Thread: Dual Battery Time Again

Just my twopence worth.

I have just fitted a large switch (marine type) connecting my two starter batteries. It has provision for OFF, BATT 1, BATT 2 and BOTH. Switch is mounted on the front of the Seat box (passenger side) and it's a simple matter of switching it over regularly eg after refuelling. About $50.

Same system I had in my Stage 1 for 20 years and the same in my 110 for the past 9 years. It works for me and I have never had a problem with it. Everything works off the battery which is selected.

When using the winch, switch to BOTH.

I know Tim is not a fan, but it works.

Tim, well maybe not a case of get what you pay for but if you pay more you get more features. It's not like a Narva VSR is cheap and nasty, it is very simple in operation so therefore cheaper.

People will have varying requirements, for me I winch with a PTO and I jump with jumper cables.

Different products for different requirements.

I'm trying to get my head around the science (and scientific responses) of how one can reasonably expect to choose a dual battery controller for use when winching. I think I got my selection right in my setup (albeit probably overkill), but my question aims to answer 'what's the minimum I can get away with'.

So I have some questions that I might pose to the masses.

1) My understanding is that if 2 batteries are combined, for say, a winching operation, then 2 x 12V batteries with a load of say 400A, will draw 200A from each battery, when they are connected in parallell. If 1 battery is connected, then this battery is required to supply the entire 400A. Is this statement correct?

2) What sort of current draw could one expect from an average winch at a average pull weight, on a reasonable number of winch rope layers? I'm led to believe a ball park figure for 9500lb at first layer could be 350A... and higher when we start talking about 4th layer on the drum, larger HP winch motors, and larger weights to move. Is this correct in other's opinion? (Has anyone put an amp meter on a winch under free-spool conditions?)

Originally Posted by Toxic_Avenger

I'm trying to get my head around the science (and scientific responses) of how one can reasonably expect to choose a dual battery controller for use when winching. I think I got my selection right in my setup (albeit probably overkill), but my question aims to answer 'what's the minimum I can get away with'.

So I have some questions that I might pose to the masses.

1) My understanding is that if 2 batteries are combined, for say, a winching operation, then 2 x 12V batteries with a load of say 400A, will draw 200A from each battery, when they are connected in parallell. If 1 battery is connected, then this battery is required to supply the entire 400A. Is this statement correct?

2) What sort of current draw could one expect from an average winch at a average pull weight, on a reasonable number of winch rope layers? I'm led to believe a ball park figure for 9500lb at first layer could be 350A... and higher when we start talking about 4th layer on the drum, larger HP winch motors, and larger weights to move. Is this correct in other's opinion? (Has anyone put an amp meter on a winch under free-spool conditions?)

your assumptions are on the money at the level that you're needing to be worried about

point 1 is on the money
point 2 is also close enough. my aldi draws 50 amps on no load startup (my clamp meter doesnt register fast enough to capture the split second startup current, this is the number that the max hold setting captures) and settles at about 30 amps when winching in with just enough tension on the cable to make it sit nicely.

IF you want the cheapest and nastiest automatic dual battery system you can get go buy a 200A relay and wire its input to a circuit feed that comes on off of the VSR (voltage sensing relay) thats in the vehicle (controls power to things like electric window heaters and seat heaters) or off of the ignition. On the VSR The aux battery will only get connected once the main battery voltage is over ~13.2v and will disconnect at about the same point. Should cost you around $30 to build it all up including the cabling.

the winching thing. This is another one of those things that the traxide does that a DCDC cant. DCDC units are one way so the amps that are in the aux battery are not available to the main to share off of the winch.

IF you're going to winch (or use large inverters) through your DBS you are essentially cutting out the ability to use most stock Deep cycle batteries as the aux as while you may not draw all of the AH while you winch you're pulling them out faster than the battery is designed to deliver them. (sciencey blurb here about amps volts plate construction heat peukets law omitted) which shortens the life of a deep cycle battery if not killing it completely.

If you want to go utterly ghetto on your DBS.. just get yourself a battery isolator switch, fit it up and wire the 2 batteries in parallel through it. just remember that you have to turn it off manually or in 3 days you'll be up agaisnt 2 flat batteries.

A couple of comments here have said that using the Traxide unit you can charge all batteries via a cable to one of the auxiliaries. And I see the same on the Def-USI-FPC diagram linked to earlier.

I had thought that optimal charging for all batteries would be achieved by charging via the cranker first. Have I got it wrong? I hope I do, as it would allow a rewiring to a far simpler and more flexible set-up.

So how would a $30 200A relay job handle winching? I'd think a lifespan of minutes if batteries are not combined, marginally longer if joined.

How would a traxide unit handle 350A if the batteries are not put into 'winch mode'? Wouldn't a large load from a big winch under sub-optimal conditions put even the most esteemed product dangerously close to it's current handling limit of 160A continuous, or make/surge current lilmit of 240A (usually this terminology is referred to as current rating for ~10seconds) ?

I know everything has a lifespan under extreme use, but the trend I have seen in my research is that 160A/240A is pretty much the biggest rating you'll get on an automotive battery controller, hence why I went for a marine grade one. On paper the Blue sea ACR I have kicks ass at 2000A cranking current (10s) or 225A continuous (24/7).
I'm looking for reasons to assure myself that I didn't spend 2x more on that unit than I needed to

Originally Posted by jc109

A couple of comments here have said that using the Traxide unit you can charge all batteries via a cable to one of the auxiliaries. And I see the same on the Def-USI-FPC diagram linked to earlier.

I had thought that optimal charging for all batteries would be achieved by charging via the cranker first. Have I got it wrong? I hope I do, as it would allow a rewiring to a far simpler and more flexible set-up.

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.

Hi Mitch, when I was developing the USI-160, I initially was going to produce a 240 amp isolator, but after quite a considerable amount of researching and testing, I found that the 160 amp version is more than adequate and to date I have never had one returned because the relays were burnout.

When you link an auxiliary battery to a cranking battery, for winching, the maximum continuos current draw from the auxiliary battery is less than 150 amps.

The actual total current draw of most winches is usually no more than 300 amps, unless the winch is forced into an overload ( stall ) situation.

My isolators will tolerate 400 amps for a very short time.

At 300 amps, the average alternator will usually provide around at least 50+ amps and the remaining 250 amps is provided by the two batteries. That means "roughly" 125 amps per battery.

Battery type and size, plus cable thickness and length, will determine how much current is drawn from each battery while winching.