CubeCell UPS:adding secondary backup battery

I’m trying to understand the correct way to add a secondary “long term” battery to a CubeCell (AB01) for a standalone LoRa sensor (target is 1y to 2y continuos working time)

one of the sensors I’m using has a high drain current (2mA) and that is a hard limit I cannot change.

On CubeCell we can user solar+lipo battery and have a decent working time, but how can I add a secondary battery (i.e. an external lithium/alkaline + regulator) that will take over when the lipo battery is empty (for example in winter time?)

right now I’m using a voltage detector directly on the lipo battery and when I sense ~3.3v I enable the voltage regulator on the external battery which is connected to V3.3 on the CubeCell pin.

but I’m worried this is not the best way of doing an UPS for CubeCell because I suspect there could be a situation where the onboard CubeCell regulator is still pumping some power.

Is there a better place/pin where to sense for power input and trigger an external power supply?



Your idea is currently only the best(Battery voltage detection).

I have submitted your question to our hardware engineer. he will Design a circuit that automatically switches the battery.

We advise you not to do this. The consequences of this may cause the battery to be charged in reverse.

Hi Jason,
thanks a lot for your reply.

I’m using an NCP300 (3.3v version) which enables/disable the MCP1625 to activate the backup UPS.
Could you please explain how it can charge the battery in reverse mode? I do not understand your point.

[NOTE: VLIPO is the Cubecell Battery header, while Vbat-bkp is the external non-rechargeable backup battery]

I’ll wait for the schematic suggestion from your engineer.

thanks a lot

Hi, your circuit is the best way base on this board. The LDO enable pin is directly connected to power input pin in our board, so we can’t turned off the LDO.

Hi Xiao,
thanks a lot for confirming this.

Is there any plan to spin another version of the Cubecell AB01/AB02 board which will support external UPS (alkaline or lipo batteries) as a secondary battery (sort of spare battery) on top of the current rechargeable one?

we just need a pin to signal when the local LDO cannot provide 3.3v from the rechargeable battery so we can enable another regulator like in my pic above.

this will extend CubeCell to use cases where we have few mA of load from external devices that cannot be turned off (draining the main battery)… or in case the rechargeable battery fails (lack of sun during the day or battery damaged after long time)


also could you post a picture of where is the enable pin of the ldo on the CubeCell AB01 (and AB02)?

if it is possible to “lift” the enable pin of the onboard LDO I might rework the board with a blue wire and disable the onboard LDO when the external battery is in use. This would solve all my concerns for my solution…

Hi, thanks for your advice. It’s a nice idea, but we will not make it in the near future. If you have enough demand, we can customize it for you.

thanks a lot for your support Xiao and Jason.

One last request: is it possible to publish the schematic of the charging/power control part of CubeCell? (the one inside the white box in the current published schematic) this way I can properly do a rework for now.

Hi Xiao, Hi Jason,

I’m looking in disabling the regulator from the lipo battery when the battery itself is low (3.4-3.3v) so that I can enable the external alkaline battery as explained before.

@JasonXu: from your picture I understand I can lift that pin and disable the regulator but I want to make sure that we are talking about the regulator ON THE LIPO BATTERY and NOT the one on the main USB input.

my sensor will be always battery powered … so I need to disable the regulator inside the “white block” of the board schematic marked as “Supply Auto Switch System”.

Could you please confirm that the “enable pin” you pointed out is indeed controlling the power from the lipo battery and not the one from Vin ? since Heltec does not publish what is inside that “supply control block” it is difficult for me to verify…

my goal is to make sure that when I turn on the external MCP1625 nothing else is active on the Cubecell board. I want to make sure that there is only one battery regulator running all the time.

thanks a lot

The LDO " ON THE LIPO BATTERY" and “main USB input” is the same one.

In your design, the 10K pullup and LED will use up the backup battery.
I recommend

  1. use NCP300 output pin to drive LED or change the LED power to Li-ion as picture show.
  2. use a higher pull-up resistor value to reduce the current draw from backup battery on MCP16251 EN pin.
  3. increase the Rled value to reduce power consumption on Li-ion battery.

awesome! crystal clear.

thanks a lot ksckung!