Thread: Dual Battery Time Again

Hi again Mitch.

A winch's positive ( + ) cable should always be connected to the cranking battery, not the auxiliary battery.

There is a very good reason for this in that the cable distance from the alternator to the winch needs to be kept as short as possible, to gain the highest voltage possible at the winch while winching.

By connecting to the auxiliary battery, you not only increase the cable distance, which can demonically increase voltage drop because of the long cable.

But the setup, like yours, then needs to go though some form of switching/isolating device, and this just adds more voltage drop to the circuit.

Originally Posted by drivesafe

Hi jc, once batteries are in a parallel setup, all batteries will be charged at the same rate, if charged by a battery charger, solar or by a DC/DC device.

All these devices are constant CURRENT charging devices

If being charged by an alternator, which is a constant VOLTAGE device, each battery will be charged at that battery's optimum charge rate, regardless of the type and state of charge of any other battery in the same circuit.

I've got an SC80 installed and for some reason I thought that the trickle charge running through the starter first would keep it in better nick than having the current come the other way via the aux and the DBS. I assume what you've said above is correct provided the starter's voltage is high enough to keep the DBS open to the aux. If it's low, as mine often is when I come home from FIFO, then how does the charge make it to the starter?

Originally Posted by Blknight.aus

relay based systems that work on overall voltage on all the batteries for disconnect/connect by means of a relay you are correct, so long as the batteries are all connected you can charge from wherever you like. For things that automatically isolate or are one way only then its a different kettle of fish and your generally better off connecting to the aux battery as it will be the one that will be at the lower state of charge most of the time.

In my case it's usually the cranker that's lower. It always loses its charge very quickly if I forget to put the charger on before I fly north. Actually, I think it's on its way out but I've not been able to prove it. I do disconnect the relay when I go away, but it doesn't seem to make a difference these days.

Originally Posted by Toxic_Avenger

In my instance, I wired the winch to the AUX battery side of the ACR, so am I right in thinking that in this instance the alternator output + battery output still has to be carried through the ACR?

correct.

3 sources of amps for the winch the allternator, the main battery and the aux battery.

your setup goes Alternator--main batt---ACR---winch---AUX battery all amps flow towards the winch

you should be looking at
Alternator-winch-main battery-ACR-aux battery

in reality you wind up with the alternator, main battery, the winch and the ACR all hooked up to the one "point"

Another thing worth noting, if you have a deep cycle battery and a cranking battery and you start putting huge load on the cranking battery is going to do more work initially than the deep cycle battery so if you have your winch hooked up to a deep cycle battery and the Main battery is trying to feed it through the ACR it may well be the case that the ACR is doing even more work than the simple maths makes it look like..

Originally Posted by drivesafe

Hi again Mitch.

A winch's positive ( + ) cable should always be connected to the cranking battery, not the auxiliary battery.

There is a very good reason for this in that the cable distance from the alternator to the winch needs to be kept as short as possible, to gain the highest voltage possible at the winch while winching.

By connecting to the auxiliary battery, you not only increase the cable distance, which can demonically increase voltage drop because of the long cable.

But the setup, like yours, then needs to go though some form of switching/isolating device, and this just adds more voltage drop to the circuit.

So other than the battery type (cranking vs deep cycle or hybrid style- I have 2x optima blue tops), what I'm reading is that attaching to the cranking battery is to 'gain the highest voltage possible' at the winch motor. This is synonymous with voltage drop, right?).

I have run 70mm2 cable from the solenoid box to the battery, and 50mm2 from the solenoid forward to the winch motor. This is 2x the conductor area that was supplied with the standard cable.

Warn recommends <1V voltage drop, which IIRC, I shot for 0.3V drop based on cable run lengths, conductor CSA, expected cable / ambient temps etc. I reason that an extra 40cm of cable from the main battery thru the ACR and battery switch will not be a significant concern given the overall conductor length in the system.

I could have attached directly to the battery (either starting battery or aux battery), but this raised concerns with potential cable damage under the vehicle, which I wish to mitigate wherever possible.

Originally Posted by Blknight.aus

correct.

3 sources of amps for the winch the allternator, the main battery and the aux battery.

your setup goes Alternator--main batt---ACR---winch---AUX battery all amps flow towards the winch

you should be looking at
Alternator-winch-main battery-ACR-aux battery

in reality you wind up with the alternator, main battery, the winch and the ACR all hooked up to the one "point"

I understand the differences between setups, but other than voltage drop (which I believe I've mitigated), what's the difference? Is it the limitations of the electrochemical reaction within the lead acid battery - ie make the connection to the winch one which is copper, not phase changing (for want of a better word) thru the battery electrolyte?

As I understand it, starting batteries are designed to deliver a big burst of power in a short time, which is what a winch needs, whereas deep cycle batteries are designed for slow draw down and recharging, such as for a fridge. Therefore, the winch should draw on the starting battery. Is that correct?

Sent from my GT-P5210 using AULRO mobile app

Originally Posted by DiscoMick

As I understand it, starting batteries are designed to deliver a big burst of power in a short time, which is what a winch needs, whereas deep cycle batteries are designed for slow draw down and recharging, such as for a fridge. Therefore, the winch should draw on the starting battery. Is that correct?

Sent from my GT-P5210 using AULRO mobile app

Correct, it the lead plate design which skews the battery towards rapid ability to carry current vs slow and low discharge. I'm running a hybrid style that is neither cranking nor deep cycle. But 750CCA and 55Ah rated... so best of both worlds.

I was led to believe that a winch needs a higher CCA rating... 200CCA wont do it... but if both batteries are the same, and within the CCA rating recommended by the winch manufacturer, and the voltage drop is within spec, then I still can't see where the problem is

the voltage drop that occurs is a natural P=V*A thing, as the volts go down the amps go up and as that happens all your nice "clacluated at 13.8V"numbers go out the window. Then things get hot and the "calcualted at 25 deg C" part of the numbers go out the window.

the wire on the alternator is the source of all your power ideally you want to keep the path from the alternator to the winch as short as feasible BUT you cant just hook the winch to the back of the alternator because the wire will not deal with 320amps coming back from the batteries to deliver the 400amps the winch wants when your alternator is pushing its 80amps worth to the winch.

And NO, you cant just fit a fat ass wire to the alternator, in some vehicles the wire size forms part of your current limiting and sensing.

Where you make the connection between the battery and the ARC for the winch, is in effect more or less irrelevant (assuming all cable is the same rating) its the side of the ARC that counts. Given your batteries are the same then the only thing that counts for you is where the alternator connects to the system while winching on one side the amps from the alternator will funnel down to the winch without having gone through the arc and on the other side the arc gets to carry the current. IF you're pushing limits that might be the difference between the ARC contacts getting hot and deforming or staying within operating tolerance.

Onto the Deep cycle VS cranking battery for winching thing. For this I'm just going to pretend that there is NO DBS and the batteries are just in parallel with the kind of cable that even I would hint at being probable overkill.

Oh, WARNING... sciencey blurby bit missing from post 45 of this thread approaching, I'll skirt it as best I can and try to avoid peuket and charging and surface effect and and and and I'll try to keep it as close to laymans while avoiding as many techy terms like thingy and insideoutsidemelty as I can. warning done with.


you're close, its not the electrochemical reaction, its the construction of the battery. Briefly yes, the electrochemical reaction does play a part but its Very minor, infact in almost but not quite all normal cases its so negligible its not even worth considering you're more likely to have more difference being caused by something else such as a pifling of an ohms worth of resistance from a bad batch of copper in a relay contact and you'll have an even bigger difference made just because no 2 batteries are ever physically or electrically identical. Thats the bit where me and drivesafe differ on the elctrochemical side of batteries, he works everything so you never have to be in the area that I'm usually getting roped into, the ragged ends of things being used to and beyond limit. where the unusual is normal and when a 1Ah difference in capacity is a big deal because your not looking at 1 out of 200 your down into the 1 out of 20 end of business. Also if you're worried about the electrochemical side unless you're being obfuscatingly difficult and trying to work out how to do it with obscure combinations like LiPO and NiCad when it comes to winching, stop. the realistic piffling of difference it does make is maybe almost not quite enough to power up the transmitter in the winch remote control.

Back to the physical stuff.

its plate surface size and acid quantity that make the difference a battery is only so big and you can either put a lot of plate surface area or a lot of acid into it.

IF you have a lot of surface area and a little bit of acid you get a lot of reaction but not for long
if you have a little bit of surface area and a lot of acid you dont get much reaction but you get it for a long time.
The voltage of the reaction is effectively the same regardless of if you have 400 molecules doing it or 400000000000000

when you dump the mass load onto your deep/crank combo the crank battery picks up the lions share of the load, that's what its designed for, the deep cycle battery does what it can and the acid start to turn back to water in both batteries. The crank battery chucks out all the magic in the acid really quickly because A, there's not as much acid to start with and B, there's more surface area for the magic to escape from.

ok So now the crank batteries running out of acid to make amps from. the deep cycle battery isn't so it starts to sacrifice itself because as the volts go down if the acid is still strong enough the reaction wants to continue so in order to produce the energy to maintain the volts the reaction happens faster, this creates heat and also damages the plates, in extreme cases it will cause the plates to buckle and can blow the internal links between the cells.

Clever people, who want to be able to winch and use a DBS that links a deep cycle to crank battery put a current limiting device between the DBS and the deep cycle battery. A short section of well protected but ventilated undersized wire will do nicely as will a handfull of other things.
I tend to use high wattage bulbs if I have to field bodge it.

Aint spark chuckin stuff fun...

Just a final note. Optima batteries are not the only choice for an AGM deep cycle battery. They actually have relatively low storage capacity for a deep cycle battery. There are a number of AGMs that have 100AH capacity or more out there and are often cheaper. So have a look around.

For example, this one has just as much storage as the most expensive Optima and is half the price http://www.ebay.com.au/itm/EXIDE-Lei...EAAOSwI-BWQb2E

This one has 110ah and over 800CCA for just over $200 http://www.ebay.com.au/itm/EXIDE-Lei...QAAOSwo0JWQb2F