Musings on LiPo protection

One of the big problems with Lithium Polymer batteries, and Lithium chemistry in general, is that if you get it wrong, it has a tenancy to bite! Overcharging, overdischarging, or piercing/scratching the cells can all lead to the lithium reacting violently and catching fire, and believe me, having used lithium, sodium, magnesium and caesium in the lab, reactive metal fires are NOT something you want happening in your radio!

Its one thing having a low voltage alarm monitoring module keeping an eye on your battery, but if your working an SSB transmitter, the current draw, and hence the cell voltages, can fluctuate rapidly, which could result in the monitor going into alarm fleetingly, and if your wearing a headset you might not hear it sounding. So, modifying the module to trip a cut-off relay is the way to go. But proper dual coil latching relays are expensive, especially with contacts rated for the sort of current I need to pass!

So, ive thought up a simple circuit that should allow me to do the cut-off with a normal non-latching relay. For this to work, the relay must be energized whilst ever the battery is at a good state of charge. Yes, this does mean that when not in use the protection circuit itself will slowly discharge the battery. To prevent this when the battery isnt being used, but still in the Clansman housing, a 'Trip' button will be provided that will de-energize the relay.

The concept is simple. A pair of NPN transistors control the relay coil. The supply to the coil comes through one half of the relays DPCO contacts. At start, the coil will have no supply. An 'Arm' button temporarily bypasses the contacts, putting bias on the transistor base and energizing the relay coil, this pulls in the contacts and the button can be released. A second transistor goes from the firsts base to ground, and is driven by the output of the alarm module. When the voltage drops to the cutoff alarm level, the alarm sounds, this transistor turns on, grounds the bias to the other transistor, which turns off, the relay de-energizes, the contacts open, and the battery output is disconnected. Attempting to re-arm the relay circuit will just result in the alarm module activating and tripping the protection again.

A 1N4148 or 1N40001 diode serves as back-EMF protection across the relay coil. A suitable fuse is also provided as close to the battery terminals as possible. A series resistor sets the voltage of the supply to the protection circuit suitable for the relay coil. It may be possible to eliminate this if a 24v relay is available.

I now need to find a suitable relay and to mock up the circuit to prove that it works as expected.