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| 18650 series discharge protection |
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| alxptt:
Hi! So I've been playing a lot with 18650 batteries, and I'm trying to design circuits that allow them to be used essentially like rechargeable alkalines. You take them out and plug them into a charger, then put them back in your thingy. The problem is, the load I'm using would just keep draining them well past the safety point of 3v or so, and also might discharge too quickly. I'm aware that there are series balancer circuits on eBay -- but are they the best solution if I intend to simply take the batteries out to charge? I'd like something as simple as possible, and they include charging circuitry that I don't need. Can anyone point me in the right direction for finding or making a simple discharge protection circuit? My additional question is, would I need balancing if all I'm doing is discharging the cells? That is, is there a risk of one cell being discharged faster than the others, or would they self-balance during discharge? If the latter is the case, is it possible that I could get away with a simple circuit that cuts the battery off at, say, 12V for a 4S battery pack? I realize this wouldn't have current limiting, but maybe I could achieve it some other way. My goal is to be as simple as possible. Any inspiration would be very appreciated! :) |
| IanB:
Why are you trying to reinvent a wheel? The market is completely saturated with the product you are contemplating. All the parts and assemblies you are talking about are made and produced in China at cost of a few cents per unit. What's more, consumer ready battery products are available from multiple sources that do everything you describe. I really don't understand your goals here. |
| alxptt:
Well, for one thing, I want to be able to understand how the components work. Simplifying my design as much as possible makes that more feasible, since I'm a beginner. Just slapping a generic module on might work, but honestly I'd rather use something I actually understand rather than a black box. |
| sleemanj:
The standard China solution is the DW01 Protection IC and 8205 Dual Mosfet, this combination provides over discharge (~2.5v) over charge (~4.35v) over **discharge** current and short circuit protection. Note that it does not protect against over charge current, just discharge, and also note that the typical over charge voltage cut off is quite high (there are different grades of DW01 with different cut offs, but from China who knows what you get). Both ICs are readily available for a few cents on the usual chinese sources, not a complicated circuit. Modules with them already made are also available, including circular ones which get welded and heat-shrunk onto 18650 cells to make them into protected 18650 cells. Ideally that is the best way, put protection circuits right on your 18650 cells. I doubt you can make a cheaper or simpler solution even if you had a couple of opamps spare in a circuit. |
| nathanpc:
You could just buy a simple DW01 module on eBay and be done with it, but apparently you really want to get a better understanding of the circuit which is a great thing. First of all, about the discharge balancing process you've mentioned. The cells are going to discharge at different rates because of the internal impedance of them, it is literally the same process that happens when charging, but in reverse this time, so when discharging a 4S pack to 12V your cells might be at different voltages and this problem just becomes aggravated when you charge the pack back up, so you do still need to balance your pack. Now to the discharge protection circuit, you can learn a lot from datasheets of ICs you might be trying to replicate. For example, if you take a look at the DW01A datasheet you'll notice that they have provided a block diagram: Of course it is a bit complex since the chip is trying to do multiple things, but if you focus on the left-hand side where the they are describing the over-discharge detector you can see that it's basically just a classic comparator circuit with a voltage reference. For example in this case you could use low-voltage opamps as comparators like the TLV272IP, or even better in this case, the MCP6292 which can work down to 2.4V. The circuit would be almost identical, but you could use a 1.8V standard red LED as a voltage reference and then just add a voltage divider to the cell so that 3V would equate to 1.8V at the input of the opamp. Then just use the output of the opamp to cut off the cell using a MOSFET. EDIT: As a simple example for you here's a very simple solution to cut off the load when the cell voltage drops below 3V, this is a very simple circuit of how to use an N-Channel MOSFET on the low-side and a nice base to start your experiments with other circuit topologies. |
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