EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Well, That Was Unexpected on April 17, 2015, 03:34:07 pm
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Apologies for the post length, this is all based on my electrical understanding. Please correct where need be, any advice greatly appreciated.
In reference to a typical SLA datasheet - I.e: http://www.master-instruments.com.au/cgi/ajax/get_file/59669/1 (http://www.master-instruments.com.au/cgi/ajax/get_file/59669/1)
The Problem: I'm trying to ascertain a \ the best way to determine how discharged an SLA battery is. In turn, allowing me to ensure the Depth of Discharge is kept under control increasing the life span of the battery.
Obviously the datasheet is helpful and used when designing a product where there are some design parameters about how long you want the battery lifespan to be and how long you want it to last before charging. However, I use SLA's for camping to power a whole host of electronics as required so my load varies and I can sometimes be required to go days without a charge. Hence, a battery meter would be great. Most meters I've seen use just the voltage which I don't like (more on this in a tick.)
The front runner for a pre designed pre made solution at the moment is this little guy: http://ki0bk.no-ip.com/~pwrgate/LLPG/Site/SLA_Batt_Mon.html (http://ki0bk.no-ip.com/~pwrgate/LLPG/Site/SLA_Batt_Mon.html)However, I don't like it because I'd like a bit more info, you can't change the low voltage point (though I'm sure this could be hacked) and, its a microcontroller solution. In regards to the last part I'd like to head down the more discrete path. Reason being that I want this thing to be rugged and able to handle going camping, hiking, accidentally getting too warm, a bit crushed etc.
The limitation and difficulties with designing such a device are that:
- The discharge characteristic curve is pretty flat. The area under the curve (i.e. integrating the function of that curve) yields the Depth of Discharge. If one were to colour in half of the area under that curve at say the 50% mark then the change in voltage would be relatively minor.
- the capacity of the battery is unknown. That is the extent it has reduced from its initial notional capacity due to age, storage conditions, nature of discharges etc etc mean that putting a figure on this is pretty much impossible to me?
If I were to use a micro controller I imagine it would be possible to continually measure the load I'm drawing and have it keep track of this over time. Inferring the amount of energy used and then giving a rough depth of discharge \ battery meter. I would just have to allow for a healthy margin of reduced battery capacity. But again its using a micro!
On a side note, this may later form part of a larger project that basically is an all in one camping power unit for my longer camping with a car trips- Multiple batteries, multiple solar inputs, inverter outputs, DC to DC converter power outputs for charging laptops etc. etc. Effectively knowing when a battery is fully charged (comparatively easy) and discharged forms a big part of this. In this situation a micro would be required so if that's pretty much the best way then so be it.
So any advice as to what other people have done?