Question on multimeters

[Jacko]

I have just gotten into using some small solar panels for my cabin , which is powering a fluorescent light. I also have a multimeter set up to check the voltage of the batteries which is doing a great job. I've had the meter set up and on 24 hrs/day for the last week or so , but now the battery in the meter is dying. I 'm wondering ,can I just attach a lead from the car batteries I'm using to power the building to supply the nine volt  juice needed by the meter?  The multimeter takes a 9 volt battery, not sure if a few more volts would hurt the damm thing. Or is there another solution ? Thanks

[TomW]

Jacko;

One comment:

Do NOT do it!!!!

It will release that necessary magic smoke from the meter.

Tom

[ghurd]

What Tom said.

And it is not the 12V that smokes it. It can not measure the battery that is powering it.

Could put in a little momentary contact switch on the meter.  Push the button and read the voltage.

Cheap meters with low batteries give some crazy readings.

G-

[joestue]

most meters use a floating supply.

Do a google search for "digital panel meters common ground"

I've found them for $9.95 before, but basic 3-1/2 digit meters are $2.95 if you find them on sale so there is no point in buying them.

If you want it to last 10 years install a lithium battery and find a meter with an auto-off option.

[tanner0441]

hi

I have a clip on DC ameter that eats batteries. I run it from a wall wart. ot two rechargable batteries if I have no mains supply.

[Jacko]

Well that answers my question , I really appreciate the replies. The meter is only a cheap one, $9.95 at Cdn Tire but it was doing a great job for me as I would just walk in the cabin, actually I call it a beach house or my shack at the beach(my wife just's calls it a shack) , look at the meter and read the voltage. Like Ghurd said , once the battery symbol started to flash, the readings were crazy.

So I have another question , if I went and bought a rechargeable battery, is there a way to rig up a small charger that works on 12 volts . I know I can plug in a store bought charger or a wall wart to the inverter and charge the battery to the meter that way, but isn't that just going around in a circle , 12 volts to 120 and then doesn't the charger just lower the voltage to the intended batteries that need charging? Electricity is not my strong point, but I'm learning, slowly , make that very slowly.

[ghurd]

We set one up with 10 AA or AAA rechargeable nicds.

Fed the AAs (at 12V) to a low drop out 9V regulator (7809), then that to the meter.

Charged them with a $12 (on sale) Harbor Freight 12V 1.5W panel (item #44768), and controlled the AA charge with a PB137ACV chip.

Pretty simple.  Nothing with more than 3 wires.

G-

[Madscientist267]

Isolation is the key.

If at any point you find either of the two supply leads from a prospective power supply that would be going to the meter with any DC continuity to either measured battery terminal, stop right there and go to plan x.

A wall wart works, although it will waste a little power, but provides the isolation to keep the meter from doing hookah dances.

[joestue]

Why do you need it on 24/7?

If you don't like using a momentary switch... you must isolate the supply, some people may have success with a wallwart but if the isolation is anything less than 10 Mohm it may affect voltage reading, not to mention the sensitivity of the damn things to EMI, you don't want an extra 4 foot antenna if you don't need one!

How about 2 square inches of solar cells and a super cap?

[kurt]

if you must have it on 24/7 then the real answer is to build a isolated power supply for it so you can run it off your 12v battery. but they do make 12v battery chargers that charge the rechargeable aa, aaa, c, and d batteries do a google products search....

[Jacko]

Thanks guys for all the wicked answers and ideas. I'm going to have to digest some of this stuff and get back to you when I update. Thanks again everyone!

[frepdx]

Make a voltage supply for you meter, don't feed it twelve volts if it's made for 9 volts.

A power supply can be as simple as a few 'dropping' diodes wired in series. Diodes have a relatively constant voltage drop that you can measure, so five cheap rectifier diodes wired in series will drop the voltage about three volts. You can use LEDs too, but measure the voltage drop as it will be different from rectifying diodes.

[ghurd]

Two problems with that.

It drops the voltage by a fixed amount, meaning the meter gets a pretty wide range of input voltage.

And it does not solve the isolation issue if it is from the same 12V battery.

G-

[windspeed]

could he use an analogue meter or a strip meter  from a motor car, no power supply needed

Windspeed

[Jacko]

I can't believe the expertise on this board, it blows my mind. If I never get this to work , it won't be because I didn't get the best help in town . I got a hold of a 1.8 watt solar panel for $5.00 , so I think I'm going to give Ghurd's method a try first . But right now , I getting ready for a overdue fishing trip in a remote location here on the island . Back in a few days , hopefully with a few salmon and sea trout. Let you guys know how it goes. Big thanks again.

Jacko

[ghurd]

AAAs are big enough, even in a super-crappy solar area.

Pretty sure we used 600maH AAs because we had them on-hand.

Could do exactly the same thing with a tiny SLA, 7AH or less, and we did.

In theory, a "7 cell 8.4V / 9V nicd" should be fine, but they didn't work out so well for us.

If the 1.8W PV looks anything like the $12 HF PV (formerly referred to as the $10 HF PV), you probably want to remove the frame to give the wire connection areas and seam between the layers of glass a good coat of some silicone.

Then reassemble it and add some more silicone at the frame and glass joint.

Rain gets at the cheap PVs pretty quick.

And under the "What?" category...

Probably want to wrap the factory PV wire with tape or something.

Most of those do not have UV (sunlight) resistant wire, as far as I can tell.

G-

[Bruce S]

G-

Just a thought here if it is one of those PVs, say a couple AAA or AA NiCds or any rechargables are used. Wire up the rechargables, and connect the $5.00 PV directly to it.

If it like those HF panels, there's a LED in it to tell of charging and I think dual use as blocking diode. The rechargables will hold the panel down to 9Vdc difference in charging current too low to worry about over charging if using the 600ma or above batts.

The old car panel meters get my vote!! can be gotten cheap even as a cluster, just won't need oil pressure guage :-D.

Seems too simple, maybe I need to flush out some old brain cells?

Cheers

Bruce S

[ghurd]

I do not trust the maH printed on most batteries.  Lucky most of my niCds are high quality, and have been tested for actual capacity.  

Direct wiring to a 1.8W panel would be fine, if the batteries are large enough to accept the daily current without overcharging.

I Tried to overcharge some ~2200maH NiMh AAs with a 100ma 12V 1.5W PV and couldn't do it under realistic conditions.

The 350maH HF AAAs NiCds would get beat to death.  

Not sure I believe the accuracy of their "700maH" AAs.

And since they are the same price, not sure why I would want their AAAs anyway!

Why I do not trust general battery info...

Their 8.4V / 9V NiCd is only rated 100maH.  They rate the "Rapid Charge" at 5 hours with "450maH".  And regular charge time as 16 hours at "150maH".  maH? What?

Just meaning I suggest people be careful.  And recycle those batteries.

G-

[Bruce S]

I agree, if you're going to buy rechargables get them from a known place.

I too make sure my NiCd/NiMh are able to handle what they read, otherwise back to the store they go.

You can now pretty much tell if the newer 9V are good by the weight, then better ones now read TRUE 9.6V and are rated at 230mAh and so far have tested true.

Cheers

Bruce S