dual batteries [one deep cycle]

Submitted: Friday, May 23, 2008 at 10:22

ThreadID: 57911 Views:2992 Replies:7 FollowUps:9

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dagwill

a [normal] battery looses voltage as it discharges,but i believe a deep cycle battery is designed to maintian full voltage as it discharges.

i have my original battery linked to a deep cycle battery in parrallell with a manual isolating switch, so that while camping i use the deep cycle only and my normal battery stays fully charged.

question....... now if i my deep cycle is half flat and i go for a drive with them both reconected wil my regulator sense that there is 12v at the battery and not recharge anything
do i need to change anything at the regulator.as a normal battery may go down to say 10 volts when half flat and so will take recharge from the alternator flat out but if the deep cycle is half flat but still showing 12v, it will only take a minimum charge

i havnt tried this in practice yet but see it as a poss. prob
any help or thoughts please
David

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Reply By: Mainey (wa) - Friday, May 23, 2008 at 11:06

Friday, May 23, 2008 at 11:06

David,
Yes, as you say, ALL batteries loose Voltage as the are used, (eg; running a fridge)
However, said simply, DC's loose Voltage 'slower' than Wetcell Cranking batteries, due to their internal design with their much thicker grid structure, (hence their very large weight difference also) very often the electrolyte is also different too.

Your question: "now if i my deep cycle is half flat and i go for a drive with them BOTH reconected, wil my regulator sense that there is 12v at the battery and NOT recharge anything"

Simple answer: no

Reasoning: both batteries will 'equalise' their respective Voltage, as it becomes only ONE large battery and the Alternator will 'see' only this lower (averaged) Voltage.
Similar to the old idea of 2 x buckets of water, one is 100% full and other 50% full, when both are connected (Eg; by a tube placed in the base of the bucket) the water level with-in the two buckets will equalise to 75% full, with the low bucket water level going up, and the full bucket water level goes down to the exact same level.

Mainey . . .

AnswerID: 305430

Follow Up By: dagwill - Friday, May 23, 2008 at 11:23

Friday, May 23, 2008 at 11:23

thanks for your answer...it was what i was hopeing for BUT the example of the buckets worries me as there not the same,normal bucket when its half empty the level is half down,allowing flow but DC bucket is emptied by throwing stones in the bucket and pushing water out the overflow the level or voltage is still roughly the same although there is only half the amount of water in it,and so it wont accept flow
dont know if i am on the right track,just thinking about it

i plan to add a solar panel to my system
im just measureing the current load and frequency of my 55ltr frige at the moment,so i can decide on the size of the panel i will need [allowing for night time as well]

thanks

FollowupID: 571431

Follow Up By: Mainey (wa) - Friday, May 23, 2008 at 11:32

Friday, May 23, 2008 at 11:32

dagwill you say:-> "but DC bucket is emptied by throwing stones in the bucket and pushing water out the overflow the level or voltage is still roughly the same although there is only half the amount of water in it"

Ok, so just don't throw the stones into the DC bucket !
With 50% less water there is DEFINATELY less Voltage !!

Mainey . . .

FollowupID: 571436

Follow Up By: Redeye - Saturday, May 24, 2008 at 09:24

Saturday, May 24, 2008 at 09:24

Mainey,

I do not see the logic. Yes the battery voltage will settle to an average of the two BUT the alternator is a constant voltage supply so will put current out until the average of the batteries equals the voltage of the alternators regulator voltage (14.x volts).

I do discharge strings of batteries on a daily basis to test their capacity. I then switch them in parallel with other strings and a constant voltage supply. The strings do restore to their original charge state given time.

Redeye