How to measure battery voltage with a good meter

[LewSchiller]

Hello,
Doing my first charge of the 48v battery from my Magnum Cruiser.
Prior to connecting the charger I wanted to know the voltage level in the battery as
it was with one "bar" left on the display. I put the meter across the + and - points on the battery plug
where it connects to the bike and got...nothing useable. I dunno...less than a volt?
I then tried connecting across the charge port connector - same thing.
So how can I measure the voltage of the battery out of the bike using a good meter?

 

[rich c]

How skilled are you with a multi meter? Using DC and the proper voltage range?

 

[LewSchiller]

Yes - skilled - fair question though. With the charger connected the +- connection ports read 51.1
With the charger disconnected...mv. The battery charge indicator shows 4 bars with the charger connected but that's probably irrelevant to the state of charge...just shows that it's charging.

 

[Trail Cruiser]

Check out this old thread. Another Magnum owner asked the same question as you.

https://electricbikereview.com/forums/threads/magnum-battery-connector.24616/#post-157521

 

[LewSchiller]

Check out this old thread. Another Magnum owner asked the same question as you.

https://electricbikereview.com/forums/threads/magnum-battery-connector.24616/#post-157521

Thanks...I have seen that thread as it comes up in search for volt meter and I have the connector he cites but inserting it into the charger socket - on the battery side - still doesn't provide a battery voltage reading. Tried an output socket test again just now as the battery has been on charge for 3 hours and same result. But I get a +- 51 volt reading with the charger connected. So I'm wondering if there's management circuitry in the battery case that's preventing a reading.
Further: In that thread he refers to measuring battery voltage at the charge connector. So I did that to no avail.
Put the battery back in the bike. Control display shows 51.1 volts. Connected the charger to the battery - shows same.
I have to be doing something wrong. In the bike the controller now shows 4 bars - full charge - but the charger is still showing red.

 

[LewSchiller]

Thanks for this!
That was the first thing I did. Pretty obvious - Positive and Negative poles. Didn't get a reading which is why I asked here.
I'll revisit my meter in the morning. Maybe it's an equipment malfunction.

 

[Deleted member 4210]

you probably already know this, but make sure the black cable is in the Com port on your meter.

Also, is your battery the newer version, where the button on the top, is now the master 'on' switch for the bike to work ? Up until mid year Magnum's batteries were such that, that top button was merely an indicator, to push down, and check the amount of battery capacity when its off the ebike. You could also use it when it is on the ebike, but you have indication on the display. But more recently, they are going to all batteries, having that button, now act as an 'on/off' switch, meaning your display or power to the ebike won't be on, until you turn that button on, and the LED's light up on it. Blix has this on their batteries, Smart Motion did too, and now Magnum is going to that.

So you may want to make sure that is 'on' if you have a more recent battery. Otherwise you wont get a reading on voltage.

I had two new Peaks sitting side by side, bought on the same order, and one had the new battery, and one had the older battery. Goofy to see at first, when they hadn't communicated this publicly to their dealers across the board. Its like random chance, learning about some of these things that the OEM's do.

 

[MarkF]

A battery at 80% discharged is 3.75 per cell. At 13 cells that would be 48.75. You don't want to see lower than this as this is 80% discharged and your battery longevity counts on it.
A fully charged battery is 4.2 per cell and at 13 cells your at 54.6.
I hope this gives some meaning to your voltage numbers.

 

[LewSchiller]

Ah Ha! Turn the battery on...of course. I put the battery back in place before turning in. I'll go out to the garage and apply this logic in the morning. Thanks Mike!
Also - Markf - thanks for the voltage specs!

 

[Trail Cruiser]

There is another thread that talks about battery state of charge and preserving life of battery.

https://electricbikereview.com/foru...-charge-battery-for-longest-life.14791/page-8

 

[AHicks]

Polarity not all that critical when taking a voltage reading. You're just going to get a positive or negative in front of the number you're really interested in. Unless you're doing something polarity critical, who cares?

My bike lives in a garage/work shop. The only time the battery is removed is when doing something like changing a controller or other major electrical work. Point being, many (most?) displays have a pretty good voltmeter built in. That's all I ever use.

I like to charge at 47-48 volts as well. There's not a lot left in them at that point, and common thinking has it that's good for the battery. I would mention though, that most controllers have a low voltage cut off built in to them (LVC), and that value is something closer to 3.0 volts per cell (40v total). That's the drop dead, don't let it get any lower than that because you're going to damage it value. Nice, because the controller's not going to let you....it's going too shut you down!

 

[MarkF]

3.7 is the absolute lowest you should go if you want your battery to live longer. If you drain your battery down to 3 volts per cell than the best advice I can give you is to charge it up as soon as possible. The battery is experiencing damage at anything below 3.7 so it helps if you get the voltage up as soon as you can to lessen the damage being done.
LVC is good and can help save you from damage. The battery voltage will drop under load so sometimes you can get away with 3. volt cutoff as the voltage will rise up to 3.7 when the load is removed. Sometimes.
Anybody who says it's ok to run your battery below 3.7 is giving bad info and should not be listen too.

 

[AHicks]

Yet thousands of these e-bike batteries are used every day without the benefit of "best battery practices" knowledge or the use of a voltmeter. The use of these not mentioned in any warranty I've seen either.

My point being, though I do practice cautions myself, running the bike per OEM directions is not going to cause the sky to fall - contrary to what some may have you think. My opinion.....

 

[LewSchiller]

Well you know what they say about new parents...first baby drops the pacifier they boil it. Third baby drops it they wipe it on their sleeve and stick it back in the kids mouth.
Turning the battery on is the trick. Embarrassing that I didn't think of it - thanks again to E-Bike Mike!
I'm at 52.4 after 3 hours charging last night. I'm going to leave it on for another hour this morning then run it later today. (I'm having the best time in a long time )
Having these voltage stats is a huge help. I do have some nerd cred ( Amateur Radio License N0LLL) but I've not been focused on Li-ion battery maintenance before this so I'm trying to get ahead of the learning curve.
Running it 80 to +- 30 seems to be the best advice I've heard thus far. Won't be hard for me to do that as I don't commute but rather pleasure/exercise ride close to home. I need to start taking notes on the voltage readings from my controller as I drop from bar to bar so I have some frame of reference.

 

[larry-new]

Arrrgh! Ok, guys, you've finally got me to do it..... 48v. batteries turned on, the one suspected to be at 50% reads 46.9...right there.
The one thought to be at 80%, still showing 4 of 5 bars on the bike...suprise! only 48.9, meaning it's about 62%. Interesting, this latter battery has been used the most, about 700 bike miles, whereas the first has barely been used at all.

You've brought out my inner metrologist you have...

 

[LewSchiller]

Arrrgh! Ok, guys, you've finally got me to do it..... 48v. batteries turned on, the one suspected to be at 50% reads 46.9...right there.
The one thought to be at 80%, still showing 4 of 5 bars on the bike...suprise! only 48.9, meaning it's about 62%. Interesting, this latter battery has been used the most, about 700 bike miles, whereas the first has barely been used at all.

You've brought out my inner meterologist you have...

Right...this is why I wanted to start taking measurements. The nerd in us is strong

 

[AHicks]

Right...this is why I wanted to start taking measurements. The nerd in us is strong

To that, I say simply, AMEN. Can't help myself some days....

 

[larry-new]

So with battery at 48.9v., two hours timed charging @2a. gives me 52.7v., or 87%. One concern for winter storage...will have to check monthly to see if % of charge holds or degrades.

 

[AHicks]

Yes, you like to use a timer, which seems like it would work well once you've gotten to know your battery. Good point how stuff could change as the temps drop.....

 

[larry-new]

My riding battery I keep at 80%, and only top off right before a ride, about two hours. Coming back with, say, 50%, I throw in three hours, which will get it to the 80%.

By the bye, I've seen a suggestion of using a USB fan (hooked thru the port) to get a battery DOWN, which makes good sense tor those of us in frigid climes whose next decent riding day might be a week or two away.