my knowledge of electrical theory is shaky at the least so perhaps someone can explain
I’ve often seen warnings and anecdotal evidence that for example starting a motorcycle with a very high capacity battery from a much larger vehicle ( same 12v) can cause problems. As far as I understand it the current drawn depends solely on the bike’s requirements/circuit resistance not what the battery can deliver ..so wheres the problem?
seen similar said about chargers for various electrical devices but surely if the voltage is correct and it can deliver at least the required amperage where’s the hidden problem?
Probably something basic I’m missing …can anyone explain before I go back to school!
thanks
Bill

Shaky here too, but surely the components aren’t smart so if the battery has a higher rating (ie, holds more amps) then too much power will flood into the circuit (ie, they don’t take what they need, they are given what the battery can deliver).

A far as I understand the number of amps flowing depends on the voltage and the resistance of the circuit.. not what can be delivered. IE Ohms law
V=IR and so I ( amps ) = voltage divided by the resistance.

number of amps flowing depends on the voltage and the resistance of the circuit.. not what can be delivered. IE Ohms law

[WILD GUESS]Thing is, the depleted battery will happily take more amps than are good for it unless the supply is restricted.[/WILD GUESS]

Glad it’s not only me that’s unsure! feel a bit better now.

A far as I understand the number of amps flowing depends on the voltage and the resistance of the circuit.. not what can be delivered. IE Ohms law
V=IR and so I ( amps ) = voltage divided by the resistance.

This is correct, with the caveat that if the circuit has such a low resistance that it tries to draw more current than the maximum the battery can deliver then the current will be that maximum.

So possibly in your motorbike starting example this is what is happening, therefore a bigger battery could deliver more current and melt/damage some components in the circuit.

Lots of amps create heat, this tends to do for cable insulation and fragile IC chips. As always it’s the current that does for things at lower voltages. At much higher voltages both will make a mess of fragile stuff.

As I understand it NewRetroTom is correct.

The maximum amps that a battery can deliver are limited by its size and chemistry – hence why a 12v car battery can give you a decent shock but those little 9v square batteries only give you a tingle on your tongue.

So if your circuit relies on the current-limiting properties of a battery then whacking in a bigger battery with a bigger current limit means more amps. (Most circuits are not like this though).

Its the internal resistance of the battery when discharged, it’ll be lower on a larger battery so draw more current when being charged – so say you fit a 40amp battery in place of a 20amp a 20amp alternator trying to charge a 40amp battery won’t last long.

NRT is correct, the car battery can supply more current to your starter circuit on a motorbike as it’s designed around a smaller Ah bike battery BUT you can use a car battery in parallel to act as a “float” to stop your bike alarm/immobiliser flattening the bike battery. Though an optimate is a better bet, but if your bike lives in a shed with no power the car battery is fine (I’ve done it for years with a variety of bikes). Just remember to unhook it before starting to stop the bike alternator from trying to charge a battery it’s not designed for.

If using it as a jump for a flattened battery use long thin wires which will act as a current limit/volt drop and leave for 20 minutes or so to put some charge into the smaller battery. Big fat jump leads will let lots of Amps flow, possibly greater than the cranking current than the bike battery is designed for and this can warp the plates inside the battery, damaging it. BATTERY FIRES ARE REALLY BAD.

As a side note we put a clip on ammeter on the battery lead of a works van and set it to measure peak current. A warm two litre Diesel engine peaked at 800A cranking current!

Broadly: NO

Most of the “don’t do this or don’t do that” warnings are ass coverings.

However, a circuit is just that, a complete circuit, and as V = IR, you have to include the internal resistance (ir) of the battery in the whole circuit, and not just the impedance of the load. (you’ll note i’ve used the term impedance rather than resistance, as we can be talking about a dynamic situation here, rather than a pure DC one)

Where the load has a high impedance, the small ir of the battery has little effect, as typical lead acid batteries have ir’s of less than 1 ohm.

BUT, a load that has very low impedance will get more current through it if a battery of lower impedance (usually a bigger capacity one!) is connected.

Secondary to this, is the fact that the voltage sag under load for a larger capacity battery is going to be smaller, in effect, that batteries dynamic ir is lower, so again, this could be a factor in setting the peak current that flows in the load.

I’m pretty sure you can’t get a shock off a 12v battery, no matter how hefty the thing.

(This is not from theoretical knowledge, this is from practical experience, I would certainly have received a belter of a shock by now as I’m a ham fisted and clumsy mechanic with several scars to prove it…)

Yes.

If you try and charge a battery with a lower impedance (i.e. a big battery) it will kill the charger unless the charger has a current limiter on it. In which case it may think it has a short circuit and cut out anyway.

You would get more amps when starting as well, but I’d be surprised if the system wasn’t designed to withstand 12V at the starter motor, and it shouldn’t be possible to melt anything in the time it takes to start.

Getting complicated now..!explains why with my very limited knowledge of electrics I couldn’t understand ..and tbh ..still a bit confused.thanks to all who tried though!

Actually..maybe it is starting to click.So the battery supplied and recommended with whatever equipment, doesn’t necessarily supply all the current a circuit could draw…with the emphasis on the COULD and it may draw more if a higher ah source is supplied .Surely though ..a fuse should be stopping this happening to avoid excessive current damaging components and circuitry if it is supplied by accident.?

I’m pretty sure you can’t get a shock off a 12v battery, no matter how hefty the thing.

Stick your tongue across both terminals and get back to us.

Stick your tongue across both terminals and get back to us.

Said I’m clumsy, not daft! (Plus, how long is your tongue??? 😯

Stick your tongue across both terminals and get back to us.

Indeed, water is a great conductor hence the tingle from a 9V battery.

It gets tricky as circuits are designed with resistance, voltage and current in mind. But current is only ever “drawn” and the voltage can be thought of as the vehicle to deliver it. So you can’t “force” current into components – they only take what they need. The problem arises when components attempt to draw more current than available, hence you can overload AC/DC adapters, etc.

So a kettle, for example, is damaged when plugged into the mains providing 13A although it only needs 1A (or whatever, just guessing here).

If the circuit is sensibly designed there should be no problem using a battery with larger capacity of a similar type. In fact you’d have to work hard to create a situation where it could matter. It’s much more likely a case of “not been tested” than actually “unsafe”.

Actually..maybe it is starting to click.So the battery supplied and recommended with whatever equipment, doesn’t necessarily supply all the current a circuit could draw…with the emphasis on the COULD and it may draw more if a higher ah source is supplied .Surely though ..a fuse should be stopping this happening to avoid excessive current damaging components and circuitry if it is supplied by accident.?

Yes, but you can’t fuse the starter as it draws hundreds of amps and relies on low resistance in the rest of the circuit.

As I said, I don’t think the starter will be a problem. You can kill a starter motor just letting it run for too long so a bigger battery just makes that a bit easier.

Actually..maybe it is starting to click.So the battery supplied and recommended with whatever equipment, doesn’t necessarily supply all the current a circuit could draw…with the emphasis on the COULD and it may draw more if a higher ah source is supplied .Surely though ..a fuse should be stopping this happening to avoid excessive current damaging components and circuitry if it is supplied by accident.?

So a kettle, for example, is damaged when plugged into the mains providing 13A although it only needs 1A (or whatever, just guessing here).

No.

The current is achieved by the voltage at the battery giving electrons energy, you then need a certain amount of energy to move though wires and other components, this gives a voltage drop over each part (every wire, component, contact, everything), in a well designed system almost all of this is over the component you actually want to do the work.

Coulombs is the flow of electrons (current x time)
Watts is the flow of power (current x voltage, or voltage drop over that component)
Joules is the energy (coulombs x volts, or current x voltage x time)

If you plug a 120v kettle into a 230v plug then what happens is the power ends up four times as much as intended (P=IV, I=V/R where R is the resistive load of the kettle and approximately constant to keep things simple, so P=V^2 / R). It can’t get rid of that much heat, so it melts/burns.

You could plug it into a 120V supply capable of supplying millions of amps and it wouldn’t matter, it would only draw what it’s resistance allows.

So a kettle, for example, is damaged when plugged into the mains providing 13A although it only needs 1A (or whatever, just guessing here).

It meant to say the kettle isn’t damaged

Agree with what you say – current is only ever drawn. The voltage, or potential difference, is the problem.

Indeed, water is a great conductor hence the tingle from a 9V battery.

Pure water is a terrible conductor.
Add in some salts, and it does conduct. General tap water is not very conductive at all.
It’s poor conductivity is used productively in large industrial water heaters, where two electrodes are put into a large tank of water. The resistance between the 2 electrodes in the water is high, so the current has to ‘work hard’ to get from one electrode to another, hence the water heats up due to the energy required to travel the short distance between the electrodes.

Remember its the Volts that jolts and the amps that cramps(mostly). Which is why we use 110V centre earth tapped (55v to earth worst potential) equipment on construction sites. When I did my apprenticeship in 87 I remember our course instructor saying that there had not been a recorded death due to 110V site equipment electric shock.

However, should any voltage punch through your relatively resistive skin and into your nice conductive electrolyte tissue and blood then a mere 30 milliamperes current flow across the heart has a good chance of killing you.

I still remember the Hawker Siddley Transformers test engineer relating the story of how he put out the apprentice on fire with extinguishers after he walked into a test area and got hit with a grid transformer arc (132KV).

kaiser – Member

As far as I understand it the current drawn depends solely on the bike’s requirements/circuit resistance not what the battery can deliver .

Up to the limit of the battery. But some shittily designed components and systems depend on the limits of the battery. And sadly quite a lot of motorbike electrics are shittily designed. So yes you can totally stick a higher cranking current battery in a motorbike and end up overheating wires, or damaging a relay or similar.

In my case, the starter overheated quickly if you cranked it too long with the higher cca battery, especially in the cold- but on the other hand, usually started faster so it was only an issue if I was doing something like starting from a dry fuel system or similiar where it had to turn over for a while. As someone noted up the page, starters are often overstressed and have a short run time by design, this really just reduces the time-to-****up. A totally durable system could do it forever but would probably weigh a ton.

You can do something like even with stock parts actually by going outside of parameters- I melted the starter cable on my mondeo by turning it over with diesel in one piston- I wanted to blow it out of the injector hole. But that strained it all way more than I expected and the smoke got out of the wire- it was pulling way more current than the designers had accounted for. (when the piston was hydrolocked, it just couldn’t turn over at all and so no issue)

I always think in water terms- it’s a bit like building your plumbing with weak pipe because you know the water pressure can’t burst it, but then they change the mains supply and suddenly you spring a leak. These metaphors usually aren’t technically correct, but they can be useful

I’m pretty sure you can’t get a shock off a 12v battery, no matter how hefty the thing.

Well, the one occasion I accidentally put a screwdriver across the terminals of a 12v car battery and got a blinding flash and a chunk blown out of one side of the screwdriver tip tends to indicate that getting a belt off a car batter would do a bit more than tingle…

Your (dry, clean) skin has a higher resistance than your screwdriver
I wouldn’t recommend experimenting, and wet skin would be another matter (body’s resistance to electricity link)

Battery terminals in my car usually have a dab of copper slip on them so would that alter the skin resistance?

You’re probably right about dry skin. I have to admit I’ve never dared try it.

Min voltage generally considered to be potentially lethal seems to be around 40-50 but even that is pretty unlikely (find a sparkie who’s never had 240v jolt). Electrodes applied to the heart could do it with less though 🙂

12v car battery certainly can shock, but it depends on skin resistance, and typically that will be too high for the voltage. However skin condition and moisture (sweaty skin etc) can provide an easy path in. Internally resistance is much lower and there it becomes dangerous as the current that can be delivered is fairly high. It takes fairly small amounts of current to kill if it gets to the heart, but from outside the body it needs power to get through the skin to get there.

Domestic 240V ac might “only” be 100A supply, and it could easily kill. EDIT RCDs generally operate in 0.2 second at 30mA, faster than the heart can beat and below the 50mA danger threshold. Car battery might be 700A supply
ac shock effect is different (alternates) to dc (continuous)
Max voltage to be classified as extra-low is 50V ac, and 120V dc
55V ac ^^^ is a UK-classification RLV (Reduced Low Voltage)

Well, the one occasion I accidentally put a screwdriver across the terminals of a 12v car battery and got a blinding flash and a chunk blown out of one side of the screwdriver

Thats not you getting a shock though (except for maybe emotionally :lol:) I really don’t think that 12v has sufficient voltage ‘pressure’ to force many amps through non metallic materials. I’ve often had a battery box on a landrover full of muddy water and it’s all continued to work perfectly, not shorting or rapidly discharging. More than can be said for the rest of the truck…

I’m pretty sure you can’t get a shock off a 12v battery, no matter how hefty the thing.

I assure you its pretty easy to do this and rather dangerous- take a wire of the neg and off the + and watch the arc you get it can melt the metal so dont use yourself as the method to make the circuit. As noted it generally wont get over your resistance – dont try this with a spark plug lead its not 12 volt and dont go near the starter cable. though you can start your car by connecting across the terminals using a metal spanner – assuming you can get access- without shock – did this on the boat when my solenoid broke.

i was wet each time I got a shock so dont fix your boat electrics in the rain no matter how urgent you are

As for batteries a larger battery will have larger cranking amps – ie it can dump more power in a short burst than a smaller battery- hence why larger cars have bigger batteries as it needs more power to start a larger engine than smaller one.

Whether your vehicle will try to do this and whether it will be sufficient to fry anything I am less convinced as I think the battery will respond to demand rather than just dump all it can irrespective of load.

I have certainly started my car from a boat battery bank with 700 amp battery bank without issue but it had no electronics on the old diesel engine – which was the same type as on the boat anyway

You won’t get a shock from 12v battery. Your internal resistance is too high. Most you’ll get is a slight tingle.

This is from experience. The spark when a battery shorts is irrelevant; you’re comparing massive impedance with zero impedance.