Accurately monitoring lead-acid battery state-of-charge would help with the long-term sustainability of off-grid renewable energy systems.
There are a number of ready-built products already available, mainly for the recreational vehicle market (eg. boats) and are relatively high cost (too high to include within a small off-grid power supply system).
There are also a number of ICs (intgrated circuits) which can perform this function. I wanted to review the few that I had heard about and see how applicable they might be for an off-grid battery monitor.
This ties in with my open-source charge controller project and also the battery monitor project, which has been on my list of things to do for a number of years. The addition of battery monitoring capability would be one of the most useful additions.
I have managed to obtain a few samples for various battery monitoring ICs and also found the data sheets for others. Here I will review the ICs I have found and include any data on testing them. If you know of any other ICs that might be useful, please add a comment.
Note: Prices were checked in November 2014 and might not be accurate.
From ST Semiconductors. £2.12 + VAT from Farnell.
These are designed for small portable consumer electronics with Li-Ion technology batteries. While not useful for a large lead-acid battery bank, this might be useful for some form of small Li-Ion solar lamp.
It measures Voltage, Current and Temperature with 12/14 bit accuracy and a 32kHz time base. Uses an I2C interface.
Current is measured through a shunt resistor, with a maximum input of +/- 80mV. Maximum battery current is 2.5A.
Maximum battery capacity is 7000mAh, hence it is not suitable for larger off-grid systems.
Edit 20/1/15: Thanks to Doug for emailing about the use of the IC in 12V battery monitoring systems with higher currents.
He mentions that the shunt resistor from the data sheet is theoretical 30mOhms, and with other shunt resistors different current ranges can be measured, as long as the maximum shunt voltage is <80mV.
The voltage can be measured with a potential divider to keep it within the range of 5.5V max.
From Texas Instruments. £5.54 + VAT from Farnell.
This IC is designed especially for Lead-Acid batteries. It uses a special ‘Impedance Track’ (proprietary) algorithm from TI. Works with batteries from 4-64V and battery banks greater than 65Ah. Uses an I2C interface. Can add an NTC thermistor for temperature measurement. Uses a 5milliOhm to 20milliOhm shunt resistor.
This device seems perfect for monitoring battery banks in the scale I am interested in.
I will build a prototype circuit board for testing and evaluating this device.
From Linear Technologies. £5.52 + VAT from Digi-Key
0-80V input voltage. 12 bit resolution for Current and Voltage. Data reported using an I2C interface. Maximum voltage across the shunt resistor is 102mV
This is not specifically a battery monitor IC, but is a power monitor. This can be used to measure power flow and hence calculate the state-of-charge of the battery. But it is only single direction, hence two would be required for correct battery monitoring.
There are a number of other ICs when you search for ‘Battery Monitor IC’, but nearly all of them relate to Li-Ion or NiMH technology and are designed for use in small personal products, such as laptops and phones. These include BQ2019, ENA2031, S-8225 and plenty of others. There are a lot of these due to the number of devices which use Li-Ion technology and the stricter charge/discharge requirements of Li-Ion technology.
There is only one dedicated lead-acid battery monitoring IC that I have found so far. Battery monitoring could also be implemented using a microcontroller, which I will also be looking into, but to get a reasonable accuracy then a better resolution ADC would be required (rather than the 10-bit ADC incorporated within most microprocessors.
I will build a test circuit and evaluate the BQ34Z110 IC as this looks the most promising.