Hello all. Looking to get some insight on how to compare the differences in voltages for charging some batteries. I had in mind to use a comparator, but most examples use a reference voltage set by some sort of resistor divider network, and I don't think that will work in this case.
So, i thought I would provide some detail on what I know, before getting to my question. I use lipo batteries in my r/c car. The ones I use are a two cell model rated at 7.4v. That is the nominal voltage.. as the individual cells can range in charge from 3.0 volts (empty) and 4.2 volts (full). That puts the pack at 6.0 volts to 8.4 volts. If you let the voltages get beyond those ranges bad things happen.
Here is a basic pack:
When you charge these batteries you apply 8.4 volts across the back, and attach a voltage monitor that watches the individual cells. It keeps them from exceeding 4.2 volts, as otherwise one cell could be 3.4 volts and the other 3.0 (bad).
Now, I would like to be able to charge two batteries in parallel. This is generally a fine practice, you keep the same voltage and just double the current. The downside is, when wired as noted below if the packs (or cells) are different states of charge in the 3.0-4.2 volt range you can get a large surge of current as they balance with their parallel cells in the neighboring pack
So, what I (think) i want to do is insert a wire-would resistor in the .5ohm range between the cells initially to keep the current flow between backs < 5Amps or so. Once the cells get balanced I want to kick in a relay to bypass the resistors for the duration of the charging process. This is what I had in mind for the final product.
So, that brings me to my questions:
I had though of using a comparator and some sort of zero crossing detection.. but I couldn't get my head around that. I have never used logic components, but look forward to trying them out. Any thoughts on a simple method to detect when the cells are withing .2 volts or so of each other and how to keep the relay 'latched' until the cycle is over?
Thanks in advance,
Rob
So, i thought I would provide some detail on what I know, before getting to my question. I use lipo batteries in my r/c car. The ones I use are a two cell model rated at 7.4v. That is the nominal voltage.. as the individual cells can range in charge from 3.0 volts (empty) and 4.2 volts (full). That puts the pack at 6.0 volts to 8.4 volts. If you let the voltages get beyond those ranges bad things happen.
Here is a basic pack:
When you charge these batteries you apply 8.4 volts across the back, and attach a voltage monitor that watches the individual cells. It keeps them from exceeding 4.2 volts, as otherwise one cell could be 3.4 volts and the other 3.0 (bad).
Now, I would like to be able to charge two batteries in parallel. This is generally a fine practice, you keep the same voltage and just double the current. The downside is, when wired as noted below if the packs (or cells) are different states of charge in the 3.0-4.2 volt range you can get a large surge of current as they balance with their parallel cells in the neighboring pack
So, what I (think) i want to do is insert a wire-would resistor in the .5ohm range between the cells initially to keep the current flow between backs < 5Amps or so. Once the cells get balanced I want to kick in a relay to bypass the resistors for the duration of the charging process. This is what I had in mind for the final product.
So, that brings me to my questions:
- Should I use two resistors as depicted, or just one for the whole pack.
- How to detect when the packs are near 0 volt differential to know when to engage the relay
I had though of using a comparator and some sort of zero crossing detection.. but I couldn't get my head around that. I have never used logic components, but look forward to trying them out. Any thoughts on a simple method to detect when the cells are withing .2 volts or so of each other and how to keep the relay 'latched' until the cycle is over?
Thanks in advance,
Rob
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