How does a battery act to suppress voltage ripple on a recified DC supply?
How does a battery act to suppress voltage ripple on a recified DC supply?
(OP)
Hello everyone.
First of all, my apologies if this is in the wrong forum and for the perhaps silly question (I have no background in electrical science/engineering as you'll see below)
Following on from friend having their vehicle ECM damaged after an autotechnician decided to disconnect the car battery whilst the engine was running, I was asked why this happened and I have absolutely no idea. I've done a bit of research based on the somewhat limited information out there, but it has opened up a whole new subject matter that I'm keen to (try and) understand.
Firstly, why would a supply voltage from the alternator change sufficiently when the battery is disconnected to damage electronic components?
Secondly, I'm interested in the technical principles/process behind how a lead-acid battery suppresses voltage ripples in the DC supply. Is the ability to suppress the voltage determined by a particular characteristic such internal resistance or capacitance? Does this characteristic change over time?
Thank you in advanced.
Adam
First of all, my apologies if this is in the wrong forum and for the perhaps silly question (I have no background in electrical science/engineering as you'll see below)
Following on from friend having their vehicle ECM damaged after an autotechnician decided to disconnect the car battery whilst the engine was running, I was asked why this happened and I have absolutely no idea. I've done a bit of research based on the somewhat limited information out there, but it has opened up a whole new subject matter that I'm keen to (try and) understand.
Firstly, why would a supply voltage from the alternator change sufficiently when the battery is disconnected to damage electronic components?
Secondly, I'm interested in the technical principles/process behind how a lead-acid battery suppresses voltage ripples in the DC supply. Is the ability to suppress the voltage determined by a particular characteristic such internal resistance or capacitance? Does this characteristic change over time?
Thank you in advanced.
Adam





RE: How does a battery act to suppress voltage ripple on a recified DC supply?
Or, if the battery was charging at the time, then perhaps a load dump - and in the worst case of also not having the battery in parallel.
Standby for others to chime in. They may have better info.
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
In terms of your battery question: You can think of a battery as a really large capacitor with a higher than normal series resistance which varies based on the SOC (State of Charge) of the battery. So like a capacitor it integrates the current reducing the ripple greatly.
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
Keith Cress
kcress - http://www.flaminsystems.com
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
Dan - Owner
http://www.Hi-TecDesigns.com
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
Z
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
Well looking it up again, currently the highest expected voltage for load-dump is 101V for 12V systems and 202V for 24V systems. I've always designed for the 200V, after reading 200V in a semiconductor appnote, and never had a product failure from load-dump.
Keith Cress
kcress - http://www.flaminsystems.com
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
Thank you for the responses. Unfortunately I only have my friend's non-technically minded word to go off regarding the lead up to the damage of the ECM so I can't say for sure that nothing else was interfered with on the vehicle prior to, or after, the ECM met it's demise. The vehicle was operating a 12 V system (Volvo V50 circa 2008 model - UK).
From the information posted, I'm quite surprised by just how high peak voltages can get under those circumstances.
Regarding the 'It was a load dump as already described, nothing to do with 'filtering' at all.'. That was my mistake for not clarifying, I was rolling two separate questions into one thread. I had tried to research what was happening prior to my post here but found most of searches were redirecting to the ripple suppression by the battery - which I found more interesting to be honest.
Thanks again,
Adam
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
If the battery happened to have been fully charged, with not much more than trickle charge current (20 mA?) going into it at the point when it was disconnected, would that still cause a load dump?
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
IRStuff,
It appears we are both a little bit off in our numbers, I've found an ST app note which states a typical load dump is around 80-100V peak I suspect that this number is going to vary greatly depending on the alternator, cable length, etc.
ST Application Note
There is also this link for a paper by an engineer in England which provides the numbers from ISO7637 (I don't have access to the standard so I can't verify it) which states for 12V systems the range is 65V to 87V and 123V to 174V for a 24V system
T.P. Jarvis Paper
I hope this helps, I know I learned a bit!
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
Dan - Owner
http://www.Hi-TecDesigns.com
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
A couple of points:
1) You realize that automotive alternators are three phase output right? That means the ripple is a very small fraction of what single phase generation would produce. Hence not a 'lot' of filtering is really needed.
2) The frequency of the aforementioned 3 phase power is crazy high compared to dawdling 60Hz from your local outlet. I think the typical gear ratio is something like 4 to one so even at idle the alternator is spinning at something like 3000 rpm and it just goes up from there.
3) The battery acts like a large slow capacitor, being peak charged by the peaks coming by on each cycle. This is all moderated thru the battery's internal resistance which is low on a car battery.
Keith Cress
kcress - http://www.flaminsystems.com
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
I've got a few scenarios I'm playing out in my head with this new insight now, but I'm struggling to decide the outcome. Please correct me on any of the following...
Let's say a vehicle battery has a such a low SoC that the starter motor won't even turn over (maybe even has a bit of permanent sulfation). As I understand, the internal resistance of the battery is relatively high when in this state. In the case of removing the battery from the vehicle during engine running, we go from a very low internal resistance device (battery) in parallel in the circuit to an infinitely high resistance 'device'. This rapid change in resistance is responsible for the load dumping event... is this correct?
If this is the case then a similar scenario would be played out but with less extreme values during jump-starting(?). I can't remember the last time I jump-started one of my cars, but this thread has got me very, very slightly nervous now!
Can a fast 'switch' to a low SoC battery in the scenarios described below cause load-dumps (or increased ripple? Since it's effectiveness of ripple suppression/rejection is affected by the internal resistance)?
1) What risk is there to automotive electronics when disconnecting jump leads following a jump-start? Will there be a notable peak voltage? Will there be increased ripple initially until the charge of the battery increases?
2) Alternatively what if a vehicle is jump-started by a standalone spare battery? Once the engine has been started and the leads are removed, would there be a load dump/spike/increased ripple in this circumstance? I'm guessing it would be similar to the above?
In both cases, is this why it is recommended to continue being connected to the donor vehicle post-jump start for a few minutes?
Hoping I haven't out-stayed my welcome with the numerous questions!
Regards,
Adam
RE: How does a battery act to suppress voltage ripple on a recified DC supply?
http://www.st.com/web/en/catalog/sense_power/FM114...