Volt meter reading fluctuates between 40 and 41

Lightning P38

Active Member
I bought a $12 digital voltmeter from Home Depot to check my battery charge. The meter reading bounced between 40 and 41. Is that due to the meter, or is the battery voltage varying that much? Or the way I am holding the probes?

The meter does not display tenths of a volt, so I am thinking the reading flips between 40 and 41 due to a rounding error?

My old meter was analog and so I don’t know much about the digital ones. No instructions came with it.

I am planning on carrying on the bike for a few trips to see how my battery voltage changes at various distances. Will a digital meter withstand the ride in a bag, with some foam wrapped around it?

Thanks.
 

Alex M

Well-Known Member
Must be one of those that sell for $4 on Chi-bay. It rounds readings to the nearest digit, if it's 40.49V-40.50V it will fluctuate btw 40V and 41V. Poor meters also have no stability, readings change as the meter heats up or for any other reason. Could also be poor contact inside of meter wire leads. Could also be poor contact between the leads and battery, i.e. user's error.

A good meter must have at least 3 decimal points. Look into brands like Fluke, Greenlee, prices upwards of $30 on Amazon. My Greenlee comes with a (removable) thick foam case similar to smartphone cases only thicker, will withstand a fall from several feet height, unfortunately it's a bulky piece, not a pocket-size like $4 meters.
 
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harryS

Well-Known Member
This one? Not enough precision. You want it to read 41.8V. Not 42V,

Well, it might work for what you want to do. The voltage will eventually say 40V and when you hit the throttle, it might drop to 38V and then climb back. A voltmeter on the battery is not that informative ,most of the time.


bad_mater.jpg
 

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Lightning P38

Active Member
This one? Not enough precision. You want it to read 41.8V. Not 42V,

Well, it might work for what you want to do. The voltage will eventually say 40V and when you hit the throttle, it might drop to 38V and then climb back. A voltmeter on the battery is not that informative ,most of the time.


View attachment 46895

That is the one in the pic.

I won’t be hooking it up to the battery for a constant read out. I just want to check the battery level every 5 or 10 miles to get used to the discharge rate. And to check it against the display battery gauge. I think it will work for what I want. When I stop, I will unplug the battery and check voltage and then compare result to kit gauge.

If the voltage range on a 36 volt battery is up to 42, and the discharged voltage is 32 volts, I have a 10 point scale to gauge the charge level. I don’t know how much a battery at rest recovers on voltage once it is partially discharged.

Basically I want to know when the battery is 50% discharged, so I can turn around and head for home!!!

I rode 8 miles the other day, and the kit display gauge did not move off the full charge mark. There are 15 marks on the display gauge, so I do not trust it at all.

Ken
 

rich c

Well-Known Member
A $12 meter is not a precision instrument. Somebody is paying the factory about $5 or less to make it, and the middle man gets the rest. So really you are asking about a $5 meter. Harbor Freight gives them away free sometimes when purchasing another item. How accurate do you think those babies are? Plus or minus 1 volt is not an issue in my book, it's just a guide.
 

Alex M

Well-Known Member
Keep it clean, use it for a week and then take it back to Home Depot :). Meanwhile, order something with at least one 2 decimal numbers in 100V range, so that it will read one decimal accurately. Preferably not with a 9V battery.