There was a time when I worked for a company that sold an IC that was intended to monitor the voltage of a rechargeable lithium battery and report on the amount of usable charge remaining. Such ICs are usually referred to as “gas-gauge” ICs, although presumably gasoline is not involved (let's hope). I sometimes had customers ask if the IC could be used to monitor a lead-acid rechargeable battery. Sadly, it could not — at least not with a practical design.
You could probably make the existing IC work, but you'd need to add a few more ICs and some trimpots, so what's the point?
Lead-acid batteries are still are used extensively — in automotive applications, of course, and in UPSs (uninterruptable power sources). The UPSs range in size from the small units used in the home for a computer to the giant units used on server farms and in telephone company central offices. So you would think someone would address the needs of designers making Pb-PbSO4 -based equipment. As a quick reminder for those of you who slept through high school chemistry class, note that
Pb + H2 SO4 ⇔ PbSO4 + H2
partially describes the reversible reaction that occurs in a lead-acid cell.
As it turns out, someone has figured out how to monitor and report the charge status of these cells. Texas Instruments has released an IC, the bq34z110. TI is targeting the applications that I had in mind: UPSs, power tools, mobile radios, and small EVs (electric vehicles).
TI says the device will work “independently of battery series-cell configurations,” and that batteries “from 4V to 64V can be supported through an external voltage translation circuit that can be controlled automatically to reduce system power consumption.” The control to which TI refers is an I2 C or an HDQ port that lets the IC talk to the host computer.
The part can be used with batteries up to and above 65A-hr capacity — though I'm not sure from the data sheet how much above 65A-hr. And the part can be used in applications where charge or discharge current is up to or above 32A — though I'm not sure from the data sheet how much above 32A. To some extent, it's a function of the low-side current sense resistor's resistance and power rating. TI recommends a resistor in the range of 5mΩ to 20mΩ.
TI says the part can estimate capacity or state of charge (SOC) and compensate for temperature, age (the battery's, not yours), and for self-discharge (in effect, the battery's internal leakage). It does this via a sophisticated algorithm that TI owns the rights to. In part, the IC is measuring the open circuit voltage (OCV) or no-load voltage of the battery and comparing that with the voltage with a known load. See their application report, “Theory and Implementation of Impedance Track Battery Fuel-Gauging Algorithm” for more details.
The IC provides a simplified readout with four LEDs; if you need more precision, there is a technique via a port expander to add five or more LEDs. An Eval board is available from TI.
An IC that monitors that state of charge would not be very useful if it drew too much current in the process of making its measurements. The part draws 140μA maximum during normal operation. If you put it into (light) sleep mode, it draws 64μA (average); in full sleep mode, it draws 19μA (average). The part is available in 14-Pin TSSOP package.
Have you tried to measure the SOC in lead-acid cells? How accurate was the method that you used?