digital voltmeter question

Robin

Robin

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I have a digital voltmeter reading the service battery volts but not the starter battery. I have just ordered a cheapish digital led voltmeter to dedicate to the starter battery alone. I will wire this to the master +ve switch and master -ve such that it will only display when this battery is 'on'. But on a longish cruise that could still be several weeks. How much would this kind of display drain the battery just by being left on? The specs do not have this information Should I perhaps put a separate switch in it's leads?


This is what I ordered:-http://www.walmart.com/ip/47609740

My interest in the starter battery volt stems from our 'ex-spurt' wiring the VSR such that the very large service battery bank has first priority over any charge input available

TIA
 
If you have a multimeter, you could connect the DVM temporarily to a 12V battery with the multimeter in between, and measure the current that it draws and calculate the amp-hours this represents over the time you are talking about. I should imagine that it is a few 10s of milli-amps, but even that will add up over a month or so.
 
RichardS said:
Can you simply swap the connections to the VSR over so that the starter battery charges first?

Perhaps it's not that simple?

Richard
Click to expand...

I had asked for it to be that way round but Mr Spurt knew better and just did it his way, plus he fitted it where I cannot get to it ( 2 hips replaced plus a stroke took away my contortionist skills) so would have to pay him to come back. BTW I think the windlass also powers off the starter battery so again another reason I want to keep an eye on it's voltage. I wish I had done the wiring myself so would know for sure the logic but had to bow to circumstances at the time. I suspect the drain for the display is truly minimal but paranoia set in after I placed the order for the voltmeter and I thought somebody would know the answer. If all else fails anyway we have a battery combiner switch and can always start up using the whole lot as one
 
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rbmatthews said:
If you have a multimeter, you could connect the DVM temporarily to a 12V battery with the multimeter in between, and measure the current that it draws and calculate the amp-hours this represents over the time you are talking about. I should imagine that it is a few 10s of milli-amps, but even that will add up over a month or so.
Click to expand...

+1

If you can borrow a simple digital multimeter, set it to Amps and measure the drain caused by the voltmeter.

I installed a huge 300W power amplifier and 12 inch subwoofer in my Son's car yesterday. (Don't ask ...... it's ridiculous when your ears start bleeding!). When I tried to connected up the +ve battery terminal the lead started sparking like a errrr.... sparkler. And that was with the amp turned off!

I put the multimeter in line series with the amp and it read 120 mA. With a 45 Ah car battery you're talking a couple of weeks (45 / 0.12 / 24) before the battery is flat. :(

I jammed on the cable, ignoring the flashing and sparking and the thing seems to work fine, so I phoned the amp manufacturer this morning "Yes Sir, 120 mA is normal for our power amp. If your Son doesn't intend to use the car for a couple of weeks just pull out the 20A fuse which is mounted on the side because there is no on/off switch, as you've noticed"

If that's today's idea of sensible amp design then I must be really getting old. :ambivalence:

Richard
 
Robin said:
I have a digital voltmeter reading the service battery volts but not the starter battery. I have just ordered a cheapish digital led voltmeter to dedicate to the starter battery alone. I will wire this to the master +ve switch and master -ve such that it will only display when this battery is 'on'. But on a longish cruise that could still be several weeks. How much would this kind of display drain the battery just by being left on? The specs do not have this information Should I perhaps put a separate switch in it's leads?

My interest in the starter battery volt stems from our 'ex-spurt' wiring the VSR such that the very large service battery bank has first priority over any charge input available

TIA
Click to expand...

Check the current it takes as suggested by the other robin but Id fit a push button or a biassed off switch . You wont need it glowing at you all the time.
 
VicS said:
Check the current it takes as suggested by the other robin but Id fit a push button or a biassed off switch . You wont need it glowing at you all the time.
Click to expand...

I think the pushbutton idea is good, I will look for one, I think I might even have a redundant one in the shower compartment though what the original owner had it wired to I have no idea, as it has no leads to it now, possibly something else he purloined before handover, unknown to me.
 
Hi Robin, have a look at the input impedance of your digital voltmeter. They are normally around 20Mohm ie 20 million ohms. Using ohms law the current I=V/R this gives a current of 0.6microamps. This is quite a negligible battery drain. I'm assuming you're not powering the voltmeter from the 12volt supply, they usually have a 9volt battery.
 
philwebb said:
Hi Robin, have a look at the input impedance of your digital voltmeter. They are normally around 20Mohm ie 20 million ohms. Using ohms law the current I=V/R this gives a current of 0.6microamps. This is quite a negligible battery drain. I'm assuming you're not powering the voltmeter from the 12volt supply, they usually have a 9volt battery.
Click to expand...
Did I confuse you? The voltmeter I'm concerned about is new yet to be panel mounted LED one so not the digital multimeter that I have which yes does run off an internal 9v cell..

I know the drain is likely miniscule but years back I thought that of a gas sniffer alarm that I eventually measured at 0.5A after it drained a battery dead over winter ( it was still 'live' with all the master switches off)
 
My Sadler 29 had an analogue voltmeter fitted by a previous owner and permanently live. I was unhappy with this aspect and utilised a redundant on/off switch on the panel to make it switchable. I never detected any battery discharge effect which could have been attributed to the voltmeter in its original configuration but I was worried about a short circuit developing. I always check the voltmeter, which is switchable between the two batteries, immediately after starting the engine in order to check that the charging circuit is working.
 
philwebb said:
Hi Robin, have a look at the input impedance of your digital voltmeter. They are normally around 20Mohm ie 20 million ohms. Using ohms law the current I=V/R this gives a current of 0.6microamps. This is quite a negligible battery drain.
Click to expand...

I think you've overlooked the fact that this is an LED display, which needs power. Typically, these little voltmeters use about 50mA, not a lot, but still about 1Ah a day. Having a pushbutton switch so that it's only on when required makes most sense.
 
A bit of an off the wall idea, but solar panels are getting cheaper, so it may be worth considering fitting some. Admittedly my needs are pretty modest, with only a couple of hundred amp-hours of battery, but 40w of panels keep my batteries topped up even through the winter, replacing my usage over weekend by the next weekend. You may well need more, but you probably have more space than I do on my 24 footer!

Do that and your battery drain worries will just go away. My controller is set to top up the starter battery then, when it burps, turn its attention to the domestic one - I'd rather the lights go a bit dim than not be able to start the engine!
 
Robin said:
I had asked for it to be that way round but Mr Spurt knew better and just did it his way, plus he fitted it where I cannot get to it ( 2 hips replaced plus a stroke took away my contortionist skills) so would have to pay him to come back. BTW I think the windlass also powers off the starter battery so again another reason I want to keep an eye on it's voltage. I wish I had done the wiring myself so would know for sure the logic but had to bow to circumstances at the time. I suspect the drain for the display is truly minimal but paranoia set in after I placed the order for the voltmeter and I thought somebody would know the answer. If all else fails anyway we have a battery combiner switch and can always start up using the whole lot as one
Click to expand...

I initially wired our system to give priority to the domestic bank. Then I changed it to give priority to engine start as that seemed to be more sensible for my needs. However, I've added a few bits since then and will probably put it back to giving domestic bank priority. So it all depends on your setup.

I now have wind-gen + 145W solar charging start and domestic via 2 outlet regulator and alternator to all banks via VSRs (actually Smartbank advanced). I'm adding another 100W solar so that means start battery should always get plenty of charge from one feed from solar/wind and remain fully charged.

However, I suspect that it isn't a big deal for me apart from when the engine is on. If VSR gives priority to domestic when I start the engine that stops me trying to charge an already well charged start battery. Solar/wind will split the charge when VSRs are not combining the batteries but I'm not certain whether that will have much impact with my simple PMW regulator.
 
Rogershaw said:
If you want a low current consumption fixed volt meter go for a LCD and not an LED. The only problem is the LCD needs a back light to see in the dark.

Like this

http://www.ebay.com/itm/DC-0-200V-B...hash=item2a060412f2:m:mdP1i-9SnrKUcPLzykWExQA

or this

http://www.ebay.com/itm/Blue-LCD-Di...r-2-Wire-MZ-/170603446906?hash=item27b8c2027a
Click to expand...

So I need to start again from the beginning and ditch what I have ordered when it arrives, nice one. Akin to well if I wanted to get to Z I would not have started out from A ;)
 
Robin said:
I had asked for it to be that way round but Mr Spurt knew better and just did it his way, plus he fitted it where I cannot get to it ( 2 hips replaced plus a stroke took away my contortionist skills) so would have to pay him to come back. BTW I think the windlass also powers off the starter battery so again another reason I want to keep an eye on it's voltage. I wish I had done the wiring myself so would know for sure the logic but had to bow to circumstances at the time. I suspect the drain for the display is truly minimal but paranoia set in after I placed the order for the voltmeter and I thought somebody would know the answer. If all else fails anyway we have a battery combiner switch and can always start up using the whole lot as one
Click to expand...

Usually these battery monitor devices have a simple push and hold switch. Push and hold, readthe volts or amps, let go. There's no point displaying current when you're not looking at it.
 
As said I think you will find it will draw 20ma from 12v system to power the LEDs. I think a push button or switch would be well worth fitting. They are quite bright in the dark. These modern DVM of course use the same wires to power as to measure so no need for a 9v battery etc. I would have chosen one with more digits and LCD http://www.banggood.com/DC-7_5-20V-...el-Volt-Meter-VoltMeter-Module-p-1003608.html but I am not suggesting you give up on what you have bought. (at higher than Chinese prices). I have a few of these little ones like you ordered just love em but havn't actually put em to use. I paid less than 2 squid.
As for the poster with the 300w amp for son's car. (Dads do crazy things for their sons) It would almost certainly have a built in switch mode power supply to provide + and - about 100 volts DC for the amp. This is probably running connected to 12v continuously. I would figure out a way to fit a power relay so only powered when radio is on.
Anyway the large filter Capacitors like several thousand microfarrad at 100v filling on initial power up would explain the large initial current and make design of a relay power on a bit difficult. olewill
 
THANKS OLewill, In hindsight I should have looked harder but came across what I ordered on Walmart's online site and I had a $10 credit note to use up so made an impulse purchase. As it happens I have an expensive panel mounted Blue seas voltmeter reading the service battery bank volts and another the shorepower AC volts both red digital LEDs so i saw this one I ordered as a close enough match bearing in mind I will locate it about 6 feet away and by the battery master switches. Since it will only power up when the engine master switch is 'on' any current drain is only of concern if we are away a few weeks with all batteries turned 'on' but not regularly starting/running the engine. Another possible option might be to connect it to the starter keyswitch or via the instrument panel lighting switch, but maybe that might read a little lower voltage due to voltage drop, it being farther away and via the engine wiring loom. In any case I don't have the meter yet so still have time to think and play with my options.. I could even just install a local easy access point ( like a socket) to just use my digital multimeter on as and when.
 
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