Is there any way to enable the LTC4015 output without a battery connected to the system?
Not easily. The LTC4015 has a battery detection algorithm which sources/sinks current from the battery input and watches for changes in voltage. With a battery connected, it is expected that the voltage will only change within a certain limit.
You would have to fool this check upon startup, and then your best bet to keep it running forever would be to configure it for a lead-acid battery since there is no timeout for that chemistry.
You might be able to reliably fool the battery detection check using an LDO on the output to create a steady voltage and emulate a battery. I see no risk in trying that. However, this is not necessarily something that we have tested or would advise.
Regardless, if you do decide to attempt this, it would be interesting to know how you did it.
Zack, thanks for the quick response.
I have one more question:
What does this controller do when a battery that is completely discharged (i.e. 0V on the battery output with the CHG/DSG FETs open) is attached?
The battery needs to see charge current in order to turn the CHG/DSG FETs back on. At this point, there would be voltage seen on the output.
As I understand it, these CHG/DSG FETs are internal to your battery pack as a form of protection, right?
If the FETs were to remain open, the LTC4015 would detect no battery and would not charge. However, the idea is that the battery detection test could pull the batteries out of their pack-protect state. That depends on the batteries, though. If that is not the case, you could consider adding a linear regulator or some other weak permanent source to pull your battery out of that state. This source could be set to output a voltage that is just enough to pull the batteries out of the state, leaving the LTC4015 to do the actual controlled charging.
I work with Jason who originally posted. If we were to use this as a Lead Acid controller, what would be the settable voltage ranges? Can we still do CC and CV?
Yes, you can configure the per-cell charge voltage in each charging operation state (absorb, equalize, CC/CV). Take a look at the VCHARGE_SETTING, VABSORB_DELTA, and VEQUALIZE_DELTA registers. The following table illustrates how they work together:
Yes, you can do CC/CV. There is also an absorb phase (which could potentially be skipped if you just wanted CC/CV). To get a feel for the lead-acid charging operation, take a look at the Charging State Diagram (Figure 8) on page 35 of the datasheet.
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