# Linking leisure batteries - same capacity?



## ChrisWade (Jul 3, 2006)

I intend to replace my dying Mastervolt AGM 160AH leisure battery, and increase capacity by connecting 2 batteries in parallel. I'm fairly sure that I've picked up from magazines that doing this requires the two batteries to be identical in capacity and preferably the same age. That was until I read Clive Mott-Gotobed's reply to a letter in the July 2010 MMM (pgs 194-5).

Quoting Clive:
"You [MMM's enquirer] said 'as I understand it; the effective capacity of the larger battery is is reduced to that of the smaller battery.' I don't know where you got this from as it's incorrect. The total ampere hour capacity will be the sum of that of the two batteries. The charger will charge both batteries according to their needs and within the constraints of the charger, no problem."

Now to quote Elecsol Batteries, which appears to conflict with Clive's advice:
Never mix battery capacities, this will cause over charging on one battery and insufficient charge on the other. Connect a 100 amph to a 100 amph. If one battery is significantly older, it can pull a newer battery down because of lead sulphation. http://www.elecsolbatteries.com/faqs/#04

I'm posting this here as I seem to recall that Clive is a regular (and much valued) contributor here on MHF. His knowledge and advice is highly regarded by this reader, and I'm interested in hearing further on this before I splash the cash. Can Clive, or others, please comment? My existing battery charger is a CBE CB522 (if this is relevant).


----------



## teemyob (Nov 22, 2005)

*Batteries*

I would make sure the batteries are both equal in AH and age.

Is there a remote control with an AAA battery and a D cell?

TM


----------



## clive1821 (Mar 27, 2010)

Well its good practice to use the same capacity batteries and of a similar age as been discussed, but there no real reason not to mix them as long as they are of a similar charge rate... eg don't mix AMG's with gell as they do have different charging requirements... I have two 90ah's one 70ar and one 120ar if i recall and all seem to be very happy.. well so far... my view is carry on... but its not a good idea to mix say 50ar with a 120ar then the charge will be effected by the lower capacity one....


----------



## ChrisWade (Jul 3, 2006)

[quoteIs there a remote control with an AAA battery and a D cell?[/quote]

Thanks for the reply, but I'm not sure what you mean.


----------



## DABurleigh (May 9, 2005)

Chris,

The advice in your OP is not inconsistent, though in the case of Elecsol's statement, differing battery capacity is not the primary reason - there are others way ahead of this per se.

I have covered the subject many times on MHF, most recently earlier this month:
http://www.motorhomefacts.com/ftopicp-855766.html#855766
but more comprehensively:
http://www.motorhomefacts.com/ftopicp-386443.html#386443
and you could consider 6V, though that may not be cost effective:
http://www.motorhomefacts.com/ftopicp-514881.html#514881

To answer your PM, Clive can be contacted via MHF here:
http://www.motorhomefacts.com/forum-userprofile-21930.html

Dave


----------



## CliveMott (Mar 10, 2008)

Clive pops up again!

This particular reader already had two wet electrolyte 12 volt batteries of 80 and 90 ampere hours. Not that different. I believe his confusion regarding the capacity of the smallest winning was based on what happens if two 6 volt batteries of different ampere hour capacities are connectred in series. Not a good idea. I believe the lack of understanding is also shared by whoever put that information on the Elecsol site mentioned. Considering his situation I took the pragmatic aproach. I would do the same.

For those without the July MMM the interchange is below.

Regards
Chive
Hi Clive,

I would like to ask your opinion/advice on linking two different leisure batteries that I own. I have read your piece on your website on this subject and to be honest I felt a little easier about linking them than before. If you would allow; I would like let you have some details and perhaps you might just let me have a quick comment?

I just taken delivery of a new 'van with a wet-cell 90ah Banner battery. I also own a separate wet-cell 80ah about 2 years old that has never seen serious use but was regularly charged and never let run down.

There is provision for the addition of a second battery. I have the cables and fuses (at both ends) in place and at this stage it only requires the insertion of the fuses to make the connections. Both locations are in the engine bay.

Essentially my query comes down to three separate questions.
Is it dangerous to connect these essentially very different batteries?
If it is not dangerous, is it harmful or risky to either of the batteries or charging equipment or other on-board equipment?
As I understand it; the effective capacity of the larger battery is reduced to that of the smaller battery. Will this have any lasting effect on the battery should I fit a new matching battery in the coming months?
Thank you for any help or advice you may let me have.

Kindest regards

I responded :-
Hi David,
Both batteries are wet electrolyte lead acid batteries and are similar in capacity. You say that the 2 year old battery is in good condition and has been kept charged on a regular basis. Therefore I see no problems at all in linking them in parallel. Make sure you fully charge both batteries first so that they will be in the same state of charge when connected in parallel.
You said "As I understand it; the effective capacity of the larger battery is reduced to that of the smaller battery." I don't know where you got this from as its incorrect. The total ampere-hour capacity will be the sum of that of the two batteries. The charger will charge both batteries according to their needs and within the constraints of the charger, no problem. So from my viewpoint all you need to do is get on with it!

Take care

Clive
http://www.motts.org/second leisiure battery.htm


----------



## CliveMott (Mar 10, 2008)

Just looked at the elecsol web site. The quote on this forum was taken out of context. 
The site said 

"For 24 volt, 36 volt and 48-volt systems, connect in series, positive-to-negative.
Never mix battery capacities, this will cause over charging on one battery and insufficient charge on the other. Connect a 100 amph to a 100 amph."

So there you go, we are all right. It was talking about SERIES connected batteries!

Regards


Chive


----------



## ChrisWade (Jul 3, 2006)

*Elecsol comment - context?*

Clive said:


> Just looked at the elecsol web site. The quote on this forum was taken out of context.


I don't think I quoted out of context, Clive. Look at it again (below, the full answer to Elecsol's FAQ No. 4).

Q.4 - HOW DO I CONNECT TWO OR MORE BATTERIES TO GIVE ME MORE CAPACITY?
A. For 240 volts through an inverter, connect the batteries in parallel, negative-to-negative and positive-to-positive. 
To increase capacity for 12 volts, connect the batteries in parallel, negative-to-negative and positive-to-positive.
For 24 volt, 36 volt and 48-volt systems, connect in series, positive-to-negative.
Never mix battery capacities, this will cause over charging on one battery and insufficient charge on the other. Connect a 100 amph to a 100 amph. If one battery is significantly older, it can pull a newer battery down because of lead sulphation.

To me it reads that Elecsol first gives instructions on connecting batteries (first 3 paras - 2 relating to parallel, 1 to series connections), and then makes a statement concerning battery sizes which appears applicable to all the above. This reading appears in keeping with the advice I'm receiving from every battery supplier I've spoken to (one of whom told me he spoke directly with you concerning my question a day or two ago, and then advised me not to connect 2 batteries of different capacities). This implies to me that Elecsol's statement about battery sizes is indeed in the context of parallel connections.

Much of this goes beyond the scope of my originating post. Initially I was hoping that your reply in MMM would allow me to connect two new batteries of totally different capacities, but I was confused by other advice. I'm grateful for your clarification.


----------



## dandywarhol (Nov 14, 2010)

Apologies for resurrecting this one but - can someone explain to me in technical terms why charging different capacity batteries is not recommended. I've read plenty about not doing it but not enough information as to "why not".

If the leisure battery was a different capacity to the starter battery (which many are) there doesn't seem to be a problem. surely the internal resistances will dictate the current demanded and when one battery is fully charged then it will not demand any more current.

Interesting to note that Elecsol state that their leisure batteries will not exceed 1.250 SG........................


----------



## DABurleigh (May 9, 2005)

I thought it had been covered adequately in a link given earlier in this thread:
http://www.motorhomefacts.com/ftopicp-386443.html#386443

To paraphrase, you may be lucky over capacity, but probably not, because by definition it will be made in different batches in the factory and won't be electrically (in terms of effective internal resistance during charge/discharge cycle) matched.

For the same reason when the batteries are adjacent and high currents are drawn, it makes sense to connect to the negative of one and the positive of the other, to maximise symmetry.

The reason it matters MUCH less for the differences between vehicle and leisure battery is due to the resistance in the long cables BETWEEN the batteries.

But technically, it is simply Ohms Law; nothing more, nothing less.

Dave


----------



## dandywarhol (Nov 14, 2010)

Having spoken to several auto electricians and fellow college lecturers I've come to the conclusion that there isn't a problem charging batteries of different capacities but when left connected together (in parallel) they will slowly discharge into one another and eventually become discharged quicker than a single battery of the combined capacity.

Thanks for your input DAB.

I'll continue to keep the dissimilar batteries at present until they both need replaced and fit an isolator/switchover switch when parked up. Need to think how to wire the 80W solar panel now................


----------

