is what I think you are proposing, you will get twice the amp/hour capacity.
When charging each battery will act as if it is the only one and accept a charging current depending on the state of charge and charging voltage, assuming that the alternator can supply enough. With 70amp alternators common this is not a problem these days. So the time to recharge wil be the same as for a single battery.

Phil I
FollowupID: 413409

Reply By: Member - Norm C (QLD) - Monday, Mar 06, 2006 at 15:39

Monday, Mar 06, 2006 at 15:39
From my experience, I think the problems associated with parrallelling non identical batteries are overstated. Yes, in an ideal situation an exact pair (brand, type, size, age) may be best, but non identical set ups seem to work fine in practice. In your case, providing both batteries are in good condition, parralling an 'old' 55 AH battery with a similar 'new' 55 AH is not a problem.

An advantage of keeping them in parrallel all the time is that they will discharge and charge at about the same rate. The problem of one discharging into the other should not be a big problem if both are in good condition.

A disadvantage is that one stuffed battery can quickly lead to two stuffed batteries.

I run two 120AH AGMs in parrallel. One is about a year older than the other. They are often (when my CT is connected) in parrallel with a standard vented 105AH deep cycle, although I normally don't leave them connected for long periods without the engine running. When engine is running all that is in parrallel with the starter battery. Had this set up for about 12 months and works fine so far.

See what other advice you get, but that is my experience.
AnswerID: 158893

Reply By: Mike DiD - Monday, Mar 06, 2006 at 16:20

Monday, Mar 06, 2006 at 16:20
Identical batteries are essentail when it is important that both always have the same state of charge e.g. two 6 volt batteries in series to deliver 12 volts.

The whole point of Dual Battery systems when camping is to allow unequal states of charge - it would be pointless making the batteries identical size.

This was extensively debated here
and the conclusion was that "identical batteries are needed in a parallel dual battery setup" is a myth that should have been buried a long time ago.

AnswerID: 158901

Reply By: waynejethro - Monday, Mar 06, 2006 at 16:47

Monday, Mar 06, 2006 at 16:47
thanks for all input, helps alot. Specially Phil and norm. that was what i was looking for. Have had consistant advice the same as you guys from the likes of Piranaha but wanted to see what actually works in the field. you make sense. was still unsure on 2 more points. Have been told that you can get upto 10 years battery life out of gel batteries, although considered rare, was wondering what was more realistic. its been suggested around 4 or 5 years ( i realize that the are a few varibles before anyone gets excited). also, is it better to leave new 55 amp hour battery permenantly in car or pull out and store and reinstall for extended stay trips, this is from the point of view of looking after the battery (life). and finally it has been suggested that gel batteries should be discharged complete every now and then as part of looking after them although this sounds wrong to me??

Thanks again for all the replys
AnswerID: 158913

Follow Up By: Mike DiD - Monday, Mar 06, 2006 at 21:43

Monday, Mar 06, 2006 at 21:43
No manufacturer of any type of Lead Acid battery suggests you will get longer life by discharging them.

For maximum life, limit charge current, limit the depth of discharge and recharge as soon as possible after discharging.

FollowupID: 413493

Reply By: Member - Norm C (QLD) - Monday, Mar 06, 2006 at 18:44

Monday, Mar 06, 2006 at 18:44
Wayne, I don't know a lot about Gel batteries. I and many others on this forum use and swear by AGMs. For some basic info on AGMs paste this link to your browser:

Also do a search on this forum. There is heaps of info on batteries, dual battery set ups, isolation, charging etc. Some of it will be a bit conflicting, but you should be able to sort the wheat from the chaff. There are a number of excellent posters on this subject with a lot more knowledge than me. Two of them are Collyn R and MileDiD, but there are others. Collyn is not posting at the moment and I get the impression Mike is becoming a bit less patient (sorry Mike), but there is heaps of great stuff available if you look from these and other great posters.

Get a glass of red and spend an hour or two using the search facility. You'll be surprised what you will learn.
AnswerID: 158931

Follow Up By: Mike DiD - Monday, Mar 06, 2006 at 21:49

Monday, Mar 06, 2006 at 21:49
I agree - go for an AGM (and do it now :-) )

Gel batteries are still a good choice for Uninterruptible Power Supplies or solar Homes, but AGMs have now become cheap enough that they are the best choice for the rugged life in 4WD.

If you can't justify an AGM, then a hybrid (Starting plus Deepcycle) wetcell such as an Exide Extreme seems to be the best value.

FollowupID: 413496

Follow Up By: Steve - Monday, Mar 06, 2006 at 21:50

Monday, Mar 06, 2006 at 21:50
good advice. Oh, and take a Panadol afterwards. I'll guarantee you'll get a bloody headache.
FollowupID: 413497

Follow Up By: Steve - Monday, Mar 06, 2006 at 21:53

Monday, Mar 06, 2006 at 21:53
after all that reading, that is. (never mind the red wine)
FollowupID: 413498

Reply By: wheeleybin - Sunday, Mar 12, 2006 at 13:13

Sunday, Mar 12, 2006 at 13:13
One battery that needs looking into is the one developed by Dr David Rand of the CSIRO which uses a new supergell paste and is made in Australia and supposedly has the lowest resistance and takes the fastest recharge and can be used as both cranking and house together.
The price is now competitive with AGM but the problem is it is not yet made in 12V
but two 6Volts in parallel will give you 200AH of capacity and they carry a five year full replacement warranty.
AnswerID: 160068

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