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Pair of leisure batteries (110Ah and 85Ah) in a motorhome



 
 
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  #1 (permalink)  
Old June 13th 04, 09:32 AM posted to uk.rec.motorcaravans
twobatteryboy
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Posts: 1
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

I'm not asking about the vehicle starter battery, but the two leisure
batteries I have fitted to the living space of my motorhome.

When I bought the MH it came with one 110Ah leisure battery, while my wallet
was out I also purchased an 85Ah leisure battery.

I now find out its not the 'done' thing to use non matching batteries but
have not been given any reason that I can clearly understand.

My questions are;

1. Is it okay to carry on as I am as I have more than sufficient power for
my needs?

2. By carrying on with my existing batteries, will I shorten the life of my
batteries, if so by what amount?

3. Should I dispose of the one battery (as I'm unlikely to be able to sell
it) and purchase another to match the remaining battery?

4. By having matching batteries, what will I gain (as I already have
sufficient power for my needs)?

5. Is there anything else I should be aware of?


Sorry to have asked so many questions, but I have searched around but not
arrived at anything conclusive or specific to my situation. May I thank
everyone for reading this and supplying any answers.

Thanks




Ads
  #2 (permalink)  
Old June 14th 04, 11:11 AM posted to uk.rec.motorcaravans
riccip
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Posts: 106
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

"twobatteryboy" wrote:

When I bought the MH it came with one 110Ah leisure battery, while my wallet
was out I also purchased an 85Ah leisure battery.

I now find out its not the 'done' thing to use non matching batteries but
have not been given any reason that I can clearly understand.


Unfortunately it is correct that you should never mix battery
capacities. A 100Ah battery, for example, should only be
connected to another 100Ah, etc. This goes the same for two
starter batteries connected together or two leisure batteries. Of
course a starter and a leisure battery should ONLY be connected
using a split charging system, or seriously dire results can
occur!

You don't say how you use these batteries or how they are wired.
They should be wired in parallel (positive to positive, negative
to negative) so they are effectively one big 12v battery.
Assuming they are then therein lies the problem.

A matched pair of batteries will charge and discharge at an equal
rate. However in your case the 85Ah battery will overcharge while
the 110Ah will have a tendency to undercharge by up to 45%. It
won't significantly harm the 110Ah but will eventually kill your
85Ah through overheating and consequent buckling of the plates.

You should remove one or the other as soon as possible. You could
use the 110 Ah battery alone or get another one to replace the
85Ah. A 110Ah battery normally provides sufficent power for a
weekend's use without any recharging so you will probably find
that's enough. You could keep the 85Ah charged up to use as a
separate back up, swapping the batteries if necessary, or you
would easily sell it on eBay for about what you paid.

You may find that due to regular undercharging your 110Ah battery
is no longer charging to capacity. You can correct this by using
a stand alone battery charger and boost charging to 14.8 volts.
Incidently this can also occur in any vehicle where a Zig-type
charger is exclusively used to charge a leisure battery due to it
cutting off prematurely, so an occasional boost charge is a good
idea anyway.

riccip
  #3 (permalink)  
Old June 14th 04, 06:41 PM posted to uk.rec.motorcaravans
Hilary[_3_]
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Posts: 26
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome


"riccip" wrote in message
news
You should remove one or the other as soon as possible. You could
use the 110 Ah battery alone or get another one to replace the
85Ah.


Perhaps you could disconnect the 85ah battery but leave it charged and
insitu as a spare for emergency use. If the lights start to fade you could
swap the connectors over?

I assume you got a receipt with the 85ah purchase. Someone might be
grateful for a half price little used battery - try Ebay you might get more
than you paid for it ( end the auction on a friday/saturday night and with a
few tinnies inside them they'll bid on anything!!)

)


  #4 (permalink)  
Old June 14th 04, 09:00 PM posted to uk.rec.motorcaravans
QrizB[_2_]
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Posts: 36
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

On Mon, 14 Jun 2004 11:11:41 GMT, riccip wrote:


You don't say how you use these batteries or how they are wired.
They should be wired in parallel (positive to positive, negative
to negative) so they are effectively one big 12v battery.
Assuming they are then therein lies the problem.

A matched pair of batteries will charge and discharge at an equal
rate. However in your case the 85Ah battery will overcharge while
the 110Ah will have a tendency to undercharge by up to 45%. It
won't significantly harm the 110Ah but will eventually kill your
85Ah through overheating and consequent buckling of the plates.


That's simply not true.

--
QrizB

"On second thought, let's not go to Z'Ha'Dum. It is a silly place."
  #5 (permalink)  
Old June 14th 04, 09:23 PM posted to uk.rec.motorcaravans
Knut Grøneng[_2_]
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Posts: 2
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

A matched pair of batteries will charge and discharge at an equal
rate. However in your case the 85Ah battery will overcharge while
the 110Ah will have a tendency to undercharge by up to 45%. It
won't significantly harm the 110Ah but will eventually kill your
85Ah through overheating and consequent buckling of the plates.


That's simply not true.


Ohh, the rule of thumb is to have metched pairs, or the discharge will drain
one and the sulfatation will stop this from beeing charged again. Eventually
this will be killed fully, and the other one suffer a heavy blow, reducing
the life of this one also. In winter time you may even get one of them
freezing to a leak - I did it myself 3 years ago with two bats one 95 and
the other 90 Ah of the same construction -leisure but diff manufacture. -
after 4 month both stopped at about 30% of capacity.

Knut


  #6 (permalink)  
Old June 14th 04, 09:39 PM posted to uk.rec.motorcaravans
QrizB[_2_]
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Posts: 36
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

On Mon, 14 Jun 2004 23:23:56 +0200, "Knut Grøneng"
wrote:

A matched pair of batteries will charge and discharge at an equal
rate. However in your case the 85Ah battery will overcharge while
the 110Ah will have a tendency to undercharge by up to 45%. It
won't significantly harm the 110Ah but will eventually kill your
85Ah through overheating and consequent buckling of the plates.


That's simply not true.


Ohh, the rule of thumb is to have metched pairs, or the discharge will drain
one and the sulfatation will stop this from beeing charged again. Eventually
this will be killed fully, and the other one suffer a heavy blow, reducing
the life of this one also. In winter time you may even get one of them
freezing to a leak - I did it myself 3 years ago with two bats one 95 and
the other 90 Ah of the same construction -leisure but diff manufacture. -
after 4 month both stopped at about 30% of capacity.


Ok, let's think about this logically. Let's say you've got a motor
caravan with a 40 Ah starter battery and a 110 Ah leisure battery, and
you charge the leisure battery using a split-charge relay. The split
charge relay works by joining the two batteries in parallel when your
alternator is running. According to you and riccip, you will be
overcharging your 40Ah starter battery by 175% and destroy it.

Now, how many people here has that happened to? Anyone?

Alternatively, you can subscribe to the school of thought that says if
you have two batteries in parallel, then wen you charge them the
bigger battery will get more currnet and the smaller one less, with
the result that both will end up equally charged.

--
QrizB

"On second thought, let's not go to Z'Ha'Dum. It is a silly place."
  #7 (permalink)  
Old June 14th 04, 10:07 PM posted to uk.rec.motorcaravans
Knut Grøneng[_2_]
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Posts: 2
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

Alternatively, you can subscribe to the school of thought that says if
you have two batteries in parallel, then wen you charge them the
bigger battery will get more currnet and the smaller one less, with
the result that both will end up equally charged.

The charging is not the big problem, but the discharge, as a the level of
discharge changes the charging that follows.
In addition to motorcaravan I also play around with an electric auto using
sealed lead acid batteries, coupled i pairs.
Just had to change a pair because one of the bats in that pair only could be
charged to 60%, the other one could after some cycles of discharge charge
get up to 90% - the rest of the pairs was ok charging above 100%.

The charging history showed that the pair had once been down to zero for
about half an hour.

Charging is driven by voltage and available amperes, good batteries should
charge almost equal regardless of ratings.
Genys and cheap chargers is ususally only limited on voltage and a high max
amp rating - no problem until you have one dead (short) bat.

Knut


  #8 (permalink)  
Old June 15th 04, 07:32 AM posted to uk.rec.motorcaravans
Andy R
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Posts: 821
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome


"twobatteryboy" wrote in message
news:[email protected]
I'm not asking about the vehicle starter battery, but the two leisure
batteries I have fitted to the living space of my motorhome.

When I bought the MH it came with one 110Ah leisure battery, while my

wallet
was out I also purchased an 85Ah leisure battery.

I now find out its not the 'done' thing to use non matching batteries but
have not been given any reason that I can clearly understand.

3. Should I dispose of the one battery (as I'm unlikely to be able to sell
it) and purchase another to match the remaining battery?


If it were me I'd get a changeover switch, keep one battery as the 'always
on' only using the other one if the main one's flat. Then just a top up
charge in the standby battery now and again or if it's been used.

Rgds

Andy R


  #9 (permalink)  
Old June 15th 04, 09:20 AM posted to uk.rec.motorcaravans
riccip
external usenet poster
 
Posts: 106
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

(QrizB) wrote:

Ok, let's think about this logically. Let's say you've got a motor
caravan with a 40 Ah starter battery and a 110 Ah leisure battery, and
you charge the leisure battery using a split-charge relay. The split
charge relay works by joining the two batteries in parallel when your
alternator is running. According to you and riccip, you will be
overcharging your 40Ah starter battery by 175% and destroy it.


Different kettle of fish mate. Like I said earlier a car
alternator is a crap leisure battery charger. They have a
regulator set at around 14V which is a less than ideal compromise
between float and boost voltages. It's so you don't flatten your
engine battery when making short trips but also it won't boil the
bugger dry on the motorway. In practice your starter battery
recharges first but won't overcharge if in good condition.

Meanwhile the alternator attempts to rapid charge the leisure
battery but it cuts out prematurely meaning most leisure
batteries charge to only around 80% of their capacity this way.
So single leisure batteries and matched pairs aren't at risk from
overcharging, and should be safely fused in case of any
electrical fault.

Alternatively, you can subscribe to the school of thought that says if
you have two batteries in parallel, then wen you charge them the
bigger battery will get more currnet and the smaller one less, with
the result that both will end up equally charged.


Someone else has already answered this point. Ask anyone who
maintains an electric milk float or forklift truck. Because their
batteries take a lot of thrape they are well aware of these
problems even using matched sets of batteries. They use charging
equalisers and shunts to compensate for differences in charging
rate.

riccip
  #10 (permalink)  
Old June 15th 04, 07:12 PM posted to uk.rec.motorcaravans
QrizB[_2_]
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Posts: 36
Default Pair of leisure batteries (110Ah and 85Ah) in a motorhome

On Tue, 15 Jun 2004 09:20:12 GMT, riccip wrote:

Someone else has already answered this point. Ask anyone who
maintains an electric milk float or forklift truck. Because their
batteries take a lot of thrape they are well aware of these
problems even using matched sets of batteries. They use charging
equalisers and shunts to compensate for differences in charging
rate.


Forklift batteries are in series, though, so you have to pass the same
current through each of 'em. Leisure ones are in parallel, so you
don't. So your comparison is't valid.

Unless we're talking about a 24 volt motor caravan, but I don't think
we are.

--
QrizB

"On second thought, let's not go to Z'Ha'Dum. It is a silly place."
 



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