12 V battery choice

[Rick Hughes]

On a separate post (Winch Project) I have been getting ideas on putting a winch in garage to pull in a boat on a twin axle trailer.

Winch I am thinking of using is a 3500LB winch which has a 1.3hp permanent magnet 12V motor. Which is aprox 1kW motor ..... Using P=VI could be drawing up to 83 Amps

However my assumption would be high current to get over initial inertia, then much lower current to keep trailer moving

12V Lead Acid batteries ... a starter battery would suit better than deep discharge, as use will be very intermittent, and then only a couple of mins at most. So something like a 30AH may be enough.

The Cold Cranking Amperage? ...low rated batteries still have around 300 A so should not have issue there.

Would something like this suit ...

want to buy the smallest for the job

[meow2222]

Motor power rating is normally run not start. Starting current is typically= many times as high. As you say its not likely to consume anywhere near 1kW= pulling a trailer.

What matters is depth of discharge. If you want it to last years, I'd avoid= more than 50% discharge, preferably less. Neither of us really knows what = watthours the winch will use, but you could estimate it as winching time x =

1kW if on a slope, or x 250w (Estimate) if on level. For 50% discharge with= a bit of added messing about getting things positioned right you can calcu= alte the estimated required capacity.

the smaller you get, the less time it'll last. Lead acids are more economic= al if you considerably oversize them.

NT

[harry]

746 watts=3D1horsepower. Such a motor would not be continuously rated. It might draw a high initial current but once the thing was moving it would be much less.

No-one can tell you any exact figures, there are too many variables. Gradient Gearing of gearbox in winch friction I mean you could do it with a ten watt motor if you were prepared to take two or three hours over the job!

[Rick Hughes]

Well the Winch has the 1.3hp motor so that much I know ... it's speed will be fixed, so I can't alter time.

My problem is I need to know what battery to go buy ..........

Perhaps I just have to guess and buy the 40AH

[Rick Hughes]

more than 50% discharge, preferably less. Neither of us really knows what watthours the winch will use, but you could estimate it as winching time x 1kW if on a slope, or x 250w (Estimate) if on level. For 50% discharge with a bit of added messing about getting things positioned right you can calcualte the estimated required capacity.

winch time assumed 5 min max, you mention winching time by 1kW .... so that would be ... 0.08 x 1000 = 6.4 but what does that tell me ? 6.4 Watt Hrs ?

How does that relate to battery size ? ... is it as simple as P/V which would give me 0.53 Which would seem a very small battery.

[Dave Liquorice]

Why not wander down toa local garage and see if they have any half decent "dead" batteries or even second user ones that they have taken out of scrapped cars. Might give you and idea of the size of battery you need rather than blindly guessing like the rest of us.

Don't forget that for 80+ amps you'll need some hefty cable to connect it up, heavy and short as possible otherwise you'll be wasting battery energy heating the cable up not pulling the boat.

[meow2222]

void more than 50% discharge, preferably less. Neither of us really knows w= hat watthours the winch will use, but you could estimate it as winching tim= e x 1kW if on a slope, or x 250w (Estimate) if on level. For 50% discharge = with a bit of added messing about getting things positioned right you can c= alcualte the estimated required capacity.

1kW =3D 12v x 83A 5mins =3D 0.083hrs So amphours used per winching operation =3D 83 x 0.083 =3D about 7Ah. So yo= u'd want at least 14Ah.

80A from a 14Ah battery would be pushing it hard, you'd get voltage drop, s= lowing the winch and prolonging time, so I'd go for something bigger.

Also don't forget a 10hour charge means you're stuck with just one winching= per 10hrs with a small battery. 30Ah would be much more capable, and used = car batteries are relatively cheap.

NT

[meow2222]

83A is comfortably within the ampacity of 2.5mm^2 T&E with all 3 conductors= connected together. If you're limiting it to 5 minutes you'd probably find= a single piece of T&E would suffice, but better to use 2 lengths, each wit= h all conductors connected together.

NT

[Bill Wright]

That's always a mistake.

Bill

[Bill Wright]

Why not run it from jump leads from your car battery and see what happens?

Bill

[Rick Hughes]

it comes with full 'chunky' cable, looks like the standard stuff they hook up batteries with

[Rick Hughes]

Not sure how that would help ... car Battery is 120 AH so all that would show is that 85AH is enough.I don't have a clamp meter so would not be able to measure current draw.

or did you mean something else ?

[Rick Hughes]

Not always ... often better to get correct sized item, over specifying can just be a waste of money. A 30AH battery is around £34 an 85AH is £125 .... if the former is big enough for the job, why pay 4 x the price you need to

[Fredxx]

I was wondering about this as well, depending on the towing car, and even then you could keep the engine going to ensure the battery is not flattened. You can buy a modest amount of cable for a 40AH battery!! And it will last longer!

[Grimly Curmudgeon]

Economies of scale for the smaller size - you could take advantage of that and buy two of them and parallel them up. You're not restricted for space, on a garage floor, I take it.

[Dave Liquorice]

Don't under estimate the affect of even a small voltage drop will have on the abilty of the whinch to perform properly. Lets say the motor pulls

50A, just 0.02 ohms will drop a volt...

[Fredxx]

Yes, at 25mm CSA, @ 0.75mR/m, I make that a combined +ve, and -ve leg of

26m, ie 90ft to drop 1V at 50Amps.

I'm very aware of voltage drop after recently using 95mm csa on a narrowboat for a 3kW inverter and a hairy second alternator.

I would recommend the OP uses flexible cable. Cable designed for welding should suffice.

[Dave Liquorice]

90' *both* legs combined so no more than 45' (13m) from source to load... Not to mention the drop across the internal resistance of the battery. I think that there would be really noticable performance loss at half that distance.

Aye.

[meow2222]

Lets see.

1mm^2 drops 44mV per amp per metre, and we can estimate under 25A draw and use a 1m cable from battery to winch. So if we limit cable Vdrop to 5% or 0.6v... Vdrop = 0.044 x 25 x 1 /squaremils sqmil = 0.044 x 25 / 0.6 = 1.8mm^2 So 2.5mm^2 T&E would be fine. If you wanted to allow for far heavier loads and stay within spec for much higher current draw, 2 pieces of 2.5mm^2, each with conductors parallelled would do that. That would be my preference.

(A 2 ton winch pulling a load up a 1 in 10 slope could shift most of 20 tonnes.)

NT

[GB]

more than 50% discharge, preferably less. Neither of us really knows what watthours the winch will use, but you could estimate it as winching time x 1kW if on a slope, or x 250w (Estimate) if on level. For 50% discharge with a bit of added messing about getting things positioned right you can calcualte the estimated required capacity.

want at least 14Ah.

slowing the winch and prolonging time, so I'd go for something bigger.

per 10hrs with a small battery. 30Ah would be much more capable, and used car batteries are relatively cheap.

I see that Eurocarparts are doing a Bosch 43 ah battery for £36 and a rather bigger one for £48. Cheap as chips! Given that, I wouldn't bother messing around with dud batteries from breaker's yards.

Alternatively, what you might do is replace your car battery with a new one, and use your working car battery for the winch. What's in your car?