deep cycle batteries

Submitted: Saturday, Jun 18, 2005 at 23:02
ThreadID: 23974 Views:1979 Replies:3 FollowUps:3
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I've got 2 deep cycle batteries linked in parallel on hte other side of the isolator solenoid from my starting battery. As a bit of a luddite, my question is this: since they have different rated amperages (120 and 75 amp-hours) will they both charge fully, and will I get the expected 195 amp-hours for running fridge, lights, etc out of them, or will it be less due to the difference in rating of the batteries?
If anyone knows the answers, I'd love to know - maybe I need to change my system?
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Reply By: V8troopie - Saturday, Jun 18, 2005 at 23:56

Saturday, Jun 18, 2005 at 23:56
It'll be less than 195Ah Alex, unless your current draw is very small indeed. Deep cycle batteries are usually rated for their C/20 rate (or C/15 or C/10) = how many amps the battery can supply for 20 (or 15 or 10) hours when fully charged. That figure is obviously larger for the bigger capacity battery, so the smaller battery gets drained well before the bigger one when you run your fridge.
When you get to recharge them, the one that's drained more will soak up most of the charge initially and the bigger battery most likely never gets fully charged.

In other words, its not a good idea what you are doing. The only way batteries in parallel are remotely happy is when both are exactly the same type and age. I do that sometimes with my boat's two indentical batteries, when I need extra power for a short time. They get charged separately from solar panels, only if I connect the 30A genny they are switched in parallel for charging and then only for the first part of the charging cycle.

AnswerID: 116312

Reply By: Member - Jeff M (WA) - Sunday, Jun 19, 2005 at 00:15

Sunday, Jun 19, 2005 at 00:15
This is a bit of a contraversial one, however from my understanding as long as there is a higher voltage than the batteries charge avaliable to them then both batteries will take what they need. If both batteries are linked in Paralell then they are both going to be at the same voltage when resting anyway as they would have equalized themselves.

ie. Battery A at 12.1V, Battery B at 12.1V Alternator supplying 14V both batteries will charge from that at their own rates. Obviously the bigger battery is going to draw more amps from the alternator than the smaller because it need more power to be put back into it.

Now the only time this becomes a problem is when the charge is stopped (ie engine is stopped). One battery may have charged up to 12.4v (probally the smaller one as it would charge faster) and the other battery may have only got to 12.2V (as it's bigger and takes longer to charge). So when the charge is stopped, the batteries will equialize again. (not real good for either as the load on the smaller one will be large as the bigger one trys to suck the life out of it and the bigger one will get a surge of power from the smaller one which then puts a strain on it.

But hey, lot's of vehicles are setup this way (including mine) and apart from decreasing the life of your batteries a small amount, it's not going to do any other harm. And IMHO the heat generated under the bonnet of a Turbo Diesel 4by working hard in an Oz summer is going to do much more damage than the above proccess anyway. But the extent of it will obviously depend on several things, a) how bigger difference there is in the batteries capacity. b) How long you reguarly charge them (drive the car) before stopping the charge.

If the batteries are similar in capacity the voltage variation between the two would not be that great anyway.

Others opinions may, no WILL differ. ;-)
AnswerID: 116314

Follow Up By: drivesafe - Sunday, Jun 19, 2005 at 00:39

Sunday, Jun 19, 2005 at 00:39
Hi Alex H, Jeff M is just about right on the money.

The only thing that I would have a difference of opinion on is the equalisation.

As Jeff put it, if the charging is stopped before both batteries are fully charged and I might add here, even when that ARE fully charged, there will most likely be a small difference in voltage levels between the two batteries. Unless there is a problem with one or both batteries, there will only be a very low current flow between the two batteries while they equalise.

If there is a problem with one of the batteries, you will soon find out as this is when the equalisation will not only be high but unless you find out about one battery having a fault, you will very quickly have two stuffed batteries but being honest, this is not something that occurs all that often.

Although not recommended, paralleling of batteries is a common practice but it is usually done using two identical batteries. Even then, using identical batteries, there is still going to be a slight difference in battery voltages.

FollowupID: 371855

Follow Up By: Flash - Monday, Jun 20, 2005 at 09:32

Monday, Jun 20, 2005 at 09:32
Jeff M
It's nice to hear someone who understands basics. There is so much "snake-oil" peddled by people who are trying to sell stuff, which then becomes the ubiquitous "urban-myth".
You are correct, and have hit the nail on the head.
Some other points to consider.....
Parallel them while the alternator is running, maybe consider disconnecting them (if different types/age) when you stop.
Deep cycle are slow to charge, and need a really good charge from solar or 240volt charger (pref. 3- stage) occasionally to get them really topped up- unless you're driving right across the country. ie: the 14.2 volts or so from the average alternator will take forever to FULLY charge them.
Consider AGM when you need to replace them. They'll accept charge very quickly AND don't mind vibration AND don't give of explosive gases.
Cheers, and well said!
FollowupID: 371988

Follow Up By: Member - Jeff M (WA) - Monday, Jun 20, 2005 at 11:41

Monday, Jun 20, 2005 at 11:41
I run two totally different batteries, an 80amp/hr Wet Cell Deep Cycle and a 770CCA 17Plate starter, but both are N70ZZ's so they both hold a similar capacity. The 17 Plate charges quickly, the deep cycle slowly but the only time that this becomes and issue is when we have really "hurt" the deep cycle camping, but usually after doing that there's a long drive home and then I charge them with the 240v jobbie once home anyway. The advantage is that the starter battery always gets up to charge quick allowing us to always have juice to crank the surf over and the deep cycle doesn't mind so much having the life sucked out of it by all my toys! ;-)

Around town the engaging of the starter motor drops the deep cycle outta the loop (so to speak) until the motor is running and the starter is back up to voltage, then links them back up again so they are both charging. Very seldom to they register a difference of more than .1V on the digital metre around town.
FollowupID: 372002

Reply By: Member - Collyn R (WA) - Sunday, Jun 19, 2005 at 11:21

Sunday, Jun 19, 2005 at 11:21
Jeff M is correct.

Each battery will draw charge current according to its needs. If you don't believe me go to my website (as above except www.) and click the Link to Ample Power Company (a leading US maker of three-stage chargers etc). You will find they say almost exactly the same.
Collyn Rivers
AnswerID: 116341

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