Thanks for the reply. So I can roughly estimate that my system is 50% DOD at around 99.2V and fully discharged at 96V?Short answer: You don't. You can't.
A bit longer answer: With some battery chemistries such as lead acid, you can make a very crude approximation, but even then you need to stop the load and wait a little. Typical guidelines for a 12V lead acid battery when idle (no load): 12.8V = full, 12.0V = empty. For lithium, forget it.
I decided when I got into my build that I didn't need any instruments that were not already in the car. I have a speedometer (so I dont spend a night in jail), a tachometer (because it was there), and the 12V original panel meter (so I can tell if the DC-DC has failed and I am running on the buffer battery) hooked up. For my SOC I just hit the trip odometer clear button when I turn the key after charging. The advantages of this are nobody who can drive a manual seems to be confused or alarmed by all the instrumentation. With LiFePO4 there doesn't seem to be a lot of reason for a voltmeter. It will tell you far to late to do much about it that you are out of energy. There doesn't seem to be a lot of reason to have an ammeter either. Amps is what you are commanding when you press the throttle. I am eventually going to turn the wattmeter technology I did in the early 90's for the electric RC aircraft fliers into a SOC meter to drive the fuel gauge but for now the odometer will tell me how far I have gone on this charge and that seems to have been good enough for the first 7 months.Hi guys, I have my car running now (finally) and currently just have a multi meter for instrumentation. I'm running 96V which fully charged is around 104.5V. How do I convert a voltage reading to know what it means in terms of 3/4 charge, 1/2 charge etc?
All of the other posters outlined why you cannot accurately do what you are talking about. My post is interpreting your question or what's behind it. You wish to cheaply and accurately determine what you have left without spending a lot of money. Assuming you own a laptop this rather cheap voltmeter has usb conversion to feed a laptop voltage information along with other useful data:Hi guys, I have my car running now (finally) and currently just have a multi meter for instrumentation. I'm running 96V which fully charged is around 104.5V. How do I convert a voltage reading to know what it means in terms of 3/4 charge, 1/2 charge etc?
A major problem with using a battery cable as a shunt is the temperature coefficient of the resistance of copper, which is about 0.4%/C. The cable can easily span a temperature range of 0C to 50C so if it is calibrated at 25C the reading can be in error by 10%. This may be OK for a rough indication, but you can get a decent 200A or 500A shunt for about $20.
The digital multimeter with USB interface may meet your needs, but even better would be a two-channel USB digital storage oscilloscope such as this, for less than $30:
But for $66 the following would be much better:
The fuel gauge function can be done by logging just ampere-hours, but it may be better to monitor watt-hours, taking the voltage into account. However, for lead-acid batteries, the effective amp-hour capacity is reduced as a function of current, so a high current drain would need to factor in the Peukert effect, and watt-hour measurement is actually even less accurate than amp-hours, since much of the wattage is lost in the battery and not measured.
This would be a fairly simple and useful project for a PIC or Arduino board. Otherwise, there are dedicated ICs that function as battery fuel gauges and charge monitors, and they are quite inexpensive:
I got a BQ34Z110PWR as well as an AMC1200SDUBR isolation amplifier from TI as free samples. But to use them you need to make a PC board and add various components, so the cost of the IC is negligible. The BQ34Z110 is about $4:
This one is designed for a single 12V battery and drives a 10-LED bargraph:
Probably beyond the scope of this post, however.