dual batteries [one deep cycle]

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

ThreadID: 57911 Views:2935 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

FollowupID: 571605

Reply By: kiwicol - Friday, May 23, 2008 at 15:47

Friday, May 23, 2008 at 15:47

one thing to remember about deep cycle batterys is that a car alternator will not fully charge it and the more you use it the less the alt can put in. Col

AnswerID: 305470

Follow Up By: Mainey (wa) - Friday, May 23, 2008 at 18:08

Friday, May 23, 2008 at 18:08

Col,
So that's why 'under charged' Deep Cycle batteries run fridges :-))

Mainey . .

FollowupID: 571502

Follow Up By: dagwill - Friday, May 23, 2008 at 19:27

Friday, May 23, 2008 at 19:27

mainey
i do apreciate your comments,make no mistake about that,but you said

With 50% less water there is DEFINATELY less Voltage !!

i believe thats one of the properties of DC, they can be 50% flat but still supply 12v

im only guessing but i imagine they need a higher voltage charge system to force the current backwards and charge them

sure a half flat DC will still run a fridge,but only half as long as it would if you fully charge it

FollowupID: 571521

Follow Up By: kiwicol - Sunday, May 25, 2008 at 10:58

Sunday, May 25, 2008 at 10:58

Mainey, yes they do, but not as long as a fully charged battery. Col

FollowupID: 571797

Follow Up By: kiwicol - Sunday, May 25, 2008 at 11:02

Sunday, May 25, 2008 at 11:02

Mainey, yes they do, but not as long as a fully charged battery. Col

FollowupID: 571798

Reply By: oldtrack123 - Friday, May 23, 2008 at 22:08

Friday, May 23, 2008 at 22:08

Hi Dagwill
I can assure you Mainy is correct !
The voltage of ALL LEAD ACID BATTERIES FALLS AS THEY DISCHARGE.
Approx 12 .7/12.8v is fully charged[on no load]
approx 11.6v is as low as you should discharge for reasonable life.
Take them down to 10.6v [FLAT]& THEY WILL NOT LAST LONG AT ALL
Paralellled batteries will equalise their charge & voltage, charging & discharging, provided some care is taken with interconnections

AnswerID: 305530

Reply By: oldtrack123 - Friday, May 23, 2008 at 22:31

Friday, May 23, 2008 at 22:31

Hi Dagwill
One of the ways to explain electrical terms is to use water anology
Voltage is electrical pressure need to cause current [amps] to flow.
water needs pressure[ psi ]to flow[ galls /litres]
A column ie a pipe 10mts high has a pressure which will cause waterto flow [gallons per minute @ the bottom as the water level drops the pressure in the pipe falls, the flow rate also falls when the pipe is empty there is no pressure & no water can flow .
Look on the battery as a column of water ,WHEN IT IS FULL [FULLY CHARGED ]IT HAS MAX PRESSURE [VOLTS] as you use the water [amps ] the pressure [volts] slowly drops until it is empty [in the case of lead acid battery @ approx 10.6v]

AnswerID: 305534

Follow Up By: oldtrack123 - Friday, May 23, 2008 at 22:47

Friday, May 23, 2008 at 22:47

Hi
using the same method of explaining,
set up two vertical pipes of different diameters conect them @ the bottom with identical size & lengths of pipe to a pump ,allthough they have different volumes & one may be half full the empty one will equalise with the other.
Fit a tap @ the pump ,turn off the pump & turn on the tap & they will drain equally .
NOW before the purists start on me , yes I know this is not totally correct due to battery internal resistance ,age, etc.
HOPEFULLY it helps Dagwill under stand electrics a little better

FollowupID: 571573

Reply By: Member - Christopher P (NSW) - Saturday, May 24, 2008 at 06:52

Saturday, May 24, 2008 at 06:52

Some people don't get it! Others do!

When you go to a petrol station you fully fill your car to drive 600klms, if your car is half full you won't go the 600klms, hence you fill up your car with petrol!

Same principle, you charge your battery fully to maintain it to go the distance, if as you state your d/c is half full when you connect it into your cars charging system it will charge back to the original fully charged state, but will take longer then your car battery, so a quick trip into town half a kay away won't do it, you need to drive an hour or so, but that depends on the state of the d/c battery.

Well thats my two cents worth! My sparkie taught me well! I hope????

AnswerID: 305555