Checking your M8 Battery

[marknorton]

A quick check for your M8 battery.

You need a voltmeter (get one from Radio Shack, Home Depot, B&Q etc) and some kitchen foil.

Put the battery on it's back with the contact slots facing you and pointing down. There are six slots, counting from the right, slots 1, 3, 5, 6 are "populated" with a contact.

Make two small "blades" out of the kitchen foil and slide into slots 1 and 5, again, counting from the right. Do NOT short the foils together, there's probably an internal fuse in there.

Measure the voltage between the two, negative to slot 1. On my 4 batteries, which are known to be good, the voltage varies from 4.14 to 4.21v.

[scott kirkpatrick]

My one battery is charging right now, but I'll check. How do you get 4.1-4.2 volts out of a 3.7 volt Li-Ion battery system? What do the other two contacts do and how do they do it? Usually there are only three contacts on a smart battery, and even then I have never figured out what they do. Does anyone know a website that shows how a smart battery is wired up?

The reason is that from time to time I have looked at the details of power management on interesting electronic devices (such as my E-1 DSLR http://www.cs.huji.ac.il/~kirk/Estim...onsumption.ppt ). There are some interesting lessons to be learned, that suggest best operating modes. But it gets much harder when there is no external DC power input which will allow you to operate under measured power without the battery. In my Ricoh GR-D the external power source (which I don't own) actually replaces the battery. In the M8 you would have to pretend to be the battery (and find a way to get the power cord through the base plate) to run on external power.

scott

[mby]

Originally Posted by scott kirkpatrick

My one battery is charging right now, but I'll check. How do you get 4.1-4.2 volts out of a 3.7 volt Li-Ion battery system? (...)

Hi Scott,

Please see for example http://xtronics.com/reference/batterap.htm for nominal vs. charge voltage.

Best regards,
Michael

[mike prevette]

The third contact is usually a monitoring point so the internal circuitry of the Li-on batteries can report back to the charger if there is a charging issue.


How to rebuild a Li-Ion battery pack

_mike

[scott kirkpatrick]

Originally Posted by mby

Hi Scott,

Please see for example Battery Backup Application Handbook for nominal vs. charge voltage.

Best regards,
Michael

That has a nice figure, but the charge voltage (14 v when you charge a lead-acid 6-cell battery) is what you measure at the charger. When I have measured the "float voltage" of a LiIon battery doing nothing, I have gotten more like 3.7 V in the past, hence my question.

scott

[scott kirkpatrick]

Originally Posted by mike prevette

The third contact is usually a monitoring point so the internal circuitry of the Li-on batteries can report back to the charger if there is a charging issue.
How to rebuild a Li-Ion battery pack

_mike

Thanks, that's a very nice reference. I like the full page of safety warnings. And it shows that if you are going for absolute maximum capacity out of a LiIon cell, you charge it all the way up to 4.2 volts and let it run down continuously to 2.5 volts. Many devices won't tolerate such a wide range at their supply, and stick closer to 3.6 or 3.7 volts. So let's see what Mark's voltage measurement gives when the battery is half discharged, or near bottom.

Most other battery systems give nearly constant output voltage under load until just before they run out, but since Li-Ion offers 3X the energy/Kg and 4X the energy/liter of the older
technologies, we should be prepared to deal with a little complexity.


scott

[marknorton]

The 4.1 - 4.2 was for batteries straight out of the charger. I've just measured the battery down to one bar on the display after yesterday's shooting and it's 3.77 volts but that is open circuit, would presumably fall under load.

If you measure the resistance between slots 1 and 6, it's about 11k ohms which might well be a thermistor.

[scott kirkpatrick]

Originally Posted by marknorton

The 4.1 - 4.2 was for batteries straight out of the charger. I've just measured the battery down to one bar on the display after yesterday's shooting and it's 3.77 volts but that is open circuit, would presumably fall under load.

4.2 down to 3.77 "float voltage" sounds like a slightly unusual way to run these batteries. I don't have camera or DVM with me right now, but will check tonight and compare with some cellphone batteries to see where they operate.

scott