Does a multimeter show amount of charge/ discharge?

[Learning Curve]

This is probably a really really dumb question, but today I got a multimeter, and I am wondering if this will give me a general idea of the state of charge or discharge of my battery?

It is a 36 volt 15 amp hour Ping Lifepo4 battery and I think the multimeter gave me a reading of 39.5 volts. I had to poke around inside the Anderson power pole connectors a while before I got any reading, but eventually did get this. Would 39.5 be about right for a almost fully charged 36 v battery? If not what would a fully charged 36 volt battery reading on a multimeter normally be?

I have never used a multimeter before and my charger fried the last time I charged the battery. It may have fried after the battery was fully charged or at some point before. It is possible it fried before it charged the battery at all. I have another charger coming, but meanwhile I don't want to leave my battery sitting with not enough of a charge for too long.

Testing the charge of the individual cells in the pack is way way beyond my abilities... I am just hoping the 39.5 volt reading might give me a bit of information on if I need to find another way to charge this while I wait for the new charger.

Sorry if this is something so basic it seems I should know the answers.

 

[e-beach]

A multi meter can tell you the volts of a pack. You can even use it while riding your bike if you know how to hook it up right. It is a tool you should learn how to use in your sleep. They are indispensable if you own an e-bike.

LiFePO4 cells usually charge to 3.65 volts per cell. 36 volt packs usually have 12 cells in series. So 3.65v x 12 = 43.8 volts fully charged. Sounds like yours is undercharged at the moment.

Why is your charger only going up to 39 volts?

 

[Learning Curve]

Thanks for the information e-beach! It is helpful to know what the voltage should be if the battery is fully charged.

And I think I read another discussion explaining what the voltage of each cell is when they are close to completely drained.... So I think knowing there is 12 cells I can figure this out.

Problem is probably the charger and I described that in another thread.

https://endless-sphere.com/forums/viewtopic.php?f=14&t=74217

The new charger should be here this week, I am just concerned about leaving my battery sit too long without being properly charged. The plug to the BMS is covered with protective layers, but if the voltage is getting low, I guess maybe I had better un peel it and unplug the BMS

 

[e-beach]

39v is ok for 36v LiFePO4. Providing all your cells are in good health. Your are going to have to learn how to periodically test them through your BMS though. Don't worry about being green, we all were at one point.

Your Ping BMS should have a connector on it with a bunch of white wires coming out of it. (they may also be red) can you take a picture of it and post it?

You may have to unrap your bms a bit to get access to testing it later, but for now can you show us the picture of the BMS.

Also, go to the other thread about your charger and lets learn about it.

 

[friendly1uk]

Look up some LiFePO4 discharge curves. Or charge curves. They both look the same anyway. Showing Voltage along one axis and charge along the other. It's per cell by that's simple maths for the owner of a multimeter.

Don't ignore the graphs and ask opinions. Opinion on this subject is very far from reality. So detached I can't settle here.

 

[amberwolf]

Assuming 12 balanced cells, then 39v puts each one about 3.25v, somewhere in the "flat" (not really but looks like it on a graph) part of the charge/discharge curve...so not easy to be sure of how much charge is left on them just by voltage. Could be somewhere near-ish full, or down toward empty.

If it drops down to around 34v or less, that's pretty well empty.

Some graphs:
https://duckduckgo.com/?q=LiFePO4+discharge+curves&iax=1&ia=images

 

[e-beach]

I have posted this video before. It is a bit long winded but comprehensive. That the time to watch the whole thing and you will be much more comfortable with a dmm.



[youtube]ZBbgiBU96mM[/youtube]

 

[999zip999]

Check your charger it should be 43volts. That's the first thing.

 

[Learning Curve]

Thanks so much for all the replies!

I did find and watch that video on using a multimeter! It was helpful!

I managed to find what I was looking for on one of Ping's webpages. (I have a Ping battery)

http://www.pingbattery.com/content/wiringguide.pdf

quote

"We recommend check its voltage at least one time every month. If voltage is lower
than its nominal voltage, please charge it.

Nominal voltages:
36V pack – 38.4V"

The page also says the nominal voltage of his batteries with different voltages.

I did look there before but didn't see it... Maybe I missed it or only part of the webpage loaded?

Anyways, it sounds like a mutimeter can show when I really need to charge the battery. And as is 39.5 I don't have anything to worry about yet, as the battery is just sitting unused waiting for the new charger.Which is what I needed to know!

Thanks again for all the information!

 

[999zip999]

I wouldn't like it lower than 37v in storage.