Monday, May 31, 2010 at 10:29
Hello ChipPunk,
it's good to see you try and get a handle on these things.
You still seem to struggle a bit though when it comes to the 'front end'.
Never mind, I'll give you some fresh ideas.
The task at hand is to get a quick indication of a shorted cell in a parallel battery configuration.
The fault may develop during charging or discharging and needs timely (within 100 hours or so) action by the OP to prevent further damage.
Looking at this failure mode, you quickly notice that the battery with the collapsed cell will draw significant current from the other battery.
So all you have to do is finding a way to monitor the inter-battery-current.
For this, remember what a Wheatstone bridge can do for you.
Then, look at the parallel configuration of the batteries with their associated line fuses which effectively form a bridge circuit.
Assuming equal voltage/current (thus resistance) balance of the batteries, and equal resistance of the (equally rated) line fuses, the bridge will be balanced. But if one battery loses a cell, the bridge will exhibit the character of an unbalanced 'quarter bridge'. The manifestation of an unbalanced bridge is of course the voltage it'll now develop.
So all you have to do is wire a voltmeter across the battery positives and you have a quick'n dirty early warning system for cell collapse.
Hope to have answered your many questions about how to go about all this?
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Best regards, Peter
If you realise that the battery configuration with the
No problem with alarm systems - I've had access to them for years - the usual (minimum) 12-bit voltage monitors (for series strings), zero-impedance current montoring, and 1kHz or DSP impedance devices.
>>>>>>>>>>>>>Good to see you seem to have a handle on these things.>>>>>>>>>>>>almost: to cut down on complexity, I suggest you monitor the total charging current (for the entire parallel configuration), and the voltage. If one cell dies, the total current will rise and stay up. The voltage reading will also be somewhat lower than normal due to small voltage drops in alternator output and in the wiring.
If you don't mind the added complexity it's certainly helpful (as you suggested above) to have the number of ammeters equalling the number of batteries in the parallel configuration. This way you can pinpoint any problems without much mucking around.
As an added bonus, individual ammeters provide an early indication/warning of developing battery faults because now you can do quick comparison checks at a glance.>>>>>>>>>>>>so now we are talking about manifestation of a dead cell during discharging (in contrast to the different indications during charging as described above):
If the failure develops during charging, and the OP won't notice it at this stage, then it'll become very obvious that something is wrong the moment charging stops. Instead of the batteries maintainingg their usual high end-of-charge-voltage for some time (load current dependent), the voltage will now initially drop rapidly and slowly begin to bottom out.
If it was for a mission critical application, I suggest to hard wire a voltmeter between each battery positive.
This would reveal any battery to battery currents by utilising the fuse resistances to become the shunt resistance for the voltmeter (hence the voltmeter then acts as a quick'n dirty inter-battery-ammeter):
Or touch each battery and compare temperatures (in this case at least - not necessarily with an engine-bay & a boot battery etc).
Is that it in a nutshell?
(Specific to this installation - ie, two "matched" batteries in similar environment (not engine bay & boot or caravan etc) and (hence) with matched interconnection - ie, diagonal load tapping.)
Luckily I avoid all those issues since my batteries are only paralleled whilst charging - hence one doesn't effect the other.
I don't even have to worry about diagonal and matched interconnections.
If one battery is really bad, I will notice that on my dash voltmeter as you say (formerly a dash mounted
ARB-Sidewider LCD unit; currently a blue-LED meter - both being connected across the main battery's terminals).
Otherwise I detect problems through other means (low voltage trip to the 2nd battery's loads; low voltage off the main battery (during cranking) etc.
I can parallel them if I want to - eg, for cranking or winching purposes.
I have no need for ammeters any of those situations - the voltmeter tells me all. (That's despite it being an old vehicle normally fitted with an ammeter. Newer vehicles have voltmeters instead, but like most oldies - I swapped my ammeter for a voltmeter. Piece of mind, and never looked back. And have avoided a few strandings!)
I do have a good charger - my alternator.
I'm happy with how it charges batteries. I think I mentioned how I considered 3 years ok for normal batteries but that I usually got more - and that was with my older "external regulator" alternators.
I only upgraded to newer integrated-regulator alternators (from the 1980s) about 2 years ago. My normal battery(s) are both 8 years old with lots of abuse - some overcharging; several flattenings (main beams on all day), etc.
I only recently replaced it/them with an old AGM.
My batteries charge at the maximum rate they and my system can handle - namely about 14.4V with no current limit.
I have yet to see the voltmeter dip below 14.4V after initial start-up with only the normal ignition etc and batteries connected.
(The battery
isolator Amperage exceeds my alternator's output.)
Although I see the advantage to a true battery charging system in a vehicle, even for those that stay at 14.4V all day without dropping to 13.8V I think batteries last long enough (except for Fords with mid engine-bay mounted batteries LOL!)
I practice, IMO the desirable ~0.2V change over a ~50°C range seems not to make a big difference (especially if the common Bosch 14.2V setting etc).
Nevertheless, I see the merits for "real" chargers with independent remote monitoring of voltage and temperature. I just don't think many would consider it worthwhile - it's a bit like using distilled water on newer flooded cells - great for longetivity, but not economically worthwhile.
Thanks for clarifying that once a fault develops, the batteries are no longer matched, and also for giving an example as to how to determine whether a mismatch has occured.
I prefer to follow what IMO is typical expert advise and NOT keep batteries paralleled except when charging, and when required.
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