Battery Powered System, Overvoltage Possible!?
Battery Powered System, Overvoltage Possible!?
(OP)
Hey everyone, first of all --- great website, long time lurker, first time poster. Thanks to all who contribute.
I have a National Instruments data acquisition device and a 900Mhz serial modem being powered by two 12V gel batteries in series. The DAQ and RF modem are in a NEMA rated enclosure and the power is brought in with a marine grade 3 prong plug/receptacle. There is a switch between the receptacle and the devices themselves. The issue I have been having is the RF modem has been intermittently failing. Inspection of the devices shows no evidence of physical damage or burned smell. The failures do not occur at any specific time (ie during battery change) or seem to follow any pattern. The OEM inspection of the failed device claims overvoltage destroyed the device. The device accepts a range of 7-28VDC, two 12V batteries at full charge should check in at around 25.5V. There has never been a failure immediately following a battery change which would stand out to me as the only way to potentially ever have an overvoltage condition but still unlikely. The DAQ accepts 11-30V and has never failed. The application has substantial intermittent impact and vibration. Both devices receive power in similar way with the ground at the receptacle and the positive coming off the fuse block. The RF device is a typical size N radio plug for power while the DAQ is a screw type terminal. What could possibly be happening to cause an overvoltage condition for that device? Is it purely happenstance that the DAQ has never failed in that same manner or perhaps the circuitry inside the device has more protection for such conditions? Is it a bad assumption to think that the voltage of a gel battery is stable even under harsh conditions? My plan at this point is to power the system via 12V to give the device a bit more of a cushion but I'm not answering the true question nor do I think that is really the cause. Perhaps you electrical gurus can help a mechanical guy stumble through this?
I have a National Instruments data acquisition device and a 900Mhz serial modem being powered by two 12V gel batteries in series. The DAQ and RF modem are in a NEMA rated enclosure and the power is brought in with a marine grade 3 prong plug/receptacle. There is a switch between the receptacle and the devices themselves. The issue I have been having is the RF modem has been intermittently failing. Inspection of the devices shows no evidence of physical damage or burned smell. The failures do not occur at any specific time (ie during battery change) or seem to follow any pattern. The OEM inspection of the failed device claims overvoltage destroyed the device. The device accepts a range of 7-28VDC, two 12V batteries at full charge should check in at around 25.5V. There has never been a failure immediately following a battery change which would stand out to me as the only way to potentially ever have an overvoltage condition but still unlikely. The DAQ accepts 11-30V and has never failed. The application has substantial intermittent impact and vibration. Both devices receive power in similar way with the ground at the receptacle and the positive coming off the fuse block. The RF device is a typical size N radio plug for power while the DAQ is a screw type terminal. What could possibly be happening to cause an overvoltage condition for that device? Is it purely happenstance that the DAQ has never failed in that same manner or perhaps the circuitry inside the device has more protection for such conditions? Is it a bad assumption to think that the voltage of a gel battery is stable even under harsh conditions? My plan at this point is to power the system via 12V to give the device a bit more of a cushion but I'm not answering the true question nor do I think that is really the cause. Perhaps you electrical gurus can help a mechanical guy stumble through this?





RE: Battery Powered System, Overvoltage Possible!?
Voltage induce failures can occur long after the fault condition has left. Most voltage failures are from a high voltage breaking down a trace, kind of like chiseling on a column that is holding up the ceiling; the chiseler may be long gone before the house collapses.
John D
RE: Battery Powered System, Overvoltage Possible!?
So a circuit is supplied more voltage than its designed for causing higher than expected current in a trace --- heat builds up, physically deforms the trace which causes higher than designed resistance resulting in further deformation overtime under original design conditions that ultimately leads to failure. Is my understanding of that condition reasonably accurate? I feel like I would be able to see a "smoking gun" if this were the case. Granted the device is on a PCB and there probably is multiple layers which may make it impossible to see physical damage.
RE: Battery Powered System, Overvoltage Possible!?
Do you actually remove the batteries from the circuit to charge? For gel cells, each battery would go up to 14.1 volts on a charger that properly limited the volts. Less sophisticated bulk chargers could be much higher. I repair a lot of stuff and seldom is there any physical sign of failure.
Maybe the manufacturer is pushing his spec a little. The modem shouldn't draw that much. A 9-12V zener in series with the modem power would get it to a lower end of the voltage range. An easy thing you could try.
RE: Battery Powered System, Overvoltage Possible!?
RE: Battery Powered System, Overvoltage Possible!?
I'd really like to uncover the true underlying cause to the overvoltage in the first place.
RE: Battery Powered System, Overvoltage Possible!?
Permanently connected in parallel.
I wonder if it's possible for them to interact badly on their primary sides when the battery switch is opened.
If that's true/possible/plausible, a procedural change might be sufficient. I.e., never open 'the switch', and have two battery connections, so the charged battery is connected before the discharged one is removed. Okay, you might want to include diodes in the battery circuit so the charged battery doesn't try to charge the discharged one.
Mike Halloran
Pembroke Pines, FL, USA
RE: Battery Powered System, Overvoltage Possible!?
Use one "12 volt" (nominal) battery.
Two gets you too close to 28 volts when they're fully charged. And obviously there's not much margin on the 28 volts.
RE: Battery Powered System, Overvoltage Possible!?
Each device is connected in parallel to a pair of 12V batteries in series, thus 24VDC nominal. The procedure is to switch off the power, now the devices see nothing, change the batteries and reswitch the power on. At no time is there ever just one battery connected to the system and each device is always powered on/off simultaneously as they are connected to the same switch.
VE1BLL:
Agreed, in hindsight it was perhaps not prudent to push the specifications to the limit. My intention at this point is to try running the system at 12VDC and see if failures persist.
All:
Could someone familiar with PCB's explain to me the physical failure mode of a device that lets assume was designed to accept 28VDC max but got say 28.2VDC. This condition would only be possible right after battery change so it would only be for a short time as the voltage would reduce over time/discharge. As I've mentioned before there has never been any correlation between battery change and failure --- it is always some time after the change.
RE: Battery Powered System, Overvoltage Possible!?
No, they see each other.
Put a scope on that connection, and open the battery switch.
Mike Halloran
Pembroke Pines, FL, USA
RE: Battery Powered System, Overvoltage Possible!?
RE: Battery Powered System, Overvoltage Possible!?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Battery Powered System, Overvoltage Possible!?
RE: Battery Powered System, Overvoltage Possible!?
John D
RE: Battery Powered System, Overvoltage Possible!?
RE: Battery Powered System, Overvoltage Possible!?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Battery Powered System, Overvoltage Possible!?
This is just one possible explanation of many.
RE: Battery Powered System, Overvoltage Possible!?
Did the modem OEM give any more detail about the failure; what failed? what pin saw overvoltage? Did a device fail or a trace on a PCB (if the latter then they have a very poorly designed PCB)?
John D
RE: Battery Powered System, Overvoltage Possible!?
It's impossible to really make good suggestions without knowing details on the failure. You shouldn't see anything other that maybe some small transient over-voltages and if those devices were well built the power supply would handle these without failure. So, I would just proceed and put the 2 batteries in parallel instead of series.
RE: Battery Powered System, Overvoltage Possible!?
Imagine a large heavy steel box that is part of a larger frame. Inside this steel box are two batteries and a stainless steel enclosure. The batteries, as mentioned before, are connected in series --- three wires: one jumping from BAT1 positive to BAT2 negative, one from BAT2 positive to plug connector, one from BAT1 negative to plug connector. The recepticle, mounted in the stainless steel box, is a plastic body with no possible electrically conductive paths to the stainless box from there I have the positive switched which then runs to a fuse block and then onto the respective devices. The grounds pass directly from the plug to the devices. Each device is mounted on what amounts to plastic standoffs so they do not contact the enclosure either. I realize I have a floating ground but how could back current possibly be in the ground wires? The prototype of the system was in a plastic enclosure and ran for a year straight without issue --- so somehow I feel that I'm getting screwed electrically here but I can't see how. I'm probably not understanding some basic rule of EE design or something.
RE: Battery Powered System, Overvoltage Possible!?
RE: Battery Powered System, Overvoltage Possible!?
Another method sometimes used is a parallel diode (normally reversed biased) that will conduct heavily if the power is reversed and will then blow a series fuse to open the circuit.
...
Zero ohm resistors should not fail under moderate over-voltage, unless there's something funny going on. The funny business might be some other upstream component failing as a dead short due to the over-voltage, and that other failure then causing high currents that damage the zero ohm resistors. But you'd have to mention the other failure. Resistors (zero ohm or otherwise) aren't going to fail directly from application of a bit more than 28 VDC.
At this point it's all guess work. If you can find a schematic diagram for the RF Modem's PS circuit then we could probably identify the design limitation.
...Or simply back away from the 28 VDC upper voltage limit, thereby leaving more time for lunch/beer.
RE: Battery Powered System, Overvoltage Possible!?
The OEM recommended that I connect ground of battery connection to the frame as their theory is somehow potential is being built up in the frame and finding its way into my system --- I just don't see how this is possible and I have some reservations about doing that because then if during battery change somehow a tool were to bridge the frame and the positive we'll have fireworks.
The zero ohm resistor was described as .020" x .040" in size, what sort of voltage/current can these handle?
RE: Battery Powered System, Overvoltage Possible!?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Battery Powered System, Overvoltage Possible!?
That is commonly referred to as a 0402 resistor. The ERJ series from Panasonic is rated for 1A and has a max overload current of 2A.
John D
RE: Battery Powered System, Overvoltage Possible!?
Switching off the power abruptly might be causing one device to inductively kick the other into a momentary over voltage.
If the input voltage range is 7 -28 volts, why are the batteries being connected in series ?
I would put them in parallel. As the battery end voltage is going to be around 10v, that will make far better use of the total battery capacity. Especially if linear voltage regulators are being used. It would effectively double the amp hour capacity.
Another suggestion might be to connect a really large electrolytic capacitor across the dc line, between the on/off switch and your two devices.
When power is switched off, the voltage on the capacitor will die away gracefully, without any sudden vicious step changes in current. A very low value resistor in the battery line could limit inrush into the electrolytic. Another non violent way charge up the big cap, might be a three position on off switch, with a suitable starting resistor in the middle position.
Last suggestion would be to fit a simple pre regulator, but I still think the most efficient solution would be to put the batteries in parallel.
RE: Battery Powered System, Overvoltage Possible!?
Warp,
Could you, in layman's terms, briefly explain what you are asserting when power is switched? I do not understand this concept of momentary overvoltage via induction. This has me thinking though, when I switch the power off --- what happens in the circuit physically from a current standpoint?
I am switching 2 of the 4 systems to parallel and hence 12V to monitor performance/life versus the two.
RE: Battery Powered System, Overvoltage Possible!?
Now if there is only one piece of equipment with an inductor in the dc path, then the voltage spike will most likely all appear across the opening switch contact.
But if you then connect a second piece of equipment controlled by that same switch, then the sudden inductive voltage kick across the switch might be enough to cause a problem.
The worst possible case would be one heavy load having a lot of series inductance, and a weak but voltage sensitive second load connected together to the same switch.
As the total load current is broken, the weak load could possibly receive a fair kick in the guts from the resultant inductive overvoltage spike.
RE: Battery Powered System, Overvoltage Possible!?
The load current per battery will be half, but at the halved discharge rate the total amp hours available to the voltage end point will more than double.
An eleven volt end point is fairly high, but because of the "S" shape of the voltage discharge curve, and it's more than doubling in width, I believe you should still gain significantly in available total equipment running hours per charge.
RE: Battery Powered System, Overvoltage Possible!?
http://
This is the TVS diode I have preliminarily selected, I was going to go with the a reverse standoff voltage of 15V. The application notes that this is for data lines...which mine is not --- what am I missing here? The specifications of the device looks like it would work nicely in my application.
I'm sort of annoyed these expensive devices don't have such protections already built in --- now I have to worry about more solder joints and durability of construction of what I make. /grumble
RE: Battery Powered System, Overvoltage Possible!?
RE: Battery Powered System, Overvoltage Possible!?
You'll be better off with a reverse-polarity-protection diode wired between the +12V supply and the load, not between the load and ground. The diode forward voltage drop could introduce enough ground shift to mess up your logic levels, and introduce ground bounce problems as the current changes (Tx to Rx, for example).
John D
RE: Battery Powered System, Overvoltage Possible!?
Thanks for the tip. So could go battery, rpp diode, tvs diode, fuse, load --- all in series on the +12V supply line?
This diode should do the trick yes? Enough forward current capacity to handle device requirements while minimizing foward voltage drop....
h
Thanks again...now I just have to put all this crap a piece of board nicely and mount it in the enclosure.
RE: Battery Powered System, Overvoltage Possible!?
My preferred option would be a big ugly electrolytic capacitor placed right across the supply.
No voltage drop, and it will gobble up any voltage spikes of either polarity without raising a sweat.
RE: Battery Powered System, Overvoltage Possible!?
An electrolytic will cause the load voltage to slowly decay when switching off, which should eliminate the voltage spike problem at sudden turn off altogether.
So the electrolytic might not need to be very large at all to do the job.
RE: Battery Powered System, Overvoltage Possible!?
---|>|-------|~~~~|----
| | |
--- | \
- --- /
--- ^ \
- / \ |
| --- |
| | |
----------------GND------
Batt RPD TVS Fuse Load
Z
RE: Battery Powered System, Overvoltage Possible!?
RE: Battery Powered System, Overvoltage Possible!?