How do you detect how fully charged a battery is?

Factual Questions
1

SSIA.

Thanks,
Rob

2

I bought a $20 battery charge detector from Radio Shack. It’s so useful, even my wife (who’s a gadget-phobe) uses it all the time. She even gets the red/black leads correct most of the time.

3

What kind of battery? Auto? NiMh? NiCad?

4

I wasn’t looking for a device to detect it, but rather how the device detected how fully charged it is. Is the technique different for different types of batteries?

Thanks,
Rob

5

a constant current is drawn from the battery and the voltage monitored, the time and the voltage changes indicate the amount of charge in the battery. each battery type and size would have their own values.

6

How accurate is it? And how do you draw a constant current?

7

It depends on the battery type. NiMH for example give nearly a full voltage reading until they die. Alkalines have a much more steady drop off.

8

Different battery chemistries use different charge endpoint detection methods. Lead-acid uses current reduction/voltage compensated for temperature. NiCd uses voltage inversion. NiMH uses the inflection point on the voltage curve.

9

How about Lithium Ion?

10

Lithium Ion batteries are generally charged with a precision constant-voltage charger set to 4.2v per cell. Once the current reaches some low value (like 3% of the rated current) the charge cycle is finished.

11

You plug it in to the device it’s designed for, and look at the battery indicator.

Seriously. There’s a monitoring circuit inside the battery pack that monitors the charge state. Some battery packs (like Dell laptop battery modules) have LEDs to display the charge state, but usually it only reports the status to the device it’s designed for.

12

How accurate is that? How does repeated charging/discharging affect it?

Thanks,
Rob

13

It varies quite a bit. What exactly are you looking for?

14

I wanted to know how well charge guages could work on electric vehicles.

15

Those cheap guages only measure the voltage. The output is analog and sometimes with red and green bands as a guide. For half the money you could have purchased a $10 digital VOM and get an accurate reading of the voltage.

The correct way to check would be to burn off the top charge of the battery with a 30 second discharge and then check voltage. You would want to know that the charged voltage of a NiCd is different from a lead acid battery and so on. I must point out that even this procedure only tells you the state of charge, not the load capacity of the battery itself. An old battery might only have a small capacity left even when fully charged. For most people, the digital voltage check is very useful.

16

If one wanted to construct a device for a Li-Ion-powered car that showed how “full” the battery is, how would you do it? Can you also tell the capacity of the battery?

I have heard that lithium batteries aren’t supposed to have a “memory” in the sense that recharging a battery that hasn’t been fully discharged will cause it to loose capacity (or whatever the term is). Is that true?

Thanks,
Rob

17

Battery charging regimes can be complicated, and even for the same chemistry type the manufacturers don’t always agree. That said, BatteryUniversity.com is the finest resource I know of - here’s some info on lithium-ion types:

Introduction

How to prolong lithium-based batteries

There’s other lithium-ion stuff on the same site, have a browse. But I think the second link is what you’re looking for sweeteviljesus.

18

Yes. Li+ cells do not have a memory effect, and so the technique of draining a battery almost completely then recharging it will do nothing to a Li+ cell but wear it out faster.