chrisp
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I have notice a few recent posts where there were problems being noticed after a flat battery and a jump start. As someone who has spent a lot of my working life in battery- and battery-charging-technologies, I thought that I’d share my views on the correct way to deal with a car with a flat battery.
I particularly wanted to share an explanation as to why ‘jump starting’ can sometimes (but not always) damage electronics in a car. If you understand how the damage occurs, you are in a much better position to prevent that damage occurring. Incorrect jump starting is a bit like playing Russian roulette, most of the time you will be fine, but sometimes things will turn out badly. I’ll try and leave you with a pretty foolproof guide on how to prevent such damage.
The role of the battery
In the battery world, a car battery is referred to as a ‘SLI battery’ - the ’SLI’ is an acronym for ‘Starting, Lighting and Ignition’ which is the three principal roles of a car battery. Most readers of this will have a good understanding of these three roles and everyone knows that a car with a flat battery won’t start.
However, there is another role for the battery which isn’t so evident, and it is to do with the recharging system. We know that the alternator recharges the battery, but what might not be so evident is that the battery is also part of the voltage regulation system. This is the reason that ‘jump starting’ can sometimes cause issues. I will explain the damage mechanism a little later.
Bootstrap - aka jump starting
A car with a flat battery is in a dilemma. It has a charging system that will recharge the flat battery but it needs the engine to be running to do the recharging. But we can’t start the engine with a flat battery. This is where jump starting can effectively ‘bootstrap’ the car in to recharging its own battery.
Jump starting - typical
Let’s run through the typical jump start that we all have done at some time and usually works out well. Firstly there are usually two cars and a set of jumper leads involved. There is a car with a flat battery and a car with a good battery. What we do is connect the two battery positives (+) to each other using the red lead, and the two battery negative (-) together using the black lead. Ideally, for safety, we’ll do the final connection away from the battery (and preferably on the flat car) as this connection may produce sparks. So far, no problem at all. We then usually start the car with the good battery (if it isn’t already running). I’d usually recommend letting the donor car idle for a few minutes as its alternator can effectively recharge the car with the flat battery. After a few minutes, attempt to start the car with the flat battery. Usually the (flat) car starts and everyone gives a sigh of relief. Crisis adverted as the car in trouble is now running. The usual instructions with jump leads are something like ’disconnect the jump leads in the reverse sequence’. Most times, everything is hunky dory and the two cars go on their merry way.
The fourth role of the battery
As mentioned earlier, the primary three purposes of the battery are SLI - Starting, Lighting, and Ignition. The alternator is set up to provide the energy that the car’s electrical system is using and to regulate the voltage. However, alternators are a little sluggish in how quickly they can adjust to changing loads on the electrical system. The battery is essentially an energy storage device that helps smooth out any short term differences between what the alternator is supplying and what the car needs. The battery is effectively acting as an ‘electrical flywheel’ in the electrical system. It effectively adds significant electrical inertial to the system and helps keep the rapid voltage fluctuations in check. This is what I’m referring to as the fourth role of the battery. The battery is also a filter (or inertia) for the electrical system.
Spikes
We have all heard of accounts or tales of cars being damaged by ‘spikes’ when jump starting. So, what are these spikes, and what causes them? And what has jump starting got to do with it? This all comes back to the charging system and the alternator-battery interaction. There are several different ways a battery might be ‘flat’. Firstly, it might have been discharged such as by leaving the headlights on when parked. It might be a weak battery on the way out, or the battery might have suddenly broken an internal connection (between any of the six internal 2V cells). All are known failure modes of a car battery. To illustrate how the spikes occur, let’s use the extreme failure mode where that battery has an open-circuit internal connection…
Jump starting - extreme case
Lets revisit a scenario where we are jump starting a car with a flat battery. But this time, and unbeknown to us, the battery has a broken inter-cell connection. So we do the usual connect up as outlined earlier. We wait a bit then start the car with the flat battery. It starts and we all give a sigh of relief. So far, so good, and everything is fine. Now we get to the part where the spikes - and damage - might occur. We disconnect the jump leads. The donor car (the car with the good battery) is fine. However, the car with the ‘flat’ but actually faulty battery is running efficiently without a battery. The ‘flat’ car’s charging system doesn’t have the ‘inertia’ of a battery so the alternator has trouble regulating the voltage.
But there is more. It’s all a bit hit-and-miss as to how much each of the two alternator were contributing to the recharging the good battery in the donor car. The worst case is if the ‘flat’ car is doing a significant part of the recharging of the donor car’s battery (maybe the donor car has a weak alternator?). When we disconnect the jump leads, the ‘flat’ car’s alternator suddenly has less load to work in to - as it’s battery is open-circuit. The flat car’s alternator takes a little time to readjust its output, but in the meantime the voltage goes high. This is referred to as a ‘load dump transient’ aka ‘voltage spike’. What damage this spike causes in the car will depend upon the size and duration of the spike, and how well the car’s electronics is designed to cope with such spikes. In an ideal world, the car will be fine.
(Image from: https://www.analog.com/en/technical...otection-for-24v-automotive-applications.html)
The important point to be aware of is that this situation is that it requires a sequence of events to occur - a ’flat’ battery that is really open-circuit, jump starting, an unfortunate combination of alternator regulator voltages, and some car electronics that is susceptible to a voltage spike, It certainly won’t happen every time. Indeed, it’ll only happen very occasionally and it might seem to be as if it is random.
Also, if a battery is really flat (like it has been sitting around unused for many months/years) it also goes in to a high-impedance state. Ironically, a very flat battery is actually very hard to recharge. This is because it exhibits a very high resistance. Such a battery isn’t strictly open-circuit, but it isn’t very effective at providing damping the car’s electrical system.
How to safely jump start
There are two methods I can recommend for starting a car with a flat battery.
Firstly - don’t jump start if you have alternative means at hand. The best way is to recharge the battery with a (bench) battery charger. This is often quicker and more practical than it initially sounds. To start a car takes about 400A for about 5 seconds - this is about 0.5 Ah. So using a very basic 4A battery charger, it would only take 7.5 minutes to provide enough charge to start the car. Put the battery on a battery charger, have a leisurely cup of coffee (15 minutes), and you are probably good to go. No need for the jump leads at all.
If you really have little option other than use jump leads, remember how the (remote, but possible) damage occurs. It‘s happens when disconnecting a car with a faulty battery while that car is running. So, do the usual jump starting bit, but don’t disconnect the jump leads while the car is running. Leave the leads connected and let the cars run for a few minutes. Stop both cars (or at very least, stop the ‘flat’ car). Disconnect the leads and try and restart the ‘flat’ car using only its own battery. If it restarts, the battery is healthy enough to work as a filter for the alternator. (Note… the restarting is effectively a ‘load test‘ of that battery). If it doesn’t restart - and especially if it seems completely dead - don’t redo the jump start. The battery needs replacing.
Simple guidelines
The above is probably all too much to remember, but two simple rules are (a) don’t disconnect the jump leads while the (flat) car is running, and (b) as long as the ‘flat‘ car can restart under it’s own battery (sans jump leads), the battery is sufficiently good enough for the recharging system (but it still might go flat again).
Final notes
(A) There are jumper leads with surge protectors. These won’t usually help all that much as it is the disconnecting of the leads that prompts the load-dump spike.
(B) there are some small portable ‘jump packs’ on the market. These are a bit safer as they are usually diode protected, so the flat car usually won’t be able to recharge them - so they shouldn’t cause the load-dump spike. However, in the case of an open-circuit battery the alternator won’t have a battery to help quell any load transients.
(C) modern cars should be able to tolerate some voltage spikes without any problems, or at least without permanent damage. However, cars are complex, and sometimes things aren’t designed as well as they could have been.
So to be safe, I would suggest still following the ‘simple guidelines above’.
Also, I haven’t really gone in to the safety considerations, nor have I gone in to possible faults with the charging system. My main intention was to provide some insights in to how to prevent damage from jump starting. After all, we do need to look after our beloved Commodores and all their electronic modules.
I particularly wanted to share an explanation as to why ‘jump starting’ can sometimes (but not always) damage electronics in a car. If you understand how the damage occurs, you are in a much better position to prevent that damage occurring. Incorrect jump starting is a bit like playing Russian roulette, most of the time you will be fine, but sometimes things will turn out badly. I’ll try and leave you with a pretty foolproof guide on how to prevent such damage.
The role of the battery
In the battery world, a car battery is referred to as a ‘SLI battery’ - the ’SLI’ is an acronym for ‘Starting, Lighting and Ignition’ which is the three principal roles of a car battery. Most readers of this will have a good understanding of these three roles and everyone knows that a car with a flat battery won’t start.
However, there is another role for the battery which isn’t so evident, and it is to do with the recharging system. We know that the alternator recharges the battery, but what might not be so evident is that the battery is also part of the voltage regulation system. This is the reason that ‘jump starting’ can sometimes cause issues. I will explain the damage mechanism a little later.
Bootstrap - aka jump starting
A car with a flat battery is in a dilemma. It has a charging system that will recharge the flat battery but it needs the engine to be running to do the recharging. But we can’t start the engine with a flat battery. This is where jump starting can effectively ‘bootstrap’ the car in to recharging its own battery.
Jump starting - typical
Let’s run through the typical jump start that we all have done at some time and usually works out well. Firstly there are usually two cars and a set of jumper leads involved. There is a car with a flat battery and a car with a good battery. What we do is connect the two battery positives (+) to each other using the red lead, and the two battery negative (-) together using the black lead. Ideally, for safety, we’ll do the final connection away from the battery (and preferably on the flat car) as this connection may produce sparks. So far, no problem at all. We then usually start the car with the good battery (if it isn’t already running). I’d usually recommend letting the donor car idle for a few minutes as its alternator can effectively recharge the car with the flat battery. After a few minutes, attempt to start the car with the flat battery. Usually the (flat) car starts and everyone gives a sigh of relief. Crisis adverted as the car in trouble is now running. The usual instructions with jump leads are something like ’disconnect the jump leads in the reverse sequence’. Most times, everything is hunky dory and the two cars go on their merry way.
The fourth role of the battery
As mentioned earlier, the primary three purposes of the battery are SLI - Starting, Lighting, and Ignition. The alternator is set up to provide the energy that the car’s electrical system is using and to regulate the voltage. However, alternators are a little sluggish in how quickly they can adjust to changing loads on the electrical system. The battery is essentially an energy storage device that helps smooth out any short term differences between what the alternator is supplying and what the car needs. The battery is effectively acting as an ‘electrical flywheel’ in the electrical system. It effectively adds significant electrical inertial to the system and helps keep the rapid voltage fluctuations in check. This is what I’m referring to as the fourth role of the battery. The battery is also a filter (or inertia) for the electrical system.
Spikes
We have all heard of accounts or tales of cars being damaged by ‘spikes’ when jump starting. So, what are these spikes, and what causes them? And what has jump starting got to do with it? This all comes back to the charging system and the alternator-battery interaction. There are several different ways a battery might be ‘flat’. Firstly, it might have been discharged such as by leaving the headlights on when parked. It might be a weak battery on the way out, or the battery might have suddenly broken an internal connection (between any of the six internal 2V cells). All are known failure modes of a car battery. To illustrate how the spikes occur, let’s use the extreme failure mode where that battery has an open-circuit internal connection…
Jump starting - extreme case
Lets revisit a scenario where we are jump starting a car with a flat battery. But this time, and unbeknown to us, the battery has a broken inter-cell connection. So we do the usual connect up as outlined earlier. We wait a bit then start the car with the flat battery. It starts and we all give a sigh of relief. So far, so good, and everything is fine. Now we get to the part where the spikes - and damage - might occur. We disconnect the jump leads. The donor car (the car with the good battery) is fine. However, the car with the ‘flat’ but actually faulty battery is running efficiently without a battery. The ‘flat’ car’s charging system doesn’t have the ‘inertia’ of a battery so the alternator has trouble regulating the voltage.
But there is more. It’s all a bit hit-and-miss as to how much each of the two alternator were contributing to the recharging the good battery in the donor car. The worst case is if the ‘flat’ car is doing a significant part of the recharging of the donor car’s battery (maybe the donor car has a weak alternator?). When we disconnect the jump leads, the ‘flat’ car’s alternator suddenly has less load to work in to - as it’s battery is open-circuit. The flat car’s alternator takes a little time to readjust its output, but in the meantime the voltage goes high. This is referred to as a ‘load dump transient’ aka ‘voltage spike’. What damage this spike causes in the car will depend upon the size and duration of the spike, and how well the car’s electronics is designed to cope with such spikes. In an ideal world, the car will be fine.
(Image from: https://www.analog.com/en/technical...otection-for-24v-automotive-applications.html)
The important point to be aware of is that this situation is that it requires a sequence of events to occur - a ’flat’ battery that is really open-circuit, jump starting, an unfortunate combination of alternator regulator voltages, and some car electronics that is susceptible to a voltage spike, It certainly won’t happen every time. Indeed, it’ll only happen very occasionally and it might seem to be as if it is random.
Also, if a battery is really flat (like it has been sitting around unused for many months/years) it also goes in to a high-impedance state. Ironically, a very flat battery is actually very hard to recharge. This is because it exhibits a very high resistance. Such a battery isn’t strictly open-circuit, but it isn’t very effective at providing damping the car’s electrical system.
How to safely jump start
There are two methods I can recommend for starting a car with a flat battery.
Firstly - don’t jump start if you have alternative means at hand. The best way is to recharge the battery with a (bench) battery charger. This is often quicker and more practical than it initially sounds. To start a car takes about 400A for about 5 seconds - this is about 0.5 Ah. So using a very basic 4A battery charger, it would only take 7.5 minutes to provide enough charge to start the car. Put the battery on a battery charger, have a leisurely cup of coffee (15 minutes), and you are probably good to go. No need for the jump leads at all.
If you really have little option other than use jump leads, remember how the (remote, but possible) damage occurs. It‘s happens when disconnecting a car with a faulty battery while that car is running. So, do the usual jump starting bit, but don’t disconnect the jump leads while the car is running. Leave the leads connected and let the cars run for a few minutes. Stop both cars (or at very least, stop the ‘flat’ car). Disconnect the leads and try and restart the ‘flat’ car using only its own battery. If it restarts, the battery is healthy enough to work as a filter for the alternator. (Note… the restarting is effectively a ‘load test‘ of that battery). If it doesn’t restart - and especially if it seems completely dead - don’t redo the jump start. The battery needs replacing.
Simple guidelines
The above is probably all too much to remember, but two simple rules are (a) don’t disconnect the jump leads while the (flat) car is running, and (b) as long as the ‘flat‘ car can restart under it’s own battery (sans jump leads), the battery is sufficiently good enough for the recharging system (but it still might go flat again).
Final notes
(A) There are jumper leads with surge protectors. These won’t usually help all that much as it is the disconnecting of the leads that prompts the load-dump spike.
(B) there are some small portable ‘jump packs’ on the market. These are a bit safer as they are usually diode protected, so the flat car usually won’t be able to recharge them - so they shouldn’t cause the load-dump spike. However, in the case of an open-circuit battery the alternator won’t have a battery to help quell any load transients.
(C) modern cars should be able to tolerate some voltage spikes without any problems, or at least without permanent damage. However, cars are complex, and sometimes things aren’t designed as well as they could have been.
So to be safe, I would suggest still following the ‘simple guidelines above’.
Also, I haven’t really gone in to the safety considerations, nor have I gone in to possible faults with the charging system. My main intention was to provide some insights in to how to prevent damage from jump starting. After all, we do need to look after our beloved Commodores and all their electronic modules.
