EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Jovana81 on March 14, 2016, 06:57:00 pm
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Hello everyone, im first time here, altho i follow YT channel for some 2 years on daily basis so i think it's ok to introduce myself a little bit. My name is Jovana, im from Serbia (so excuse me if i do some chinglish here) :) Anyway, i design some hobby kind of stuff when someone want it for free, nothing professional (yet) :) My real job is distribution of high current actuators, motors and relays for some local company... enough off topic. i got those images of some heavy lead-acid baterry charger (0-30 V, 25 A), and what required from me to do, is implementation of capacity test option for the given battery, so full charge, full discharge (i mean not full, but to low voltage cut-off, let's say 10V), and full charge again would be entire process. on 2 pics u can see device, and on third schematic which i plan to use, with electro-mechanical clock parallel to load, for knowing time without constantly monitoring the discharge process :) Do you guys think i can add or remove something to improve it? Thanks, and best regards!
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Whilst I've never designed anything that uses >5A I think you may have to make things a bit more complicated (or rather 'less simple') than the diagram you provided. That circuit will turn on Q2 when the button is pressed, and will keep the load switched in until the battery's voltage drops to a preset value (between 4.7V and 10V) when it will disconnect the load.
What would probably be a better idea is a programmable dummy load that discharges the battery with a constant current, a timer (could be a mechanical clock?) and cut-off circuit similar to the one you provided. So I guess that circuit *could* be used to switch the constant current load. Its capacity will depend on the current during discharge, and you will have to set the cut-off voltage to a standard voltage that lead acid batteries use as their cut-off. Because you want to know the batteries capacity at say 0.5C (C = capacity in amp-hours) as well as lower currents, thats a lot of current, so your load will have to dissipate quite a bit of energy. As an example, my colleague makes dummy loads for discharge tests, and these are banks of 20W resistors, water cooled... the actual electronics is very simple, just uses an opamp to sense the voltage across one of these resistors and drives some heavy MOSFET's.
In terms of complexity, it isn't that complex, but in terms of components, you'll need some pretty beefy resistors and transistors for high current discharges. You coud discharge at 2A, but this won't tell you that much, and it'll take a very long time :)
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Thanks for reply, but if i instead of high power ceramic resistors put some car light main beam (or high beam in some countries), 60W @12V 5 amp roughly, could that bulb represent load?
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Meaning, if the battery rated 55 Ah sustain 5 A load for 11 Hours (perfect conditions) its good battery, less capacity means less good battery and so on... basically all i need is that same instruments on this charger read current and voltage, and its simple cuz its connected on same gretz bridge, which i can disconnect with added relays in this scheme (charge/discharge) mode with same 5 pin 12 V relays. but will this LM358 do well cut off V and constant current load, or i should add one UA741 for cut off threshold?
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I don't see anything wrong with using the LM358. Its output can get very near to 0v (its negative rail) so its fine for driving low side transistors, although you'll probably be better off with a power MOSFET, as bipolar transistors would require quite a bit of base current to get them to conduct 5A (power NPN darlington would need >20mA to switch 5A).
I think you could make a constant current sink with an opamp (say LM358, its fine, nothing fast here), an N-Channel MOSFET, a current sense resistor, and a voltage reference. Although zeners are cheap and easy, voltage references are cheap enough, so might as well go more accurate. That will drive your 5A headlight, with a fixed current of.. well 5A, or whatever you choose to set it as. Then use your cut off circuit to drive a relay that powers all this. A SPDT relay you can use one for the CC load, and one for a timer, which means your timer can be simple.
For more complexity, just a relay switching the battery's anode between the charger, and the CC load, so you can cycle it. In that case your cut-off circuit would also work (again, with a voltage reference instead of zener, all driving the relay)/
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Thank you very much, now i think i can manage this as very cheap and very useful upgrade... cheers :)