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Battery monitor/Alarm


Hi Tim

Just noticed this element of your query which no-one seems to have addressed yet…

Below 2V LiFePO4 cells risk irreversible damage. Recommended minimum working voltage is 2.5V. However voltage drops very rapidly as the cell nears exhaustion. The discharge curve is very flat, so voltage is a bit of a blunt tool for monitoring state of charge but setting the alarm at the point where the discharge curve starts to steepen should be about right. This will actually vary a bit with battery, loading conditions etc. but I’m going to start at 2.8V and see how that goes with my FT857.


I’m going to start at 2.8V and see how that goes with my FT857.

11.2 V is plenty for an FT-857. Mine works perfectly down to 10.5 V.



Hi Karl

Been running some tests today with my (fairly recent) FT857D using a variable bench power supply and it doesn’t switch itself off on receive until supply voltage is down to 9V, similarly for transmit though power output starts reducing below 11V.

How it’ll behave with the LiFePO4 battery will have to wait until I can get out into the field…

73 de Paul G4MD


No prob if they’re 4S packs. Below 8V is the time to start worrying :wink:


I use this gadjet, displays voltage cell and total voltage and allows configure a buzz alarma when the voltage arrives at 9v, p.e.

Only use for Lipo batteries.


The LED displays on these monitors look quite bright in the photos, has anyone measured the current they take? (can the brightness be turned down, or off altogether?)
This is probably insignificant in the model world, where motors pull a lot of amps, and the expected run time is minutes rather than hours.
On a summit, where you have turned off display backlights, chosen to use latching relays etc to save every last milliamp, and hope to be able to activate for several hours in total, it might be more of a consideration?

Just thinking aloud, but comments welcome!



Dunno, but the dB is amazing when they go orf on a summit


I"ll second that. They are LOUD and obnoxious. They will for sure get your attention on the summit.
Tim - K5DEZ


Mine remembers the set voltage - and they are very loud. There does not seem to be a way of changing brightness or turning the display off - not that I have tried very hard. Probably the biggest danger is leaving it connected to a discharged battery as it will take it back past the point of no return. Weight minimal :slight_smile: Voltage seems to mostly agree with the charger and a multimeter. 73 Paul


The Hasmat delivery has finally arrived after many delays.
The new LiFe batteries now have connectors and are on charge in a fireproof bag, and the monitors have been tested and set to 2.8v

Early indications show the couple of £ spent on each one is a worthwhile investment.
Easy to use, line up the black (neg) wire on the balance plug with the left pin on the monitor, and it bursts into life, it also tests the alarm piezo’s too - beware, they are loud.

On switch on the monitor confirms cell configuration, and then cycles through each cell before displaying the pack voltage.

There is a tiny switch on the opposite end of the monitor, that when pressed, beeps (loud) and shows the alarm voltage. Further presses of the button allow you to set the cell alarm voltage. Mine are set to 2.8v at present.

Whilst connected the display continues to cycle and the LED’s are illuminated.
Removing the balance connector appears to be the only way to turn it off.
Once reconnected, the settings appear to be correct.
I have added a label to remind me to press the button to check! - the factory setting is 3.3v per cell.

Hope this helps somebody save an expensive battery.

Next job is to build a housing for the new 8400’s, then hopefully be heard with the HF packer.





If you bring your smartphone, here is a bluetooth monitor that works great:

It doesn’t have an alarm but you can see exactly what your battery is doing. Only draws 8mA. Admittedly my power generator is much larger (.58kWh) and I have a solar charging setup, this allows me to watch net power use/charge. It tells me battery capacity percentage and estimated time remaining, with low battery cutoff. Very slick. Most important, it is also RF quiet. (I have no stake in this product, just a very happy user.)