Connection of new batteries to house bank - best method to avoid current surge?

SteveSarabande said:
It is only the same as connecting jump leads to a car with a flat battery.
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Instructions for using jump-leads between two cars always tell you to connect the positive terminals first and then the negatives. I've never understood what difference it makes whether you connect the positives or negatives first. Can anyone elucidate - or is it old baloney?
 
SteveSarabande said:
It is only the same as connecting jump leads to a car with a flat battery.
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PaulRainbow said:
Exactly !
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And when doing that there is a recommended procedure/ sequence to follow, The most important part of which is that the final connection to be made ( and the first to be broken) is a negative connection to a suitable point that is away from the battery. The object being to avoid a spark on a battery terminal which could ignite any hydrogen.

The chances of that happening are very small but it can and does happen. The consequences of a battery exploding in one's face are potentially so severe that the risk is not worth taking!
 
MonniotC said:
Instructions for using jump-leads between two cars always tell you to connect the positive terminals first and then the negatives. I've never understood what difference it makes whether you connect the positives or negatives first. Can anyone elucidate - or is it old baloney?
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If you connect the negative first, both cars bodies become part of the circuit. If you then touch the jump positive on the car you are about to connect you will get big sparks! Doing it to he other way round prevents this problem.
 
VicS said:
And when doing that there is a recommended procedure/ sequence to follow, The most important part of which is that the final connection to be made ( and the first to be broken) is a negative connection to a suitable point that is away from the battery. The object being to avoid a spark on a battery terminal which could ignite any hydrogen.

The chances of that happening are very small but it can and does happen. The consequences of a battery exploding in one's face are potentially so severe that the risk is not worth taking!
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Hydrogen is only released when batteries are being charged. Since the OP is installing additional batteries why would he be charging at the same time? He has already stated that the new batteries are charged and the existing batteries are somewhat less charged and presumably the existing battery box has no residual hydrogen lurking awaiting to explode on you! Since hydrogen is lighter than air it is unlikely.
 
VicS said:
And when doing that there is a recommended procedure/ sequence to follow, The most important part of which is that the final connection to be made ( and the first to be broken) is a negative connection to a suitable point that is away from the battery. The object being to avoid a spark on a battery terminal which could ignite any hydrogen.
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Not quite. When connecting jump leads the point of connecting the negative last (disconnecting first) is to prevent accidental short circuits, as described by pyrojames. There will be no hydrogen present at either vehicle.

The chances of that happening are very small but it can and does happen. The consequences of a battery exploding in one's face are potentially so severe that the risk is not worth taking!
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In a jump start scenario there is no hydrogen present. In the OPs case, there is no hydrogen present.

Typical issues with exploding car batteries are when a fast charger is disconnected without turning it off. A spark can occur when the lead is removed, which will ignite the hydrogen produced by charging the battery (especially if it is overcharged, or very fast charged). A car battery is mostly in a reasonably well vented place (the engine bay), and most modern batteries are "sealed", so the chance of an explosion may not be great. Back in the days of open cell batteries, when we used to charge them with the tops off, the danger was very real. You could smell the gases and a spark from a live charger cable would ignite them, resulting in a serious explosion, sufficient to destroy the battery and create a nice shower of acid.
 
My Jaguar battery is in an enclosure adjacent to and within the boot and with no ventilation facility. Seems to be fine ...it's 14 years old now and never been charged from an external source and I've never topped it up and I've never heard any explosions.
 
Why do you think think they use big ugly 40mm sq cables on good Jump Leads ! Its the high current and resulting heat generated in the cables.

The current flow when the two batteries are connected will follow Ohms law as someone stated earlier.

I=V/R The current will be equal V/R. (In this case V= the potential difference between the two Banks Say 3 Volts if the existing bank is well discharged. )
But the only (R) resistance in the circuit is the internal battery resistance which is very low by design.
So you still have the possibility of very high current and a lot of power transferring.
Will it cause damage? probably not.

Why not reduce the risk by charging up up the existing bank in advance?

Kinsale 373
 
Kinsale373 said:
1. Why do you think think they use big ugly 40mm sq cables on good Jump Leads ! Its the high current and resulting heat generated in the cables.

The current flow when the two batteries are connected will follow Ohms law as someone stated earlier.

I=V/R The current will be equal V/R. (In this case V= the potential difference between the two Banks Say 3 Volts if the existing bank is well discharged. )
But the only (R) resistance in the circuit is the internal battery resistance which is very low by design.
So you still have the possibility of very high current and a lot of power transferring.
Will it cause damage? probably not.

Why not reduce the risk by charging up up the existing bank in advance?

Kinsale 373
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Do you think that perhaps you need fat jump-lead cables on the off-chance you want them to turn a starter motor and starters use lots of amps? But if you want to caution the OP not to install his new batteries whilst simultaneously trying to start his engine, do feel free. Personally, I doubt he's that stupid. Or has enough hands.

The someone who mentioned Ohm's law earlier was me. And you're misapplying it. The 'V' is the difference in potential between the new batteries and the old. In other words, probably less than 1V, as I also mentioned before.

How do you reduce a risk that doesn't exist?
 
Kinsale373 said:
Why do you think think they use big ugly 40mm sq cables on good Jump Leads ! Its the high current and resulting heat generated in the cables.

The current flow when the two batteries are connected will follow Ohms law as someone stated earlier.

I=V/R The current will be equal V/R. (In this case V= the potential difference between the two Banks Say 3 Volts if the existing bank is well discharged. )
But the only (R) resistance in the circuit is the internal battery resistance which is very low by design.
So you still have the possibility of very high current and a lot of power transferring.
Will it cause damage? probably not.

Why not reduce the risk by charging up up the existing bank in advance?

Kinsale 373
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I had always assumed that the cables were sized to take the high current draw when the starter motor was engaged on the vehicle with the flattened battery. Current flow when making the connection between the batteries always appears to be minimal.

I often leave some boat batteries on charge at home and have an older one on the other side of the garage. I don't lug it over to do the odd top up, just connect with a cable which happens to have an in-line 10A fuse. The fuse has never blown when connecting the old flat battery to a charged one prior to turning on the charger. I'm not really concerned about the old battery but it does light an LED if there's a power cut and gives enough light to start a generator.
 
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As a teenager I went to collect a freshly charged battery from a quite open space in a workshop & knocked a metal ladle off of a shelf which hit the terminals. The spark ignited some hydrogen & the battery exploded in my face as I leaned over it to pick it up. Only the smart action of my mother grabbing a screaming teenager & shoving my face in a washing up bowl of water, then under a running tap saved my sight. I spent several days with my eyes bandaged before the hospital could remove them. The skin on my face was scarred for some time.

I would suggest that the advice to charge prior to connection should be taken with care. Certainly I now wear goggles when playing with such things.

I have also managed to weld my metal watch strap to the starter, which gave the wrist quite a burn as well!!!!!
 
If I may digress just a little. I replaced my two 120amp hr house batteries last year.

I think by far the biggest risk is carrying the damm things across a car park, down a ramp and along the pontoon. Nearly pulled my arms out. I will not even mention getting them on board and down into the box in the saloon then lugging the old ones back to the car :-)
 
Rhoda Rose said:
If I may digress just a little. I replaced my two 120amp hr house batteries last year.

I think by far the biggest risk is carrying the damm things across a car park, down a ramp and along the pontoon. Nearly pulled my arms out. I will not even mention getting them on board and down into the box in the saloon then lugging the old ones back to the car :-)
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You are not wrong there.
The electrical risks are small, but can be reduced to zero.
There's no Health and Safety at Play Act, so amateurs can take what risks they like.
 
MonniotC said:
Instructions for using jump-leads between two cars always tell you to connect the positive terminals first and then the negatives. I've never understood what difference it makes whether you connect the positives or negatives first. Can anyone elucidate - or is it old baloney?
Click to expand...

It's like disconnecting the ground connection first; the chances of a spectacular short circuit are reduced. If you connected the grounds first then dropping the live as you approached the second battery would be dangerous because touching any part of the second car's bodywork would close the circuit. If you connect the lives first then you have to hit the live terminal on the second battery if you drop the ground jump lead, which is a much smaller target.
 
Kinsale373 said:
Why do you think think they use big ugly 40mm sq cables on good Jump Leads ! Its the high current and resulting heat generated in the cables.
Click to expand...

Yeah, but that high current is to run the starter. You rarely get more than a small fat spark when connecting up.

The current flow when the two batteries are connected will follow Ohms law as someone stated earlier.
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Only up to a point. The internal resistance of a discharging lead-acid battery is very low but the effective internal resistance of a charging one is much higher. Or, to put it another way, voltage in the battery being charged builds very rapidly. Ohm's Law doesn't apply to non-linear circuits.

Daydream believer said:
As a teenager I went to collect a freshly charged battery from a quite open space in a workshop & knocked a metal ladle off of a shelf which hit the terminals. The spark ignited some hydrogen & the battery exploded in my face as I leaned over it to pick it up.
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Are you sure that it was hydrogen already present which caused the explosion and not the short circuited battery? VicS is the man for this, but I think that a shorted battery produces hydrogen.
 
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