# Hi. Simple Newbie question about what batteries I need



## alburglar (Dec 14, 2011)

I found this on ebay, but it doesn't list any specs and I don't really know what I'm looking for.

http://www.ebay.co.uk/itm/Lightweig...arts_Vehicles_CarParts_SM&hash=item20bfb18ddd

found some
Peak Current - 500A 
Capacity - 12Ah 
Weight - 2.3kg 
Terminals - M8 female 
Dimensions - 180mm(l) x 75mm(w) x 165mm (h)

http://vamosbatteries.com/


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## dtbaker (Jan 5, 2008)

alburglar said:


> Hello folks,
> I am going to run a completely stand alone reverse system in a car I'm building but I'm struggling on sufficient power supply. I Have a starter motor from a ford mondeo (average sized saloon car). This motor engages on a flywheel on the rear transmission on the car to reverse it.



huh?
what are your goals here? why? how are you controlling it? 

from what you've written so far, I doubt anyone can tell what you are trying to do, or why....


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## EVfun (Mar 14, 2010)

It is unlikely a car starter will move a car for minutes at a time without failing within a few such abuses. Car starter motors where not designed for efficiency so they really should never run for more than 30 seconds, tops. 

With a proper drive system 1 Optima yellow top battery could run you around in reverse for at least 10 minutes.


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## alburglar (Dec 14, 2011)

Ok great, Thanks for that. I had looked at the optima yellow top battery along with the red and blue top, I think. They are all a similar price, so which one would be best? 

...and to answer dtbaker:

The goal is to reverse! lol. I have a motor bike engined mini on a chain drive to a sprocket bolted to a diff. Motor bike gearboxes have no reverse.
The flywheel for reverse is bolted to the diff too. I have a floor mounted push button wired through a relay so it will only activate when the gearbox is in neutral. You simply push the button with your foot and it engages with the flywheel and moves the car backwards.

I will post a pic up when I'm at home.


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## alburglar (Dec 14, 2011)

The switch is to be replaced by a relay to test and then wired in properly.








Is the powercap useful at 2.0 farad or is it a waiste of space?


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## Ace_bridger (Nov 22, 2011)

I love what you're doing, I can't help with advice but think it's great.

Would I be way off if I guess that your nationality is British? It sounds like some great British eccentricity!!

Keep it up,

Yours,

A British eccentric!!


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## Woodsmith (Jun 5, 2008)

Are you aiming for many very short periods of reverse or are you thinking of one prolonged period of reverse?

For short intermittent bursts the starter will probably survive but not be efficient so waste a lot of energy.
For a long period of reverse the motor will not be good enough and you would be better off replacing it with a proper DC motor and maybe running at a higher voltage to keep current down.

Is this a road vehicle or a competiton vehicle?


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## RobSmith (Aug 13, 2010)

Hi,
I bet it is a competition vehicle. I was looking into building a Sports 1000 years ago with a BMW K100 bike engine. Unfortunately the Rules state (something like this...) The drive must include a reverse gear and it must be possible for it to be operated by the driver whilst normally seated.
The wording was something like that and there were a few Sports 1000 cars with starter motors on the backs of gearboxes engaging on ring gears bolted to the front end of the propshaft. (We also had to run a live rear axle)

There was a compromise between battery and starter abuse and weight.

Edit: Handbrake and Yoko A*** tyres so maybe some rally class? )

Edit 2: Just re-read your post... reversing for 2 to 5 minutes...gosh thats a long time. From memory I think an average starter is about 1.4hp so 1kw ish. 5 minutes at 1kw is 1/12th kw/h. 1kW at 12v is about 83amps.
Maybe someone who knows about batteries can add the last bit

Rob


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## alburglar (Dec 14, 2011)

Hi Guys,
Well yes, the british eccentrcity appears to have shone through! It is a fast road/track orientated car, so yes short periods of reverse mainly. I certainly don't intend to be reversing it constantly for 10 minutes, it just needs to be use-able in real world manouvres. Like backing up to get out of the garage and three point turns etc, but the option to have to reverse back down a small country lane to the nearest passing point has to be available.

I understand the limitations of a starter motor, but the main reason for choosing one is because they are 'pre-engaged', as in it is not mechanically connected to the drivetrain any time, until I press the button to reverse. Therefore it provides no drag on the drive train whilst going forwards and receives no wear when going forwards.


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## RobSmith (Aug 13, 2010)

alburglar said:


> Hi Guys,
> Well yes, the british eccentrcity appears to have shone through! It is a fast road/track orientated car, so yes short periods of reverse mainly. I certainly don't intend to be reversing it constantly for 10 minutes, it just needs to be use-able in real world manouvres. Like backing up to get out of the garage and three point turns etc, but the option to have to reverse back down a small country lane to the nearest passing point has to be available.
> 
> I understand the limitations of a starter motor, but the main reason for choosing one is because they are 'pre-engaged', as in it is not mechanically connected to the drivetrain any time, until I press the button to reverse. Therefore it provides no drag on the drive train whilst going forwards and receives no wear when going forwards.


One of the Sports 1000 cars had the motor pivoted and then the gears brought into mesh by moving the motor..... not elegant at all but allowed him to use a non-starter motor motor.
Tilton, Bosch and probably all the others do starters with integral gearboxes to reduce the load on the motor when turning over big or high compression engines. I guess they might be found on big diesels road cars too.
Rob


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## RobSmith (Aug 13, 2010)

I'd guess the capacitor is not doing much for you.
I bet it empties almost immediately with the startup current of the motor.

Thats just a feeling...I don't realy know much about capacitors.

Rob


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## RobSmith (Aug 13, 2010)

The motor might well be ok for a while if cold to start with.
It looks like you have that installed ok so maybe just try it out with some ludicrous batteries installed and reverse it around a car park for a bit.
One good thing is that the motor might be cool to start with.
The starter motor failures I had to deal with were due to heat being conducted through from hot gearboxes and radiated heat from exhaust systems in the back of LMP cars. If the heat didn't kill thm the vibration did. They often ran two starter motors as it doubled the chance of having one that worked.

Rob


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## EVfun (Mar 14, 2010)

alburglar said:


> Ok great, Thanks for that. I had looked at the optima yellow top battery along with the red and blue top, I think. They are all a similar price, so which one would be best?


It is actually not the top color that is important. Optimas have either a light gray or dark gray case. The dark gray cases are SLI batteries (Starting Lighting Ignition) batteries and have a short life if deep cycled. All the yellow tops are light gray cased -- they are deep cycle. The blue top batteries come in both types/case colors. All the red tops come with a dark gray case and are SLI batteries. 

If you choose a deep cycle blue top I do not recommend you use the studs for the type of current you are drawing. You should use proper crimped terminals attached to the automotive posts to withstand the amps. Lead loosens up on its own slowly (creeps) and even a little heat greatly accelerates this. A bad connection while drawing a couple hundred amps is a good way to melt a terminal off the battery.


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## alburglar (Dec 14, 2011)

Ok Thanks.
I think I'm getting the hang of this, but bare with me:
So if an average starter is about 1.4hp so 1kw ish. 1kW at 12v is about 83amps. 5 minutes is 1/12th kw/h. Therfore, in theory, I would need an 83Ah battery to run for an hours worth of use (excluding the big current draw from initial start-up). To get 5 minutes use out of it is 1/12th of 83 which is 6.9Ah, which is feck all, but the battery needs to have the big numbers for the initial startup.
So the optima yellow top 4.2 is 55Ah and is designed for starting and heavy use - so I'm gonna go for that when funds allow, it being nearly chrimbo and all.
The man from vamos batteries, that make the lithium battery, has been very helpful. His current battery is 12Ah and he is developing an 18Ah one. The 12ah weighs 2.3 kgs and is pretty small, so a bank of 3 18Ah's lithiums would be great if money was no object. They drop in power to all most nothing when cold tho (around 0 degrees). But it's pretty warm in my mini with the engine going!

I may keep the power cap as it has a handy digital voltage read-out.


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## EVfun (Mar 14, 2010)

If the starter motor is making 1.4 HP that would be an output of 1044 watts (that is a big if -- series wound motors will make a lot of peak power if bogged down and very little power at high rpm.) I doubt a starter motor is more than 50% efficient so you are looking at around 1088 input watts. It is unlikely the battery will be a full 12 volts under load, perhaps 11.5 volts. So you would be looking at about 182 amps. 5 minutes is 908 amp minutes or 15 amp hours. 

Now to look at the battery. Lead acid batteries suffer from what is known as Peukert's exponent, resulting in a reduction in available capacity at higher amp loads. The Optima is a 55 amp hour battery at the 20 hour rate (2.75 amps for 20 hours.) It is pretty well known to be a 33 amp hour battery at the 100 amp rate (100 amps for 20 minutes.) It might be a sustain 180 amps for 10 minutes. You will be using about 1/2 the capacity in 5 minutes. Trying to use the bottom half of the capacity is likely to result in some pretty severe sag when the motor is first engaged because the starting current will be much higher than the running current. The Optima is a good battery, most flooded lead has an even steeper reduction in available capacity at high discharge rates. 

Peukert's Law explained. Golf cart lead has a Peukert's exponent of around 1.2. Optima deep cycle batteries have a Peukert's exponent of around 1.1.


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## Ace_bridger (Nov 22, 2011)

Just a thought...we know that the top speed of a motor is dictated by battery voltage. The higher the voltage the higher the top speed. So, would it be worth testing the starter motor at a voltage lower than 12V?

The starter motor will not last, in my opinion, as they're built for high bursts of power at high speed for short periods.

If there is still sufficient torque at lower voltage then it might last longer and give a more appropriate performance.

You could try three lithium batteries in series to get just under 10V.

I might be talking jibberish but I'd welcome comments on the theory.

Good luck.


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## RobSmith (Aug 13, 2010)

I'd go for enough time to get you a three point turn. If you are reversing a long way then you have lost an aweful lot of time that once and spectators can maybe give you a shove. Unless Health and Safety have now stepped in to stop that. If you are carrying excess weight it will cost you time all the time.

Rob


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## Ace_bridger (Nov 22, 2011)

What about running the starter motor on something less than 12V. The motor is designed to run at 12V giving high speed and high torque for short bursts.

Perhaps there's mileage in running it off 3 lithium cells...this would give just under 10V and might give a more suitable speed?

Might be jibberish...might be genius!!! 

Good luck with whatever solution you find.

Oh, and lithium would save precious pounds in your lightweight sporty sportster!!


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## alburglar (Dec 14, 2011)

It appears that my gripper diff is providing alot of drag during 3 point turns.
I am leaning towards buying two lithium battery's 12Ah and 500A each and trying it, and then adding another in parrallel if that doesn't work! It's a toss up between weight and cost, but if I add a whole lump of weight to the car, it kind of defeats the object of it. But at £260 a battery that is expensive. However, I'm building a car that I can jump in on a sunny sunday and take for a blast along the b roads and that will be equally at home on a track day. So the expense seems justified to save weight. But if the gripper diff is what demands the battery oomph in 3 point turns, should I fit a Quaife ATB?, which wont cause the drag,then one lithium battery would probably be adequate. And that could be achieved for a little bit more money.
Decisions!!!!!


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## Rational (Nov 26, 2011)

alburglar said:


> Hello folks,
> Regards Alastair


Put your relevant equations into a spreadsheet and start tweaking values depending on constraints and desires. The posts above will help you with this.

To save your energy, first find out if your design goal is 'overconstrained", e.g. in the equation hp = torque x RPM/5252, you want 5252 RPM, 2 lb-ft of torque but only want to buy 1 hp.

For sure, Reality with a capital R sucks. But learning how to negotiate with reality is a valuable skill which many people have never learned.


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