How can I turn off the battery charging circuit in the Dash 1.2? I’ve pulled my hair out trying to effectively comprehend the API reference, but it’s a bit confusing with the 1.1 and 1.2 documents listed in the same general section – there’s the occasional blue box telling me that (some? all? calls) only work on HW 1.1, and other sections referencing the (no longer present on 1.2) hardware jumpers.
Background: I’m looking to power the Dash occasionally via a non-lithium battery source through the battery port, and also to run it most of the time via the USB port. So, I don’t want the Dash 1.2 attempting any charging activity, but I would happily like to monitor the voltage at the battery port and use the battery port as my “backup” power source.
I hope that makes sense, and I’d be grateful for any hints or help!
While waiting for Hologram’s reply, will a diode at the wire’s positive side of the non-lithium battery help you in your usecase? (To prevent charging voltage to reach the battery)
Just take note of the 0.7V drop across the diode.
I’ve read API:
• Charger.begin() - automatically starts even without calling this method (starting 0.9.2)
• Charger.end() - not deprecated in 1.2?
If voltmeter is at hand, maybe you can tinker on this method call.
Sorry for the confusion. Dash 1.2 charging is managed by the new charger IC without any software control. There is software reporting of charge and input power status. This was how the 1.1 was intended to work, with the software control only added to address flaws in the charging circuit. The Charger class can only be used to get the battery voltage and percentage on the 1.2. We don’t recommend attaching any other power source to the battery terminals except for a Li-Ion. See the charger datasheet for details on charge behavior. MCP73871 - Battery Management and Fuel Gauges - Battery Management and Fuel Gauges - Battery Chargers
Thanks, Arnold. I was able to run a quick test with 3 x 1.5 AA alkalines and a 1N5822 diode on the positive lead. The diode dropped about 0.3V out of the circuit, so with brand new batteries, I was seeing 4.2V at the JST. It booted and supported the board perfectly. Unfortunately (based on Erik’s reply), it looks the charger can’t be controlled via SW, and my beloved jumpers are no more on this version, so no HW control, either.
How much current does Dash 1.2 draw? I recall 2A as being the upper limit, but I’ve never seen anything that high in my testing. Any thoughts as to how I can keep current draw to a minimum (and still have modem functionality)? Big cap?
Since the new board is 3G capable, expect modem current requirement higher than the previous board.
2Amp is max during the < 1s modem transmission (send data). I also can’t catch this surge in my Volt/Amp meter.
My U201 board has 220uF/16V electrolytic on the external power line and it’s holding up well even at low voltage.
By the way, have you tried using L01 pin with 6V (4 alkalines)? Or with a 9V battery + LM7805 + 220UF. Just make sure that your USB is not connected to Computer when connecting battery to L01.
I was able to get decent results on the Dash 1.1 by powering at L01 using 3 x AA (Energizer brand) lithium cells – they’re a bit higher voltage than alkalines, coming in at about 4.9V fresh out of the bubble pack. The board and modem would hang in consistently down to 3.5V (usually a bit below). Dash 1.2 (as you and I mentioned) is more sensitive to low voltage.
@MichaelM What about a DC boost converter like Adafruit 1903? It’s a little tricky to monitor the battery voltage though, especially above 3.3V.
If I use the JST connector instead (and don’t connect USB/Vin), what’s the max voltage for VBATT? The new datasheet only lists the max current. Is the modem always voltage-regulated? (I think this was a jumper setting in Dash 1.1.)
I’m also interested in using alkaline batteries (3xD or 3xAA, e.g., Adafruit 3287) and monitoring the voltage. Nominally alkaline voltage is 1.5x3=4.5V but a fresh, unloaded battery can be slightly higher.
I considered that Adafruit boost converter, but it’s only 0.5A. Seems a bit light for SMS transmission, when the current can (briefly) spike as high as 2A.
Based on the description and datasheet, the 0.5A converter has an internal 2A switch (~2.5A peak limiting), and the output current varies with the input voltage, e.g., 0.5A+ at 1.8V and ~2A above 4.6V.
