An E-meter measures the state of charge of batteries.
In a nutshell, it measures the current flow into and out of the battery pack. The thing is that the effective capacity of a lead-acid battery is dependent on the rate at which it's discharged. A simplified example:
A 100 Ah (amp-hour) battery can provide 1 amp for 100 hours, or 2 amps for 50 hours, or 5 amps for 20 hours. That's the theory. In actuality, the more current you draw, the lower the actual capacity you get out. So, you may get 1 amp for 100 hours, but 5 amps for only 19 hours, and instead of 50 amps for 2 hours, you'd only get 50 amps for 1 hour. This is known as the Peukert's Law.
Anyhow, an e-meter not only measures the total amps that go into (charging) and out of (discharging) a battery, but takes into account the rate that it's used, and uses Peukert's empirical formula to calculate the effective charge, giving a more accurate indication of the battery's true state of charge (SOC).
Short version: Battery Fuel Gauge.
A commonly used E-Meter for electric vehicles it the Xantrex Link-10. http://www.xantrex.com/web/id/237/p/1/pt/5/product.asp The Link 10 is a state-of-the-art battery bank monitor that uses sophisticated microprocesor technology to report all significant battery information.
Xantrex also make the Link-20 and the XBM e-meters.
- Build it yourself battery monitor for multi-battery packs http://home.earthlink.net/~evtkw/ Includes plans schematics and details - by Tim Wong
- Gordon Stallings battery monitor with schematics http://genki.home.ionet.net/BattMon/BattMonArticle.html
- PakTrakr http://www.paktrakr.com/index.html monitors individual batteries in packs of multiple batteries
- Smart Gauge (UK) http://www.smartgauge.co.uk/ Great technical info on how battery monitors work.
- http://www.rotordesign.com/s10/cellmon/cellmon1.pdf schematic for battery low voltage threshold indicator