25th August 2004 11:19 AM
DS2438 with 12V lead acid battery
I was just wondering whether anyone had tried using a DS2438 1-Wire Smart Battery Monitor chip with a 12V lead acid battery. The chip includes an 8 bit internal current accumulator, a 16-bit charging current accumulator and a 16-bit discharing current accumulator which can be used to calculate the remaining capacity inside a battery in Ampere-Hours. This current accumulator, however, is designed for batteries which have only a maximum of 2.5Ah capacity so I was wondering whether it was possible to maybe cheat the system into thinking it was a smaller capacitance battery. I already know how to convert the voltage to a measurable range (as the DS2438 requires 1.5-10v input) but am not sure about the current.
One idea I was toying with was to perhaps have a long variable for charge inside the master (in this case a PIC18F252 microcontroller) and every, say, 10 minutes read the values of the charging currents/discharging currents and add/subtract from the internal variable accordingly, thus hopefully enabling me to establish the remaining capacity.
The system I have uses a PIC18F252 microcontroller (programmed in CCS PIC C) hosting a Dallas 1-Wire bus to which a DS2438 battery monitor chip is connected. This chip is monitoring the status of a 12V, 84Ah Lead Acid battery from which the system (amongst other things) is powered.
Any thoughts/ideas would be appreciated.
29th August 2006 11:21 AM
Sealed Lead Acid Battert capacity monitor
I was very interseted to read your thread, although it is a long time ago.
I am developing a battery backup system composed of 5 sla batteries
12V 150AH type. I need to indicate the State of Charge of the battery pack.
I will require one DS2438 per battery and communicate via SMBus to a PIC18 type device.
You have used this device with a high Ah rating.
What kind of development time are we talking any tips?
Any help much appreciated
29th August 2006 12:45 PM
monitoring charge and discharge current is a matter of a known resistance and an op-amp ... or a pair of those with diodes, so you can tell if current is flowing out versus in.
the reason you won't easily find any chips offering this ability built in is related to the power levels involved. The manuf. assume if you need 150AH of batteries, you might try to draw 75A of current (for example) ... you'll need a very small resistance to avoid losing a lot of power at 75A, which means the voltage drop should also be very small.
for SLA batteries, I believe the 'industry' recommends testing the no-load terminal voltage as an indication of the state of charge. most SLA powered UPS offer a run-time calibration so the onboard processor can map the voltage curve over time, for the batteries installed. This way the processor has a pretty accurate idea of the capacity remaining at any given time.
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