Running air compressor off deep cycle or cranking battery

Submitted: Wednesday, Jan 16, 2013 at 22:47
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Which is better assuming both are being charged while the compressor is used?
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Reply By: BluePrint Industries Pty Ltd - Wednesday, Jan 16, 2013 at 23:08

Wednesday, Jan 16, 2013 at 23:08
Well that would also depend on a few things as well

1. The the size of compressor,

2. Whats the current draw? "Being charged" Does this mean engine running, and therefore supplying the current, or is it a battery charger / or solar panel thats putting in a few amps but the compressors is drawing a lot.


For example a small one drawing say 5 or 10 amps would be ok on either for a short run, but a bigger one like our BP160A series which draw around 42 Amps at full load will significantly drain the battery very quickly.

Assuming Engine running, then I would say it wouldn't really matter as the majority of the current is coming from the cars alternator.

We recommend that people always have their motor running while inflating tyres, as this keeps the voltage up around 13V which reduces the current draw,and also helps the motor run cooler.

3. How many tires or how long you intend to run it?

4. What size are the tyres.

5. bigger compressors normally have shorter leads to reduce the voltage drop across the wire, so if your 2nd battery was in the rear of the car running it off the 2nd battery would give you more reach with your hose. Or you could run it off the main battery and use a longer hose.

So I would suggest it wouldn't really matter as long as you were putting as much current back in as the compressor was drawing.



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Follow Up By: garbage - Wednesday, Jan 16, 2013 at 23:53

Wednesday, Jan 16, 2013 at 23:53
Thanks for the detailed reply!

Size of the compressor I'm not sure. It's the ABR one that Derek sells. My deep cycle is being charged on mains by a Ctek charger. The starter would be charged by the altenator while the car is running. I have stock tyres on my Prado, again not sure what size.

From the replies, it seems the power required to run the compressor is literally coming directly from either the altenator or the Ctek charger?
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Follow Up By: Sand Man (SA) - Thursday, Jan 17, 2013 at 07:47

Thursday, Jan 17, 2013 at 07:47
Garbage,

Just remember that the alternator (when engine is running) is putting out something like 60-80 amps,while a charger would be giving more like 20-30 amp max (usually less for the average charger) and it is easier to see why most use the cranker with engine running while running a compressor, or other high current device such as a winch.

Bill


I'm diagonally parked in a parallel Universe!

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Follow Up By: garbage - Thursday, Jan 17, 2013 at 14:39

Thursday, Jan 17, 2013 at 14:39
Thanks Sandman! I was unaware of that! :)
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Reply By: AlbyNSW - Wednesday, Jan 16, 2013 at 23:29

Wednesday, Jan 16, 2013 at 23:29
I run mine off the cranking battery and leave motor running when using the compressor
I would rather save the amps in the deep cycle for my refrigeration
AnswerID: 502750

Reply By: Crackles - Wednesday, Jan 16, 2013 at 23:42

Wednesday, Jan 16, 2013 at 23:42
Doesn't matter. While the engine is charging both batteries are joined together anyway so power is either provided by the alternator or shared by the two batteries.
Cheers Craig...............
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Follow Up By: Allan B (Member, SunCoast) - Wednesday, Jan 16, 2013 at 23:48

Wednesday, Jan 16, 2013 at 23:48
Exactly. Simple as that.
Cheers
Allan

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Follow Up By: Member - Captain (WA) - Thursday, Jan 17, 2013 at 00:18

Thursday, Jan 17, 2013 at 00:18
And... if you run it off your AUX battery, if by chance you do run the compressor while the car is off, you know you will have enough charge to re-start the car. May not always be the case if you have a big compressor and pump up a convoy of vehicles ;)

Cheers

Captain
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Follow Up By: Ross M - Thursday, Jan 17, 2013 at 10:23

Thursday, Jan 17, 2013 at 10:23
Crackles.
Not so.
The batteries on his vehicle are not really connected to each other at all.
One is being fed from a charger running on a power source which just happens to be the main battery.
If the Ctek can deliver 20 amps max and the comp runs at 40 amps, then the aux battery will be going flat as you use it. This will be lowering the voltage of the aux and making the compressor run slow and hot and likely to overheat and let the smoke out of it.

Seeing there is a loss in a Ctek anyway, it is not the most efficient method either as it will be running flat out.

Some one said above, the alternator is charging at 60 to 80 amps. This is also a bit misleading. The alternator has a regulator and it will deliver, up to it's maximum, the amount of current required to keep the battery charged to a certain voltage level.
So if the engine is running and the vehicle requires 10 amps and the compressor uses 40 amps, then the alt will be outputting 50 amps to maintain charge level. The alt won't be pouring out a high amperage charge just for the fun of it.

A compressor running off the main battery is the most efficient method with no unexpected loadings and losses.

Ross M
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Follow Up By: garbage - Thursday, Jan 17, 2013 at 10:40

Thursday, Jan 17, 2013 at 10:40
Thank you Ross! :) Will use the cranking battery in that case.
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Follow Up By: Allan B (Member, SunCoast) - Thursday, Jan 17, 2013 at 10:50

Thursday, Jan 17, 2013 at 10:50
Ross, you make some good points. However it is not absolutely clear how Garbage's aux battery is being charged. Quite possibly by a Ctek DC-DC charger with a 20A capacity, but not necessarily so. If it were a simple solenoid-type isolated as both Crackles and I have assumed then the compressor current would be supplied from the alternator even with the compressor connected to the aux battery.

Garbage, could you advise just what battery isolator system you have?

But Ross, you have caused me to reappraise my system and have identified just this problem. I originally had a solenoid isolator to the aux battery with the compressor connected there for wiring convenience. When I upgraded to two aux batteries each with 20A DC-DC chargers I neglected to redirect the compressor to the cranking battery. Seems I have some more wiring to do! Thanks mate.

Cheers
Allan

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Follow Up By: Echucan Bob - Thursday, Jan 17, 2013 at 11:26

Thursday, Jan 17, 2013 at 11:26
I agree with Ross. Not all of us have connections to the auxiliary battery capable of handling 40 amps, or of delivering that current without a significant voltage drop. You need to assume the worst case scenario that the aux battery delivers no current and the full load comes through your charging system (be it a DC to DC charger, solenoid, Redarc or just a long wire). Attaching the compressor as close to the alternator (ie the positive post of the cranking battery), with the engine running, is the way to go.
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Follow Up By: Member - Captain (WA) - Thursday, Jan 17, 2013 at 11:51

Thursday, Jan 17, 2013 at 11:51
Actually, I don't think there is a single right answer! As so often happns, there are so many variables that one has to weigh up the compromises and do what suits their circumstances best.

If using a solenoid type isolator and you have good size cable, then connecting to the AUX battery will give the advanatge of not running down the STARTER battery if the compressor is run with the vehicle off. And the AUX battery still gets full alternator current if the vehicle is running.

But if a dc-dc charger with a capacity lower than the compressor is used for charging the AUX battery, there are certainly advantages to using the STARTER battery. But one should make sure its not used without the vehicle running!

While everyone has the intention of never running the compressor without the engine going, I have seen people using a "tyre" compressor to pump up air mattresses while setting up camp (engine off) and then needing a jump start the next day :(

My preference is to connect accessories to the AUX battery and keep the STARTER just for starting, but understand that people have different reasons for doing different things.

Cheers

Captain
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Follow Up By: Ross M - Thursday, Jan 17, 2013 at 13:29

Thursday, Jan 17, 2013 at 13:29
Captain
He did however, say he was charging the aux from a Ctek charger.
He did mention mains charger but even if a mains charger, which is no good in the bush, OR a DC/DC unit the facts remain the same.
He did not at any time mention a solenoid type connection, that would change the game plan somewhat.

Because he is unlikely to be airing down on the caravan park it would be reasonable to assume for him and other who might use a DC/DC unit again, the facts remain the same.
On a hot day, had some of those recently the compressor is already at 35Deg C and has less duty cycle before the smoke comes out, cos it will get hotter. Full alt voltage will keep the cooling aspect more under control, so running off aux, if a few tyres or gear needs compressor work, is better.

If the use of a compressor for an airbed flattens the main so you don't start next morning, then your main battery is too small or stuffed. The untouched aux can then help to stat you in the morning.

It is all a balance of use and storage,

Ross M
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Follow Up By: Member - Captain (WA) - Thursday, Jan 17, 2013 at 16:00

Thursday, Jan 17, 2013 at 16:00
Ross M,

Original post did not mention what type of charger, was only in a followup later.

If you want to run you compressor off the STARTER battery, go ahead. I will stick to using the batteries for what they are for - STARTER for starting and AUX for accessories.

Everyones uses and requirments are different, no single generic correct answer and will depend on the compromises made.

Cheers

Captain
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Reply By: ABR - SIDEWINDER - Thursday, Jan 17, 2013 at 09:29

Thursday, Jan 17, 2013 at 09:29
Hi Garbage

Thank you for buying one of our compressors.

The ABR MK4 draws 40A at 13V and will take approx 2 minutes to inflate a standard tyre from 18 psi to 40 psi or just over a minute 18psi to 34 psi.

Please always let the car idle while using the compressor, off either the main battery or aux battery.

Regards

Derek from ABR
AnswerID: 502771

Follow Up By: Ross M - Thursday, Jan 17, 2013 at 10:41

Thursday, Jan 17, 2013 at 10:41
Derek
The running of the compressor off the aux battery supplied with a 20 amp Ctek will be reducing the available voltage at the battery while in use do to the mismatch of use and charge rates. I'm surprised you endorsed the aux as also ok. Yes it will run a compressor but not the optimum by a long chalk.

Therefore, having the car idle as the comp runs off the Ctek and aux battery is achieving absolutely nothing and the aux system is going backwards at a rate of a constant 20 amps.

If the aux battery has run a fridge and lights at night or all night, it's charge level will already be down and unless you upgrade the Ctek to a 60 amp model you aren't going to make hay at all.

The idling vehicle only provides a beneficial maintenance of charge/voltage level to the compressor IF it is connected to the same battery as the alternator is also directly connected.

Everyone need to think about the operation of the system and component performance so they can work out what is best, after all that is what the OP asked the question for in the first place, to find out which is best.

Yours, in amps/volts and inflated tyres.

Ross M
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Follow Up By: Allan B (Member, SunCoast) - Thursday, Jan 17, 2013 at 10:56

Thursday, Jan 17, 2013 at 10:56
Ross, quite so but Garbage did say "My deep cycle is being charged on mains by a Ctek charger".
Note that he said "mains". It is not yet clear if, or how, his aux battery is connected to the alternator. Perhaps he could advise?
Cheers
Allan

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Follow Up By: garbage - Thursday, Jan 17, 2013 at 11:25

Thursday, Jan 17, 2013 at 11:25
My deep cycle isn't currently connected to the altenator as I haven't yet put in a dual battery system. So at the moment, it sits in my garage on float charge from the Ctek (can't remember which one, but it is the second smallest Ctek available).

So I think the answer is that the compressor should be connected to any battery as long as it is being charged by the altenator and the car is idling?
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Follow Up By: Allan B (Member, SunCoast) - Thursday, Jan 17, 2013 at 11:35

Thursday, Jan 17, 2013 at 11:35
Ah yes Garbage, thanks.
Definitely use the cranking battery. Probably the best connection even if you did have a solenoid isolator as, after all, there is no point in unnecessarily loading a solenoid with the compressor current.
Cheers
Allan

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Follow Up By: ABR - SIDEWINDER - Thursday, Jan 17, 2013 at 11:38

Thursday, Jan 17, 2013 at 11:38
Hi Garbage

"So I think the answer is that the compressor should be connected to any battery as long as it is being charged by the altenator and the car is idling?"

Correct, I don't recommend the ctek charger method.

Regards

Derek from ABR
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Follow Up By: Ross M - Thursday, Jan 17, 2013 at 13:37

Thursday, Jan 17, 2013 at 13:37
Allan B
Yes, I did read the mains charger bit, while that might be so, the situation of mains and Ctek while compressing is not usual and most if not all situations such as ours, is to use the comp while out and about.
So my comments tried to address both situations.

Either way if Ctek mains or DC/DC Ctek the battery is still going to go backwards re. charge level. How much depends on compressor load and charger ability and the time it is running for.
Yes, lots of variables there.

Ross M
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Reply By: olcoolone - Thursday, Jan 17, 2013 at 18:20

Thursday, Jan 17, 2013 at 18:20
End of the day it isn't really going to matter what battery you run it off.

Air compressors are only used for a short amount of time and the current draw is not going to be high enough to cause a concern.

If your compressor draws 40 amps and you use it for 20 minutes blowing up some tyres.... your only going to be drawing 13 amps, not really enough to kill any good battery.

It's always advisable to keep your engine running and the rate your discharging you battery at will be less then it's getting charged at.

Simple answer is..... what ever battery you can get to the easiest to connect to.

AnswerID: 502794

Reply By: Member - PJR (NSW) - Thursday, Jan 17, 2013 at 19:26

Thursday, Jan 17, 2013 at 19:26
A bit like my good mate above. Anything that is an accessory that I have installed, like a winch or compressor, and any fridge etc ( ie not belonging to the car) we run of the auxiliary pair of batteries. We run three 100AH All rounders with one kept as crank and the other two isolated through a 200 amp Redarc isolator.

Phil
AnswerID: 502799

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